Galaxy formation and evolution

Abstract:

We exploit the deep and extended far-IR data sets (at 70, 100 and 160 μm) of the Herschel Guaranteed Time Observation (GTO) PACS Evolutionary Probe (PEP) Survey, in combination with the Herschel Multi-tiered Extragalactic Survey data at 250, 350 and 500 μm, to derive the evolution of the rest-frame 35-, 60-, 90- and total infrared (IR) luminosity functions (LFs) up to z ~ 4.We detect very strong luminosity evolution for the total IR LF (LIR α (1 + z)3.55 ± 0.10 up to z ~ 2, and α (1 + z)1.62 ± 0.51 at 2 < z 4) combined with a density evolution (α (1 + z)-0.57 ± 0.22 up to z ~ 1 and α (1 + z)-3.92 ± 0.34 at 1 < z 4). In agreement with previous findings, the IR luminosity density (ρIR) increases steeply to z ~ 1, then flattens between z ~ 1 and z ~ 3 to decrease at z 3. Galaxies with different spectral energy distributions, masses and specific star formation rates (SFRs) evolve in very different ways and this large and deep statistical sample is the first one allowing us to separately study the different evolutionary behaviours of the individual IR populations contributing to ρIR. Galaxies occupying the well-established SFR-stellar mass main sequence (MS) are found to dominate both the total IR LF and ρIR at all redshifts, with the contribution from off-MS sources (≥0.6 dex above MS) being nearly constant (~20 per cent of the total ρIR) and showing no significant signs of increase with increasing z over the whole 0.8 < z < 2.2 range. Sources with mass in the range 10 ≤ log(M/M⊙) ≤ 11 are found to dominate the total IR LF, with more massive galaxies prevailing at the bright end of the high-z (2) LF. A two-fold evolutionary scheme for IR galaxies is envisaged: on the one hand, a starburst-dominated phase in which the Super Massive Black Holes (SMBH) grows and is obscured by dust (possibly triggered by a major merging event), is followed by an AGN-dominated phase, then evolving towards a local elliptical. On the other hand, moderately star-forming galaxies containing a low-luminosity AGN have various properties suggesting they are good candidates for systems in a transition phase preceding the formation of steady spiral galaxies. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

The first deep blank-field 450 μm map (1σ ≈ 1.3 mJy) from the Submillimetre Common- User Bolometer Array-2 SCUBA-2 Cosmology Legacy Survey (S2CLS), conducted with the James Clerk Maxwell Telescope (JCMT) is presented. Our map covers 140 arcmin2 of the Cosmological Evolution Survey field, in the footprint of the Hubble Space Telescope (HST) Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey. Using 60 submillimetre galaxies detected at ≥3.75σ, we evaluate the number counts of 450-μm-selected galaxies with flux densities S450 > 5 mJy. The 8 arcsec JCMT beam and high sensitivity of SCUBA-2 now make it possible to directly resolve a larger fraction of the cosmic infrared background (CIB, peaking at λ ~ 200 μm) into the individual galaxies responsible for its emission than has previously been possible at this wavelength. At S450 > 5 mJy, we resolve (7.4 ± 0.7) x 10-2 MJy sr-1 of the CIB at 450 μm (equivalent to 16 ± 7 per cent of the absolute brightness measured by the Cosmic Background Explorer at this wavelength) into point sources. A further ~40 per cent of the CIB can be recovered through a statistical stack of 24 μm emitters in this field, indicating that the majority (≈60 per cent) of the CIB at 450 μm is emitted by galaxies with S450 > 2 mJy. The average redshift of 450 μm emitters identified with an optical/near-infrared counterpart is estimated to be 〈z〉 = 1.3, implying that the galaxies in the sample are in the ultraluminous class (LIR ≈ 1.1 x 1012 L⊙). If the galaxies contributing to the statistical stack lie at similar redshifts, then the majority of the CIB at 450 μm is emitted by galaxies in the luminous infrared galaxy (LIRG) class with LIR > 3.6 x 1011 L⊙. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We present Jansky Very Large Array observations of the cold (CO (1→0)) molecular gas in IRAS F10214+4724, a lensed ultraluminous infraRed galaxy (ULIRG) at z = 2.3 with an obscured active nucleus. The galaxy is spatially and spectrally well resolved in the CO (1→0) emission line. The total intensity and velocity maps reveal a reasonably ordered system; however, there is some evidence for minor merger activity. A CO (10) counter-image is detected at the 3σ level. Five of the 42 kms-1 channels (with >5σ detections) are mapped back into the source plane and their total magnification posterior probability distribution functions are sampled. This reveals a roughly linear arrangement, tentatively a rotating disc. We derive a molecular gas mass of Mgas = 1.2 ± 0.2 × 1010 M, assuming a ULIRG LCOto- Mgas conversion ratio of α = 0.8 M (K km s-1 pc2)-1 that agrees well with the derived range of α = 0.3-1.3M (K km s-1 pc2)-1 for separate dynamical mass estimates at assumed inclinations of i = 90°-30°. The lens modelling and CO (1→0) spectrum asymmetry suggest that there may be substantial (factor 2) preferential lensing of certain individual channels; however, the CO (1→0) spatially integrated channel flux uncertainties limit the significance of this result. Based on the AGN and CO (1→0) peak emission positions and the lens model, we predict a distortion of the CO spectral line energy distribution where higher order J lines that may be partially excited by AGN heating will be preferentially lensed owing to their smaller solid angles and closer proximity to the AGN and therefore the cusp of the caustic. Comparison with other lensing inversion results shows that the narrow-line region and AGN radio core in IRAS F10214+4724 are preferentially lensed by a factor of >3 and 11, respectively, relative to the molecular gas emission. This distorts the global continuum emission spectral energy distribution and strongly suggests caution in unsophisticated uses of IRAS F10214+4724 as an archetype high-redshift ULIRG. We explore two large velocity gradient models, incorporating spatial CO (10) and CO (32) information and present tentative evidence for an extended, low-excitation, cold gas component that implies that the total molecular gas mass in IRAS F10214+4724 is a factor of 2 greater than that calculated using spatially unresolved CO observations. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

In this work, we explore the impact of the presence of an active galactic nucleus (AGN) on the mid- and far-infrared (IR) properties of galaxies as well as the effects of simultaneous AGN and starburst activity in the same galaxies. To do this, we apply a multicomponent, multiband spectral synthesis technique to a sample of 250 μm selected galaxies of the Herschel Multitiered Extragalactic Survey (HerMES), with Infrared Spectrograph (IRS) spectra available for all galaxies. Our results confirm that the inclusion of the IRS spectra plays a crucial role in the spectral analysis of galaxies with an AGN component improving the selection of the best-fitting hot dust (torus) model. We find a correlation between the obscured star formation rate, SFRIR, derived from the IR luminosity of the starburst component, and SFRPAH, derived from the luminosity of the PAH features, LPAH, with SFRFIR taking higher values than SFRPAH. The correlation is different for AGN- and starburst-dominated objects. The ratio of LPAH to that of the starburst component, LPAH/LSB, is almost constant for AGN-dominated objects but decreases with increasing LSB for starburst-dominated objects. SFRFIR increases with the accretion luminosity, Lacc, with the increase less prominent for the very brightest, unobscured AGN-dominated sources. We find no correlation between the masses of the hot (AGN-heated) and cold (starburstheated) dust components. We interpret this as a non-constant fraction of gas driven by the gravitational effects to the AGN while the starburst is ongoing. We also find no evidenceof the AGN affecting the temperature of the cold dust component, though this conclusion is mostly based on objects with a non-dominant AGN component. We conclude that our findings do not provide evidence that the presence of AGN affects the star formation process in the host galaxy, but rather that the two phenomena occur simultaneously over a wide range of luminosities. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

By combining optical and near-IR observations from the Hubble Space Telescope with near-IR photometry from the Spitzer Space Telescope, it is possible to measure the rest-frame UV- optical colours of galaxies at z = 4-8. The UV-optical spectral energy distribution of star formation dominated galaxies is the result of several different factors. These include the joint distribution of stellar masses, ages and metallicities (solely responsible for the pure stellar spectral energy distribution), and the subsequent reprocessing by dust and gas in the interstellar medium. Using a large cosmological hydrodynamical simulation (MassiveBlack-II), we investigate the predicted spectral energy distributions of galaxies at high redshift with a particular emphasis on assessing the potential contribution of nebular emission. We find that the average (median) pure stellar UV-optical colour correlates with both luminosity and redshift such that galaxies at lower redshift and higher luminosity are typically redder. Assuming that the escape fraction of ionizing photons is close to zero, the effect of nebular emission is to redden theUV-optical 1500-Vω colour by, on average, 0.4mag at z=8 declining to 0.25 mag at z = 4. Young and low-metallicity stellar populations, which typically have bluer pure stellar UV-optical colours, produce larger ionizing luminosities and are thus more strongly affected by the reddening effects of nebular emission. This causes the distribution of 1500-Vω colours to narrow and the trends with luminosity and redshift to weaken. The strong effect of nebular emission leaves observed-frame colours critically sensitive to the redshift of the source. For example, increasing the redshift by 0.1 can result in observed-frame colours changing by up to ̃0.6. These predictions reinforce the need to include nebular emission when modelling the spectral energy distributions of galaxies at high redshift and also highlight the difficultly in interpreting the observed colours of individual galaxies without precise redshift information. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.