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Register a new userThe Evolving Interstellar Medium of Star-Forming Galaxies, as traced by $texttt{Stardust}$
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We analyse the far-infrared properties of $sim$ 5,000 star-forming galaxies at $z<4.5$, drawn from the deepest, super-deblended catalogues in the GOODS-N and COSMOS fields. We develop a novel panchromatic SED fitting algorithm, $texttt{Stardust}$, that models the emission from stars, AGN, and infrared dust emission, without relying on energy balance assumptions. Our code provides robust estimates of the UV-optical and FIR physical parameters, such as the stellar mass ($M_*$), dust mass ($M_{rm dust}$), infrared luminosities ($L_{rm IR}$) arising from AGN and star formation activity, and the average intensity of the interstellar radiation field ($langle U rangle$). Through a set of simulations we quantify the completeness of our data in terms of $M_{rm dust}$, $L_{rm IR}$ and $langle U rangle$, and subsequently characterise the distribution and evolution of these parameters with redshift. We focus on the dust-to-stellar mass ratio ($f_{rm dust}$), which we parametrise as a function of cosmic age, stellar mass, and specific star formation rate. The $f_{rm dust}$ is found to increase by a factor of 10 from $z=0$ to $z=2$ and appears to remain flat at higher$-z$, mirroring the evolution of the gas fraction. We also find a growing fraction of warm to cold dust with increasing distance from the main sequence, indicative of more intense interstellar radiation fields, higher star formation efficiencies and more compact star forming regions for starburst galaxies. Finally, we construct the dust mass functions (DMF) of star-forming galaxies up to $z=1$ by transforming the stellar mass function to DMF through the scaling relations derived here. The evolution of $f_{rm dust}$ and the recovered DMFs are in good agreement with the theoretical predictions of the Horizon-AGN and IllustrisTNG simulations.
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We report ALMA observations of the neutral atomic carbon transitions [CI] and multiple CO lines in a sample of $sim30$ main sequence galaxies at $zsim1$, including novel information on [CI](2-1) and CO(7-6) for 7 of such normal objects. We complement our observations with a collection of $>200$ galaxies with coverage of similar transitions, spanning the $z=0-4$ redshift interval and a variety of ambient conditions from local to high-redshift starbursts. We find systematic variations in the [CI]/IR and [CI]/high-$J$ ($J=7$) CO luminosity ratios among the various samples. We interpret these differences as increased dense molecular gas fractions and star formation efficiencies in the strongest high-redshift starbursts with respect to normal main sequence galaxies. We further report constant $L_{rm [CI]2-1}$/$L_{rm [CI]1-0}$ ratios across the galaxy populations and redshifts, suggesting that gas temperatures $T_{rm exc}$ traced by [CI] do not strongly vary. We find only a mild correlation with $T_{rm dust}$ and that, generally, $T_{rm exc} lesssim T_{rm dust}$. We fit the line ratios with classical PDR models, retrieving consistently larger densities and intensities of the UV radiation fields in submm galaxies than in main sequence and local objects. However, these simple models fall short in representing the complexity of a multiphase interstellar medium and should be treated with caution. Finally, we compare our observations with the Santa Cruz semi-analytical model of galaxy evolution, recently extended to simulate submm emission. While we confirm the success in reproducing the CO lines, we find systematically larger [CI] luminosities at fixed IR luminosity than predicted theoretically. This highlights the necessity of improving our understanding of the mechanisms regulating the [CI] emission on galactic scales. We release our data compilation to the community.
We present new results from near-infrared spectroscopy with Keck/MOSFIRE of [OIII]-selected galaxies at $zsim3.2$. With our $H$ and $K$-band spectra, we investigate the interstellar medium (ISM) conditions, such as ionization states and gas metallicities. [OIII] emitters at $zsim3.2$ show a typical gas metallicity of $mathrm{12+log(O/H) = 8.07pm0.07}$ at $mathrm{log(M_*/M_odot) sim 9.0-9.2}$ and $mathrm{12+log(O/H) = 8.31pm0.04}$ at $mathrm{log(M_*/M_odot) sim 9.7-10.2}$ when using the empirical calibration method. We compare the [OIII] emitters at $zsim3.2$ with UV-selected galaxies and Ly$alpha$ emitters at the same epoch and find that the [OIII]-based selection does not appear to show any systematic bias in the selection of star-forming galaxies. Moreover, comparing with star-forming galaxies at $zsim2$ from literature, our samples show similar ionization parameters and gas metallicities as those obtained by the previous studies using the same calibration method. We find no strong redshift evolution in the ISM conditions between $zsim3.2$ and $zsim2$. Considering that the star formation rates at a fixed stellar mass also do not significantly change between the two epochs, our results support the idea that the stellar mass is the primary quantity to describe the evolutionary stages of individual galaxies at $z>2$.
The spectral index of synchrotron emission is an important parameter in understanding the properties of cosmic ray electrons (CREs) and the interstellar medium (ISM). We determine the synchrotron spectral index ($alpha_{rm nt}$) of four nearby star-forming galaxies, namely NGC 4736, NGC 5055, NGC 5236 and NGC 6946 at sub-kpc linear scales. The $alpha_{rm nt}$ was determined between 0.33 and 1.4 GHz for all the galaxies. We find the spectral index to be flatter ($gtrsim -0.7$) in regions with total neutral (atomic + molecular) gas surface density, $Sigma_{rm gas} gtrsim rm 50~M_odot pc^{-2}$, typically in the arms and inner parts of the galaxies. In regions with $Sigma_{rm gas} lesssim rm 50~M_odot pc^{-2}$, especially in the interarm and outer regions of the galaxies, the spectral index steepens sharply to $<-1.0$. The flattening of $alpha_{rm nt}$ is unlikely to be caused due to thermal free--free absorption at 0.33 GHz. Our result is consistent with the scenario where the CREs emitting at frequencies below $sim0.3$ GHz are dominated by bremsstrahlung and/or ionization losses. For denser medium ($Sigma_{rm gas} gtrsim rm 200~M_odot pc^{-2}$), having strong magnetic fields ($sim 30~mu$G), $alpha_{rm nt}$ is seen to be flatter than $-0.5$, perhaps caused due to ionization losses. We find that, due to the clumpy nature of the ISM, such dense regions cover only a small fraction of the galaxy ($lesssim5$ percent). Thus, the galaxy-integrated spectrum may not show indication of such loss mechanisms and remain a power-law over a wide range of radio frequencies (between $sim 0.1$ to 10 GHz).
We present results from Subaru/FMOS near-infrared (NIR) spectroscopy of 118 star-forming galaxies at $zsim1.5$ in the Subaru Deep Field. These galaxies are selected as [OII]$lambda$3727 emitters at $zapprox$ 1.47 and 1.62 from narrow-band imaging. We detect H$alpha$ emission line in 115 galaxies, [OIII]$lambda$5007 emission line in 45 galaxies, and H$beta$, [NII]$lambda$6584, and [SII]$lambdalambda$6716,6731 in 13, 16, and 6 galaxies, respectively. Including the [OII] emission line, we use the six strong nebular emission lines in the individual and composite rest-frame optical spectra to investigate physical conditions of the interstellar medium in star-forming galaxies at $zsim$1.5. We find a tight correlation between H$alpha$ and [OII], which suggests that [OII] can be a good star formation rate (SFR) indicator for galaxies at $zsim1.5$. The line ratios of H$alpha$/[OII] are consistent with those of local galaxies. We also find that [OII] emitters have strong [OIII] emission lines. The [OIII]/[OII] ratios are larger than normal star-forming galaxies in the local Universe, suggesting a higher ionization parameter. Less massive galaxies have larger [OIII]/[OII] ratios. With evidence that the electron density is consistent with local galaxies, the high ionization of galaxies at high redshifts may be attributed to a harder radiation field by a young stellar population and/or an increase in the number of ionizing photons from each massive star.
We present VLT/SINFONI near-infrared (NIR) integral field spectroscopy of six $z sim 0.2$ Lyman break galaxy analogs (LBAs), from which we detect HI, HeI, and [FeII] recombination lines, and multiple H$_2$ ro-vibrational lines in emission. Pa$alpha$ kinematics reveal high velocity dispersions and low rotational velocities relative to random motions ($langle v/sigma rangle = 1.2 pm 0.8$). Matched-aperture comparisons of H$beta$, H$alpha$, and Pa$alpha$ reveal that the nebular color excesses are lower relative to the continuum color excesses than is the case for typical local star-forming systems. We compare observed HeI/HI recombination line ratios to photoionization models to gauge the effective temperatures (T$_{rm eff}$) of massive ionizing stars, finding the properties of at least one LBA are consistent with extra heating from an active galactic nucleus (AGN) and/or an overabundance of massive stars. We use H$_2$ 1-0 S($cdot$) ro-vibrational spectra to determine rotational excitation temperature $T_{rm ex} sim 2000$ K for warm molecular gas, which we attribute to UV heating in dense photon-dominated regions. Spatially resolved NIR line ratios favor excitation by massive, young stars, rather than supernovae or AGN feedback. Our results suggest that the local analogs of Lyman break galaxies are primarily subject to strong feedback from recent star formation, with evidence for AGN and outflows in some cases.
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