Research @ KICP
February 17, 2010
New Population of High-Redshift Star-Forming Galaxies Discovered with the South Pole Telescope
by The SPT group at KICP
The 10-meter South Pole Telescope (SPT), equipped with a sensitive, large-format bolometer array camera at its focus, is being used to map large areas of the southern sky to exquisite precision at millimeter (mm) wavelengths. Although the primary goal of the survey is to find galaxy clusters through the Sunyaev-Zel'dovich (SZ) effect [Galaxy clusters discovered with the South Pole Telescope], there are many other signals of interest in the SPT maps. In particular, the sensitivity and angular resolution of the SPT with its current camera, which are key to conducting a successful SZ cluster survey, also make it an excellent tool for detecting extragalactic sources of emission. An unexpected result of searching the multi-band SPT data for extragalactic sources was the discovery of the new population of mm-bright star-forming galaxies. These sources are much brighter than the known population of submillimeter-selected galaxies, and they appear to lie at high redshift, meaning that they must either be intrinsically very luminous at millimeter wavelenghts --- more so than predicted by standard models of galaxy formation --- or be strongly gravitationally lensed by intervening structure.
In maps made from data taken in the current SPT configuration, extragalactic sources are detectable down to unprecedentedly low flux levels for a large survey at mm wavelengths. For example, the WMAP satellite has mapped the entire sky up at frequencies up to 90 GHz (wavelengths down to 3 mm), but only to source flux levels of approximately 1 Jansky (Jy). Instruments such as the AzTEC camera on the ASTE telescope have mm-wave source sensitivity of a few milliJansky (mJy) but have only mapped a few square degrees of sky. In contrast, the SPT has already mapped over 800 square degrees of sky with sensitivity of a few to ten mJy in three wavelength bands.
Based on previous results from radio surveys and small mm-wave surveys, we expect the extragalactic sources that we detect in the SPT maps to be dominated by one of two distinct sources of emission: 1) synchrotron emission from active galactic nuclei (AGN); 2) thermal emission from dust grains heated by starlight to temperatures of tens of Kelvin. Though some galaxies will exhibit both of these types of emission, in general the dust emission is expected to come from a population of very distant (high-redshift, z >~ 2) galaxies that are undergoing a prodigious burst of star formation, while the AGN are expected to reside in a population of galaxies that peaks at a lower redshift (z ~= 1).
With an instrument that surveys the sky in a single frequency band, it would be impossible to distinguish between these two sources of emission. However, with its multiple observing bands, the current SPT camera allows us to tell these sources apart based on the frequency dependence of the emission. At mm wavelengths, the thermal emission from dust increases with increasing observing frequency, while the synchrotron emission from AGN does the opposite. Using this behavior to separate SPT-detected sources into two families, Vieira et al. (2009) recently reported source counts as a function of brightness for dust-dominated and synchrotron-dominated sources from an initial, small (100-square-degree) subset of the SPT survey data.
Dusty-galaxy source counts as a function of flux measured by SPT at 1.4 mm, compared to model predictions. Nearby sources detectable by IRAS have been removed from both the data and the models. The large discrepancy between data and models above 15-20 mJy shows that the SPT is detecting a new population of mm-bright sources.
SPT measurements of the AGN population as a function of source brightness are consistent with extrapolations from lower frequencies, but the SPT-detected dusty sources include an unexpected population of very bright sources. These sources are significantly brighter than those discovered by AzTEC and sub-millimeter instruments such as SCUBA, and most of them do not have counterparts in catalogs compiled from Infrared Astronomy Satellite (IRAS) data, indicating they are not members of the population of nearby (low-redshift) starburst galaxies known as ultra-luminous IR galaxies (ULIRGs). Analysis of IRAS maps at the locations of these new SPT sources shows that they must either be at redshifts similar to or higher than the z >~ 2 population seen by AzTEC and SCUBA or have dust that is far colder than expected.
If these sources are indeed at high redshift, they represent an exciting new population. For such bright sources to exist at such great distances, they must either be much more intrinsically bright than most galaxy formation models predict or have been strongly gravitationally lensed by intervening matter. Either way, this new population presents an opportunity to learn a great deal about the early stages of galaxy formation and evolution.
Scatter plot of IRAS 100-micron flux vs. SPT 1.4-mm flux for the dusty population detected by SPT, along with lines of constant flux ratio predicted by modified blackbody models at different combinations of dust temperature and redshift. This shows that the sources detected in SPT but not in IRAS must either be at moderate to high redshift or have anomalously cold dust.
Efforts are now underway to observe these sources in many different regions of the electromagnetic spectrum, from radio to optical. These multi-wavelength follow-up observations will help pin down the nature of this exciting new population of sources discovered by the SPT.