Tuesday, December 15, 2009

The Era of Baby Galaxies

Hello loyal reader.  So, I haven't written to this blog in over three months.  I promise - there is a reason.  Right about then, I started working on a really cool research project.  I thought I would finish it quickly, so I told myself when I did, I would write up a blog post about it.  Days turned into weeks and weeks into months, but finally its finished.  If you really want to torture yourself, you can read the paper here:

But, its in scientist-speak, so you can save yourself the trouble, and read my non-scientist friendly summary :)  And, if you want to skip straight to the pretty pictures, go here:

This all started back in May of this year when NASA serviced the Hubble Space Telescope, or HST (the mission was called Servicing Mission 4 - though it was actually the fifth mission to HST).  The shuttle Atlantis performed the mission, and they did a number of tasks, including fixing the main camera (the Advanced Camera for Surveys -- or ACS -- which had broken over a year earlier) and installing the new camera, Wide Field Camera 3 (or WFC3 in astronomer - speak).  Although WFC3 can also take visible-light images, its real power lies in its capability to detect light just bluer than the eye can see -- the near-ultraviolet -- and just redder than the eye can see -- the near-infrared.

One reason why this was so exciting, was that we believed that WFC3 would give us the capability to discover the most distant galaxies.  This is precisely because we know the Universe is expanding.  The further away a galaxy is, the faster it appears to be receding from us. Due to the Doppler effect, this recessional velocity actually stretches out the light we see from these galaxies, such that they appear redder. The further away they are, the faster they're moving and thus the redder they appear.  Us astronomers call this redshift, and we give it a number.  The higher the redshift, the more distant a galaxy is.  Until recently, the most distant galaxies known have redshifts of ~ 6.5 - 7.  We are thus seeing these galaxies as they were about 12.5 billion years ago.  The Universe is 13.6 billion years old, thus we were seeing them as they were only a billion years after the Big Bang.

Just a few months after WFC3 was installed, HST spent 60 orbits (~ 60 hours) staring at one area in the sky known as the Hubble Ultra Deep Field.  These observations were proposed by Dr. Garth Illingworth, a professor at UC Santa Cruz, but the data became publicly available immediately.  The area in this deep field is tiny, only 3% the angular extent of a full moon, but the image can see objects one billion times fainter than the human eye!  A number of papers came out within week, publishing the first discovery of large numbers of galaxies at redshifts > 7.  This was not previously possible, because we did not have the necessary instruments.  These galaxies are so distant that their visible light has been shifted completely to the infrared.  It is difficult to observe in the infrared from the ground since the sky actually glows at those wavelengths.  So, we need to go to space, and thats what WFC3 has given us.

We examined these data, and searched for galaxies which are moving so fast, that their light has been shifted nearly out of the optical and in the infrared.  To do this, we used the new WFC3 data in conjunction with existing ACS optical data, and looked for objects which were apparent in the WFC3 infrared image, but either very red or nonexistent in the ACS optical image.  We found 35 such objects, and from their colors they appear to be at redshifts from 7 - 8.  The light we see left these objects 13 billion years ago, which corresponds to a time when the Universe was only 700 million years old!!

Our goal was to do a detailed analysis to understand the physical make-up of these galaxies.  We have been studying in detail over the last few decades or so what galaxies at lower redshifts look like.  By comparing to higher redshift, we can really gain a sense of how galaxies are evolving, which we can turn around and use to probe the conditions at the beginning of the Universe.

The first interesting thing we wanted to look at was the dust and metal content of these galaxies.  In the Big Bang, only hydrogen and helium were formed.  Any elements heavier than that we astronomers call "metals", and when we talk about metallicity, we mean how many atoms of metal elements exist when compared to the number of hydrogen atoms.  Although we haven't seen them yet, the first stars by definition should have had no metals in them.  These stars should be blazing hot, and galaxies composed of these stars will appear extremely blue.  Similarly, dust, which is material between stars in galaxies composed of silicon and other metals, is also interesting to study, as it typically forms in the supernovae (the deaths of massive stars) as well as in the atmospheres of very old stars.  Thus, if you see dust or non-zero metallicity in a galaxy, it is a good indicator that a previous generation of stars has existed, and thus you're not quite seeing the first galaxies.

To assess the properties of the galaxies in our sample, we compared them to typical local star-forming galaxies.  What we found was that our galaxies at redshifts of 7-8 are significantly bluer than typical local galaxies.  This implies that they are some combination of younger, dustier and lower metallicity.  This is very interesting, as in Dr. Casey Papovich's thesis work (along with a number of other studies) we learned that galaxies at a redshift of 3 are very similar to local star-forming galaxies.  The time elapsed from now until redshift of 3 is ~ 12 billion years, and only another billion years goes by from redshift 3 to redshift 7, thus it is very striking that galaxies have evolved significantly in such a short period of time!!!

Since our galaxies are bluer, could it mean that we've finally found the first galaxies in the universe?  Well, I said above that we compared them to "typical" local galaxies.  What we did next was to compare them to some of the bluest galaxies we see nearby.  And, we found that our high redshift galaxies appear very similar in color to locally very blue galaxies!  What does this mean?  Well, these galaxies have almost no dust, and very low metallicities.  But, they do not have zero metallicities, so we conclude that these distant galaxies, while fairly primitive in their composition, are not the first-ever galaxies in the universe.

From their brightness, we are also able to measure the amount of stellar mass they contain.  We find that they are from 1 - 10% as massive as the Milky Way.  To put this into context, galaxies at a redshift of 3 are about as massive as the Milky Way (maybe a tiny bit less), so again we have discovered strong evolution in typical galaxies over only one billion years in cosmic time.  Putting all of the pieces above together, it really does look like at redshifts of 7-8, we are really probing the era of baby galaxies.  The larger, Milky Way size galaxies that are common at lower redshifts are nowhere to be seen, and everything we see appears to be fairly young and primitive.  But, we have not yet found the infant galaxies, containing the metal-free stars, but with the launch of the James Webb Space Telescope in 2014/2015 we should come awfully close.

One other interesting thing we investigated was the effect these galaxies would have on their environment.  We know that soon after the Big Bang (a few hundred thousand years), the Universe was filled with neutral hydrogen.  When we look between galaxies today, there is still hydrogen, but it is ionized, meaning that its electron has been stripped away, so the space between galaxies is filled with a proton and electron soup (what we would call a plasma).   One of the burning questions in astronomy is, what caused the reionization of the intergalactic gas?  One theory is that young galaxies in the early universe did it, but that was far from determine.  In order for this to happen, very energetic ultraviolet light has to be able to "escape" from a galaxy in order to ionize the intergalactic hydrogen.  When we look at the Milky Way, only a few percent of this light can escape.  So, if we look at our galaxies, if only a few percent of the energetic light escapes, they are not close to being able to reionize the intergalactic gas.  But, as we learned in our study, our galaxies are NOT like the Milky Way.  The most important difference is that they have little-to-no dust.  Dust can block this energetic light from escaping, so if you take it away, then the light can escape, and these galaxies can explain reionization!  When we actually look at how bright our galaxies are, we find that if on average 50% of the energetic light escapes, typical galaxies are the dominant source behind reionization.  We do not (yet) have a way to directly measure this escape fraction, but the fact that these galaxies appear fairly primitive in their physical make-up implies that they likely have large escape fractions.

1 comment:

  1. Reading this makes me awestruck. It never ceases to amaze me what you and Keely have accomplished and the level at which you function on a daily basis to not only grapple with all of this, but pushing forward with research that is always one step ahead in asking the next question...and persisting in your search for answers. At the most basic level reading this touched my soul with the magic and mystery that is the universe.