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

Thursday, October 10, 2013

Our Blog Featured on Learning Space

Last week, Janine Pforr and I participated in a live Google Hangout on Learning Space,with Nicole Gugliucci and Georgia Bracey from CosmoQuest. We discussed the CANDELS survey, public outreach, and this blog. Take a look below!

Wednesday, October 2, 2013

An Update on Mature Galaxies in the Early Universe

One of the goals of CANDELS is to document how galaxy shapes evolved over about 13 billion years of history.  Avid readers of this blog might recall our posting about the presence of mature galaxies 11 billion years ago. In that post, we explained that a beautiful montage of galaxies that accompanied a recent European Space Agency press release gave a misleading impression of the scientific finding.

We're delighted to say that the ESA press officer understood our concerns immediately and offered to produce a version with images of galaxies that are from the CANDELS data themselves. That image is shown below. The leftmost panel is identical with the original press release, but the center panel now uses images of galaxies at redshifts between z=0.3 and 0.7 and the rightmost panel uses galaxies between z=2 and 2.7.

An updated image of the evolution of the Hubble sequence over the past 11 billion years. From ESA.

At each epoch, the illustration is intended to show the Hubble tuning fork. The four leftmost postage-stamps show early-type galaxies, known as ellipticals and lenticulars (or S0 galaxies). The tuning fork then splits to show the late-type spiral galaxies. The top four images show normal spiral galaxies while the bottom four panels show barred spirals (galaxies with a bar-like feature in the middle).

While this is still not a perfect representation of what is happening to galaxy shapes, there are several things to notice.

  • The galaxies 4 billion years ago and 11 billion years ago were smaller. Alas, this image still doesn't have the relative scales exactly correct between the epochs, but the sense of the evolution from smaller to larger is consistent with detailed measurements.
  • The early-type galaxies 11 billion years ago look sort of similar to those today, albeit smaller. They had about the same round, ball-like shape with nearly uniform color. They tend to be redder than the late-type galaxies.
  • The late-type galaxies 4 billion years ago look pretty similar to today. They have spiral features and you can find examples that have bars in their centers.
  • The late-type galaxies 11 billion years ago look a bit different. It is very hard to find any convincing examples of barred spirals, and the ones that don't have bars look more disordered than their present-day counterparts.
  • The greenish tint of the galaxies in the center panel isn't real. These images are composites through different filters, and the color balance has not been tweaked in a meaningful way. When measured in detail, galaxies in the past tended to be bluer than today, which is a result of the fact that they contained more hot, young stars and less dust.
While the illustration here gives a better illustration of how galaxies are transforming their shapes, it doesn't illustrate some of the more interesting features like bright clumps or mergers. Also it doesn't tell you anything about the relative numbers of galaxies of different shapes. It turns out that the early-type galaxies were quite rare 11 billion years ago, while they are much more common today, for example. 

Finally, in spite of our quibbles about the artwork, we would like to give a shout-out to the Hubble outreach team at ESA for phenomenal work over the years. Most recently, they were recognized for their efforts with a Parsec award "Fact behind the Fiction" award for their brilliant Hubblecast series.