Extragalactic Astronomy

I went to Professor Siana's colloquium last week, and I decided to take notes so that I could write a blog about it. Hopefully I don't butcher this too badly...

Before the presentation, the title, Finding the Ultra-Faint Galaxies that Ionized the Universe already had me thinking. I remember learning about the ionization of the universe a few years ago, but I hadn't thought about it since.

The first slide showed the Andromeda Galaxy in the ultraviolet spectrum, and Professor Siana explained that we were able to study Andromeda very well, since it is our closest neighboring galaxy. But how are we able to study farther, older galaxies, especially since really old galaxies are thirty times smaller than they are now? And also, how did we go from being so tiny to the size that we are today? (I guess I'm using the galactic 'we', here...) And on top of all that, he explained that by measuring redshift, we can determine that galaxies also used to make more stars half a universe age ago. The three steps to this process are to first find galaxies at each epoch, then determine their luminosities over all wavelengths, which will yield their star formation rate, and finally add all of the galaxies. Simple, right? Well, not really, but you get the idea.

To categorize galaxies into epochs, their flux densities were plotted versus their observed wavelengths in angstroms. (Galaxies which are observed below a certain wavelength, also known as a "rigbert', spelling phonetically, are able to ionize hydrogen.) In the infrared, we lose data, since the galaxies are already "red" and shifting redder, specifically at the Lyman break, at around one-thousand angstroms. In the graph, redshift is denoted by the letter 'z' and breaks up and defines epochs. To give an idea of the z-scale, the absolute most distant galaxies observed are around ten.

The second step involves luminosity functions which convert luminosity to star formation density. Using these, it has been determined that at ten billion years ago there was a peak in star formation. One of the next slides asked, "How Low Can You Go?" and was referring to detecting faint galaxies and went on to explain their importance. We now know that there was a host of galaxies which provided gamma ray bursts which enriched the intergalactic medium with heavy elements as a function of time, and carbon was plentiful everywhere. When the universe was about a third of a million years old, it was ionized; on its one hundred-million-year birthday, it became neutral; and four hundred million years later, it became ionized again. What's up with that?

Maybe stars ionized it. The research shows that at z=7, everything was ionized. Because of this, were all galaxies free to roam? Sources of ionizing background don't cancel out the sinks. So we use gravitational lensing, which was theorized first and then detected and confirmed in 1919, to find some hiding galaxies indirectly. We got to see some pretty cool famous lensed galaxies on the next slide, such as CB58 from the nineties, the Cosmic Horseshoe, the Cosmic Eye, and even an app called GravLens HD, designed by Eli Rykoff, which could turn any image into a lensed "galaxy," and Professor Siana told us about how he got to do some spectra with Spitzer in 2008 on some aromatic hydrocarbons. Some of the pros of gravitational lensing are magnification and higher spacial resolution, and some of the cons are that we are constrained by the accuracy of the lens model, and that high magnification over large areas complicate mass profiles, like when dealing with galaxy clusters, for example. The last con presents a real problem. The newest deep field photo is of Abell A1689 (z=2), and is being taken of the greatest area with large magnification in a single pointing over 60 orbits of UV imaging using the Hubble telescope, which has produced half of the data so far. There are 30+ galaxies in the optical/near infrared from this imaging. We got to see a four orbit image (z=2) using color selection, by his own grad student Alavi. There's a lot of star formation we've been missing due to dust obscuration, which scatters blue light, and we must correct for how "red" a galaxy "looks," and we need to know the spectral slope. The new information from this deep imaging is giving us this data.

Future work: Professor Siana and will work on Deep Keck Spectroscopy to observe rest wavelengths, gas inflow and outflow, hot gas explodes out of young dward galaxies, circumgalactic medium, ionizing photon escape fraction, ionizing radiation which must escape galaxies Fesc>0.2, photoionization cross sections, and gas opaque at NHI>2E+17cm-2. He's also going to be looking for a massive amount of gas to condense and cool and then get the heck out of the way!

My favorite quote from the presentation is: "The UV escape fraction remains problematic." -X Fan (I guess he's not a fan anymore.)

Anyway, I did not fully understand everything that was presented in the talk, but I feel like I understood it better than I was able to explain it. If anyone else who went to the colloquium wants to add anything, or if there are any corrections to be made, please let me know!