No abstract

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

We use deep Herschel observations taken with both PACS and SPIRE imaging cameras to estimate the dust mass of a sample of galaxies extracted from the GOODS-S, GOODS-N and the COSMOS fields. We divide the redshift-stellar mass (M star )-star formation rate (SFR) parameter space into small bins and investigate average properties over this grid. In the first part of the work we investigate the scaling relations between dust mass, stellar mass and SFR out to z = 2.5. No clear evolution of the dust mass with redshift is observed at a given SFR and stellar mass. We find a tight correlation between the SFR and the dust mass, which, under reasonable assumptions, is likely a consequence of the Schmidt-Kennicutt (S-K) relation. The previously observed correlation between the stellar content and the dust content flattens or sometimes disappears when considering galaxies with the same SFR. Our finding suggests that most of the correlation between dust mass and stellar mass obtained by previous studies is likely a consequence of the correlation between the dust mass and the SFR combined with the main sequence, i.e., the tight relation observed between the stellar mass and the SFR and followed by the majority of star-forming galaxies. We then investigate the gas content as inferred from dust mass measurements. We convert the dust mass into gas mass by assuming that the dust-to-gas ratio scales linearly with the gas metallicity (as supported by many observations). For normal star-forming galaxies (on the main sequence) the inferred relation between the SFR and the gas mass (integrated S-K relation) broadly agrees with the results of previous studies based on CO measurements, despite the completely different approaches. We observe that all galaxies in the sample follow, within uncertainties, the same S-K relation. However, when investigated in redshift intervals, the S-K relation shows a moderate, but significant redshift evolution. The bulk of the galaxy population at z ∼ 2 converts gas into stars with an efficiency (star formation efficiency, SFE = SFR/M gas , equal to the inverse of the depletion time) about 5 times higher than at z ∼ 0. However, it is not clear what fraction of such variation of the SFE is due to an intrinsic redshift evolution and what fraction is simply a consequence of high-z galaxies having, on average, higher SFR, combined with the super-linear slope of the S-K relation (while other studies find a linear slope). We confirm that the gas fraction ( f gas = M gas /(M gas + M star )) decreases with stellar mass and increases with the SFR. We observe no evolution with redshift once M star and SFR are fixed. We explain these trends by introducing a universal relation between gas fraction, stellar mass and SFR that does not evolve with redshift, at least out to z ∼ 2.5. Galaxies move across this relation as their gas content evolves across the cosmic epochs. We use the 3D fundamental f gas -M star -SFR relation, along with the evolution of the main sequence with redshift, to estimate the evolution of ...

We present detailed clustering measurements from the 2dF Quasi‐Stellar Object Redshift Survey (2QZ) in the redshift range 0.8 < z < 2.1. Using a flux‐limited sample of ∼14 000 objects with effective redshift zeff= 1.47, we estimate the quasar projected correlation function for separations 1 < r/h−1  Mpc < 20. We find that the two‐point correlation function in real space is well approximated by a power law with slope γ= 1.5 ± 0.2 and comoving correlation length r0= 4.8+0.9−1.5 h−1 Mpc. Splitting the sample into three subsets based on redshift, we find evidence for an increase of the clustering amplitude with look‐back time. For a fixed γ, evolution of r0 is detected at the 3.6σ confidence level. The ratio between the quasar correlation function and the mass autocorrelation function (derived adopting the concordance cosmological model) is found to be scale‐independent. For a linear mass‐clustering amplitude σ8= 0.8, the ‘bias parameter’ decreases from b≃ 3.9 at zeff= 1.89 to b≃ 1.8 at zeff= 1.06. From the observed abundance and clustering, we infer how quasars populate dark matter haloes of different masses. We find that 2QZ quasars sit in haloes with M > 1012 M⊙ and that the characteristic mass of their host haloes is of the order of 1013 M⊙. The observed clustering is consistent with assuming that the locally observed correlation between black hole mass and host galaxy circular velocity is still valid at z > 1. From the fraction of haloes which contain active quasars, we infer that the characteristic quasar lifetime is tQ∼ a few × 107 yr at z∼ 1 and approaches 108 yr at higher redshifts.

Aims. We exploit deep observations of the GOODS-N field taken with PACS, the Photodetector Array Camera and Spectrometer, onboard of Herschel, as part of the PACS evolutionary probe guaranteed time (PEP), to study the link between star formation and stellar mass in galaxies to z ∼ 2. Methods. Starting from a stellar mass -selected sample of ∼4500 galaxies with mag 4.5 μm < 23.0 (AB), we identify ∼350 objects with a PACS detection at 100 or 160 μm and ∼ 1500 with only Spitzer 24 μm counterpart. Stellar masses and total IR luminosities (L IR ) are estimated by fitting the spectral energy distributions (SEDs). Results. Consistently with other Herschel results, we find that L IR based only on 24 μm data is overestimated by a median factor ∼ 1.8 at z ∼ 2, whereas it is underestimated (with our approach) up to a factor ∼ 1.6 at 0.5 < z < 1.0. We then exploit this calibration to correct L IR based on the MIPS/Spitzer fluxes. These results clearly show how Herschel is fundamental to constrain L IR , and hence the star formation rate (SFR), of high redshift galaxies. Using the galaxies detected with PACS (and/or MIPS), we investigate the existence and evolution of the relations between the SFR, the specific star formation rate (SSFR=SFR/mass) and the stellar mass. Moreover, in order to avoid selection effects, we also repeat this study through a stacking analysis on the PACS images to fully exploit the far-IR information also for the Herschel and Spitzer undetected subsamples. We find that the SSFR-mass relation steepens with redshift, being almost flat at z < 1.0 and reaching a slope of α = −0.50 +0.13 −0.16 at z ∼ 2, at odds with recent works based on radio-stacking analysis at the same redshift. The mean SSFR of galaxies increases with redshift, by a factor ∼15 for massive M > 10 11 M galaxies from z = 0 to z = 2, and seems to flatten at z > 1.5 in this mass range. Moreover, the most massive galaxies have the lowest SSFR at any z, implying that they have formed their stars earlier and more rapidly than their low mass counterparts (downsizing).