Monday, September 30, 2013

Astronomer of the Month: Dan McIntosh

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 Dan McIntosh

Tell us a little about yourself!

My name is Daniel McIntosh, Assistant Professor of Physics & Astronomy at U.Missouri-Kansas City starting Fall 2008. I was born in Columbus, OH (my parents met at Ohio State U.).  After moving around quite a bit early on, I spent my teenage and young adult years in Los Angeles. I attended LA Valley Community College and then UCLA, then I went to the University of Arizona for grad. school. I was a postdoc researcher at University of Massachuselts-Amherst for 7 years before moving to K.C. I have two great, college-age children who were born while I was still an undergraduate. I am married to a wonderful person and a fellow academic who shares my sense of adventure.

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

I am interested in the growth of galaxies over cosmic time.  In particular, I'm interested in the role of galaxy interactions and collisions, and I want to understand the physical processes that make the largest galaxies spheroidal in shape and dead in activity.  My role within CANDELS has mainly been in helping the Morphology Working Group set up the visual classification infrastructure. Additionally, I am mentoring two students who are investigating the evolution of spheroidal and disturbed galaxies from z=2.5 to today.

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

It wasn't until I was taking calculus and physics in college that it occurred to me that I might actually set my goal to become an astronomer. Yet, I always loved science and I found math to be fun from my earliest memories. When I was five I said I wanted to be an archaeologist (I really meant paleontologist but didn't know that word). I loved learning about the planets in elementary school and by the time I was 12 my uncle had given me his old 3-inch refractor and my grandparents had given me a subscription to Sky & Telescope. I read a lot of popular science books on astronomy and watched every documentary with the word 'space' or 'universe' in its title, but I could not imagine actually being an astronomer -- it seemed too much like a dream. When my college calculus teacher recommended The Mechanical Universe & Beyond -- an acclaimed video instructional series for freshman physics at Caltech -  the light bulb finally went off for me when I learned that the math I was enjoying was at the heart of the clockwork of the universe.

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

In retrospect, my largest obstacle was gaining the confidence to finish research projects.  I had no trouble with finding interesting projects, coming up with questions, and doing the hard work. What I got stuck on, and what can still trip me up, is knowing when you have done enough and have enough to say something of interest. It is important to gain this confidence and to avoid perfectionism. My advice to overcome this hurdle is: make a plan, use time management practices to assess your plan's progress regularly, and do not work in isolation. It is important to realize that all questions are valid; it is a trap to think that your question is dumb or that you 'should know the answer'. Keep on asking questions -- it is at the heart of what we do!
Who has been your biggest scientific role model and why? 

That's a tough question! I have several.  Pieter van Dokkum (Yale) for clarity of writing and productivity. Two close friends, Eric Bell (Michigan) for always asking interesting and tough questions and Stephane Courteau (Queens) for his enthusiasm for mentoring students. Finally, Carl Sagan and Neil deGrasse Tyson for their ability to communicate science to the world.

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

I love my career. As a professor, I have many hats, but teacher and scientist are foremost and on equal footing in my mind. I think that where these two intersect is my favorite aspect;I love working along side and mentoring student researchers from first-time undergraduates to experienced graduate students. I am also passionate about communicating scientific ideas and concepts to the general public whether informally or in a big 'Astro 100' classroom.

What motivates you in your research? 

I enjoy the process of seeking answers to questions -- it is lifelong learning in a nutshell. And I feel that there are so many unanswered questions about galaxies; the more we learn the more questions come to mind. I find this tremendously exciting.

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

The Hubble Space Telescope, of course!  It has revolutionized our understanding of the universe and made the past 20 years a very exciting time in astronomy.

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

I want to continue to inspire students for as long as I can. Having a positive impact on a young person's life is what it means to be a professor. And I want to continue to make my small contributions to improving our overall understanding -- after all, science is a collective endeavor.
If you could have any astronomy related wish, what would it be? 

That our politicians would collectively (all parties!) wake up and realize that federal investments in all basic science like astronomy research and facilities have always paid huge dividends to our country's status and economy, and thereby the small current investment should be simply doubled rather than further constricted. This would solve hundreds of issues and help our country in countless positive ways that are difficult to predict but are clearly important.

What is your favorite, most mind-boggling astronomy fact? 
That we are star stuff -- that the atoms that make up me and my loved ones were formed long before our world and Sun.

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

I really appreciate the public's engagement in astronomy; I think it is a wonderful thing.

Tuesday, September 24, 2013

UKY Public Lecture

On August 27, Sandy Faber presented a public lecture at the University of Kentucky. This lecture was scheduled to take place during the CANDELS team meeting and many CANDELS team members attended as well as the general public. We recorded Sandy's lecture and you can watch it here:

Tuesday, September 17, 2013

Measuring All the Light Since the Big Bang with Gamma Rays

How much light has been emitted by all galaxies since the cosmos began?

Most photons (particle of light) from ultraviolet to far infrared wavelengths ever radiated by all galaxies that ever existed throughout cosmic history are still speeding through the Universe today. If we could carefully measure the number and energy (wavelength) of those photons — not only at the present time, but also back in time — we would learn important secrets about the nature and evolution of the Universe, including how similar or different ancient galaxies were compared to the galaxies we see today.
That bath of ancient and young photons suffusing the Universe today is called the extragalactic background light (EBL). An accurate measurement of the EBL is as fundamental to cosmology as measuring the heat radiation left over from the Big Bang (the cosmic microwave background) at radio wavelengths. A new paper, called “Detection of the Cosmic γ-Ray Horizon from Multiwavelength Observations of Blazars,” by my former PhD student Alberto Dominguez and six coauthors including me (Joel Primack) was recently published in the Astrophysical Journal. This pathbreaking paper — based on observations spanning wavelengths from radio waves to very energetic gamma rays, obtained from several NASA spacecraft and several ground-based telescopes — describes the best measurement yet of the evolution of the EBL over the past 5 billion years.
This figure illustrates how energetic gamma rays (dashed lines) from a distant blazar strike
photons of extragalactic background light (wavy lines) and produce pairs of electrons and
positrons. By measuring how much the gamma rays of various energies have been absorbed
by this process we can determine the amount of extragalactic background light.
Directly measuring the EBL by collecting its photons with a telescope, however, poses towering technical challenges — harder than trying to see the dim band of the Milky Way spanning the heavens at night from midtown Manhattan. Earth is inside a very bright galaxy with billions of stars and glowing gas. Indeed, Earth is inside a very bright solar system: sunlight scattered by all the dust in the plane of Earth’s orbit creates the zodiacal light radiating across the optical spectrum down to long-wavelength infrared. Therefore ground-based and space-based telescopes have not succeeded in reliably measuring the EBL directly.

So, astrophysicists developed an ingenious work-around method: measuring the EBL indirectly through measuring the attenuation of — that is, the absorption of—very high energy gamma rays from distant blazars. Blazars are supermassive black holes in the centers of galaxies with brilliant jets directly pointed at us like a flashlight beam. Not all the high-energy gamma rays emitted by a blazar, however, make it all the way across billions of light-years to Earth; some strike a hapless EBL photon along the way. When a high-energy gamma ray photon from a blazar hits a much lower energy EBL photon, both are annihilated and produce two different particles: an electron and its antiparticle, a positron, which fly off into space and are never heard from again. Different energies of the highest-energy gamma rays are waylaid by different energies of EBL photons. Thus, measuring how much gamma rays of different energies are attenuated or weakened from blazars at different distances from Earth indirectly gives a measurement of how many EBL photons of different wavelengths exist along the line of sight from blazar to Earth over those different distances.

Observations of blazars by NASA’s Fermi Gamma Ray Telescope spacecraft for the first time detected that gamma rays from distant blazars are indeed attenuated more than gamma rays from nearby blazars, a result announced on November 30, 2012, in a paper published in Science, as theoretically predicted.
Now, the big news — announced in our Astrophysical Journal paper — is that the evolution of the EBL over the past 5 billion years has been measured for the first time. That’s because looking farther out into the Universe corresponds to looking back in time. Thus, the gamma ray attenuation spectrum from farther distant blazars reveals how the EBL looked at earlier eras.

This was a multistep process. First, the coauthors compared the Fermi findings to intensity of X-rays from the same blazars measured by X-ray satellites Chandra, Swift, Rossi X-ray Timing Explorer, and XMM/Newton and lower-energy radiation measured by other spacecraft and ground-based observatories. From these measurements, Dominguez et al. were able to calculate the blazars’ original emitted, unattenuated gamma-ray brightnesses at different energies.
The coauthors then compared those calculations of unattenuated gamma-ray flux at different energies with direct measurements from special ground-based telescopes of the actual gamma-ray flux received at Earth from those same blazars. When a high-energy gamma ray from a blazar strikes air molecules in the upper regions of Earth’s atmosphere, it produces a cascade of charged subatomic particles. This cascade of particles travels faster than the speed of light in air (which is slower than the speed of light in a vacuum). This causes a visual analogue to a “sonic boom”: bursts of a special light called Čerenkov radiation. This Čerenkov radiation was detected by imaging atmospheric Čerenkov telescopes (IACTs), such as HESS (High Energy Stereoscopic System) in Namibia, MAGIC (Major Atmospheric Gamma Imaging Čerenkov) in the Canary Islands, and VERITAS (Very Energetic Radiation Imaging Telescope Array Systems) in Arizona.

Comparing the calculations of the unattenuated gamma rays to actual measurements of the attenuation of gamma rays and X-rays from blazars at different distances allowed Dominquez et al. to quantify the evolution of the EBL — that is, to measure how the EBL changed over time as the Universe aged — out to about 5 billion years ago (corresponding to a redshift of about z = 0.5). Five billion years ago is the maximum distance we are able to probe with our current technology. There are blazars farther away, but we are not yet able to detect them because the high-energy gamma rays they are emitting are too attenuated by EBL when they get to us — so weakened that our instruments are not sensitive enough to detect them. This measurement is the first statistically significant detection of the so-called “Cosmic Gamma Ray Horizon” as a function of gamma-ray energy. The Cosmic Gamma Ray Horizon is defined as the distance at which roughly one-third (or, more precisely, 1/e – that is, 1/2.718 – where e is the base of the natural logarithms) of the gamma rays of a particular energy have been attenuated.

This latest result confirms that the kinds of galaxies observed today are responsible for most of the EBL over all time. Moreover, it sets limits on possible contributions from many galaxies too faint to have been included in the galaxy surveys, or on possible contributions from hypothetical additional sources (such as the decay of hypothetical unknown elementary particles).

(Note: this blog post was coauthored by Joel Primack and Trudy E. Bell.)

Thursday, September 12, 2013

Hubble's Law and Hubble's Legacy

In astronomy, everything in the Universe is moving relative to everything else. The Earth moves around the Sun, the Sun around the Milky Way, the Milky Way moves relative to the other Local Group galaxies, and the Local Group relative to more distant galaxies and galaxy clusters. Within the large-scale cosmic web we find bulk motions in every direction on the sky. 

Such motions can be measured using a variety of techniques depending on the objects of interest. For galaxies, this is typically achieved through the identification of known "lines" in their spectra, which shift from where they should be, where we measure them at rest in the lab. This is simply a "light" version of the known Doppler Effect for sound, where, for example, the pitch of a train goes up when approaching, then down when moving away, compared to the pitch you hear when you're on it.

In astronomy, the degree of this shift is known as either redshift (for galaxies moving away from us) or blueshift (for galaxies moving towards), as explained previously.

It was then a curious set of observations in the early 1900's that revealed that the majority of objects outside our own galaxy (then called nebulae, now known as other galaxies) were all moving away from us (i.e. redshifted), and in approximate proportion to their distance. This was explicitly seen in the pioneering work of Vesto Slipher in 1917, Knut Lundmark in 1924, and Edwin Hubble in 1929, amongst others of the time. 

In the figure below we reproduce the original "discovery" plot by Edwin Hubble, which has since come to be known as Hubble's Law, written as:

galaxy recession velocity = H0 x galaxy distance

The proportionality constant, H0, is called Hubble's constant and was determined by fitting a straight line through the data. Hubble estimated H0 = 500 km/s/Mpc at the time.

Edwin Hubble's "discovery" plot from 1929, showing that the distance to a galaxy (on the x-axis) is correlated with the speed at which it's moving away from us (called redshift, on the y-axis). Such a distance-redshift relation is strong evidence supporting the idea that the Universe is expanding.

The observation that every distant galaxy in the Universe appears to be red and not blueshifted is itself remarkable. In effect, it tells us that the motions of all galaxies beyond our local volume are in a direction away from us, and Hubble's Law tells us that the further away a galaxy is, the faster its moving away. This was, in essence, the first observational evidence of an expanding Universe!

That the Universe could be expanding was predicted by Einstein's equations of general relativity, as many of you may know. A somewhat crazy idea when first proposed, Einstein himself was unsatisfied with the concept of a dynamic space-time, which led him to update his equations with the famous cosmological constant, Lambda.

Although Hubble is solely credited with the discovery of his Law, closer examination of the literature shows a more complex history with no one single eureka moment by any individual. In fact, the redshifts that Hubble used above were entirely borrowed from Slipher's earlier work, and the distances that Hubble measured himself were unfortunately significantly flawed. The modern value of H0 is 67.3 km/s/Mpc, measured to about 2% accuracy by the Planck cosmic microwave background satellite.

As Edinburgh Royal Observatory Professor John Peacock recently argued, Hubble was perhaps somewhat fortunate to be able to demonstrate the relation given the data he had on hand at the time. However he was already an important figure in the community, very good at promoting the result, and the community of the day was equally as excited to accept it.

Regardless, suffice it to say that once the Law was established its ramifications changed our understanding of the Universe. The measurement of the Hubble expansion (and repeated confirmation over the years) heralded in the age of modern cosmology. It underpins our modern cosmological paradigm. And it is a key component to many of the observations and results that CANDELS produces using the Hubble Space Telescope.

More discussion of the Hubble constant and its use (and misuse) in astronomy data analysis can be found in my recently published paper, "Damn you little h!".

Tuesday, September 10, 2013

Drinking Bourbon, Talking Astronomy: the CANDELS Lexington Meeting

CANDELS team.  Photo courtesy: D. Kocevski

This year was my first ever CANDELS team meeting, and I didn't know quite what to expect. I knew there would plenty of discussion about galaxies, but after that I was unsure.  

I was nervous to give a talk to my largest audience yet. I was giving a presentation on my research about the structure and shape of galaxies with an active galactic nucleus (for more information on "AGN" check out this post) when these galaxies live either really close to other galaxies or far away from other galaxies. I eventually settled into a nice fluid delivery pretty quickly and realized quickly that 20 minutes really isn't that long.

Astronomers busy listening and working.
Photo courtesy: D. Kocevski
In addition to my talk there were many, many others. On subjects ranging from supernovae to large scale computer simulations of the formation and evolution of galaxies. Some of the most interesting presentations dealt with something called "candelizing" images. This process takes simulated galaxies and blurs them to a point where they resemble real galaxies that would be observed in CANDELS images (hence candelizing). When a galaxy simulation is performed, theoretically all knowledge about it is known.  

These "candelized" images can help with the visual classifications of galaxies, both by professional astronomers (in CANDELS) and amateur astronomers (through Galaxy Zoo). We can classify the simulated galaxies based on what they look like after "candelization" and then compare that to how they look in perfect resolution. This way we can use typical observational techniques when analyzing these galaxies, except that now we have the "correct answer" for all the physical attributes of the galaxy.

I singled out just one specific discussion but there were plenty more interesting talks.  After listening to all the talks I certainly learned a lot.  

Throughout the conference we splintered off into break-out sessions where experts in a very specific subfield of astronomy (such as AGN, spectrometry, galaxy clustering, data products, etc.) discussed the major problems and upcoming team papers. (During the AGN session it was fun to see my paper advertised as forthcoming very soon!)  There was also a small group discussing education and public outreach.  I learned we have an iPad app displaying the universe from very large scales down to the size of the Milky Way. The app both looks really cool visually and full of interesting facts. Be on the lookout in the app store for it in the near future.

Another grad student asked why isn't all research like the conference. Where we are constantly in contact with all of our collaborators. While I think it would be awesome for a day or two, I think everyone needs personal time to work on all their pet projects best done alone.

Basement cellar bourbon barrels, some aging over 20 years.
Photo courtesy: D. Kocevski
The (non-astronomy) highlight of the conference was definitely the bourbon distillery tour.  We visited Buffalo Trace's distillery in Frankfort, KY and got to see where "the magic happens".  We saw hundreds of barrels. (some of which have been aging longer than me!  I was afraid to ask how much those bottles of bourbon ran.)

Bottling by hand.  Photo courtesy: D. Kocevski
The tour guide also showed us the bottling plant, which is all done by hand. I asked one of the workers how many bottles a day they can fill and package and he said nearly 2000. If you're a fan of bourbon (or just seeing how things are made) I would highly recommend visiting the distillery.

After a week of astronomy my brain was full.  I don't think I could remember one more thing.  I had a great time and look forward to the next time we can all get together and talk about astronomy for a week straight.

Friday, September 6, 2013

In Search of the First Galaxies

The quest for finding the first galaxies is motivated by at least two main goals: 1) these galaxies are likely the building blocks for present-day galaxies, and 2) they are responsible for making the Universe transparent to light, a period commonly referred to as the reionization epoch. This epoch is one of the most important periods in the history of the universe  because it was when most of the neutral hydrogen in the Universe was evaporated. This event dramatically changed the  Universe forever. Before diving into the method of searching first galaxies, here is a brief summary of what the  Universe was like during the early stage of its lifetime.

 Color composite image of the three  newly discovered galaxy candidates at redshift ~7 (at this redshift the age of the universe is merely 800 million years). These galaxies were selected using a medium-band imaging survey called the Fourstar Galaxy Evolution Survey (zFOURGE). For more details about this discovery, please see Tilvi et al 2013
Immediately after the Big Bang the Universe was too hot and chaotic for electromagnetic radiations to escape. It was only after about 400,000 years that the Universe expanded and cooled enough (to about 3000 Kelvin) for free electrons to be able to combine with protons to form neutral hydrogen. Due to this attraction of electrons to protons, enough empty was created for light to escape and travel to large distances (and eventually reaching us) without bouncing back and forth, for the first time. This is what we see today as the microwave background radiation (light emitted in the microwave part of the electromagnetic spectrum). Even by this time there were no stars and no galaxies in the Universe. The Universe went into the "Dark ages" during which it was full of neutral hydrogen and we would have to wait until about 100-500 Myrs after the Big Bang for the first stars and first galaxies to form. 

The intense radiation from the first stars and galaxies likely ionized the neutral hydrogen and the Universe changed from an opaque to a transparent Universe. This important milestone is referred to as the "reionization epoch" which most likely occurred between 100 - 1000 Myrs after the Big Bang. Searching for the first galaxies and understanding how and exactly when did this dense fog of neutral hydrogen evaporate are among the frontiers of modern observational cosmology.

Lyman-break (dropout) selection method: Due to the presence of
neutral hydrogen in the intergalactic-medium, flux shortward (blueward)
of the Lyman-break gets attenuated cause a sharp flux-drop. This
break shifts towards longer wavelengths for more and more 

distant galaxies. For redshift~7 galaxies, the flux drops in the z' 
band while this object is detected in longer wavelength bands
(Tilvi et al 2013).
In order to study the early universe, there are  a few different probes that can be used -- one of them is star-forming galaxies. These galaxies provide a unique probe of the reionization epoch because such galaxies can be observed to very large distances or at very high redshifts -- redshift being a proxy to distance. Astronomers now have some observational evidence that the reionization of the universe occurred somewhere at redshift z > 6 at which the age of the universe was less than one billion years. Identifying galaxies at such large distances (or high redshifts) has been only possible recently due to the advent of near-infrared detectors that are efficient in detecting photons that are highly redshifted due to their extremely large distances in the expanding universe. Taking advantage of redshifts and some other galaxy properties, astronomers have devised a clever way of identifying high-redshift galaxies -- in an expanding Universe, the farther a galaxy is, the more its light shifted in wavelength (towards redder or longer wavelengths).

One of the popular methods of identifying extremely distant galaxy candidates is to use a technique called Lyman-break selection method  in which galaxies are selected based on strong absorption of galaxy's blue light (light at wavelengths shortward of Lyman-break wavelength) caused due to the presence of neutral hydrogen in the vicinity of the galaxy. This break gets shifted towards longer wavelengths for more distant galaxies. The figure to the right illustrates this technique --- a combination of broad-band filters (middle panel) used to identify z~7 galaxy candidates. The dropout candidate galaxy (top panel) drops out of the z' filter and is visible in all filters that are redward of about 9000Angstrom. The bottom panel shows a filter set consisting of medium-band filters that provide a better spectral resolution compared with broad-band only filter sets.

T-dwarf contamination: A T-dwarf template spectrum is overlaid on 
the medium-band photometry of a candidateT-dwarf. As can be seen,
 the medium-band photometry nicely traces the absorption features 
of T-dwarfs thereby helping to minimize the contamination of high-redshift
galaxy candidates by dwarfs in the Milky Way (Tilvi et al 2013). 
While the dropout method described above has been used quite extensively to identify galaxy candidates at relatively large distances, there are other objects (e.g., brown dwarfs within the Milky Way) that can mimic the Lyman-break drop. While such brown dwarfs can normally be identified based on their compact sizes using the excellent resolution of the Wide Field Camera on the Hubble Space Telescope, using only object sizes to distinguish stars can eliminate some genuine galaxy candidates because high redshift galaxies tend to be compact. Fortunately, medium-band filters (filters that allow photons with a smaller wavelength coverage compared with the broad-band filters) provide a great way to identify features that are associated with brown dwarfs. The figure below shows an example of a T-dwarf that mimics a dropout galaxy candidate. As can be seen the absorption features (or drop in the fluxes) are nicely traced by the medium-band filters.

Artist's conception of a Tdwarf (Credit: V. Tilvi).

Signatures of the First Galaxies
Defining a 'First Galaxy' is somewhat nebulous. Compared to our own Milky Way Galaxy, we expect the first galaxies to be small, less massive, lacking large quantities of heavier elements (often called metals in astronomy), and nearly dust free because these galaxies haven't had enough time to make all this stuff. But, whether the mass of the first galaxies could be comparable to the star-cluster masses and when we start differentiating between star-clusters and first galaxies is unclear. Some theoretical models suggest that the metal content of first galaxies could be anywhere between 0.000001 -0.0001 times that of the Sun, and that first galaxies might get 'polluted' within few tens of million years due to the supernovae explosions from massive stars, thus making it not dust-free. A better way to answer this question is to first find a 'first galaxy,' which should be very different from a normal star-forming galaxy (e.g. our Milkyway)  and then we will better know what the first galaxies look like.

Currently there are several candidate galaxies beyond redshift ~7 which are possibly among the first galaxies to have formed in the Universe. One of the major hurdles for finding these candidates and then confirming their real distances using spectroscopy is their faintness due to extremely large distances. Fortunately, newly installed near-infrared camera WFC3 on the Hubble Space Telescope has allowed astronomers to find candidate galaxies even up to a redshift of about 12 (at this redshift the Universe is merely 380 Myrs old). Confirming such candidates however, requires extremely deep spectroscopy (a method to measure accurate distances) which is currently not feasible from the ground-based observatories. We will likely have to wait until James Webb Space Telescope to be launched within the next few years.

Wednesday, September 4, 2013

Team Meeting Tweets

The CANDELS team meeting is complete! One way that astronomers can listen in on conferences that they cannot attend in person is to follow them on Twitter. We've tried this for a couple of our meetings and it can be really useful. Basically, anyone that is interested can write snippets about the meeting that they think are interesting and share them on Twitter with a common hash tag (in this case, #CANDELS2013). We have saved all of our team meeting tweets and shared them using Storify below.

In the coming days we will be writing a few posts about different aspects of the meeting. Until then, take a look at what we had to talk about during the meeting itself!