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Register a new userCosmic evolution of star-forming galaxies to $z simeq 1.8$ in the faint low-frequency radio source population
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We study the properties of star-forming galaxies selected at 610 MHz with the GMRT in a survey covering $sim$1.86 deg$^2$ down to a noise of $sim$7.1,$mu$Jy / beam. These were identified by combining multiple classification diagnostics: optical, X-ray, infrared and radio data. Of the 1685 SFGs from the GMRT sample, 496 have spectroscopic redshifts whereas 1189 have photometric redshifts. We find that the IRRC of star-forming galaxies, quantified by the infrared-to-1.4 GHz radio luminosity ratio $rm{q_{IR}}$, decreases with increasing redshift: $rm{q_{IR},=,2.86pm0.04(1,+,z)^{-0.20pm0.02}}$ out to $z sim 1.8$. We use the $rm{V/V_{max}}$ statistic to quantify the evolution of the co-moving space density of the SFG sample. Averaged over luminosity our results indicate $rm{langle V/V_{max} rangle}$ to be $rm{0.51,pm, 0.06}$, which is consistent with no evolution in overall space density. However we find $rm V/V_{max}$ to be a function of radio luminosity, indicating strong luminosity evolution with redshift. We explore the evolution of the SFGs radio luminosity function by separating the source into five redshift bins and comparing to theoretical model predictions. We find a strong redshift trend that can be fitted with a pure luminosity evolution of the form $rm{L_{610,MHz},propto,(,1+,z)^{(2.95pm0.19)-(0.50pm0.15)z}}$. We calculate the cosmic SFR density since $rm{z sim 1.5}$ by integrating the parametric fits of the evolved 610,MHz luminosity function. Our sample reproduces the expected steep decline in the star formation rate density since $rm{z,sim,1}$.
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We present an analysis of the chemical abundance properties of $approx$650 star-forming galaxies at $z approx0.6-1.8$. Using integral-field observations from the $K$-band Multi-Object Spectrograph (KMOS), we quantify the [NII]/H$alpha$ emission-line ratio, a proxy for the gas-phase Oxygen abundance within the interstellar medium. We define the stellar mass-metallicity relation at $z approx0.6-1.0$ and $z approx1.2-1.8$ and analyse the correlation between the scatter in the relation and fundamental galaxy properties (e.g. H$alpha$ star-formation rate, H$alpha$ specific star-formation rate, rotation dominance, stellar continuum half-light radius and Hubble-type morphology). We find that for a given stellar mass, more highly star-forming, larger and irregular galaxies have lower gas-phase metallicities, which may be attributable to their lower surface mass densities and the higher gas fractions of irregular systems. We measure the radial dependence of gas-phase metallicity in the galaxies, establishing a median, beam smearing-corrected, metallicity gradient of $ Delta Z/ Delta R=0.002 pm0.004$ dex kpc$^{-1}$, indicating on average there is no significant dependence on radius. The metallicity gradient of a galaxy is independent of its rest-frame optical morphology, whilst correlating with its stellar mass and specific star-formation rate, in agreement with an inside-out model of galaxy evolution, as well as its rotation dominance. We quantify the evolution of metallicity gradients, comparing the distribution of $Delta Z/ Delta R$ in our sample with numerical simulations and observations at $z approx0-3$. Galaxies in our sample exhibit flatter metallicity gradients than local star-forming galaxies, in agreement with numerical models in which stellar feedback plays a crucial role redistributing metals.
We have measured the radial profiles of isophotal ellipticity ($varepsilon$) and disky/boxy parameter A$_4$ out to radii of about three times the semi-major axes for $sim4,600$ star-forming galaxies (SFGs) at intermediate redshifts $0.5<z<1.8$ in the CANDELS/GOODS-S and UDS fields. Based on the average size versus stellar-mass relation in each redshift bin, we divide our galaxies into Small SFGs (SSFGs), i.e., smaller than average for its mass, and Large SFGs (LSFGs), i.e., larger than average. We find that, at low masses ($M_{ast} < 10^{10}M_{odot}$), the SSFGs generally have nearly flat $varepsilon$ and A$_4$ profiles for both edge-on and face-on views, especially at redshifts $z>1$. Moreover, the median A$_4$ values at all radii are almost zero. In contrast, the highly-inclined, low-mass LSFGs in the same mass-redshift bins generally have monotonically increasing $varepsilon$ with radius and are dominated by disky values at intermediate radii. These findings at intermediate redshifts imply that low-mass SSFGs are not disk-like, while low-mass LSFGs appear to harbour disk-like components flattened by significant rotation. At high masses ($M_{ast} > 10^{10}M_{odot}$), highly-inclined SSFGs and LSFGs both exhibit a general, distinct trend for both $varepsilon$ and A$_4$ profiles: increasing values with radius at lower radii, reaching maxima at intermediate radii, and then decreasing values at larger radii. Such a trend is more prevalent for more massive ($M_{ast} > 10^{10.5}M_{odot}$) galaxies or those at lower redshifts ($z<1.4$). The distinct trend in $varepsilon$ and A$_4$ can be simply explained if galaxies possess all three components: central bulges, disks in the intermediate regions, and halo-like stellar components in the outskirts.
We study the cosmic evolution of radio sources out to $z simeq 1.5$ using a GMRT 610 MHz survey covering $sim$1.86 deg$^2$ of the ELAIS N1 field with a minimum/median rms noise 7.1/19.5,$mu$Jy / beam and an angular resolution of 6,arcsec. We classify sources as star forming galaxies (SFGs), radio-quiet (RQ) and radio-loud (RL) Active Galactic Nuclei (AGN) using a combination of multi-wavelength diagnostics and find evidence in support of the radio emission in SFGs and RQ AGN arising from star formation, rather than AGN-related processes. At high luminosities, however, both SFGs and RQ AGN display a radio excess when comparing radio and infrared star formation rates. The vast majority of our sample lie along the $rm{SFR - M_{star}}$ main sequence at all redshifts when using infrared star formation rates. We derive the 610 MHz radio luminosity function for the total AGN population, constraining its evolution via continuous models of pure density and pure luminosity evolution with $rm{Phi^{star},propto,(,1+,z)^{(2.25pm0.38)-(0.63pm0.35)z}}$ and $rm{L_{610,MHz},propto,(,1+,z)^{(3.45pm0.53)-(0.55pm0.29)z}}$ respectively. For our RQ and RL AGN, we find a fairly mild evolution with redshift best fitted by pure luminosity evolution with $rm{L_{610,MHz},propto,(,1+,z)^{(2.81pm0.43)-(0.57pm0.30)z}}$ for RQ AGN and $rm{L_{610,MHz},propto,(,1+,z)^{(3.58pm0.54)-(0.56pm0.29)z}}$ for RL AGN. The 610 MHz radio AGN population thus comprises two differently evolving populations whose radio emission is mostly SF-driven or AGN-driven respectively.
We present the results of a study investigating the dust attenuation law at $zsimeq 5$, based on synthetic spectral energy distributions (SEDs) calculated for a sample of N=498 galaxies drawn from the First Billion Years (FiBY) simulation project. The simulated galaxies at $zsimeq 5$, which have M$_{1500} leq -18.0$ and $7.5 leq rm{log(M/M}_{odot}rm{)} leq 10.2$, display a mass-dependent $alpha$-enhancement, with a median value of $[alpha/rm{Fe}]_{z=5}~simeq~4~times~[alpha/rm{Fe}]_{Z_{odot}}$. The median Fe/H ratio of the simulated galaxies is $0.14pm0.05$ which, even including the effects of nebular continuum, produces steep intrinsic UV continuum slopes; $langle beta_{i} rangle = -2.4 pm 0.05$. Using a set of simple dust attenuation models, in which the wavelength-dependent attenuation is assumed to be of the form $A(lambda) propto lambda^{n}$, we explore the parameter values which best reproduce the observed $z=5$ luminosity function (LF) and colour-magnitude relation (CMR). We find that a simple model in which the absolute UV attenuation is a linearly increasing function of log stellar mass, and the dust attenuation slope ($n$) is within the range $-0.7 leq n leq-0.3$, can successfully reproduce the LF and CMR over a wide range of stellar population synthesis model (SPS) assumptions. This range of attenuation curves is consistent with a power-law fit to the Calzetti attenuation law in the UV ($n=-0.55$), and other similarly `grey star-forming galaxy attenuation curves recently derived at $zsimeq2$. In contrast, attenuation curves as steep as the Small Magellanic Cloud (SMC) extinction curve ($n=-1.24$) are formally ruled out. Finally, we show that our models are consistent with recent 1.3mm ALMA observations of the Hubble Ultra Deep Field (HUDF), and predict the form of the $zsimeq5$ IRX$-beta$ relation.
We present radio observations of ultraluminous infrared galaxies (ULIRGs) using the Giant Metrewave Radio Telescope (GMRT) and combine them with archival multi-frequency observations to understand whether ULIRGs are the progenitors of the powerful radio loud galaxies in the local Universe. ULIRGs are characterized by large infrared luminosities ($L_{IR}>$10$^{12}$L$odot$), large dust masses ($sim10^{8}M_{odot}$) and vigorous star formation (star formation rates $sim$10-100 $M_{odot}~$yr$^{-1}$). Studies show that they represent the end stages of mergers of gas-rich spiral galaxies. Their luminosity can be due to both starburst activity and active galactic nuclei (AGN). We study a sample of 13 ULIRGs that have optically identified AGN characteristics with 1.28~GHz GMRT observations. Our aim is to resolve any core-jet structures or nuclear extensions and hence examine whether the ULIRGs are evolving into radio loud ellipticals. Our deep, low frequency observations show marginal extension for only one source. However, the integrated radio spectra of 9 ULIRGs show characteristics that are similar to that of GPS/CSS/CSO/young radio sources. The estimated spectral ages are 0.4 to 20 Myr and indicate that they are young radio sources and possible progenitors of radio galaxies. Hence, we conclude that although most ULIRGs do not show kpc scale extended radio emission associated with nuclear activity, their radio spectral energy distributions do show signatures of young radio galaxies.
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