Endeavour's Last Adventure

Endeavour's first launch in 1992
image credit: NASA

In today's post I'd like to tell you about a little piece of history that I witnessed yesterday and although it seems like this has nothing to do with CANDELS, let me tell you otherwise. 

Yesterday, the space shuttle Endeavour flew piggyback on a special Boeing 747 over Tucson, AZ, at a height of about only 1500 feet (yes, that's pretty low!). This fly-by is one of many that Endeavour has made and will make on its way across the country from Houston to its final destination at the California Science Center.

Fly-by of Endeavour over Tucson, AZ
image credit: Janine Pforr

The Endeavour is one of the 5 NASA orbiters that ever made it into space. The others are Discovery, Atlantis, Challenger and Columbia. The Endeavour was built in 1987 (and 1988, '89, '90, and '91, as it took some years to built it) after the loss of Challenger during a launch accident. In May 1992 it took off for its first flight into space. Between then and its last flight in May 2011, Endeavour spent nearly 300 days in space while carrying out 25 missions. During this time, Endeavour orbited the earth 4671 times and traveled for 122,883,151 miles. In comparison, if you were to drive once around the Earth in your car (if that were possible), you would have only traveled 24,901 miles and would have to make the same journey another 4934 times to reach the same mileage. Or in other words Endeavour traveled 1.3 times the distance between the Earth and the Sun in its 19 years of service. It was planned that the last ever space shuttle mission would be carried out by Endeavour, but then it was beat by Atlantis and one last mission. During its time in space Endeavour met the MIR, the Russian Space Station, and helped transport men and material to built the International Space Station (ISS). It also carried the first female African-American astronaut Mae Jemison.

But there are a lot of other special facts about Endeavour. It was the first space shuttle that received its name from school children through a naming contest. They chose the name of Captain James Cook's ship "Endeavour" with which he crossed the South Pacific in the 18th century to observe the Transit of Venus in Tahiti. James Cook was not only an explorer by sea, but also an explorer of space as an amateur astronomer!

Endeavour piggyback on the special Boeing 747, Tucson, AZ
image credit: Janine Pforr

But I promised you at the beginning of this post that I would tell you the connection between Endeavour and CANDELS. Although Endeavour did not take the Hubble Space Telescope, with which CANDELS observes the night sky, into space it was the space shuttle that carried out the first servicing mission for Hubble. During this mission in 1993 the astronauts on board of Endeavour repaired the famous mirror problem which had left the HST's performance well below optimal. The astronaut crew installed a new wide field camera (number 2) which corrected the problem and in essence provided Hubble with a pair of glasses. So if it weren't for Endeavour and the other space shuttles (and of course the many astronauts, ground personell and scientists), we might not be able to carry out the research that we do with CANDELS and which relies on the excellent images taken with a space-based telescope!

So you can imagine that I was pretty excited to learn that Endeavour will fly over my head, also because I have never seen a space shuttle this close or on the back of a Boeing 747 for that matter. Many people assembled on the outside grounds of the University of Arizona to witness this spectacle around 11:15 am local time and everyone applauded the shuttle on its last travel. Today, the 747+Endeavour package will do several fly-overs across California, for example NASA's Jet Propulsion Laboratory and the San Francisco Bay area, before landing around noon in LA from where it will make a road trip to the California Science Center. Save final travels to you Endeavour and a well-deserved retirement!

The Dawn of Galaxies

One of the most exciting areas of astrophysics today is understanding how the very first stars and galaxies lit up the Universe.  This happened during the Epoch of Reionization (EoR), which was highlighted in the latest Astronomy Decadal Survey report New Worlds, New Horizons as the area of astronomy with the greatest discover potential in the next decade.  It is a central goal of CANDELS to probe galaxies in the EoR.

The Universe began in a hot Big Bang.  Early on, it was too hot for the protons and electrons to combine into atoms, because the high temperature made particles smash into each other too often.  So the Universe after about three minutes consisted of a fully ionized plasma -- that is, an admixture of positively-charged hydrogen and helium nuclei, and negatively-charged electrons.  Finally, after about 380,000 years, the Universe became cool enough that protons and electrons could bind together into neutral hydrogen atoms.  Thus began the Cosmic Dark Ages, so called because no sources of light were present, and all of the cosmos was enshrouded in a fog of neutral hydrogen and helium gas.

The Dark Ages lasted until a few hundred million years after the Big Bang.  It was then that the very first sources of light appeared, providing energy that ate away at the neutral hydrogen fog.  And so the Universe became ionized again, slowly and inhomogeneously, with electrons and protons being separated by energetic photons emitted by the earliest stars and galaxies.  We call this process cosmic re-ionization.  The Epoch of Reionization lasted until about one billion years after the Big Bang, and left the bulk of the Universe fully ionized and transparent as we see it today.  The EoR is the last major phase transition that the Universe undergoes, and it is the frontier of galaxy evolution studies today.

CANDELS has already detect a few EoR galaxies, which is pretty exciting in of itself.  But that's only part of the job.  What we really want is to understand what these galaxies look like, how they got there, and what they imply for the process of reionization. This is a job for the CANDELS theory crew.

So what do we want to figure out?

The biggest questions here are among the most basic:
a) What are the sources responsible for reionization? and
b) What is the topology of reionization?

While we know what galaxies look like today, there are good reasons to think that the first galaxies responsible for reionization might have looked quite different.  For one thing, like in a new house, there hasn't been much time for dust to accumulate. This is critical, because it turns out that star formation as we know it today requires dust as a catalyst. So how can stars form in the first galaxies with little or no dust?

The answer is: very slowly. But the stars that do form can be incredibly massive -- perhaps hundreds of times heavier than the Sun! These first stars, known as Population III stars, are copious emitters of ionizing radiation that can eat away at the cold fog of neutral hydrogen. On the flip side, because they form slowly, they are rare, so it's unknown whether there will be enough of them to power reionization.  Current thinking says probably not, but don't bet your first-born on it.

  This animation of a simulation from John Wise shows heavy elements (yellow) surrounding Population III stars after they have exploded.  

Moreover, these massive Population III stars have a short life, exploding in spectacular hypernovae after just a few million years. The details of these explosions are crucial:  Heavy elements like carbon, oxygen, and silicon are catalyzed in copious amounts during their short lives, and the hypernova could disperse them widely to form dust that quickly transitions star formation to the more familiar Population II (dust-catalyzed) mode. On the other hand, if the star collapses directly to a massive black hole, it would suck most of these heavy elements into oblivion, and the Population III era would continue for longer. Since Population III stars are no longer around today, it is difficult to see how they work in detail, and insights from models are often all we have to go on.

Even after the Population III epoch ends, it remains unclear whether there are enough stars to power re-ionization. CANDELS, as impressive as it is, only allows us to view the brightest of reionization-epoch galaxies -- Hubble cannot directly detect the fainter galaxies (this is what JWST will do). If there are not enough galaxies to provide the reionizing photons needed, it may indicate that there are more exotic contributors such as early black holes or an unexpected preponderance of Population III stars. Yet many models indicate that these faint galaxies are so numerous, that they actually dominate the radiation output! Clearly, tallying the total photon budget from CANDELS galaxy counts remains a poorly constrained yet critical aspect for understanding the sources of re-ionization.

This movie of a simulation from Tiziana di Matteo shows an evolving cube of the cosmos as sources begin to ionize the surrounding gas, eventually leaving a transparent Universe after about 1 billion years. Note the complex topology of filaments and sheets that houses early galaxy formation; this is known as the Cosmic Web.

As if those uncertainties aren't enough, there is the issue of topology. Topology refers to the spatial distribution, in this case of the protons and electrons.  While radiation from early galaxies can ionize hydrogen, the Universe is still sufficiently dense that the dissociated protons and electrons can quickly re-join back into hydrogen. This is a process known as recombination.

While recombinations are (cosmically) rare today, nature has perversely arranged the timescales for reionization and recombination to be annoyingly comparable during the EoR. This means one has to understand the spatial clustering or topology of protons and electrons, in order to know how often after being so cruelly separated, they will bump into each other again and re-discover their lost electrochemical bonds of love. This can be quantified by the clumping factor of protons and electrons.  If the clumping factor is high, it requires many photons to ionize a single atom, since protons and electrons remain close enough to recombine again after being ionized. If clumping is low, a single photon might be enough to keep an atom ionized. Hence we not only have to count how many photons are being emitted, but we also have to understand matter clumping in order to know how effective each photon is at reionizing the cosmic fog.

With all this uncertain physics flying around, it's not surprising that the EoR represents one of the most difficult modeling problems in astronomy today. Our most sophisticated simulations include all the complex processes we use to model galaxy formation at later epochs, plus the dispersal of heavy elements via outflows along with radiative transfer -- the emission and propagation of photons from cosmic sources. This last aspect is particularly challenging, requiring massive supercomputers to move not only mass but light around the simulated cosmos.

CANDELS theorists have developed a remarkable simulation code, called MARCH, capable of handling all these physical effects with essentially no simplifying approximations. Using MARCH, we have been able to show that the clumping factor is around 3, in contrast to earlier estimates of 10-30, and that CANDELS is directly detecting the sources that provide about one-quarter of the photons needed for reionization. While these results are encouraging, there remain many uncertainties in such calculations, particularly the escape fraction, i.e. the number of ionizing photons that escape from within galaxies. There is a long way to go before we can confidently model all the processes going on during the EoR.

Nonetheless, the EoR remains one of the most vibrant and revolutionary areas of study in the CANDELS team. CANDELS data provides the boundary conditions for theorists' models, while the models inform the interpretation of the observations. The recent CANDELS team meeting in Santa Cruz enabled the High-Redshift Working Group to assess where we stand now in both observations and theory, and how to best proceed in concert. Together, we are shining a new light on the Cosmic Dark Ages by peering boldly into the dawn of galaxies.

The 3rd CANDELS team meeting in a nutshell

Last week about 90 of the CANDELS team members came together at the University of California in Santa Cruz for the third annual team meeting. Throughout the week we provided some snippets of the meeting but let me wrap up the meeting with a summary.

As is typical for collaboration meetings, the week consisted of break out or splinter sessions and plenary sessions. Plenary sessions are attended by everyone and meant to update the team on the activities of the several working groups and the status of the survey itself. Splinter sessions on the other hand provide the working groups with an opportunity to get into more detail of on-going projects and plan future work. Most surveys undertaken by a large collaboration have different working groups. The working groups offer a communication platform to team members with a specific science interest to discuss their work among a smaller group and focus on their specific issues. Within CANDELS we have several working groups such as Multi-wavelength cataloging, structure and morphology, high redshift science, AGN science, Theory, Supernova science, UV science, SED-fitting and photometric redshifts, Education and Public Outreach, Junior Scientists and many more. Many of these groups had splinter sessions during this year's team meeting. 

The CANDELS team at the 3rd annual meeting in Santa Cruz, California. Image credit & copyright: Dale Kocevski

The team meeting started off with a splinter session for the SED-fitting and photometric redshift working group. Bahram Mobasher and Tomas Dahlen who are leading this working group gave us an overview of the recent photometric redshift and stellar mass estimations. Several working group members, myself included, presented their current work in this area in the morning. Most of the afternoon was then spent discussing these results and making plans on how to proceed and turn these works into publications. 

Tuesday was the first day of plenary sessions. As we told you in last Wednesday's post, on this day we mainly heard progress reports from the working group leaders on the achievements and goals of the working groups. We heard that the collaboration as a whole has already published 13 papers and 9 have been submitted and are in the refereeing process (scientific papers for publication are submitted to a journal and then evaluated by a referee who provides suggestions for improvement and clarification before the paper is published). Sandy Faber, one of the principal investigators for CANDELS, summarized the most interesting science that has come out of the survey so far. In Karina Caputi's paper a new population of very red, and dust obscured galaxies has been discovered of which you will hear more about in a future post. Another science highlight was the very heavily star-forming dwarf galaxies Arjen van der Wel told you about in his previous post. Of course there were many other interesting results discussed!

As leader of the education and public outreach working group I summarized details on this blog and provided some statistics on the number of viewers and readers we have already! I also listed some ideas for the future which we later discussed in our own splinter session on Thursday. We will tell you about these soon here, too!

As is tradition during conferences and team meetings, we also had a team dinner. Ours happened Tuesday evening during which some team members were honored for their special contributions to the team. And now we also finally have a team logo! You can read more about it in this post from last week. 

Speed brainstorming breakout session, image credit: Dale Kocevski

Wednesday was the second day of plenary sessions. We heard about the HST mosaics for the CANDELS fields and observation scheduling from Anton Koekemoer and Norman Grogin and about other catalogues for galaxy properties that have been produced by other team members. But it's not all about science with CANDELS data alone! Part of the morning was dedicated to observing proposals to use facilities other than the Hubble Space Telescope that are planned within the team. But all the good science needs some organising, too! So some of Wednesday was spent on planning out the telecon schedule, announcing up-coming meetings of some of the working groups, and suggesting a time plan for papers in progress. After all there is still a lot more science to do with CANDELS!

While the senior members of the team had their own executive council dinner, the Junior Scientists (i.e., students and postdocs) finished off the day with their own social event including wine and cheese.  This gave us a chance to get to know each other better and bring up any issues without senior members present. We also got some job-finding and application advice from some members of the team that just recently obtained permanent jobs and made the jump from a postdoctoral position to a faculty job. We could ask questions and got some real insights, and all of that while munching cheese, nibbling on some fruit and sipping wine!

Two happy principal investigators at the end of the meeting!
Harry Ferguson and Sandy Faber, Image credit: Dale Kocevski

Thursday and Friday were completely devoted to splinter sessions for different working groups. During the splinter sessions most time was spent on short talks and lots and lots of discussion on future projects. I attended the session for Structure and Morphology and led the session for the Education and Public Outreach working group. You already read a good summary of both of these last Friday. In parallel, the working groups for Star formation rates, AGN, UV science and spectroscopy met. Unfortunately, one cannot be in several places at the same time! Dale Kocevski will give you a summary of the AGN working group in a few days time. During the session for the newly formed star formation rate working group a lot of different star formation rate estimators were discussed. Since this working group is not very old yet, a lot of the session time was dedicated to discussion on possible projects and science that needs to be done. The UV working group discussed how to use the remaining observation time and planned the observation scheduling. They also addressed the data reduction for the UV data. In the high-redshift session team members presented their results on some of the most distant galaxies and how to decode their properties while during the spectroscopy session we got an overview over what spectral data is available in the CANDELS fields from other surveys. 

As another tradition, during the lunch break on Thursday, a team photo was taken. We already showed you the team photo from last year's meeting in Edinburgh, I won't tell you in which post, but can you find it? This year's photo is shown as the first photo above. This time, almost everyone looked into the camera smiling! That's quite hard to achieve with nearly 100 people.

Finally, the meeting was finished off with a few drinks after the last sessions before everybody got on their way back to their institutes to do all that amazing science that has been discussed and planned during the meeting. We are curious to hear about the results and to meet everyone again next year

Splinter Session Science

CANDELS astronomers listening to talks. Image credit: Janine Pforr

With the plenary sessions of Tuesday and Wednesday behind us, Thursday was a busy day of working group splinter sessions. Several of the different working groups got together in smaller numbers to discuss specific topics. This morning started off with the Structure and Morphology Working Group Session that I organized. To kick off the session I gave an overview talk about the status of the group, the papers we have been working on, and the catalogs we have produced. After this, we had a fun series of quick talks (ten minutes total for each, including discussion!). The purpose of these talks was to give the entire group a flavor of what everyone else is working on and the status of various projects. They also allowed us a chance to ask questions and discuss some interesting scientific topics. There were 17 of these talks in total, spanning a wide range of topics.

Our group will meet again this morning, this time to discuss future research topics. In particular, we want to decide upon the most important scientific questions that we should be answering in the next year with our rich set of data. The science that can be done is unlimited, so a discussion like this really helps us to focus on what would be the most interesting and the most useful to the scientific community.

During the afternoon, there were three concurrent sessions. It was tough to decide which one to go to! I chose to go to the session on Education and Public Outreach, lead by Janine Pforr. During this session, we mapped out strategies for how to proceed with this blog in the future. We brainstormed about possible post topics that would be interesting to our readers as well as ways to increase our readership. We also discussed some other project ideas and started to get organized about what information is needed and who could be appointed to coordinate. We think we came up with a lot of great ideas and you'll be hearing more about them in the future!

While we were discussing EPO, the star formation rate indicators group was also meeting. Unfortunately, I had to miss it but I got to hear a lot about it from my collaborators and it seems that they had some very interesting discussions on how to best measure the rates at which galaxies form stars. The stars that influence star formation rates the most are massive stars. Massive stars are very bright in ultraviolet light. You might guess that if we could measure the total ultraviolet light from a galaxy, then we would be able to measure the total star formation rate. This is only half true. It turns out that galaxies are not that simple. A lot of them also have dust, and that dust hides the ultraviolet light from galaxies -- just like a hazy day when light from the Sun is blocked. So if we only use ultraviolet light, we will underestimate the star formation rate. Fortunately, the hidden ultraviolet light isn't lost. It goes toward heating the dust and then that dust emits light in the infrared. Because of this, astronomers try to gather both ultraviolet and infrared light from galaxies to recover the true star formation rate of galaxies. This method is just an example. There are many other ways that have been proposed to measure star formation rates. Yesterday's discussion certainly helped us to get one step closer to obtaining the real number of newly born stars in distant galaxies. New data sets in the CANDELS fields, including far-infrared imaging from the Herschel Space Observatory and near-infrared spectroscopy from the WFC3 grism, will be immensely useful for addressing this question.

The third group that met was the theory working group. The group discussed some of their own data products, including catalogs based on simulations. These simulations are very useful for observers, so one of the main goals of this session was to discuss the best way to combine theoretical models with observations from CANDELS. 

Today, the last day of our meeting, several other groups will be meeting (AGN, UV, high redshift galaxies) to have similar discussions about their science results so far and their plans for the future. It's hard to believe that the meeting is almost over!

Day 2 of the CANDELS Team Meeting: Working Groups and the Social Side of Meetings

Yesterday's agenda at the team meeting was packed with a lot of interesting science! It was the first of two days of plenary sessions, meaning that for these two days everyone will be here and meet in one room. For the other days, we are meeting in smaller groups to discuss specific topics in detail. The bulk of today's agenda included summary talks from each of the working group leaders discussing what the different groups have accomplished so far and what their goals are for the future. As mentioned in our last post, CANDELS has several working groups on different science topics that work together to produce necessary data sets and write papers. For example, I lead the morphology working group and so gave a talk about the papers we have published so far and the data products we produced. It was quite useful to hear what all of the individual groups have been up to and what each is planning for the next year. CANDELS has published a lot of papers so far, but there is so much more we would like to do. The plans are ambitious, and there is a delicate balance between the important tasks we must each undertake (such as producing catalogs for the team to use) and writing papers.

We also had a very fruitful discussion at the end of the day about how various aspects of the team organization works, for example, how each of the groups communicates about their results to the rest of the team. We got to hear what works and what doesn't, and brainstorm ideas for ways to improve how the team interacts with each other.

After all of the talks were over, the team gathered together at a nice restaurant by the water in Santa Cruz for our team dinner. It is typical at a meeting like this to plan a group dinner on one of the evenings so that everyone has a chance to socialize and get to know each other a little better while talking about the meeting in an informal setting. This is often a lot of fun! The evening was made even more interesting by some announcements from Sandy Faber and Harry Ferguson, the principal investigators of the project.

The new CANDELS logo, designed by
Dale Kocevski and Nina McCurdy

Since the formation of the team back in 2009, various ideas have been tossed around for a team logo. Having a logo for a collaboration such as this is great because it gives the team a visual representation of what they do and can be used in various places. For example, logos are often used in presentations or included on posters. In order to come up with a good one, and have a little bit of fun in the process, Harry and Sandy suggested having a logo contest. So, everyone was free to submit their ideas for logos and encouraged to be creative. Many ideas were submitted and we all voted. Tonight, the winning logo was announced. The designers of our new logo are Dale Kocevski and Nina McCurdy. The logo itself is shown to the right. It depicts the Hubble Space Telescope overlayed on an HST image with the CANDELS team name written around in a circle. We are all quite excited to have a new logo to start using! What do you think?

Dale Kocevski, smiling at the team dinner after
receiving his awards

In addition to the announcement of the logo contest winners, Sandy and Harry gave out several unexpected awards to various team members. Dale Kocevski received an award for the most popular CANDELS paper on Vox Charta (a website used by astronomy departments for discussing new papers as they come out). Congratulations Dale! Two different awards were given for important contributions that have benefited the entire team: Audrey Galametz for the huge effort that has gone toward creating multiwavelength catalogs, and Tomas Dahlen and Bahram Mobasher for their work on comparing photometric redshifts produced by many different groups. Karen Pena was awarded for her incredible efforts in organizing this great meeting and Adriano Fontana for the greatest contribution to CANDELS team resources. And finally, this very blog received an award for its contribution to Education and Public Outreach.

In many ways, our meeting has just begun! Today we will have a discussion about our big picture science goals for the future and the papers that only a project like CANDELS can write.

Astronomers deep in scientific discussion over lunch
Photos taken by Janine Pforr

Kicking off the CANDELS 2012 Team Meeting in Santa Cruz

As I type this, CANDELS team members from all over the world are arriving in Santa Cruz, California for the third annual team meeting. Over 85 astronomers have registered and will be meeting for the entire week at the University of California, Santa Cruz campus starting this morning. And what a week we have planned! The schedule is packed full of talks about CANDELS science and discussion of plans for the future. In addition, we will have social events throughout the week, such as a team dinner on Tuesday evening and a special event for the Junior scientists on the team on Wednesday.

The week will be broken up into plenary talk sessions that everyone is attending and breakout sessions where smaller groups of us can discuss particular topics in detail. For example, I will be leading sessions for the Morphology working group on Thursday and Friday mornings. We will be discussing the papers we have written so far and then we will discuss those we are currently working on and plan to write in the future. There will be other sessions on AGN, high-redshift galaxies, theory, and various other topics.

All this week, and probably into next week, we will be sharing various aspects of this meeting here. Not only will we tell you about some of the science we are discussing, we will also describe some of our future plans and social events. For a real-time look at our meeting, search for #CANDELS2012 on Twitter. @CANDELS_team will be posting, as well as myself and other team members.

Astronomer of the Month: Boris Häußler

Each month we will highlight a member of the CANDELS team by presenting an interview introducing them and what it's like to be an astronomer. This month's Astronomer is Boris Häußler.

Tell us a little about yourself!

My name is Boris Häußler (Haeussler) and I'm a Postdoc (research assistant) at the University of Nottingham. I was born in Karlsruhe, Germany, a long time ago and lived there until I had finished school and had studied physics at the university for 2 years. I then changed over to the University of Heidelberg to be able to take on astronomy as a side topic in my studies (Karlsruhe is only really famous for solid state physics and although that's what my dad does, I don't get it). In Germany, physics studies end with a 12 months thesis, which I carried out at MPIA with Klaus Meisenheimer and Hans-Walter Rix. I got a bit lucky there because the satellite mission that I wanted to work on was cancelled on my third day, so, looking for a new project for me to work on, Hands-Walter mentioned this new HST project that he had started and so I slipped into GEMS.  Best thing that ever happened to me, I guess. After my Diploma, I continued working on GEMS during my PhD and I managed to get involved into STAGES, a sister project of GEMS, which basically gave me my first postdoc in 2007 in Nottingham with Meghan Gray, where I continued to work on STAGES for 3 years. I also met Steven Bamford who, close to the end of my project, started a project (MegaMorph) that was basically trying to enhance exactly what I had done for years, so that gave me my second postdoc. I am now starting to look for a new position and I will see where the wind blows me.

What is your specific area of research? What is your role within the CANDELS team? 

Generally, I work on galaxy evolution. More specifically, I have worked on blue spheroidal galaxies (which we think are an intermediate step between galaxy mergers, which turn galaxies into ellipticals, but still recent enough to contain young, blue stars) and the dependence (basically none) of galaxy boxyness/diskiness on galaxy environment. Technically, I have worked on simulating galaxy/survey images to then test galaxy profile fitting codes. I am currently developing a new technique that allows GALFIT (the actual fitting software) to use multi-band data simultaneously, thus down-weighting image noise and returning good values for many more galaxies as previously possible.
 

This technique enables us to do research in many areas of galaxy evolution that were previously not possible due to lack of a good code and good fitting values. I won't tell which ones, though, I want to do that work myself. As I am 100% occupied with the development of this new code, I have not actually done much work with CANDELS. I have created some images for people to test their codes on and helped to figure out some biases seen in the fitting data. Other than that, my main contribution is that I got the CANDELS and the GALAXY ZOO teams together (as I am a member of both), so we will have CANDELS galaxies classified by the public. Together with Jennifer Donley, I am also the junior scientists representative in the CANDELS team. 

What made you want to become an astronomer? At what age did you know you were interested in astronomy? 

I was always good at physics at school, it came naturally that I did something in that direction. At university I noticed that solid state physics wasn't quite my thing. I had a subscription to a physics magazine and I noticed that I only ever read the astronomy articles and then stored it away. I had also always enjoyed star gazing and astronomy pictures, so the decision to do astronomy at least as a side topic was an easy one. After my change to the University of Heidelberg, I did this and then decided to do my diploma thesis on an astronomical topic. Once I was in research, there was no way back for me because I thoroughly enjoyed it.

What obstacles have you encountered on your path to becoming an astronomer and how did you overcome them? 

I haven't encountered many obstacles yet. My career so far has been pretty straight forward and lucky. The job market is a bit of an annoyance, especially for longer-term jobs, which I have not gotten yet, but I stay optimistic about it. If a scientific career does not work out, I am very keen on outreach jobs as well. Ideally, I'd like to do a bit of both.

Who has been your biggest scientific role model and why? 

I don't really have a role model. I have more than a few people that I do NOT want to be like, but of course I am not going to mention anyone here. If I HAD to pick one, I could mention Carl Sagan. Although, being German, I didn't known of him when I was a kid, I found him very inspiring once I discovered him. He had a very easy but still accurate way of explaining things to non-specialists and I think this is an invaluable skill. Richard Feynman was the same way.

What is it like to be an astronomer? What is your favorite aspect? 

Being an astronomer is the coolest job in the world! It's fun to simply 'find out stuff' and look at things that no one has seen before. My favourite aspect would possibly be that I largely have freedom in what (and when!) I am doing. Also, traveling to nice places for conferences is fantastic. 

What motivates you in your research? 

As I said, 'finding out new things' is pretty cool. I currently run an outreach project here at Nottingham where we go to schools and the pupils reactions to astronomy and the shining eyes (at least for some kids) are a real motivation.

What is your favorite astronomical facility? (This could include telescopes or super computers, for example) 

Gosh! That's tough! I haven't visited many. Of the ones I have used, I would say UKIRT, mainly because of its location on Hawaii. Gemini next door is pretty cool, LBT is a great project, and VLT simply amazes me every time. I think in the future, the E-ELT will blow us away!

Where do you see yourself in the future? What are your career aspirations? 

That's a difficult question. Ideally, I would stay in a scientific research career, but with time for outreach activities on the side. If I had to choose between the two, my decision usually jumps from one to the other over a timescale of a year or so. Both are fun and I want to continue doing both. Certainly, in 5 years time, I see myself in a longer-term position, but I am open to where that would be. 

If you could have any astronomy related wish, what would it be? 

I would wish that telescopes of all kinds are built within their forecasted timeline and budget, because it would make the whole experience a LOT cheaper and faster. More telescopes mean more data to work with. 

What is your favorite, most mind-boggling astronomy fact? 

The emptiness of space. Once you have seen how empty space is even in a crowded place like the solar system, you cannot forget that. The universe is VAST and we're only a tiny living being on a tiny speck of dust.

Is there anything else you would like for the public to know about you or astronomy in general? 

Not really, I have to get back to work, I have a paper to write!

The Role of Mergers in Galaxy Evolution

Disk Galaxy: NGC 3370
Credit: NASA/ESA

When we look around us in what we astronomers call "the nearby Universe", most of the galaxies that we see can be divided into two basic groups. There are the "disk" galaxies, sometimes called "spiral" or "late type" galaxies, which are flat like a saucer. We of course live in a disk galaxy, and our nearby companion, the Andromeda Galaxy (M31), is one also.

Giant Elliptical galaxy M87
Credit: NASA/ESA

Then there are the "elliptical" or "early type" galaxies. These look more like round balls of stars from any angle, though they can be slightly flattened. Have a look at this previous post for more information and more  pictures of disk galaxies and elliptical galaxies. Also see this recent post for a discussion of how we measure and quantify galaxy type or morphology. Although it is not apparent just from looking at the images of these galaxies, disk and elliptical galaxies are different in several other ways besides their morphology. Disk galaxies also contain cold gas, which provides fuel for new stars, while elliptical galaxies don't have much gas and contain very few young stars. The motions, or orbits, of the stars within these galaxies are also very different. In disk galaxies, the stars and gas move around the galaxy on regular, nearly circular orbits, with smaller up and down motions, like animals on a merry-go-round. In elliptical galaxies, the stars move around with more random motions like a swarm of bees.

Ever since astronomers first noticed that galaxies came in these different types (which goes all the way back to Edwin Hubble), they have been wondering why. Are these galaxies different because they had different properties from birth? Or could something happen to galaxies to make them one way or the other -- were they shaped by their environment or even perhaps by a traumatic event? This is sometimes called the "Nature or Nurture" debate.

An important clue came from galaxies that don't fall neatly into either of these categories, like the ones shown above. They aren't very common, but we see them often enough to know that they could be telling us something important. These strange-looking galaxies tell us that sometimes, galaxies can interact and even collide. See this previous post introducing galaxy mergers.

Computer simulation of a merger of two disk galaxies
Image Credit: Cox et al. 2008, MNRAS, 384, 386

This inspired theorists to try work out in more detail just what would happen to galaxies if indeed they did interact with one another. We set up "particles" that represent stars and gas in two disk galaxies, for example similar to the Milky Way and M31. We also include the "halos" (extended spherical envelopes) of dark matter that we now believe surround all galaxies, and make up most of their mass (why we believe that is another topic for another day). Fortunately, we think that dark matter interacts with itself and with normal matter according to the usual laws of gravity, and doesn't feel any other forces, so it is actually relatively easy to program a computer to predict what it will do (even though we don't know what it is). Then we set the galaxies on a collision course and use a supercomputer to compute what would happen to the stars, gas, and dark matter as the galaxies move towards one another and eventually begin to interact. There are several nice animations of these kinds of simulations in previous blog posts -- here and here.

The picture above shows a time sequence of snapshots from such a computer simulation of a merger of two nearly equal-mass galaxies. The color scale shows the density of the stars, and the little number in the top left of each panel is the time that has elapsed since the beginning of the merger, in billions of years (Gigayears). The dotted line shows the trajectory of the orbit. The first thing you probably notice is the long streams of stars that are drawn out on both sides. These are called "tidal tails" and are caused by the same kind of tidal forces that the Moon exerts on the Earth (only of course much, much stronger). You might also notice that the centers of the galaxies seem to get denser and more compact. By the end of the simulation, 6 billion years later (remember we think the Universe is about 13.5 billion years old), the two galaxies have merged into one and the remaining galaxy no longer looks like a nice thin disk of stars -- it's a much rounder structure, more like the elliptical galaxies that we saw above.

What is actually happening here? There is a lot of space between stars in galaxies relative to the size of the stars, so the stars themselves do not collide with one another. However, gravity can perturb those nice circular orbits that the disk stars were on. Basically some of the energy from the galaxies' motions relative to one other gets transferred to the stars, scrambling the orbits and making the stars move around more randomly.

Rate of new stars born as function time during a galaxy merger. Image credit: Patrik Jonsson

The gas that was in those two disk galaxies is also dramatically affected by the interaction. The gas gets driven into the nuclei of the galaxies, and when gas gets dense, it can form new stars more efficiently. So the rate of new starbirth goes way up as the galaxies interact and merge. I've shown a little graph here showing the rate at which new stars are being born as a function of time, along with pictures of the merging system along the way. As you can see, the rate of new stars being born spikes up as the galaxies start to interact, and peaks when the galaxies coalesce. It then dies off again as the gas gets used up. In addition, some of the massive stars start to explode as supernovae, which deposits a large amount of energy in the gas. This can heat the gas up and blow it away, removing the fuel for further star formation. Observational studies have shown that galaxies in close pairs do seem to be forming stars more efficiently than isolated galaxies, which seems to support this picture. CANDELS will allow us to further study the connection between mergers and star formation, which will provide important tests for theories of galaxy formation.

Artist's depiction of an accretion disk around a black hole
Image Credit: A. Hobart (CXC)

There is another very intriguing possible consequence of galaxy interactions. If there are massive Black Holes lurking in the centers of the progenitor galaxies, the strong torques during the merger could funnel the gas so close to them that it would begin to be accreted onto the Black Holes. As gas approaches very close to the Black Hole, it forms a hot dense structure called an "accretion disk", which can glow very brightly (I've shown an artist's rendition here). These accreting Black Holes are called Active Galactic Nuclei (AGN), or Quasars, and have been the subject of other posts. Computer simulations of galaxy mergers suggest that these events could cause the Black Holes to grow very rapidly. Theorists have further suggested that the energy radiated by the accreting Black Hole could heat up the remaining gas and even blow most of it out of the galaxy! There is a beautiful animation of a simulation that tries to model that process here.

To sum up, we think that mergers can change disk galaxies from flat to round, scramble their stars from regular circular orbits to random orbits, and maybe can activate black holes that blow away their gas and shut off their star formation. So perhaps all galaxies were born as disks and some get transformed into ellipticals by mergers. It's a nice story, but a number of open questions remain. Do we see enough mergers? Could there be other processes that are important? If mergers cause black holes to shine, why don't we see AGN preferentially in disturbed-looking galaxies? CANDELS is helping us to answer these questions.

Summary of the General Assembly of the International Astronomical Union in Bejing

The International Astronomical Union XXVIII general assembly took place in Beijing, China during the last two weeks of August. The IAU General Assembly is perhaps one the largest astronomical meetings, gathering hundreds to thousands of astronomers working in many different topics: from stars and planets to galaxies, the interstellar medium, or cosmology. It only happens once every three years (can't wait for Hawaii 2015) and it holds roughly 12 different simultaneous sessions every day ranging from small joint discussions and special sessions to the large symposia. Here I'll try to present a brief (and extremely biased) summary of my experience there. Other astronomers with different perspectives are encouraged to complement my side of the story!

Banner for the IAU General Assembly Meeting in Beijing, China

The most interesting meeting for me was the "The intriguing life of massive galaxies" which included both an observational and theoretical perspective on the formation and evolution of the most massive galaxies. This symposium took place only during the second week, but there were other very interesting discussions during the first week such as special sessions on UV-emission in galaxies, cosmic evolution in galaxy clusters, and secular evolution. The latter featured very interesting discussions on the processes driving morphological transformations of disk galaxies and also quenching mechanisms related with these transformations. I specially enjoyed John Kormendy's summary on bulges (classical ellipticals), pseudo-bulges (bulges within spiral galaxies) and the processes involved in their formation, namely dissipational processes for the bulges and secular processes (bars, rings) for the latter, and their kinematic properties: pseudo-bulges are fast-rotators whereas bulges are mainly dispersion dominated (check his last paper and the 2004 review for more details). There were also observational results from the SAURON and ATLAS3D teams showing the motion of these structures for galaxies in the local universe. A quick note from the UV-emission special session, G. Bruzual presented some results from the recently updated version of their stellar population synthesis models (commonly used for SED fitting). Among the upgrades they will include improved libraries in the UV region of the spectra and a revised treatment of the thermally pulsating AGB phase to reduce its contribution to the NIR emission, bringing the values closer to their 2003 models.

Let's go back to the symposium on massive galaxies, where I spent most of my time. The session started with some talks about recent observational results on z~7-10 galaxies. Bouwens and Stark talked about the weak evolution of the specific star formation rate (sSFR, the star formation rate divided by the stellar mass) with redshift, and their efforts to improve their results including new data from deepest IRAC surveys. We certainly missed our CANDELS experts here because that was all the mention of very high redshift galaxies for the remainder of the week. On the cosmic-noon side, S. Wuyts, I. Trujillo and E. Daddi presented summaries on our current knowledge of the structural, kinematic, and star formation properties of galaxies at z~2 and the evolution of these down to z~0. Let's see if I can summarize some of the highlights mentioned in their talks (and many others) without making a horribly long post!

The most popular figure of the meeting was, without discussion, the stellar mass-size relation. Although it was intended to depict the sizes of all massive galaxies, it was almost exclusively used to describe the remarkable size evolution of non-star forming galaxies. Since the notion that the small sizes of these galaxies are robustly measured is now widely accepted, most of the discussion revolved around the possible growing mechanism, with minor merging being the most frequently advocated. Nonetheless, several speakers acknowledged that current measurements of the merger ratio seem to be insufficient to explain the observed size growth from z~2 to 1. It was also highlighted in the closing summary that the size evolution of the massive non-star forming population does not involve a single population, but a mixture of older and recently quenched (star formation has recently been halted) galaxies that are being continuously added, therefore making the interpretation even more complicated.

The second most popular figure (or referred term) was the so-called main-sequence that refers to the correlation between stellar mass and SFR. This relation has been observed up to z~3 with little change in the slope and an evolving normalization towards higher SFRs at early times. The importance of this relation was highlighted by both observers and theorists. The first showed increasing observational evidence of a secular, steady growth, phase for galaxies in the main-sequence co-existing with a starburst phase for galaxies with enhanced star-formation efficiencies (forming more stars per volume of gas), probably indicative of differences in the interstellar medium. Several talks highlighted the importance of the slope of the main-sequence and the relative fraction of galaxies in the starburst phase to characterize the evolution of the stellar mass function at both the most and least massive ends. A slope of unity implies that the galaxy mass function (the number of galaxies with a given stellar mass in a given volume of the Universe) simply shifts to higher masses without changing its shape, typically characterized by a Schechter function. Recent CANDELS results from Wuyts, Targett, and Kartaltepe were mentioned in the context of studying the evolutionary mechanisms that move galaxies from the main-sequence to the quiescent red-sequence after perhaps a brief transit through the starburst phase.  

Regarding talks on galaxy structure and kinematics, we saw really impressive results from SINS and the more recent MASSIV surveys using the NIR/IFU SINFONI to make velocity maps of star-forming massive galaxies up to z~2.  These surveys provide evidence of rotating disk structures in most of these galaxies, although the increased velocity dispersions suggest a more turbulent star-formation (along with short lived clumps) different from the more ordered disks observed at z<1. On the quiescent side, CANDELS results from Van der Wel, Bruce, and Mozena seem to identify a population of passive disks co-existing with the compact spheroids. This could place strong constraints in the quenching mechanisms and transformation processes, as we know that the majority of passive galaxies will become large ellipticals. Different velocity dispersion measurements for small samples of quiescent galaxies at z > 1 confirm that in fact these are 2 to 5 times denser than their local analogs as suggested by their small sizes. Van de Sande presented some new results from their XSHOOTER survey including a galaxy with a velocity dispersion above 400km/s! 

Before making this post too long, I'd like to finish this brief overview with a quick note on the theory side and the models for galaxy formation. Talks from Oser, Naab, and Johansson discussed what they call a two-phase formation scenario, which consists of the in-situ formation of a massive compact galaxy at z>~2 followed by a continuous growth in mass and size as a result of mergers and satellite accretion. Gabor and also Peng and Lilly focused on a more general process controlling galaxy growth and quenching through the evolution of the dark matter halo (its mass and temperature) and the galaxy environment. In this regard, and to conclude on the observational side, Duc showed some extremely deep amazing images from the Virgo NGVS survey reaching down to 29 mag/arcsec. With this kind of data it is possible to study galaxy-galaxy interactions with a high level of detail: enormous tidal features, minor merging, and even satellites of satellites around massive galaxies!

All in all, the IAU general assembly provides a wonderful venue to catch up on the most recent updates for your favourite topic, and also a fantastic opportunity to discover interesting results in other related topics. The large number of attendants ensures that you'll meet a lot of different people and make new connections, so I strongly recommend the experience. Don't miss the next one in Hawai'i 2015!