Sample Of Star-forming Galaxies Research Articles

Trading oxygen for iron. I. The [O/Fe]-specific star formation rate relation of galaxies

Our current knowledge of the star-forming metallicity of galaxies relies primarily on gas-phase oxygen abundance measurements. However, these do not always allow an accurate description of differences in stellar evolution and feedback, which are driven by variations in iron abundance. alpha -elements (such as oxygen) and iron are produced by sources that operate on different timescales and the link between them is not straightforward. We explore the origin of the O/Fe --specific SFR (sSFR) relation, linking chemical abundances to galaxy formation timescales. This relation is adhered to by star-forming galaxies across redshifts according to cosmological simulations and basic theoretical expectations. Its apparent universality makes it suitable for trading the readily available oxygen for iron abundance. We show that the relation is determined by the relative iron production efficiency of core-collapse and type Ia supernovae and the delay-time distribution of the latter ---uncertain factors that could be constrained empirically with the O/Fe ---sSFR relation. We compile and homogenise a literature sample of star-forming galaxies with observational iron abundance determinations to place first constraints on the O/Fe --sSFR relation over a wide range of sSFR. The relation shows a clear evolution towards lower O/Fe with decreasing sSFR and a flattening above log$_ $(sSFR/yr)>-9. These results are broadly consistent with expectations, but better constraints are needed to inform the models. We independently derive the relation from old Milky Way stars and find remarkable agreement between the two, as long as the recombination-line absolute oxygen abundance scale is used in conjunction with stellar metallicity measurements.

Exploring the hardness of the ionising radiation with the infrared softness diagram

Aims. We explored the softness parameter in the infrared, ηIR′, whose main purpose is the characterisation of the hardness of the incident ionising radiation in emission-line nebulae. This parameter is obtained from the combination of mid-infrared wavelength range transitions corresponding to consecutive ionisation stages in star-forming regions. Methods. We compiled observational data from a sample of star-forming galaxies (SFGs), including luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs), to study the ηIR′ and its equivalent expression in two dimensions, the softness diagram. We compared them with predictions from photoionisation models to determine the shape of the ionising continuum energy distribution in each case. We also used the measured emission-line ratios as input for HCMISTRY-TEFF-IR, a code that performs a Bayesian-like comparison with photoionisation model predictions in order to quantify the equivalent effective temperature (T*) and the ionisation parameter. Results. We found similar average values within the errors of ηIR′ in (U)LIRGs (−0.57) in the rest of the SFGs (−0.51), which could be interpreted as indicative of a similar incident radiation field. This result is confirmed from the analysis using HCM-TEFF-IR, which simultaneously points to a slightly lower, although similar within the errors, T* scale for (U)LIRGs, even when a higher dust-to-gas mass ratio is considered in the models for these objects. These derived T* values are compatible with the ionisation from massive stars, without any need of harder ionising sources, both for (U)LIRGs and the rest of the SFGs. However, the derived T* in (U)LIRGs do not show any correlation with metallicity. This could be interpreted as a sign that their similar average T* values are due to the attenuation of the energetic incident flux from massive stars by the heated dust mixed with the gas. This is supported by the known very large amounts of small grains associated with the very high star formation rates measured in galaxies of this type.

MIGHTEE-H i: H i galaxy properties in the large-scale structure environment at z ∼ 0.37 from a stacking experiment

ABSTRACT We present the first measurement of H i mass of star-forming galaxies in different large scale structure environments from a blind survey at z ∼ 0.37. In particular, we carry out a spectral line stacking analysis considering 2875 spectra of colour-selected star-forming galaxies undetected in H i at 0.23 < z < 0.49 in the COSMOS field, extracted from the MIGHTEE-H i Early Science data cubes, acquired with the MeerKAT radio telescope. We stack galaxies belonging to different subsamples depending on three different definitions of large-scale structure environment: local galaxy overdensity, position inside the host dark matter halo (central, satellite, or isolated), and cosmic web type (field, filament, or knot). We first stack the full star-forming galaxy sample and find a robust H i detection yielding an average galaxy H i mass of $M_{\rm H \, {\small I}}=(8.12\pm 0.75)\times 10^9\, {\rm M}_\odot$ at ∼11.8σ. Next, we investigate the different subsamples finding a negligible difference in MH i as a function of the galaxy overdensity. We report an H i excess compared to the full sample in satellite galaxies (MH i = (11.31 ± 1.22) × 109, at ∼10.2σ) and in filaments (MH i = (11.62 ± 0.90) × 109. Conversely, we report non-detections for the central and knot galaxies subsamples, which appear to be H i-deficient. We find the same qualitative results also when stacking in units of H i fraction (fH i). We conclude that the H i amount in star-forming galaxies at the studied redshifts correlates with the large-scale structure environment.

Enhanced Subkiloparsec-scale Star Formation: Results from a JWST Size Analysis of 341 Galaxies at 5 < z < 14

We present a comprehensive search and analysis of high-redshift galaxies in a suite of nine public JWST extragalactic fields taken in Cycle 1, covering a total effective search area of ∼358arcmin2 . Through conservative (8σ) photometric selection, we identify 341 galaxies at 5 < z < 14, with 109 having spectroscopic redshift measurements from the literature, including recent JWST NIRSpec observations. Our regression analysis reveals that the rest-frame UV size–stellar mass relation follows Reff∝M*0.19±0.03 , similar to that of star-forming galaxies at z ∼ 3, but scaled down in size by ∼0.7 dex. We find a much slower rate for the average size evolution over the redshift range, R eff ∝ (1 + z)−0.4±0.2, than that derived in the literature. A fraction (∼13%) of our sample galaxies are marginally resolved even in the NIRCam imaging (≲100 pc), located at ≳1.5σ below the derived size–mass slope. These compact sources exhibit a high star formation surface density ΣSFR > 10 M ⊙ yr−1 kpc−2, a range in which only <0.01% of the local star-forming galaxy sample is found. For those with available NIRSpec data, no evidence of ongoing supermassive black hole accretion is observed. A potential explanation for the observed high [O iii]-to-Hβ ratios could be high shock velocities, likely originating within intense star-forming regions characterized by high ΣSFR. Lastly, we find that the rest-frame UV and optical sizes of our sample are comparable. Our results are consistent with these early galaxies building up their structures inside out and being yet to exhibit the strong color gradient seen at lower redshift.

Molecular Outflow in the Reionization-epoch Quasar J2054-0005 Revealed by OH 119 μm Observations

Molecular outflows are expected to play a key role in galaxy evolution at high redshift. To study the impact of outflows on star formation at the epoch of reionization, we performed sensitive Atacama Large Millimeter/submillimeter Array observations of OH 119 μm toward J2054-0005, a luminous quasar at z = 6.04. The OH line is detected and exhibits a P-Cygni profile that can be fitted with a broad blueshifted absorption component, providing unambiguous evidence of an outflow, and an emission component at near-systemic velocity. The mean and terminal outflow velocities are estimated to be v out ≈ 670 and 1500 km s−1, respectively, making the molecular outflow in this quasar one of the fastest at the epoch of reionization. The OH line is marginally spatially resolved for the first time in a quasar at z > 6, revealing that the outflow extends over the central 2 kpc region. The mass outflow rate is comparable to the star formation rate ( Ṁout/SFR∼2 ), indicating rapid (∼107 yr) quenching of star formation. The mass outflow rate in a sample of star-forming galaxies and quasars at 4 < z < 6.4 exhibits a positive correlation with the total infrared luminosity, although the scatter is large. Owing to the high outflow velocity, a large fraction (up to ∼50%) of the outflowing molecular gas may be able to escape from the host galaxy into the intergalactic medium.

Direct T e-based Metallicities of z = 2–9 Galaxies with JWST/NIRSpec: Empirical Metallicity Calibrations Applicable from Reionization to Cosmic Noon

We report detections of the [O iii]λ4364 auroral emission line for 16 galaxies at z = 2.1–8.7, measured from JWST/NIRSpec observations obtained as part of the Cosmic Evolution Early Release Science (CEERS) survey program. We combine this CEERS sample with 9 objects from the literature at z = 4−9 with auroral-line detections from JWST/NIRSpec and 21 galaxies at z = 1.4−3.7 with auroral-line detections from ground-based spectroscopy. We derive electron temperature (T e) and direct-method oxygen abundances for the combined sample of 46 star-forming galaxies at z = 1.4−8.7. We use these measurements to construct the first high-redshift empirical T e-based metallicity calibrations for the strong-line ratios [O iii]/Hβ, [O ii]/Hβ, R23 = ([O iii]+[O ii])/Hβ, [O iii]/[O ii], and [Ne iii]/[O ii]. These new calibrations are valid over 12+log(O/H) = 7.4−8.3 and can be applied to samples of star-forming galaxies at z = 2−9, leading to an improvement in the accuracy of metallicity determinations at Cosmic Noon and in the Epoch of Reionization. The high-redshift strong-line relations are offset from calibrations based on typical z ∼ 0 galaxies or H ii regions, reflecting the known evolution of ionization conditions between z ∼ 0 and z ∼ 2. Deep spectroscopic programs with JWST/NIRSpec promise to improve statistics at the low and high ends of the metallicity range covered by the current sample, as well as to improve the detection rate of [N ii]λ6585 and thus allow the future assessment of N-based indicators. These new high-redshift calibrations will enable accurate characterizations of metallicity scaling relations at high redshift, improving our understanding of feedback and baryon cycling in the early Universe.

Decoding the IRX–β dust attenuation relation in star-forming galaxies at intermediate redshift

Aims. We aim to understand what drives the IRX–β dust attenuation relation at intermediate redshifts (0.5 &lt; z &lt; 0.8) in star-forming galaxies. We investigate the role of various galaxy properties in shaping this observed relation. Methods. We used robust [O II]λ3727, [O III]λλ4959,5007, and Hβ line detections of our statistical sample of 1049 galaxies to estimate the gas-phase metallicities. We derived key physical properties that are necessary to study galaxy evolution, such as the stellar masses and the star formation rates, using the spectral energy distribution fitting tool CIGALE. Equivalently, we studied the effect of galaxy morphology (mainly the Sérsic index n and galaxy inclination) on the observed IRX–β scatter. We also investigated the role of the environment in shaping dust attenuation in our sample. Results. We find a strong correlation with respect to the IRX–β relation on the gas-phase metallicity in our sample, as well as a strong correlation with galaxy compactness characterized by the Sérsic indexes. With higher metallicities, galaxies move along the track of the IRX–β relation towards higher IRX. Correlations are also seen with the stellar masses, specific star formation rates, and the stellar ages of our sources. Metallicity is strongly correlated with the IRX–β scatter, which also results from the presence of older stars and higher masses at higher beta values. Galaxies with higher metallicities show higher IRX and beta values. The correlation with specific dust mass strongly shifts the galaxies away from the IRX–β relation towards lower β values. We find that more compact galaxies go through a greater amount of attenuation than less compact galaxies. There is a subtle variation in the dust attenuation scatter between edge-on and face-on galaxies, but the difference is not statistically significant. Galaxy environments do not significantly affect dust attenuation in our sample of star-forming galaxies at intermediate redshift.

Ionized gas kinematics and chemical abundances of low-mass star-forming galaxies at z  ∼  3

Context. Feedback from massive stars plays a crucial role in regulating the growth of young star-forming galaxies (SFGs) and in shaping their interstellar medium (ISM). This feedback contributes to the removal and mixing of metals via galactic outflows and to the clearance of neutral gas, which facilitates the escape of ionizing photons. Aims. Our goal is to study the impact of stellar feedback on the chemical abundances of the ISM in a sample of SFGs with strong emission lines at z ∼ 3. Methods. We selected 35 low-mass SFGs (7.9 &lt; log(M⋆/M⊙) &lt; 10.3) from deep spectroscopic surveys based on their CIII]λ1908 emission. We used new follow-up near-infrared (NIR) observations to examine their rest-optical emission lines and to identify ionized outflow signatures through broad emission line wings detected after Gaussian modeling of [OIII]λλ4959,5007 profiles. We characterized the gas-phase metallicity and carbon-to-oxygen (C/O) abundance of the galaxies using a Te-based method via the OIII]λ1666/[OIII]λ5007 ratio and photoionization models. Results. We find line ratios and rest-frame equivalent widths (EWs) characteristic of high-ionization conditions powered by massive stars. Our sample displays a mean rest-frame EW([OIII]λ5007) of ∼560 Å, while about 15% of the SFGs show EW([OIII]λλ4959,5007)  &gt;  1000 Å and EW(CIII])  &gt;  5 Å, closely resembling those now seen in epoch of reionization (EoR) galaxies with the James Webb Space Telescope. We find high Te values, which imply low gas-phase metallicities 12+log(O/H) ∼ 7.5–8.5 (mean of 17% solar) and C/O abundances from 23% to 128% solar, with no apparent increasing trend with metallicity. Our sample follows the mass-metallicity relation at z ∼ 3, with some galaxies showing lower gas-phase metallicities. This results in significant deviations from the fundamental metallicity relation. From our [OIII]λλ4959,5007 line profile modeling, we find that 65% of our sample shows an outflow component, which is found both blue- or redshifted relative to the ionized gas systemic velocity, and the mean maximum velocities are vmax ∼ 280 km s−1. We find a weak correlation between vmax and the star formation rate surface density (ΣSFR) of vmax = (2.41 ± 0.03) × ΣSFR(0.06 ± 0.03). Moreover, we find that the mass-loading factor μ of our galaxy sample is typically lower than in more massive galaxies from the literature, but it is higher than in typical local dwarf galaxies. In the stellar mass range covered by our sample, we find that μ increases with ΣSFR. This suggests that for a given stellar mass, denser starbursts in low-mass galaxies produce stronger outflows. Our results complement the picture drawn by similar studies at lower redshift, suggesting that the removal of ionized gas in low-mass SFGs driven by stellar feedback is regulated by their stellar mass and by the strength and concentration of their star formation, that is, ΣSFR.

Star formation history of ≤ z ≤ mass-selected galaxies in the ELAIS-N1 Field

ABSTRACT We measure the specific star formation rates (sSFRs) of K-band selected galaxies from the European Large Area ISO Survey North 1 by stacking Giant Metrewave Radio Telescope data at 610 MHz. We identify a sample of star-forming galaxies (SFGs), spanning ${0.1\le \, {z}\, \le \, 1.5}$ and $\rm {10^{8.5}\lt \, {{\mathit{ M}}_{\star }}/{{M}_{\odot }}\lt 10^{12.4}}$, using a combination of multiwavelength diagnostics obtained from the deep LOw Frequency ARray Two-metre Sky Survey multiwavelength catalogue. We measure the flux densities in the radio map and estimate the radio SFR in order to probe the nature of the galaxies below the noise and confusion limits. The massive galaxies in our sample have the lowest sSFRs, which is in agreement with previous studies. For the different populations, we show that the sSFR–mass relation steepens with redshift, with an average slope of $\rm {\langle \beta _{All} \rangle \, =\, -0.49\pm 0.01}$ for the whole sample, and $\rm {\langle \beta _{SFG} \rangle \, =\, -0.42\pm 0.02}$ for the SFGs. Our results indicate that galaxy populations undergo ’downsizing’, whereby most massive galaxies form their stars earlier and more rapidly than low-mass galaxies. Both populations show a strong decrease in their sSFR towards the present epoch. The sSFR evolution with redshift is best described by a power law ${(1\, +\, {z})^{n}}$, where $\rm {\langle {\mathit{ n}}_{ALL}\rangle \sim 4.94\pm 0.53}$ for all galaxies and $\rm {\langle {\mathit{ n}}_{SFG}\rangle \sim 3.51\pm 0.52}$ for SFGs. Comparing our measured sSFRs to results from literature, we find a general agreement in the sSFR–M⋆ plane.

Estimating Molecular Gas Content in Galaxies from Polycyclic Aromatic Hydrocarbon Emission

Emission from polycyclic aromatic hydrocarbons (PAHs), a commonly used indicator of star formation activity in galaxies, also has the potential to serve as an effective empirical tracer of molecular gas. We use a sample of 19 nearby galaxies with spatially resolved mid-infrared Spitzer spectroscopy, multiwavelength optical and mid-infrared imaging, and millimeter interferometric CO(1–0) maps to investigate the feasibility of using PAH emission as an empirical proxy to estimate molecular gas mass. PAH emission correlates strongly with CO emission on subkiloparsec scales over the diverse environments probed by our sample of star-forming galaxies and low-luminosity active galactic nuclei. The tight observed correlation, likely a consequence of photoelectronic heating of the diffuse interstellar gas by the PAHs, permits us to derive an empirical calibration to estimate molecular gas mass from the luminosity of PAH emission that has a total scatter of only ∼0.2–0.25 dex. Mid-infrared bands sensitive to PAH emission (e.g., the Spitzer/IRAC4 and WISE/W3 filters) can also be used as a highly effective substitute for this purpose.

A Selection of Hα Emitters at z = 2.1–2.5 Using the K s -band Photometry of ZFOURGE

Large and less-biased samples of star-forming galaxies are essential to investigate galaxy evolution. Hα emission line is one of the most reliable tracers of star-forming galaxies because its strength is directly related to recent star formation. However, it is observationally expensive to construct large samples of Hα emitters by spectroscopic or narrowband imaging survey at high redshifts. In this work, we demonstrate a method to extract Hα fluxes of galaxies at z = 2.1–2.5 from K s broadband photometry of ZFOURGE catalog. Combined with 25–39 other filters, we estimate the emission-line fluxes by SED fitting with stellar population models that incorporate emission-line strengths. 2005 galaxies are selected as Hα emitters by our method and their fluxes show good agreement with previous measurements in the literature. On the other hand, there are more Hα luminous galaxies than previously reported. The discrepancy can be explained by extended Hα profiles of massive galaxies and a luminosity dependence of dust attenuation, which are not taken into account in the previous work. We also find that there are a large number of low-mass galaxies with a much higher specific star formation rate (sSFR) than expected from the extrapolated star formation main sequence. Such low-mass galaxies exhibit larger ratios between Hα and UV fluxes compared to more massive high sSFR galaxies. This result implies that a “starburst” mode may differ among galaxies: low-mass galaxies appear to assemble their stellar mass via short-duration bursts, while more massive galaxies tend to experience longer-duration (>10 Myr) bursts.

The main sequence of star-forming galaxies across cosmic times

ABSTRACT By compiling a comprehensive census of literature studies, we investigate the evolution of the main sequence (MS) of star-forming galaxies (SFGs) in the widest range of redshift (0 &amp;lt; z &amp;lt; 6) and stellar mass (108.5–1011.5 M⊙) ever probed. We convert all observations to a common calibration and find a remarkable consensus on the variation of the MS shape and normalization across cosmic time. The relation exhibits a curvature towards the high stellar masses at all redshifts. The best functional form is governed by two parameters: the evolution of the normalization and the turnover mass (M0(t)), which both evolve as a power law of the Universe age. The turn-over mass determines the MS shape. It marginally evolves with time, making the MS slightly steeper towards z ∼ 4–6. At stellar masses below M0(t), SFGs have a constant specific SFR (sSFR), while above M0(t) the sSFR is suppressed. We find that the MS is dominated by central galaxies. This allows to turn M0(t) into the corresponding host halo mass. This evolves as the halo mass threshold between cold and hot accretion regimes, as predicted by the theory of accretion, where the central galaxy is fed or starved of cold gas supply, respectively. We, thus, argue that the progressive MS bending as a function of the Universe age is caused by the lower availability of cold gas in haloes entering the hot accretion phase, in addition to black hole feedback. We also find qualitatively the same trend in the largest sample of star-forming galaxies provided by the IllustrisTNG simulation. Nevertheless, we still note large quantitative discrepancies with respect to observations, in particular at the high-mass end. These can not be easily ascribed to biases or systematics in the observed SFRs and the derived MS.

On the Triggering of Extreme Starburst Events in Low-metallicity Galaxies: A Deep Search for Companions of Green Peas

Green pea galaxies are starbursting, low-mass galaxies that are good analogues to star-forming galaxies in the early universe. We perform a survey of 23 green peas using the Multi Unit Spectroscopic Explorer (MUSE) Integral Field Unit spectrograph on the Very Large Telescope to search for companion galaxies. The survey reaches an average point-source depth of ∼10−18 erg cm−2 s−1 for emission lines. The MUSE field of view allows us to probe a 1 × 1 arcmin2 field around these galaxies and to search their surroundings for faint companions that could have interacted with them and induced their starburst episodes. We search for companions using a variety of methods including template matching to emission- and absorption-line spectra. When restricting the search to the same physical area (R = 78 kpc) for all galaxies, we find that the fraction of green pea galaxies with companions is . We define a control sample of star-forming galaxies with the same stellar masses and redshifts as the green peas, but consistent with the star formation main sequence. We find that green pea galaxies are as likely to have companions as the control sample; for which the fraction of objects with companions is . Given that we do not find statistical evidence for an elevated companion fraction in the green peas in this study, we argue that the “companions” are likely unrelated to the bursts in these galaxies.

Mass–metallicity and star formation rate in galaxies: A complex relation tuned to stellar age

Context. In this work we study the stellar mass–metallicity relation (MZR) of an extended sample of star-forming galaxies in the local Universe and its possible dependence on the star formation rate (SFR). Aims. We selected a sample of approximately 195 000 Sloan Digital Sky Survey (SDSS) star-forming galaxies up to z = 0.22 with the aim of analysing the behaviour of the MZR with respect to SFR whilst taking into account the age of their stellar populations. Methods. For the first time, with this sample, we obtained aperture corrected oxygen and nitrogen-to-oxygen abundances (O/H and N/O, respectively) and SFR using the empirical prescriptions from the Calar Alto Legacy Integral Field Area (CALIFA) survey. To perform this study we also make use of the stellar mass of the galaxies and the parameter Dn(4000) as a proxy for the age of the stellar population. Results. We derive a robust MZR locus, which is found to be fully consistent with the ‘anchoring’ points of a selected set of well-studied nearby galaxies for which the chemical abundance has been derived using the direct method. We observe a complex relation between MZR and SFR across the whole range of galaxy mass and metallicity, where the slope changes seen in the O/H–SFR plane present a pattern that seems to be tuned to the stellar age of the galaxies, and therefore stellar age has to be taken into account in the stellar mass–metallicity–SFR relation. Conclusions. In order to provide an answer to the question of whether or not the MZR depends on the SFR, it is essential to take into account the age of the stellar populations of galaxies. A strong dependence of the MZR on SFR is observed mainly for star-forming galaxies with strong SFR values and low Dn(4000). The youngest galaxies of our SDSS sample show the highest SFR measured for their stellar mass.

Are the host galaxies of long gamma-ray bursts more compact than star-forming galaxies of the field?

Context. Long gamma-ray bursts (GRBs) offer a promising tool for tracing the cosmic history of star formation, especially at high redshift, where conventional methods are known to suffer from intrinsic biases. Previous studies of GRB host galaxies at low redshift showed that high surface density of stellar mass and high surface density of star formation rate (SFR) can potentially enhance the GRB production. Evaluating the effect of such stellar densities at high redshift is therefore crucial to fully control the ability of long GRBs for probing the activity of star formation in the distant Universe. Aims. We assess how the size, stellar mass, and star formation rate surface densities of distant galaxies affect the probability of their hosting a long GRB, using a sample of GRB hosts at z &gt; 1 and a control sample of star-forming sources from the field. Methods. We gathered a sample of 45 GRB host galaxies at 1 &lt; z &lt; 3.1 observed with the Hubble Space Telescope WFC3 camera in the near-infrared. Our subsample at 1 &lt; z &lt; 2 has cumulative distributions of redshift and stellar mass consistent with the host galaxies of known unbiased GRB samples, while our GRB host selection at 2 &lt; z &lt; 3.1 has lower statistics and is probably biased toward the high end of the stellar mass function. Using the GALFIT parametric approach, we modeled the GRB host light profile with a Sérsic component and derived the half-light radius for 35 GRB hosts, which we used to estimate the star formation rate and stellar mass surface densities of each object. We compared the distribution of these physical quantities to the SFR-weighted properties of a complete sample of star-forming galaxies from the 3D-HST deep survey at a comparable redshift and stellar mass. Results. We show that similarly to z &lt; 1, GRB hosts are smaller in size and they have higher stellar mass and star formation rate surface densities than field galaxies at 1 &lt; z &lt; 2. Interestingly, this result is robust even when separately considering the hosts of GRBs with optically bright afterglows and the hosts of dark GRBs, as the two subsamples share similar size distributions. At z &gt; 2, however, GRB hosts appear to have sizes and stellar mass surface densities more consistent with those characterizing the field galaxies. This may reveal an evolution with redshift of the bias between GRB hosts and the overall population of star-forming sources, although we cannot exclude that our result at z &gt; 2 is also affected by the prevalence of dark GRBs in our selection. Conclusions. In addition to a possible trend toward a low-metallicity environment, other environmental properties such as stellar density appear to play a role in the formation of long GRBs, at least up to z ∼ 2. This might suggest that GRBs require special environments to enhance their production.

Atomic Gas Dominates the Baryonic Mass of Star-forming Galaxies at z ≈ 1.3

We present a comparison between the average atomic gas mass, 〈M Atom〉 (including hydrogen and helium), the average molecular gas mass, 〈M Mol〉, and the average stellar mass, 〈M ⋆〉, of a sample of star-forming galaxies at z ≈ 0.75–1.45 to probe the baryonic composition of galaxies in and during the epoch of peak star formation activity in the universe. The 〈M Atom〉 values of star-forming galaxies in two stellar-mass-matched samples at z = 0.74–1.25 and z = 1.25–1.45 were derived by stacking their Hi 21 cm signals in the GMRT-CATz1 survey. We find that the baryonic composition of star-forming galaxies at z ≳ 1 is dramatically different from that at z ≈ 0. For star-forming galaxies with 〈M ⋆〉 ≈ 1010 M ⊙, the contribution of stars to the total baryonic mass, M Baryon, is ≈61% at z ≈ 0, but only ≈16% at z ≈ 1.3, while molecular gas constitutes ≈6% of the baryonic mass at z ≈ 0, and ≈14% at z ≈ 1.3. Remarkably, we find that atomic gas makes up ≈70% of M Baryon in star-forming galaxies at z ≈ 1.3. We find that the ratio 〈M Atom〉/〈M ⋆〉 is higher at both z ≈ 1.3 and at z ≈ 1.0 than in the local universe, with 〈M Atom〉/〈M ⋆〉 ≈ 1.4 at z ≈ 1.0 and ≈ 4.4 at z ≈ 1.3, compared to its value of ≈0.5 today. Further, we find that the ratio 〈M Atom〉/〈M Mol〉 in star-forming galaxies with 〈M ⋆〉 ≈ 1010 M ⊙ is ≈2.3 at z ≈ 1.0 and ≈5.0 at z ≈ 1.3. Overall, we find that atomic gas is the dominant component of the baryonic mass of star-forming galaxies at z ≈ 1.3, during the epoch of peak star formation activity in the universe.

The Low-Redshift Lyman Continuum Survey

Aims. Combining 66 ultraviolet (UV) spectra and ancillary data from the recent Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier studies, we form a statistical sample of star-forming galaxies at z ∼ 0.2 − 0.4 with which we study the role of cold interstellar medium (ISM) gas in the leakage of ionizing radiation. We also aim to establish empirical relations between the H I neutral and low-ionization state (LIS) absorption lines with different galaxy properties. Methods. We first constrain the massive star content (stellar ages and metallicities) and UV attenuation by fitting the stellar continuum with a combination of simple stellar population models. The models, together with accurate LyC flux measurements, allow us to determine the absolute LyC photon escape fraction for each galaxy (fescabs). We then measure the equivalent widths and residual fluxes of multiple H I and LIS lines, and the geometrical covering fraction of the UV emission, adopting the picket-fence model. Results. The LyC escape fraction spans a wide range, with a median fescabs (0.16, 0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in the LyC (1σ upper limits of fescabs ≲ 0.01 for non-detections, typically). The H I and LIS line equivalent widths scale with the UV luminosity and attenuation, and inversely with the residual flux of these lines. Additionally, Lyα equivalent widths scale with both the H I and LIS residual fluxes, but anti-correlate with the corresponding H I or LIS equivalent widths. The H I and LIS residual fluxes are correlated, indicating that the neutral gas is spatially traced by the low-ionization transitions. We find that the observed trends of the absorption lines and the UV attenuation are primarily driven by the geometric covering fraction of the gas. The observed nonuniform gas coverage also demonstrates that LyC photons escape through low-column-density channels in the ISM. The equivalent widths and residual fluxes of both the H I and LIS lines strongly correlate with fescabs: strong LyC leakers (highest fescabs) show weak absorption lines, low UV attenuation, and large Lyα equivalent widths. We provide several empirical calibrations to estimate fescabs from UV absorption lines. Finally, we show that simultaneous UV absorption line and dust attenuation measurements can, in general, predict the escape fraction of galaxies. We apply our method to available measurements of UV LIS lines of 15 star-forming galaxies at z ∼ 4 − 6 (plus 3 high-z galaxy composites), finding that these high-redshift, UV-bright galaxies (MUV ≲ −21) may have low escape fractions, fescabs ≲ 0.1. Conclusions. UV absorption lines trace the cold ISM gas of galaxies, which governs the physics of the LyC escape. We show that, with some assumptions, the absolute LyC escape can be statistically predicted using UV absorption lines, and the method can be applied to study galaxies across a wide redshift range, including in the epoch of cosmic reionization.

The molecular gas properties in local Seyfert 2 galaxies

Aims. We present a multiwavelength study of the molecular gas properties of a sample of local Seyfert 2 galaxies to assess if, and to what extent, the presence of an active galactic nucleus (AGN) can affect the interstellar medium (ISM) properties in a sample of 33 local Seyfert 2 galaxies. Methods. We compare the molecular gas content (MH2) derived from new and archival low-J CO line measurements of a sample of AGN and a control sample of star-forming galaxies (SFGs). Both the AGN and the control sample are characterized in terms of host-galaxy properties, for example stellar and dust masses (M⋆ and Mdust, respectively) and the star formation rate (SFR). We also investigate the effect of AGN activity on the emission of polycyclic aromatic hydrocarbon (PAH) molecules in the mid-infrared (MIR), a waveband where the dust-reprocessed emission from the obscured AGN contributes the most. Result. The AGN hosted in less massive galaxies (i.e., M⋆ &lt; 1010.5 M⊙; Mdust &lt; 107.5 M⊙) show larger molecular gas contents with respect to SFGs that have the same stellar and dust masses. When comparing their depletion times (tdep ∝ MH2/SFR), AGN show tdep ∼ 0.3 − 1.0 Gyr, similar to the times observed in the control sample of SFGs. Seyfert 2 galaxies show fainter PAH luminosity the larger the dominance of the nuclear activity in the MIR. Conclusions. We find no clear evidence for a systematic reduction in the molecular gas reservoir at galactic scales in Seyfert galaxies with respect to SFGs. This is in agreement with recent studies that show that molecular gas content is only reduced in sub-kiloparsec-sized regions, where emission from the accreting supermassive black hole dominates. Nonetheless, we show that the impact of AGN activity on the ISM is clearly visible as a suppression of the PAH luminosity.

The VIMOS Ultra Deep Survey: The reversal of the star-formation rate − density relation at 2 &lt; z &lt; 5

Utilizing spectroscopic observations taken for the VIMOS Ultra-Deep Survey (VUDS), new observations from Keck/DEIMOS, and publicly available observations of large samples of star-forming galaxies, we report here on the relationship between the star-formation rate (SFR) and the local environment (δgal) of galaxies in the early universe (2 &lt; z &lt; 5). Unlike what is observed at lower redshifts (z ≲ 2), we observe a definite, nearly monotonic increase in the average SFR with increasing galaxy overdensity over more than an order of magnitude in δgal. The robustness of this trend is quantified by accounting for both uncertainties in our measurements and galaxy populations that are either underrepresented or not present in our sample (e.g., extremely dusty star-forming and quiescent galaxies), and we find that the trend remains significant under all circumstances. This trend appears to be primarily driven by the fractional increase of galaxies in high-density environments that are more massive in their stellar content and are forming stars at a higher rate than their less massive counterparts. We find that, even after stellar mass effects are accounted for, there remains a weak but significant SFR–δgal trend in our sample implying that additional environmentally related processes are helping to drive this trend. We also find clear evidence that the average SFR of galaxies in the densest environments increases with increasing redshift. These results lend themselves to a picture in which massive gas-rich galaxies coalesce into proto-cluster environments at z ≳ 3, interact with other galaxies or with a forming large-scale medium, subsequently using or losing most of their gas in the process, and begin to seed the nascent red sequence that is present in clusters at slightly lower redshifts.

Infrared-radio relation in the local Universe

Context.The Square Kilometer Array (SKA) is expected to detect high-redshift galaxies with star formation rates (SFRs) up to two orders of magnitude lower thanHerschelsurveys and will thus boost the ability of radio astronomy to study extragalactic sources. The tight infrared-radio correlation offers the possibility of using radio emission as a dust-unobscured star formation diagnostic. However, the physics governing the link between radio emission and star formation is poorly understood, and recent studies have pointed to differences in the exact calibration required when radio is to be used as a star formation tracer.Aims.We improve the calibration of the relation of the local radio luminosity–SFR and to test whether there are nonlinearities in it.Methods.We used a sample ofHerschelAstrophysical Terahertz Large Area Survey (H-ATLAS) sources and investigated their radio luminosity, which was derived using the NRAO VLA Sky Survey (NVSS) and Faint Images of the Radio Sky at Twenty-cm (FIRST) maps. We stacked the bins of infrared luminosity and SFR and accounted for bins with no detections in the stacked images using survival analysis fitting. This approach was tested using Monte Carlo simulations.Results.After removing sources from the sample that have excess radio emission, which is indicative of nuclear radio activity, we found no deviations from linearity of the mean relations between radio luminosity and either SFR or infrared luminosity.Concluisions.We analyzed the link between radio emission and SFR or infrared luminosity using a local sample of star-forming galaxies without evidence of nuclear radio activity and found no deviations from linearity, although our data are also consistent with the small nonlinearity reported by some recent analyses. The normalizations of these relations are intermediate between those reported by earlier works.