Galaxy formation and evolution

Abstract:

Using Herschel Photodetector Array Camera (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) observations of Lockman Hole-North and Great Observatories Origins Deep Survey-North (GOODS-N) as part of the Herschel Multi-tiered Extragalactic Survey (HerMES) project, we explore the far-infrared (IR) properties of a sample of mid-IR-selected starburst-dominated ultraluminous infrared galaxies (ULIRGs) at z~ 2. The selection of the sample is based on the detection of the stellar bump that appears in the spectral energy distribution of star-forming galaxies at 1.6 μm. We derive robust estimates of infrared luminosities (LIR) and dust temperatures (Td) of the population and find that while the luminosities in our sample span less than an order of magnitude (12.24 ≤ log(LIR/L·) ≤ 12.94), they cover a wide range of dust temperatures (25 ≤Td≤ 62 K). Galaxies in our sample range from those that are as cold as high-z submillimetre galaxies (SMGs) to those that are as warm as optically faint radio galaxies (OFRGs) and local ULIRGs. Nevertheless, our sample has median Td= 42.3 K, filling the gap between SMGs and OFRGs, bridging the two populations. We demonstrate that a significant fraction of our sample would be missed from ground-based (sub)mm surveys (850-1200 μm), showing that the latter introduce a bias towards the detection of colder sources. We conclude that Herschel observations confirm the existence of high-z ULIRGs warmer than SMGs, show that the mid-IR selection of high-z ULIRGs is not Td dependent, reveal a large dispersion in Td of high-z ULIRGs and provide the means to characterize the bulk of the ULIRG population, free from selection biases introduced by ground-based (sub)mm surveys. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

We use the Herschel-Astrophysical Terahertz Large Area Survey (ATLAS) science demonstration data to investigate the star formation properties of radio-selected galaxies in the GAMA-9h field as a function of radio luminosity and redshift. Radio selection at the lowest radio luminosities, as expected, selects mostly starburst galaxies. At higher radio luminosities, where the population is dominated by active galactic nuclei (AGN), we find that some individual objects are associated with high far-infrared luminosities. However, the far-infrared properties of the radio-loud population are statistically indistinguishable from those of a comparison population of radio-quiet galaxies matched in redshift and K-band absolute magnitude. There is thus no evidence that the host galaxies of these largely low-luminosity (Fanaroff-Riley class I), and presumably low-excitation, AGN, as a population, have particularly unusual star formation histories. Models in which the AGN activity in higher luminosity, high-excitation radio galaxies is triggered by major mergers would predict a luminosity-dependent effect that is not seen in our data (which only span a limited range in radio luminosity) but which may well be detectable with the full Herschel-ATLAS data set. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

We present the first results of a new Keck spectroscopic survey of UV faint Lyman break galaxies in the redshift range 3 < z < 7. Combined with earlier Keck and published European Southern Observatory (ESO) VLT data, our spectroscopic sample contains more than 600 dropouts offering new insight into the nature of sub-L* sources typical of those likely to dominate the cosmic reionization process. In this first paper, in a series discussing these observations, we characterize the fraction of strong Lyα emitters within the continuum-selected dropout population. By quantifying how the 'Lyα fraction', xLyα, varies with redshift, we seek to constrain changes in Lyα transmission associated with reionization. In order to distinguish the effects of reionization from other factors which affect the Lyα fraction [e.g. dust, interstellar medium (ISM) kinematics], we study the luminosity and redshift-dependence of the Lyα fraction over 3 ≲z≲ 6, when the intergalactic medium (IGM) is known to be ionized. These results reveal that low-luminosity galaxies show strong Lyα emission much more frequently (xLyα= 0.47 ± 0.16 at MUV=-19) than luminous systems (xLyα= 0.08 ± 0.02 at MUV=-21), and that at fixed luminosity, the prevalence of strong Lyman α emission increases moderately with redshift over 3 < z < 6 (d xLyα/d z= 0.05 ± 0.03). Based on the bluer mean UV slopes of the strong Lyα emitting galaxies in our data set (〈Β〉Lyα-〈Β〉noLyα=-0.33 ± 0.09 at MUV=-20.5) we argue that the Lyα fraction trends are governed by redshift and luminosity-dependent variations in the dust obscuration, with likely additional contributions from trends in the kinematics and covering fraction of neutral hydrogen. Using the limited infrared spectroscopy of candidate z{reversed tilde} 7 galaxies, we find a tentative decrease in the Lyα fraction by a factor of >1.9 with respect to the predicted z{reversed tilde} 7 value, a result which, if confirmed with future surveys, would suggest an increase in the neutral fraction by this epoch. Given the abundant supply of z and Y drops now available from deep Hubble WFC3/IR surveys, we show it will soon be possible to significantly improve estimates of the Lyα fraction using optical and near-infrared multi-object spectrographs, thereby extending the study conducted in this paper to 7 ≲z≲ 8. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

One of the most fundamental problems in modern astrophysics concerns the formation of galaxies and clusters of galaxies. The Dutch-European radio telescope LOFAR will open up the last unexplored window of the electromagnetic spectrum for astrophysical studies and make important contributions to our knowledge of the structure formation in the universe. LOFAR's world-class observational capabilities will be used to survey the entire Northern low-frequency sky at a number of key frequencies. Studies of the most distant radio galaxies, clusters of galaxies and the cosmic star formation history and the exploration of new parameter space for serendipitous discovery were the four key topics that drove the areas, depths and frequency coverage of the proposed surveys. In addition to the key topics, the LOFAR surveys will provide a wealth of unique data for a huge number of additional important topics, including: detailed studies of AGN, and AGN physics, AGN evolution and black hole accretion history, nearby galaxies, strong gravitational lenses, cosmological parameters and large-scale structure formation, and Galactic radio sources. In this contribution we will first briefly discuss the scientific topics that have driven the design of the surveys. Subsequently we will present the design of the surveys. We will then briefly report on commissioning work carried out to prepare the instrument and the software pipelines for carrying out these surveys. At the end we will elaborate on LOFAR studies on clusters and show some first LOFAR results related to the nearby rich cluster Abell 2256. With at the time of writing only 15 out of the planned 36 Dutch stations working and several aspects of the calibration pipleline not fully functional, the obtained 135 MHz image already is among the deepest ever produced at low frequencies. The central halo of A2256 is well detected, illustrating the potential of LOFAR to map diffuse steep spectrum radio emission.

Abstract:

The reliability of infrared (IR) and ultraviolet (UV) emissions to measure star formation rates (SFRs) in galaxies is investigated for a large sample of galaxies observed with the Spectral and Photometric Imaging Receiver (SPIRE) and the Photodetector Array Camera and Spectrometer (PACS) instruments on Herschel as part of the Herschel Multi-Tiered Extragalactic Survey (HerMES) project. We build flux-limited 250-jj.m samples of sources at redshift z < 1, crossmatched with the Spitzer/MIPS and GALEX catalogues. About 60 per cent of the Herschel sources are detected in UV. The total IR luminosities, LIR, of the sources are estimated using a spectral energy distribution (SED) fitting code that fits to fluxes between 24 and 500 μm. Dust attenuation is discussed on the basis of commonly used diagnostics: the LIR/LUV ratio and the slope, β, of the UV continuum. A mean dust attenuation AUV of ≃3 mag is measured in the samples. LIR/LUV LIR. Galaxies with LIR > 1011 L⊙ and 0.5 < z < 1 exhibit a mean dust attenuation AUV of about 0.7 mag lower than that found for their local counterparts, although with a large dispersion. Our galaxy samples span a large range of β and LIR/LUV values which, for the most part, are distributed between the ranges defined by the relations found locally for starburst and normal star-forming galaxies. As a consequence the recipe commonly applied to local starbursts is found to overestimate the dust attenuation correction in our galaxy sample by a factor of ~2-3. The SFRs deduced from LIR are found to account for about 90 per cent of the total SFR; this percentage drops to 71 per cent for galaxies with SFR < 1 M⊙ yr-1 (or LIR < 1010L⊙). For these faint objects, one needs to combine UV and IR emissions to obtain an accurate measure of the SFR. © 2010 The Authors. Journal compilation © 2010 RAS.