* Astronomy

Scientists said Wednesday that they have found the most distant galaxy yet, nearly 13 billion light-years away, in a discovery that could help explain how stars were formed at the dawn of time.

The galaxy, named IOK-1, is so far away that the light waves that reached Earth depict it as the system of stars existed shortly after the Big Bang created the universe 13.66 billion years ago.
That period, known to astronomers as the Dark Ages, saw the formation of the first stars and galaxies from elementary particles. Scientists had been unable to directly observe that time period until now.

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Position(2000): RA = 132359.8 Dec = 272456

Astronomers using the Subaru telescope in Hawai'i have looked 60 million years further back in time than any other astronomers, to find the most distant known galaxy in the universe. In doing so, they are upholding Subaru's record for finding the most distant and earliest galaxies known. Their most recent discovery is of a galaxy called I0K-1 that lies so far away that astronomers are seeing it as it appeared 12.88 billion years ago.
This discovery, based on observations made by Masanori Iye of the National Astronomical Observatory of Japan (NAOJ), Kazuaki Ota of the University of Tokyo, Nobunari Kashikawa of NAOJ, and others indicates that galaxies existed only 780 million years after the universe came into existence about 13.66 billion years ago as a hot soup of elementary particles.
To detect the light from this galaxy, the astronomers used Subaru telescope's Suprime-Cam camera outfitted with a special filter to look for candidate distant galaxies. They found 41,533 objects, and from those identified two candidate galaxies for further study using the Faint Object Camera and Spectrograph (FOCAS) on Subaru. They found that IOK-1, the brighter of the two, has a redshift of 6.964, confirming its 12.88 billion-light-year distance.

The discovery challenges astronomers to determine exactly what happened between 780 and 840 million years after the Big Bang. IOK-1 is one of only two galaxies in the new study that could belong to this distant epoch. Given the number of galaxies that have been discovered from 840 million years after the Big Bang, the research team had expected to find as many as six galaxies at this distance. The comparative rarity of objects like IOK-1 means that the universe must have changed over the 60 million years that separate the two epochs.
The most exciting interpretation of what happened is that we are seeing an event known to astronomers as the reionization of the universe. In this case, 780 million years after the Big Bang, the universe still had enough neutral hydrogen to block our view of young galaxies by absorbing the light produced by their hot young stars. Sixty million years later, there were enough hot young stars to ionize the remaining neutral hydrogen, making the universe transparent and allowing us to see their stars.
Another interpretation of the results says that there were fewer big and bright young galaxies 780 million years after the Big Bang than 60 million years later. In this case, most of the reionization would have taken place earlier than 12.88 billion years ago.

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-- Edited by Blobrana at 21:13, 2006-09-13

Title: The quest for Type 2 quasars: Chandra observations of luminous obscured quasars in the Sloan Digital Sky Survey
Authors: Authors: C. Vignali (1,2), D.M. Alexander (2), A. Comastri (2) ((1) Dipartimento di Astronomia, Universita` di Bologna, Italy; (2) INAF - Osservatorio Astronomico di Bologna, Italy; (3) Department of Physics, Durham University, UK)

We report on new Chandra exploratory observations of six candidate Type 2 quasars at z=0.49-0.73 selected among the most (OIII) luminous emitters from the Sloan Digital Sky Survey (SDSS). Under the assumption that (OIII) is a proxy for the intrinsic luminosity of the central source, their predicted rest-frame X-ray luminosities are L(2-10keV)~10^45 erg/s. For two of the targets, the photon statistics are good enough to allow for basic X-ray spectral analyses, which indicate the presence of intrinsic absorption (~10^{22-23} cm^-2) and luminous X-ray emission (L_X>10^44 erg/s). Of the remaining four targets, two are detected with only a few (3-6) X-ray counts, and two are undetected by Chandra. If these four sources have the large intrinsic X-ray luminosities predicted by the (OIII) emission, then their nuclei must be heavily obscured (N_H>few 10^23 cm^-2) and some might be Compton thick (N_H>1.5 10^24 cm^-2). We also present the results for two Type 2 quasar candidates serendipitously lying in the fields of the Chandra targets, and provide an up-to-date compilation of the X-ray properties of eight additional SDSS Type 2 quasars from archival Chandra and XMM-Newton observations (five with moderate-quality X-ray data). The combined sample of 16 SDSS Type 2 quasars (10 X-ray detections) provides further evidence that a considerable fraction of optically selected Type 2 quasars are obscured in the X-ray band (at least all the objects with moderate-quality X-ray spectra), lending further support to the findings presented in Vignali, Alexander and Comastri (2004a) and unification schemes of Active Galactic Nuclei, and confirms the reliability of (OIII) emission in predicting the X-ray emission in obscured quasars.

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This is the final talk of Symposium 235 on Galaxy Evolution, but there will be much more other things going on tomorrow that are worth writing about.

Downsizing was mentioned frequently during the last days and in the presented survey, red, emission-line-less cluster-galaxies are used to measure the "fossil record" of galaxies, i.e. the old stars. The thousands of spectra are sorted by velocity dispersion (a measure of the total mass) and stacked together to get high quality average spectra with many spectral features that can be analysed to get ages of the stellar population.
They find that the smaller galaxies have smaller ages, i.e. downsizing. The age-spread is much larger at low masses than at the high-mass end. There was no morphological selection but of course it is ellipticals that dominate the sample. The S0-type galaxies are slightly younger than Es with the same sigma, but this trend is weaker than the trend with sigma itself.
Comparisons with the total dynamical mass (also using Sauron-data) there is little room for dark matter (25%).
The ages also correlate with environment, i.e. distance from cluster centre (16% change). Again, this is not a strong trend. Metallicity does not show a trend, but alpha-enhancement does. The tilt of the distribution in a colour-magnitude diagram comes half from ages, half from metallicity.
By calculating backward, how te CMD would have looked for these galaxies at some earlier time, they can be compared to CMDs at some redshift.

Source (IAU Blog)

A survey of galaxies observed along the sightlines to quasars and gamma-ray bursts--both extremely luminous, distant objects--has revealed a puzzling inconsistency. Galaxies appear to be four times more common in the direction of gamma-ray bursts than in the direction of quasars.

Quasars are thought to be powered by accretion of material onto supermassive black holes in the centres of distant galaxies. Gamma-ray bursts, the death throes of massive stars, are the most energetic explosions in the universe. But there is no reason to expect galaxies in the foreground to have any association with these background light sources.

"The result contradicts our basic concepts of cosmology, and we are struggling to explain it" - Jason X. Prochaska, associate professor of astronomy and astrophysics at the University of California, Santa Cruz.

Prochaska and graduate student Gabriel Prochter led the survey, which used data from NASA's Swift satellite to obtain observations of the transient, bright afterglows of long-duration gamma-ray bursts (GRBs). They described their findings in a paper accepted for publication in Astrophysical Journal Letters. The paper, which could have strange cosmological implications, has been a source of significant debate among astronomers throughout the world.
The study is based on a fairly straightforward concept. When light from a GRB or a quasar passes through a foreground galaxy, the absorption of certain wavelengths of light by gas associated with the galaxy creates a characteristic signature in the spectrum of light from the distant object. This provides a marker for the presence of a galaxy in front of the object, even if the galaxy itself is too faint to observe directly.
Prochter and Prochaska analysed 15 GRBs in the new study and found strong absorption signatures indicating the presence of galaxies along 14 GRB sightlines. They had previously used data from the Sloan Digital Sky Survey (SDSS) to determine the incidence of galaxies along the sightlines to quasars. Based on the quasar study, they would have predicted only 3.8 galaxies instead of the 14 detected along the GRB sightlines.
The quasar analysis was based on more than 50,000 SDSS observations, so the data for quasars are much more robust statistically than the data for GRBs. Nevertheless, the probability that their results are just a statistical fluke is less than about one in 10,000.
The researchers examined three potential explanations for the inconsistency. The first is obscuration of some quasars by dust in galaxies. The idea is that if a quasar is behind a dusty galaxy it wouldn't be seen, and this could skew the results.

"The counter argument is that with this huge database of quasar observations, the effect of dust has been well characterised and it should be minimal" - Jason Prochaska.

Another possibility is that the absorption lines in the GRB spectra are from gas ejected by the GRBs themselves, rather than from gas in intervening galaxies. But in nearly every case when researchers have taken a closer look in the direction of the GRB, they have in fact found a galaxy at the same position as the gas.
The third idea is that the intervening galaxy may act as a gravitational lens, enhancing the brightness of the background object, and that this effect is somehow different for GRBs than for quasars. Although Prochaska said he prefers this explanation, several factors make strong lensing of the GRBs seem unlikely.

"Those who know more about gravitational lensing than I do tell me it's unlikely to be the answer" - Jason Prochaska.

The paper, a draft of which has been posted on an Internet server for several weeks, has stimulated widespread discussion and at least one new paper proposing a potential explanation. But so far the findings remain perplexing.

"A lot of people have been scratching their heads, and most hope that it goes away. The GRB sample is small, so we would like to triple or quadruple the number in our analysis. That should happen during Swift's extended mission, but it will take time." - Jason X. Prochaska.

In addition to Prochaska and Prochter, the authors of the paper include Hsiao-Wen Chen of the University of Chicago; Joshua Bloom and Ryan Foley of UC Berkeley; Miroslava Dessauges-Zavadsky of the Geneva Observatory; Sebastian Lopez of the University of Chile; Max Pettini of Cambridge University; Andrea Dupree of the Harvard-Smithsonian Centre for Astrophysics; and Puragra GuhaThakurta, professor of astronomy and astrophysics at UC Santa Cruz.
Data used in this study were obtained at the W. M. Keck Observatory, the Gemini Observatory, the Very Large Telescope at the Paranal Observatory, and the Magellan Observatory. Support for this research was provided by the National Science Foundation and NASA.

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