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Register a new userDeviations from passive evolution - star formation and the UV excess in z~1 radio galaxies
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Added by
Katherine J. Inskip
Publication date
2005
fields
Physics
and research's language is
English
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Galaxy colours are determined for two samples of 6C and 3CR radio sources at z~1, differing by a factor of ~6 in radio power. Corrections are made for emission line contamination and the presence of any nuclear point source, and the data analysed as a function of both redshift and the radio source properties. The galaxy colours are remarkably similar for the two populations, and the UV excess evolves with radio source size similarly in both samples, depsite the fact that the alignment effect is more extensive for the more powerful 3CR radio galaxies. These results seem to suggest that the alignment effect at these redshifts does not scale strongly with radio power, and is instead more closely dependent on galaxy mass (which is statistically comparable for the two samples). However, it is likely that the presence of relatively young (< several 10^8 years old) stellar populations has considerably contaminated the K-band flux of these systems, particularly in the case of the more powerful 3CR sources, which are ~0.5mag more luminous than the predictions of passive evolution models at z~1. The higher luminosity of the 3CR alignment effect is balanced by emission at longer wavelengths, thereby leading to comparable colours for the two samples.
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Using the Herschel Space Observatory we have observed a representative sample of 87 powerful 3CR sources at redshift z < 1. The far-infrared (FIR, 70-500 micron) photometry is combined with mid-infrared (MIR) photometry from the Wide-Field Infrared Survey Explorer (WISE) and catalogued data to analyse the complete spectral energy distributions (SEDs) of each object from optical to radio wavelength. To disentangle the contributions of different components, the SEDs are fitted with a set of templates to derive the luminosities of host galaxy starlight, dust torus emission powered by active galactic nuclei (AGN) and cool dust heated by stars. The level of emission from relativistic jets is also estimated, in order to isolate the thermal host galaxy contribution. The new data are in line with the orientation-based unification of high-excitation radio-loud AGN, in that the dust torus becomes optically thin longwards of 30 micron. The low excitation radio galaxies and the MIR weak sources represent MIR- and FIR-faint AGN population different from the high-excitation MIR-bright objects; it remains an open question whether they are at a later evolutionary state or an intrinsically different population. The derived luminosities for host starlight and dust heated by star formation are converted to stellar masses and star formation rates (SFR). The host-normalized SFR of the bulk of the 3CR sources is low when compared to other galaxy populations at the same epoch. Estimates of the dust mass yield a 1--100 times lower dust/stellar mass ratio than for the Milky Way, indicating that these 3CR hosts have very low levels of interstellar matter explaining the low level of star formation. Less than 10% of the 3CR sources show levels of star formation above those of the main sequence of star forming galaxies.
We use measurements of the stellar mass function, galaxy clustering, and galaxy-galaxy lensing within the COSMOS survey to constrain the stellar-to-halo mass relation (SHMR) of star forming and quiescent galaxies over the redshift range z=[0.2,1.0]. For massive galaxies, M*>~10^10.6 Msol, our results indicate that star-forming galaxies grow proportionately as fast as their dark matter halos while quiescent galaxies are outpaced by dark matter growth. At lower masses, there is minimal difference in the SHMRs, implying that the majority low-mass quiescent galaxies have only recently been quenched of their star formation. Our analysis also affords a breakdown of all COSMOS galaxies into the relative numbers of central and satellite galaxies for both populations. At z=1, satellite galaxies dominate the red sequence below the knee in the stellar mass function. But the number of quiescent satellites exhibits minimal redshift evolution; all evolution in the red sequence is due to low-mass central galaxies being quenched of their star formation. At M*~10^10 Msol, the fraction of central galaxies on the red sequence increases by a factor of ten over our redshift baseline, while the fraction of quenched satellite galaxies at that mass is constant with redshift. We define a migration rate to the red sequence as the time derivative of the passive galaxy abundances. We find that the migration rate of central galaxies to the red sequence increases by nearly an order of magnitude from z=1 to z=0. These results imply that the efficiency of quenching star formation for centrals is increasing with cosmic time, while the mechanisms that quench the star formation of satellite galaxies in groups and clusters is losing efficiency.
We use a new stacking technique to obtain mean mid IR and far IR to far UV flux ratios over the rest near-UV/near-IR color-magnitude diagram. We employ COMBO-17 redshifts and COMBO-17 optical, GALEX far and near UV, Spitzer IRAC and MIPS Mid IR photometry. This technique permits us to probe infrared excess (IRX), the ratio of far IR to far UV luminosity, and specific star formation rate (SSFR) and their co-evolution over two orders of magnitude of stellar mass and redshift 0.1<z<1.2. We find that the SSFR and the characteristic mass (M_0) above which the SSFR drops increase with redshift (downsizing). At any given epoch, IRX is an increasing function of mass up to M_0. Above this mass IRX falls, suggesting gas exhaustion. In a given mass bin below M_0 IRX increases with time in a fashion consistent with enrichment. We interpret these trends using a simple model with a Schmidt-Kennicutt law and extinction that tracks gas density and enrichment. We find that the average IRX and SSFR follows a galaxy age parameter which is determined mainly by the galaxy mass and time since formation. We conclude that blue sequence galaxies have properties which show simple, systematic trends with mass and time such as the steady build-up of heavy elements in the interstellar media of evolving galaxies and the exhaustion of gas in galaxies that are evolving off the blue sequence. The IRX represents a tool for selecting galaxies at various stages of evolution.
Combining the catalogue of galaxy morphologies in the COSMOS field and the sample of H$alpha$ emitters at redshifts $z=0.4$ and $z=0.84$ of the HiZELS survey, we selected $sim$ 220 star-forming bulgeless systems (Sersic index $n leq 1.5$) at both epochs. We present their star formation properties and we investigate their contribution to the star formation rate function (SFRF) and global star formation rate density (SFRD) at $z < 1$. For comparison, we also analyse H$alpha$ emitters with more structurally evolved morphologies that we split into two classes according to their Sersic index $n$: intermediate ($ 1.5 < n leq 3 $) and bulge-dominated ($n > 3$). At both redshifts the SFRF is dominated by the contribution of bulgeless galaxies and we show that they account for more than 60% of the cosmic SFRD at $z < 1$. The decrease of the SFRD with redshift is common to the three morphological types but it is stronger for bulge-dominated systems. Star-forming bulgeless systems are mostly located in regions of low to intermediate galaxy densities ($Sigma sim 1 - 4$ Mpc$^{-2}$) typical of field-like and filament-like environments and their specific star formation rates (sSFRs) do not appear to vary strongly with local galaxy density. Only few bulgeless galaxies in our sample have high (sSFR $>$ 10$^{-9}$ yr$^{-1}$) and these are mainly low-mass systems. Above $M_* sim 10^{10}$ M$_{odot}$ bulgeless are evolving at a normal rate (10$^{-9}$ yr$^{-1} <$ sSFR $<$10$^{-10}$ yr$^{-1}$) and in the absence of an external trigger (i.e. mergers/strong interactions) they might not be able to develop a central classical bulge.
We study the ultraviolet to far-infrared (hereafter UV-to-IR) SEDs of a sample of intermediate redshift (0.2 < z < 0.7) UV-selected galaxies from the ELAIS-N1 and ELAIS-N2 fields by fitting a multi-wavelength dataset to a library of GRASIL templates. Star formation related properties of the galaxies are derived from the library of models by using the Bayesian statistics. We find a decreasing presence of galaxies with low attenuation and low total luminosity as redshift decreases, which does not hold for high total luminosity galaxies. In addition the dust attenuation of low mass galaxies increases as redshift decreases, and this trend seems to disappear for galaxies with M* > 10^11 M_sun. This result is consistent with a mass dependent evolution of the dust to gas ratio, which could be driven by a mass dependent efficiency of star formation in star forming galaxies. The specific star formation rates (SSFR) decrease with increasing stellar mass at all redshifts, and for a given stellar mass the SSFR decreases with decreasing redshift. The differences in the slope of the M*--SSFR relation found between this work and others at similar redshift could be explained by the adopted selection criteria of the samples which, for a UV selected sample, favours blue, star forming galaxies.
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