Tuesday, December 17, 2013

Project Astro Kick-off at Donaldson Elementary School

Like last year, I and many other astronomers around the US are participating in Project Astro. Project Astro partners a teacher with an astronomer. We introduced Project Astro in this previous blog post and told you about the workshop that is held at the beginning of each new school year at the National Optical Astronomy Observatory in Tucson, Arizona. In short: The goal is for the astronomer to help the teacher to bring astronomy and science closer to the children in the classroom. Various activities support this goal.

This year, I am partnered up with Donaldson Elementary School's 4th grade classes. Donaldson Elementary is a public school in the North West of Tucson, Arizona. Each of its 4th grade classes consists of about 30 children. Last week, I visited the 2 classes for the first time. When I entered the class room, the children looked at me with big, excited eyes. As a welcome present they had prepared a book for me with drawings of what they think a female astronomer/scientist looks like. It was really touching and their art work was great. Their female astronomers came in all shapes and sizes and forms and colours. I particularly liked the one that put me on the moon!

What does a female scientist/astronomer look like? Here are a few example images of what the children thought before meeting me. Image credit: Janine Pforr, drawings from Donaldson Elementary 4th grade pupils.

We started off with just a general question and answer session, so the children could get to know me a little better and ask anything they wanted to know. One hand after another shot up. And boy, did they ask tough questions! For example: "What's left after a Supernova?" or "How did the Universe start?" or "How long can a person live in space?" or "What is the biggest star?" (You can check for a list of the largest known stars here, they are nearly 2000 times larger than our Sun). It was great and I was really impressed with their sheer endless curiosity.

Phases of the moon. Try to find the right order!
One solution is below, but no early peeking!!
Image Credit: "The Universe at your Fingertips"
and the Astronomical Society of the Pacific.
After that, we started our first activity, the phases of the moon. At the outset I had every child draw how they see the moon. Some drew full moons, some half-moons, some crescent moons. All had craters on them. Some added aliens and the American flag and the moon lander module. We discussed how everyone's drawing looked different and then I distributed little cards that have different moon phases on them, which the kids had to order. Naturally, most of them ordered the pictures from smallest crescent moon to full moon, thus creating half a cycle. The pictures however showed a full cycle. By drawing their attention to the details in the pictures, i.e. craters and mares, I had them rethink their choice and order. With a little help all of them managed to get the order properly and we talked about how often the moon cycles through its phases in a year and how long one cycle roughly is. Next time, I think we'll talk about what causes the moon phases and the seasons.

That day, the kids were very sad to see me leave, but I am sure we will have a great next visit.

With all the curiosity and interest of the children during my first visit, I have to say that I had one special highlight of the day. During recess, one of the little girls came up to me and asked if I was a scientist, which I confirmed. And then she said she wanted to become a scientist, too, when she was grown up. I thought that was very sweet and also exactly what many of us want to achieve with outreach events like this. We want to interest more children in astronomy in particular and science in general and encourage young girls especially that math and engineering and science is for them, too. 

One possible solution for the right order of the moon phases. Image Credit: "The Universe at your Fingertips" and the Astronomical Society of the Pacific.

Friday, December 13, 2013

The Geminid Meteor Shower

We may be in the grip of some Arctic weather here in the US right now, but if you can stand to venture outside then the skies have a treat in store this week, in the shape of the Geminid meteor shower. Hyped as the best meteor shower of any given year, the Geminids reaches its peak this weekend (13/14 December).

So what is a meteor?

They’re also known colloquially as ‘shooting stars,’ but have nothing to do with stars. A lot of things in astronomy that are basically the same are given different names depending on how and where we see them. Space debris is no different. A small, solid body moving within the Solar System is known as a meteoroid. If that meteoroid happens to cross paths with the Earth, it burns up in the atmosphere, creating the distinctive streak that we know as a meteor. In exceptional cases, a large rock might not entirely burn up, and survives intact to hit the ground. The solid remains that hit the Earth’s surface are known as meteorites.

Space is full of debris. A quick look at the crater-covered moon is a good indicator of what the Earth might look like if we didn’t have the atmosphere to burn up most of what might impact us, and erosion on the ground to smooth over the damage caused by those that do. On any clear night, if you watch a patch of sky for long enough, chances are you’ll see a meteor.

What makes meteor showers different to these random occurrences is that they’re highly concentrated – a lot of meteors all originating in the same place – and that they occur regularly, at the same time each year. This is because the Earth is moving through space as it orbits the Sun, carving out the same path every year, and so at the same time each year we hit the same particularly intense patches of debris.

Most of these debris patches have been left by comets. Comets have been observed for as long as astronomical observations have been recorded, and often viewed as divine messengers or omens. One of the earliest recorded sightings was in China in 240BC. The same comet was also recorded by the Babylonians and in medieval Europe, and is even featured on the Bayeux Tapestry. It wasn’t until 1705 that Edmund Halley realised these sightings were of the same object: it now bears his name, Halley’s Comet, and is due to pass the Earth again in 2061.

Comets are small bodies made up of ice, rock and dust, thought to originate in the outer reaches of the Solar System. Some of these ‘dirty snowballs’ are pushed towards the centre of the Solar System, where they enter into highly elliptical orbits that see them pass close to the Sun before shooting off back to the outer reaches of the Solar System. As they approach the center of the Solar System, radiation from the Sun causes some of the material in the comet to melt and vaporize; this gives rise to the characteristic tail. Consequently, the tail always points away from the Sun.

Forging paths through the entire length of the Solar System is dangerous work, and not all comets survive the journey intact. In 1994, Comet Shoemaker-Levy 9 collided spectacularly with Jupiter. More recently, you may have seen Comet ISON in the news as it ventured into the inner Solar System. ISON made its closest approach to the Sun (called perihelion) on November 28th 2013, but is believed to have disintegrated as it whipped around the Sun.

Those that do survive their journeys, however, are not good at cleaning up after themselves. There are still debris trails from several comets that have crossed the path of the Earth’s orbit in the past, and each time the Earth reaches that point in its orbit – once per year – we collide with this debris, which burns up in the atmosphere to produce the streaks of light we call meteors. Comet Halley mentioned above actually intersects the Earth’s orbit twice, and its trail is believed to give rise to both the Eta Aquarids in May and the Orionids in late October.

The Geminids are unusual for a meteor shower in that the origin is not actually a comet, but an asteroid known as 3200 Phaethon. The asteroid is on an unusual orbit that brings it closer to the Sun than Mercury, and it sheds enough material to generate the most intense meteor shower of the year. The video below from NASA Science Casts explains more about the origin of the Geminids.


Meteors can be seen all over the sky, but most will appear to originate at a single point, known as the radiant. For the Geminids, this radiant is in the constellation Gemini (which gives the shower its name), close to the star Castor. This effect is caused by the fact that the Earth is moving into the debris; this is the same effect used to demonstrate spaceships moving at faster-than-light speeds in science fiction.

The Eastern sky as seen from Austin, Texas at 9pm on Friday December 13th 2013.
The Geminid meteors appear to radiate from the constellation Gemini, near to the
star Castor. Gemini can be most easily located by finding Orion with its distinctive belt.
A little way over Orion's left shoulder (the red star Betelgeuse) are the two bright
stars Castor and Pollux. (Image credit: Stellarium)
The Geminid meteor shower will peak on Friday and Saturday nights (December 13th – 14th), but meteors can be seen for a few days either side. The best thing about meteor showers is that no equipment is required (save something to keep you warm) – just pick a dark location and lie back so that you can see as much of the sky as possible.
The Geminids regularly peak in intensity around mid-December and seem to have increased in strength in the past years. This year, astronomers expect 120-160 meteors per hour during the peak, which would be early in the morning on Dec. 14. However, the moon is close to  full during the peak so only the brightest meteors will be easy to spot. For the truly dedicated, the best time to watch is an hour before dawn, when the moon will have set leaving the sky much darker. Reports say that we can still expect around 50 per hour under the best observing conditions (clear skies, no light pollution, etc.) and we may even be able make out their different colors (mainly white and yellow and a few being blue, green or red).

And if you’re not brave enough to venture out into the cold, you can even watch online. Now that’s astronomy for the 21st century.

Thursday, December 5, 2013

Astronomer of the Month - Benjamin Weiner

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 Benjamin Weiner.

Benjamin Weiner at Las Campanas in Chile
Tell us a little about yourself!

My name is Benjamin Weiner.  I'm an Associate Astronomer (research scientist) at Steward Observatory, which is the Department of Astronomy at the University of Arizona, and operates several mountaintop observatories. I was born in California but grew up in Pittsburgh.  I went to Swarthmore College, took a couple of years off, then got a PhD in physics/astronomy from Rutgers. I have lived in Pennsylvania, Massachusetts, New Jersey, California (both halves), Maryland and Arizona. Away from work I like to spend time hiking, running, and riding my bike.

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

I work on several different projects -- the one most closely related to CANDELS is the nature and properties of star-forming galaxies at redshifts 1-2, 7-10 billion year ago, essentially adolescent galaxies. By "nature" I mean we try to figure out what is the amount of gas and stars in these galaxies, how fast are they forming stars, what are the velocities of the gas - are they rotating like the Milky Way disk or more chaotic. I also study the winds (gas outflows) driven by galaxies and the link between galaxies and the circumgalactic gas probed by quasar absorption lines.  And the properties of dwarf satellites of low-redshift present-day galaxies.  I have also built instrumentation for ground-based telescopes, and software for reducing data.

Within CANDELS my role is to lead the data reduction and science efforts from the grism spectroscopy observations that we do with Hubble. The main goal of these observations is to get spectra of distant supernovae and their host galaxies, but we also get spectra of other galaxies in the same field.

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

I was interested in astronomy as a grade-schooler, although since I grew up in a city, didn't really get a good look at the night sky very often unless we went out of town -- I have a distinct memory of being wowed by a clear night sky with many stars when I was about 16, from a small town in Pennsylvania. I remember borrowing a small telescope from my high school physics teacher, and us trying to photograph a partial eclipse. Also of the Pennsylvania Governor's School for the Sciences summer school taking us to the Allegheny Observatory where we got to look at Saturn through a pretty big telescope. Even with pictures from space probes, there is no substitute for seeing the thing directly. However, I did physics in college, not astronomy, and didn't really seriously consider astronomy until my first year in grad school, when I was less motivated by the physics classes. My friend Julianne  suggested, "You should try astronomy, it's more interesting and easier." So I did.

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

When I started really doing research, rather than taking classes and doing homework, I worried about having to create my own new projects and problems to work on. It turns out that wasn't so hard. It's much harder to manage all of the projects and to know when to declare one "good enough," finish it, and move on.
Who has been your biggest scientific role model and why? 

I have never really identified people as role models, but one of the astronomers I learned the most from, just from informal talking, is Steve Shectman. I admire Vera Rubin's persistence and enthusiasm, George Preston's humor, and Jim Peebles's graciousness to younger people.

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

My favorite aspect is that we get to engage our curiosity, and that we have to confront whatever it is that the universe has decided should be physical fact. We can try to construct ideas or test certain theories, and sometimes they're right, but often nature just doesn't work the way we expect or throws us a surprise.

My less favorite aspect -- aside from the office politics that go with any job -- is that for a discipline that grew from staring at the sky, we spend an awful lot of time inside, under artificial lights.

What motivates you in your research? 

I like the challenge of figuring out original ways to design observations or studies to understand astronomical objects or test models. We can't do traditional lab-style  "experiments" in astronomy since everything is far away, so you have to measure effects with what nature allows you to detect. And sometimes a clever observational design means you don't need the biggest telescope or the most resources.

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

Benjamin Weiner walking up to the Magellan Telescopes
at Las Campanas in Chile.
I like going to the telescope, which for me is the most romantic, and also most scenically beautiful, part of doing astronomy. It's hard to pick one favorite, but some of mine are the observatories at Las Campanas in Chile, the VLA in New Mexico, and the MMT on Mount Hopkins here in Arizona. I would really like to visit Antarctica someday but don't currently work in the areas of astronomy that would get me there.

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

Right now I'm mostly doing research on relatively short-term projects, but I would like to do more teaching or outreach and work on a longer term project, possibly a telescope or a survey. Both of those are planning to build things for the future, more so than hopping from one project and grant proposal to the next.
If you could have any astronomy related wish, what would it be? 

I'd like to be able to see up close and in detail some of the things we study -- a supermassive black hole's accretion region, or a distant galaxy, who wouldn't?  But astronomy is not just a collection of objects, but a system of knowledge created by people, and if I could have one wish it would be that those people behave more decently to each other.

If I could have a second wish it would be that our society would value education, research, and knowledge more highly.
What is your favorite, most mind-boggling astronomy fact? 
That we can deduce anything at all about the nature and physics of stars, gas, dust, galaxies -- even planets around other stars -- that are so far away and that we'll never be able to touch or see in any more detail. Much of what I do is spectroscopy, and it's always hard to explain because it doesn't make pretty pictures. In 1859, Kirchhoff and Bunsen used the then-new spectrograph to show that the Sun was made of the same elements that exist on Earth. It both blows my mind that we can know that, and that we've only known it for 150 years.

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

I wish we could better communicate how science is imperfect, but generally correct or workable, especially in the long run. I think the popular conception of science is too biased toward lone geniuses making huge discoveries and overturning all of what was known before. In reality we make much slower progress, and "scientific revolutions" are based on a slow accumulation of facts and ideas that gradually build a new consensus. Even Einstein was no Einstein; he built on the work of people before him and discussed many ideas with his contemporaries.

Science doesn't know or predict everything, but the consensus is usually pretty damn reliable, especially for well studied subjects. You have to understand that science can be imperfect and largely reliable at the same time, otherwise you fall prey to hucksters claiming that scientists have got it all wrong and are covering it up. Anti-evolutionists, quack medicine scammers, and climate change denialists use these arguments, which are based on misrepresenting how science is an imperfect process that nevertheless produces useful results.

I'm concerned that an easily distracted culture that values only short term returns isn't supporting the resources and education we need to build a long-lasting healthy society.  For example, the Pennsylvania Governor's School that I mentioned going to many years back, was cancelled several years ago due to state budget cuts. It was just revived through heroic fundraising and donation efforts by some dedicated alumni. But that's not a long term solution.  The society at large, through the state, needs to make it a priority to have an educated population, to value the work of teachers and people who create knowledge. Education takes people and facilities and you just can't build a stable program by cheaping out or depending on charity every year.

Wednesday, November 27, 2013

Why Should We Study Astronomy?

In this video blog post, Jonathan Trump from the Pennsylvania State University, discusses why we should study Astronomy. This is our first video blog post - let us know what you think about the video and this format in the comments!

Tuesday, November 5, 2013

Galaxy Evolution and Gravitational Waves, Part II

This post is a continuation from Part I last week.

It turns out that there is another way to detect gravitational waves (GWs) without laser interferometers such as LIGO: once again, through pulsars, except this time in a way that directly demonstrates the presence of gravitational waves nearby the earth. (And it will be particularly suited for supermassive black hole GW detection, as I’ll explain later.) This method is entirely different from the indirect method I spoke of earlier that won Russell Hulse and Joseph Taylor the Nobel Prize in Physics in 1993, but it too relies on the fact that pulsars emit very regular pulses. Imagine a distant (single) pulsar rotating like a lighthouse and sending trains of regular pulses propagating through space and eventually reaching us. Because the timing, and thus the distance, between lightspeed-propagating pulses is very regular, these interstellar radio waves themselves are like a very long ruler! Should there be a stretching of space at or around the earth, a careful pulsar astronomer will find that the number of pulses received per second (typically on the order of a thousand per second) decreases slightly, then increases slightly, as a gravitational wave stretches the fabric of spacetime back and forth. (Note that the gravitational wave I’m referring to otherwise has nothing to do whatsoever with the pulsars being used to detect it.) This method, when it uses multiple pulsars, is referred to as a pulsar timing array (PTA), where the word “array” refers to the pulsars timed regularly in order to look for this slight change in arrival time. NANOGrav (North American Nanohertz Observatory for Gravitational waves) is such an effort underway using the Arecibo Observatory along with the Green Bank Observatory in West Virginia. NANOGrav is also part of a larger international effort known as the International Pulsar Timing Array, using eight radio telescopes. (Just as a miles-long laser, in the case of LIGO can be referred to as an “observatory”, so can forty pulsars spread around the Milky Way galaxy!)

Diagram of a pulsar timing array such as NANOGrav.
Each line of sight to a particular pulsar (yellow) functions
as a "lever arm" with which to measure waves in space-time
(i.e. the hills and valleys in the green grid).
As said before, individual pairs of merging supermassive black holes are unlikely to produce gravitational waves we can detect here on earth. But it turns out that the sum total of all merging black holes throughout a giant volume of space stretching back to about one half of the universe’s elapsed history, produces a combined GW background that can be detected. Thus, there would be gravitational waves coming equally from all directions in the sky. (This is the gravitational wave equivalent to the extragalactic background light or EBL, which is the uniform “glow” in the night sky due to numerous unresolved individual galaxies. CANDELS has also contributed to studying the EBL, as described here. See also the recent posting on the EBL.) 

What does this have to do with galaxies? A lot, because it turns out that there is a very close correlation between the rates at which smaller galaxies merge to form larger galaxies, and the (relative) rates at which small supermassive black holes (SMBHs) merge to become more massive (single) SMBHs. Because most galaxies have a SMBH at the center, if two galaxies merge, then eventually their central black holes will also undergo a merger. If one wants to predict the expected gravitational wave background, then, one needs to look carefully at what galaxy evolution simulations have to say. Carefully adding up the number of galaxy mergers over cosmic time leads to a prediction of how intense the gravitational wave background due to SMBHs should be.

As it turns out, doing this calculation reveals that the strongest part of the gravitational wave background should be at long periods – where a single oscillation takes greater than five years! Fortunately, pulsar timing arrays such as NANOGrav will be able to observe these very slowly vibrating (i.e. “nanohertz”) waves. This is because laser interferometers cannot remain stably phase locked for decades at a time. Millisecond pulsars, however, often do remain stable clocks (and thus rulers) for this length of time. Should the SMBH gravitational wave background be detected, it will be a direct measurement of the galaxy merger rate across cosmic time. This information could then be used to go back and make an even better computer simulation of galaxy evolution.
The Green Bank Telescope in Green Bank, WV.
Along with the Arecibo Observatory, astronomers
hope to detect gravitational waves emitted by
supermassive black holes in the centers of merging galaxies

In other words, all roads lead back to galaxies. At the time of this writing, a new study has been released from the Parkes Pulsar Timing Array in Australia, which uses its pulsar timing measurements to rule out models of galaxy evolution in which galaxy (and thus SMBH) mergers happened very late in cosmic history. (See here: http://arxiv.org/abs/1310.4569) It will be interesting to see what new limits on gravitational waves will come about in the next few years. Or even better, a detection!

Friday, November 1, 2013

Kitt Peak REU and Working with CANDELS

Hi! My name is Kirsten Blancato and I am an astronomy and physics major at Wellesley College. This past summer, I had the exciting privilege of being a part of the 2013 Kitt Peak National Observatory Research Experience for Undergraduates (REU) program. For twelve weeks, I worked with CANDELS team member Jeyhan Kartaltepe at the National Optical Astronomy Observatory in Tucson, Arizona.

Working at the National Observatory was truly an amazing experience. I have been in interested in astronomy since I was a child and being able to study astronomy in college has been a dream come true. I was thrilled when I was accepted to work at NOAO the summer after my sophomore year. I had always heard about the amazing work going on at Kitt Peak and could not wait to be there.

Kitt Peak from the Mayall 4-meter telescope
Image credit: Kirsten Blancato
In addition to working on a research project with an astronomer at NOAO, the REU program included many activities including field trips to observatories, observing time at Kitt Peak, and many interesting astronomy lectures. We spent a week in New Mexico, where we visited the National Solar Observatory at Sacramento Peak, the Sloan Digital Sky Survey at Apache Point Observatory, and my favorite of all - The Very Large Array.  Each student also had five nights of observing on Kitt Peak. For a few days at a time we would live on the mountain, observing at night and sleeping during the day.  We were able to observe with the 2.1 meter telescope both in the optical and in the IR.  It was great to meet so many different types of astronomers both while observing and visiting observatories.

But for a majority of the twelve weeks, I was working at the NOAO offices in Tucson with Jeyhan Kartaltepe. I did not know much about extragalactic astronomy before this summer, but was immediately amazed by CANDELS and all of the great science that is being done by the team members. My project focused on high redshift galaxies and how morphology can be used to identify mergers at high redshifts.  Galaxy mergers are thought to play a critical role in galaxy evolution. In the early universe, when everything was closer together, there were many young disk galaxies. As time went on, galaxy collisions eventually formed elliptical galaxies, which are much more common in the local universe. But before we can better understand how galaxies have evolved since the beginning of the universe, we need to be able to identify galaxy mergers at higher redshifts. And this is what my summer work focused on!

Images of high redshift galaxy mergers
Image Credit: Jeyhan Kartaltepe
Using many different data sets from CANDELS, I compiled a data set of around 22,000 galaxies from the GOODS-S, UDS, and COSMOS fields. For each of the 22,000 galaxies, we have visual classification information, redshifts, and several different image statistics. The main problem with identifying high redshift galaxy mergers is that mergers features are much fainter at these distances, making it much harder to see features such as tidal arms. While automated methods for picking out mergers have been developed for low redshift (z < 1) galaxies, more work needs to be done to develop a successful automated method that picks out high redshift (z > 1) mergers. This summer, we looked at how well visually classified mergers and other morphological features were picked out by the different image statistics.

I had a great time learning about galaxy mergers and evolution and at the end of the summer realized that twelve weeks goes by pretty fast.  After the twelve weeks, I had more questions and more things I wanted to explore than I did at the beginning of the summer. I am definitely excited to hear about all of the science that will result from CANDELS and NOAO in the future.

After my summer at NOAO and working with CANDELS, I am very excited to continue on in astronomy. This spring, I will be studying astrophysics abroad at the University of St. Andrew's in Scotland and will then return to Wellesley for my senior year.  After that, I definitely plan to attend graduate school in astronomy.

Wednesday, October 30, 2013

Galaxy Evolution and Gravitational Waves, Part I

Much has been said on previous blog posts here about how computer models of galaxy evolution, which are being refined and improved by the Hubble Space Telescope’s observations collectively known as CANDELS, contribute to our overall understanding of the universe. The numbers, types, shapes, sizes, and large-scale clustering properties of galaxies throughout cosmic history can at times be predicted with an astonishing accuracy. When predictions from computations of galaxy evolution do not line up well with observations, then, happily, we have a clue that something remains to be discovered. (A now classic example is the so-called missing satellite problem: simulations predict many more dwarf galaxies surrounding other galaxies than are actually observed. Are they as yet undiscovered, or is our understanding of dark matter structures – on which galaxy simulations rely – extremely flawed?)

The Arecibo Telscope in Arecibo, Puerto Rico. Russell Hulse and 
Joseph Taylor used this facility to make the first indirect
detection of gravitational waves in 1975. A direct detection
would help astronomers learn about how often galaxies merge
throughout the history of the universe.
A very nice spinoff of having a digital universe of growing, colliding, re-shaping, and color-changing galaxies is that these simulations can be used to make predictions in an entirely different area of physics: gravitational waves.

A gravitational wave is an oscillation in the fabric of spacetime itself, which, after its initial production, propagates away and has nothing further to do with the merging objects that made it. The wave travels through space on its own at the speed of light, stretching and bending everything in its path. (Not to worry, though: the stretching and bending is on a scale smaller than the nucleus of an atom, which is exactly why gravitational waves are difficult to detect!) The waves are emitted whenever a very massive object exhibits quadrupolar motion – which essentially means rotating motion with a lot of heaviness on the outer rim of whatever is rotating. (Thus, the rotating, spherical Sun produces no gravitational waves, whereas two stars orbiting one another closely do.) Ordinary binary stars are not dense enough and close enough together to produce any noticeable gravitational waves. It generally takes pairs of extremely dense objects – white dwarfs, neutron stars, and black holes – to inspiral very close to one another and merge with enough spherically asymmetric rotation for gravitational waves to come about. Generally, then, if a gravitational wave were detected here on Earth, that means it probably originated from a pair of extreme remnants of stars, whirling toward one another.

Gravitational waves have not been directly detected yet. However, there is indirect evidence that they exist. Hulse and Taylor won the Nobel Prize in Physics in 1993 for their discovery of a pulsar system using the Arecibo Observatory in Puerto Rico. (A pulsar is a rapidly rotating neutron star, emitting a very regular beam of radio pulses.) The pulsar had a companion (non-pulsating) neutron star and the two objects orbited each other closely. This was inferred by the sharp regularity of the pulses, after modeling the small changes in that regularity due to the otherwise invisible companion. However, the orbits did not fit the pattern that one would expect from ordinary, Newtonian gravity. The two objects had slightly decaying orbits, which indicated that energy was continually being dissipated. It turns out that the rate of energy loss was exactly that expected if the system were emitting gravitational waves. Look at how tiny the uncertainties are in Fig 1!

Figure 1: Deviations of the Hulse-Taylor pulsar system (black data
points) from Newtonian predictions (horizontal line). Thirty years later,
        the system continues to follow the predictions of general relativity. 
(From Weisberg and Taylor 2005.)
Here on earth, though, how does one go about directly detecting gravitational waves? The now “classic” method (I use quotation marks because this is a very new field of physics) uses laser interferometry. Facilities such as LIGO (Laser Interferometric Gravitational wave Observatory) in Washington state and Louisiana, as well as the Virgo observatory in Italy, use a very long laser beam to detect changes in length less than the radius of an atomic nucleus. (As a side note, astronomy today uses the term "observatory" somewhat loosely – apart from detecting length changes on earth due to astrophysical sources, the experiment doesn’t “see” anything in the sense that an optical telescope does.) Were a gravitational wave to pass the earth once these facilities are completely operating in 2017, the length measured by the laser interferometer would oscillate back and forth between 1 + 10-20 and 1 – 10-20 times its original length. The hard part of making such an observatory work is sorting out these vibrations from distant trains, from earthquakes on the other side of the world, from wind vibrations, and from a very long list of other irritating sources. A successful detection, however, would mean hundreds or thousands of these tiny oscillations a second, due to a gravitational wave tracing back to a particular pair of rapidly inspiraling pair of neutron stars (for instance).

However, detecting a particular merging pair of supermassive black holes with a laser interferometric observatory is not terribly likely, for the following reason. A supermassive black hole is one of the most exotic objects in the universe (by supermassive, astronomers mean anywhere from a million to about a billion solar masses worth of material in a single black hole. This can be a small but significant fraction of an entire galaxy, which typically weighs a hundred billion solar masses). Typically a supermassive black hole lies in the center of a galaxy. In fact, our own Milky Way galaxy very likely has one at its center: Saggitarius A, which is the equivalent mass of four million suns. But, given only one or two SMBHs per galaxy, and given the fact that mergers between supermassive black holes are rare (on the order of a million in the entire observable universe per year), one would have to look very far to find a pair (probably beyond a redshift of about 0.4). And chances are, it would be so far away that the merger’s gravitational waves would be too weak for us to detect. (As for merging white dwarfs and neutron stars, there are numerous sources nearby in our own galaxy, and so there will probably be plenty of gravitational wave sources to detect with LIGO and Virgo.)

Fortunately, more distant galaxies (far enough away that that we see them at about half the universe's age, due to the finite speed of light) merged more often. As a result, supermassive black hole mergers were also more common. This provides a way to detect gravitational waves from these mergers, which also happens to utilize pulsars. More about this in part two of this post.

Friday, October 25, 2013

The Universe Explored by Herschel

The Herschel Space Observatory
Image credit: ESA (Image by AOES Medialab);
Background: HST, NASA/ ESA/ STScI
After nearly four years of operation, on April 29, 2013 the Herschel Space Observatory finished its mission. However, this was not the end of its job. The wealth of data from Herschel is still being analyzed and new results are being discovered. A conference, "The Universe Explored by Herschel," was held in the Netherlands from Oct 15-18 to discuss exciting results that have been made with Herschel.

Herschel is a space telescope that was launched on May 14, 2009. Its mirror is 3.5m which makes it the largest space telescope launched so far. It observed the sky in the infrared. Why do astronomers care about infrared? Infrared allows astronomers to detect dust emission that is caused by objects embedded in the dust, observe low temperature objects like some planets, and study high redshift galaxies. Herschel could observe the wavelength range of 60-670 microns in the infrared. This range covers a part of the infrared bump of dust emission in galaxies and many important infrared spectral lines that play a role in determining gas properties in the interstellar medium. CANDELS also has 85-670 micron deep imaging data taken with Herschel in the GOODS-S, GOODS-N, COSMOS, and UDS fields. With these data sets, we can detect the dust emission in distant galaxies.

During the conference, I mainly attended the sessions related to galaxy formation and evolution. Infrared galaxies were common in the early universe and they dominated the star formation history. We discussed how infrared galaxies were formed and evolved -- how much dust these galaxies have, what is the dust temperature, how many stars they made at a given period (star formation rates), why some of them have intense star formation compared with typical star-forming galaxies, how they stop making stars, what kind of shapes they have, what mechanisms heat the dust, etc. 

View of a canal in Leiden. Image Credit: H. Inami
From the CANDELS team, Jeyhan Kartaltepe displayed a poster (shown below) presenting interesting results on the morphology of galaxies detected with Herschel. In particular, she explored the role that galaxy mergers play in high redshift ultraluminous infrared galaxies. David Rosario discussed the connection between active galactic nuclei and star formation using the Herschel data. I presented infrared luminosity functions built with our own Herschel data, and star formation rate comparisons based on the infrared luminosity and the optical/near-infrared spectral energy distribution fitting, which Janine Pforr has been working on. Without Herschel, it would have been difficult to answer these questions accurately. Although Herschel is not observing anymore and many interesting discoveries have been made, astronomers are still working hard to explore the infrared universe using its data.

Image Credit: J. Kartaltepe