Unprecedented number of gas-rich galaxies along single sightline
22 October 2013
Absorption of the 21-cm spin-flip transition of hydrogen by galaxies intervening the lines-of-sight to high redshift radio sources (quasars) can provide important insight into star formation rates and galaxy evolution at a time when chemical abundances were markedly different to the present day. Although there are currently 12,000 strong intervening HI absorbers known, detected through the ultra-violet Lyman-alpha transition redshifted into the optical band at z >1.7, only 40 of these systems have been detected in 21-cm absorption. While Lyman-alpha traces all of the neutral gas, 21-cm traces the cool, star-forming component of this, and so is of particular importance in determining the star formation history of the Universe.
The authors had previously shown that the traditional selection of the optically identified absorbers biases against the intervening galaxies which contain the most dust, required to protect the cool phase of the atomic and molecular gas against the harsh UV environment. Searches for this material must therefore be radio biased. However, given that redshifts are usually obtained from good optical spectra means that we do not have a frequency at which to tune the radio receiver (see figure).
In their recent publication, the authors have therefore embarked upon a program of spectral scans, with the Green Bank Telescope (GBT), towards optically faint, but radio bright, objects in the search for any intervening galaxies responsible for the obscuration of the optical light from the background quasar. From the first of these, the extremely red (V - K = 10.26) MG J0414+0534, they have so far detected four gas-rich intervening systems, in addition to the previously known 21-cm absorption in the quasar host galaxy at z = 2.64. This is an unprecedented number of galaxies detected along a single sight-line and could have profound implications for how the Universe is populated by objects invisible to even the largest optical telescopes. Although equipped with wide-band spectrometers, the GBT is a single dish, susceptible to severe radio frequency interference (RFI). The Australian Square Kilometre Array Pathfinder, ASKAP, however, while equipped with a similarly wide instantaneous band-width, will have the further advantages of being an interferometer in an RFI quiet environment, thus being ideal in determining whether such a high occurrence of intervening galaxies is common.
by S. Curran