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Register a new userThe KMOS$^{rm 3D}$ Survey: Investigating the Origin of the Elevated Electron Densities in Star-Forming Galaxies at $1lesssim{z}lesssim{3}$
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We investigate what drives the redshift evolution of the typical electron density ($n_e$) in star-forming galaxies, using a sample of 140 galaxies drawn primarily from KMOS$^{rm 3D}$ ($0.6lesssim{z}lesssim{2.6}$) and 471 galaxies from SAMI ($z<0.113$). We select galaxies that do not show evidence of AGN activity or outflows, to constrain the average conditions within H II regions. Measurements of the [SII]$lambda$6716/[SII]$lambda$6731 ratio in four redshift bins indicate that the local $n_e$ in the line-emitting material decreases from 187$^{+140}_{-132}$ cm$^{-3}$ at $zsim$ 2.2 to 32$^{+4}_{-9}$ cm$^{-3}$ at $zsim$ 0; consistent with previous results. We use the H$alpha$ luminosity to estimate the root-mean-square (rms) $n_e$ averaged over the volumes of star-forming disks at each redshift. The local and volume-averaged $n_e$ evolve at similar rates, hinting that the volume filling factor of the line-emitting gas may be approximately constant across $0lesssim{z}lesssim{2.6}$. The KMOS$^{rm 3D}$ and SAMI galaxies follow a roughly monotonic trend between $n_e$ and star formation rate, but the KMOS$^{rm 3D}$ galaxies have systematically higher $n_e$ than the SAMI galaxies at fixed offset from the star-forming main sequence, suggesting a link between the $n_e$ evolution and the evolving main sequence normalization. We quantitatively test potential drivers of the density evolution and find that $n_e$(rms) $simeq{n_{H_2}}$, suggesting that the elevated $n_e$ in high-$z$ H II regions could plausibly be the direct result of higher densities in the parent molecular clouds. There is also tentative evidence that $n_e$ could be influenced by the balance between stellar feedback, which drives the expansion of H II regions, and the ambient pressure, which resists their expansion.
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We analyse the velocity dispersion properties of 472 z~0.9 star-forming galaxies observed as part of the KMOS Redshift One Spectroscopic Survey (KROSS). The majority of this sample is rotationally dominated (83 +/- 5% with v_C/sigma_0 > 1) but also dynamically hot and highly turbulent. After correcting for beam smearing effects, the median intrinsic velocity dispersion for the final sample is sigma_0 = 43.2 +/- 0.8 km/s with a rotational velocity to dispersion ratio of v_C/sigma_0 = 2.6 +/- 0.1. To explore the relationship between velocity dispersion, stellar mass, star formation rate and redshift we combine KROSS with data from the SAMI survey (z~0.05) and an intermediate redshift MUSE sample (z~0.5). While there is, at most, a weak trend between velocity dispersion and stellar mass, at fixed mass there is a strong increase with redshift. At all redshifts, galaxies appear to follow the same weak trend of increasing velocity dispersion with star formation rate. Our results are consistent with an evolution of galaxy dynamics driven by disks that are more gas rich, and increasingly gravitationally unstable, as a function of increasing redshift. Finally, we test two analytic models that predict turbulence is driven by either gravitational instabilities or stellar feedback. Both provide an adequate description of the data, and further observations are required to rule out either model.
To investigate the growth history of galaxies, we measure the rest-frame radio, ultraviolet (UV), and optical sizes of 98 radio-selected, star-forming galaxies (SFGs) distributed over $0.3 lesssim z lesssim 3$ and median stellar mass of $log(M_star/ rm M_odot)approx10.4$. We compare the size of galaxy stellar disks, traced by rest-frame optical emission, relative to the overall extent of star formation activity that is traced by radio continuum emission. Galaxies in our sample are identified in three Hubble Frontier Fields: MACSJ0416.1$-$2403, MACSJ0717.5+3745, and MACSJ1149.5+2223. Radio continuum sizes are derived from 3 GHz and 6 GHz radio images ($lesssim 0$$.6$ resolution, $approx0.9, rm mu Jy, beam^{-1}$ noise level) from the Karl G. Jansky Very Large Array. Rest-frame UV and optical sizes are derived using observations from the Hubble Space Telescope and the ACS and WFC3 instruments. We find no clear dependence between the 3 GHz radio size and stellar mass of SFGs, which contrasts with the positive correlation between the UV/optical size and stellar mass of galaxies. Focusing on SFGs with $log(M_star/rm M_odot)>10$, we find that the radio/UV/optical emission tends to be more compact in galaxies with high star-formation rates ($rm SFRgtrsim 100,M_odot,yr^{-1}$), suggesting that a central, compact starburst (and/or an Active Galactic Nucleus) resides in the most luminous galaxies of our sample. We also find that the physical radio/UV/optical size of radio-selected SFGs with $log(M_star/rm M_odot)>10$ increases by a factor of $1.5-2$ from $zapprox 3$ to $zapprox0.3$, yet the radio emission remains two-to-three times more compact than that from the UV/optical. These findings indicate that these massive, {radio-selected} SFGs at $0.3 lesssim z lesssim 3$ tend to harbor centrally enhanced star formation activity relative to their outer-disks.
We present the results of a new study of dust attenuation at redshifts $3 < z < 4$ based on a sample of $236$ star-forming galaxies from the VANDELS spectroscopic survey. Motivated by results from the First Billion Years (FiBY) simulation project, we argue that the intrinsic spectral energy distributions (SEDs) of star-forming galaxies at these redshifts have a self-similar shape across the mass range $8.2 leq$ log$(M_{star}/M_{odot}) leq 10.6$ probed by our sample. Using FiBY data, we construct a set of intrinsic SED templates which incorporate both detailed star formation and chemical abundance histories, and a variety of stellar population synthesis (SPS) model assumptions. With this set of intrinsic SEDs, we present a novel approach for directly recovering the shape and normalization of the dust attenuation curve. We find, across all of the intrinsic templates considered, that the average attenuation curve for star-forming galaxies at $zsimeq3.5$ is similar in shape to the commonly-adopted Calzetti starburst law, with an average total-to-selective attenuation ratio of $R_{V}=4.18pm0.29$. We show that the optical attenuation ($A_V$) versus stellar mass ($M_{star}$) relation predicted using our method is consistent with recent ALMA observations of galaxies at $2<z<3$ in the emph{Hubble} emph{Ultra} emph{Deep} emph{Field} (HUDF), as well as empirical $A_V - M_{star}$ relations predicted by a Calzetti-like law. Our results, combined with other literature data, suggest that the $A_V - M_{star}$ relation does not evolve over the redshift range $0<z<5$, at least for galaxies with log$(M_{star}/M_{odot}) gtrsim 9.5$. Finally, we present tentative evidence which suggests that the attenuation curve may become steeper at log$(M_{star}/M_{odot}) lesssim 9.0$.
Using integral field spectroscopy we investigate the kinematic properties of 35 massive centrally-dense and compact star-forming galaxies (${log{overline{M}_*}}=11.1$, $log{(Sigma_mathrm{1kpc})}>9.5$, $log{(M_ast/r_e^{1.5})}>10.3$) at $zsim0.7-3.7$ within the KMOS$^mathrm{3D}$survey. We spatially resolve 23 compact star-forming galaxies (SFGs) and find that the majority are dominated by rotational motions with velocities ranging from {$95-500$ km s$^{-1}$}. The range of rotation velocities is reflected in a similar range of integrated H$alpha$ linewidths, $75-400$ km s$^{-1}$, consistent with the kinematic properties of mass-matched extended galaxies from the full KMOS$^mathrm{3D}$ sample. The fraction of compact SFGs that are classified as `rotation-dominated or `disk-like also mirrors the fractions of the full KMOS$^mathrm{3D}$ sample. We show that integrated line-of-sight gas velocity dispersions from KMOS$^mathrm{3D}$ are best approximated by a linear combination of their rotation and turbulent velocities with a lesser but still significant contribution from galactic scale winds. The H$alpha$ exponential disk sizes of compact SFGs are on average $2.5pm0.2$ kpc, $1-2times$ the continuum sizes, in agreement with previous work. The compact SFGs have a $1.4times$ higher AGN incidence than the full KMOS$^mathrm{3D}$ sample at fixed stellar mass with average AGN fraction of 76%. Given their high and centrally concentrated stellar masses as well as stellar to dynamical mass ratios close to unity, the compact SFGs are likely to have low molecular gas fractions and to quench on a short time scale unless replenished with inflowing gas. The rotation in these compact systems suggests that their direct descendants are rotating passive galaxies.
We present the discovery and spectrophotometric characterization of a large sample of 164 faint ($i_{AB}$ $sim$ $23$-$25$ mag) star-forming dwarf galaxies (SFDGs) at redshift $0.13$ $leq z leq$ $0.88$ selected by the presence of bright optical emission lines in the VIMOS Ultra Deep Survey (VUDS). We investigate their integrated physical properties and ionization conditions, which are used to discuss the low-mass end of the mass-metallicity relation (MZR) and other key scaling relations. We use optical VUDS spectra in the COSMOS, VVDS-02h, and ECDF-S fields, as well as deep multiwavelength photometry, to derive stellar masses, star formation rates (SFR) and gas-phase metallicities. The VUDS SFDGs are compact (median $r_{e}$ $sim$ $1.2$ kpc), low-mass ($M_{*}$ $sim$ $10^7-10^9$ $M_{odot}$) galaxies with a wide range of star formation rates (SFR($Halpha$) $sim 10^{-3}-10^{1}$ $M_{odot}/yr$) and morphologies. Overall, they show a broad range of subsolar metallicities (12+log(O/H)=$7.26$-$8.7$; $0.04$ $lesssim Z/Z_{odot} lesssim$ $1$). The MZR of SFDGs shows a flatter slope compared to previous studies of galaxies in the same mass range and redshift. We find the scatter of the MZR partly explained in the low mass range by varying specific SFRs and gas fractions amongst the galaxies in our sample. Compared with simple chemical evolution models we find that most SFDGs do not follow the predictions of a closed-box model, but those from a gas regulating model in which gas flows are considered. While strong stellar feedback may produce large-scale outflows favoring the cessation of vigorous star formation and promoting the removal of metals, younger and more metal-poor dwarfs may have recently accreted large amounts of fresh, very metal-poor gas, that is used to fuel current star formation.
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