Star Formation Rates Of Galaxies Research Articles

XGASS: passive discs do not host unexpectedly large reservoirs of cold atomic hydrogen

ABSTRACT We use the extended GALEX Arecibo SDSS Survey (xGASS) to quantify the relationship between atomic hydrogen (H i) reservoir and current star formation rate (SFR) for central disc galaxies. This is primarily motivated by recent claims for the existence, in this sample, of a large population of passive discs harbouring H i reservoirs as large as those observed in main-sequence galaxies. Across the stellar mass range 109 < M*/M⊙ < 1011, we practically find no passive (≳2σ below the star forming main sequence) disc galaxies with H i reservoirs comparable to those typical of star-forming systems. Even including H i non-detections at their upper limits, passive discs typically have ≥0.5 dex less H i than their active counterparts. We show that previous claims are due to the use of aperture-corrected SFR estimates from the MPA/JHU SDSS DR7 catalogue, which do not provide a fair representation of the global SFR of H i-rich galaxies with extended star-forming discs. Our findings confirm that the bulk of the passive disc population in the local Universe is H i-poor. These also imply that the reduction of star formation, even in central disc galaxies, has to be accompanied by a reduction in their H i reservoir.

A panchromatic spatially resolved analysis of nearby galaxies – I. Sub-kpc-scale main sequence in grand-design spirals

ABSTRACT We analyse the spatially resolved relation between stellar mass (M⋆) and star formation rate (SFR) in disc galaxies (i.e. the main sequence, MS). The studied sample includes eight nearby face-on grand-design spirals, e.g. the descendant of high-redshift, rotationally supported star-forming galaxies. We exploit photometric information over 23 bands, from the UV to the far-IR, from the publicly available DustPedia data base to build spatially resolved maps of stellar mass and SFRs on sub-galactic scales of 0.5–1.5 kpc, by performing a spectral energy distribution fitting procedure that accounts for both the observed and obscured star formation processes, over a wide range of internal galaxy environments (bulges, spiral arms, and outskirts). With more than 30 000 physical cells, we have derived a definition of the local spatially resolved MS per unit area for discs, log (ΣSFR) = 0.82log (Σ*) − 8.69. This is consistent with the bulk of recent results based on optical IFU, using the H α line emission as an SFR tracer. Our work extends the analysis at lower sensitivities in both M⋆ and SFR surface densities, up to a factor of ∼10. The self-consistency of the MS relation over different spatial scales, from sub-galactic to galactic, as well as with a rescaled correlation obtained for high-redshift galaxies, clearly proves its universality.

Thermal Regulation and the Star-forming Main Sequence

Abstract We argue that the interplay between cosmic rays, the initial mass function (IMF), and star formation plays a crucial role in regulating the star-forming “main sequence.” To explore these phenomena we develop a toy model for galaxy evolution in which star formation is regulated by a combination of a temperature-dependent IMF and heating due to starlight, cosmic rays, and (at very high redshift) the cosmic microwave background. This produces an attractor, near-equilibrium solution which is consistent with observations of the star-forming main sequence over a broad redshift range. Additional solutions to the same equations may correspond to other observed phases of galaxy evolution, including quiescent galaxies. This model makes several falsifiable predictions, including higher metallicities and dust masses than anticipated at high redshift and isotopic abundances in the Milky Way. It also predicts that stellar mass-to-light ratios are lower than produced using a Milky Way–derived IMF, such that inferences of stellar masses and star formation rates for high redshift galaxies are overestimated. In some cases, this may also transform inferred dark matter profiles from core-like to cusp-like.

Properties of simulated galaxies and supermassive black holes in cosmic voids

ABSTRACT Cosmic voids, the underdense regions of the cosmic web, are widely used to constrain cosmology. Voids contain few, isolated galaxies, presumably expected to be less evolved and preserving memory of the pristine Universe. We use the cosmological hydrodynamical simulation Horizon-AGN coupled to the void finder vide to investigate properties of galaxies in voids at z = 0. We find that, closer to void centres, low-mass galaxies are more common than their massive counterparts. At a fixed dark matter halo mass, they have smaller stellar masses than in denser regions. The star formation rate of void galaxies diminishes when approaching void centres, but their specific star formation rate slightly increases, suggesting that void galaxies form stars more efficiently with respect to their stellar mass. We find that this cannot only be attributed to the prevalence of low-mass galaxies. The inner region of voids also predominantly hosts low-mass black holes (BHs). However, the BH mass-to-galaxy mass ratios resemble those of the whole simulation at z = 0. Our results suggest that even if the growth channels in cosmic voids are different from those in denser environments, voids grow their galaxies and BHs in a similar way. While a large fraction of the BHs have low Eddington ratios, we find that $\text{$\sim$} 20{{\ \rm per\ cent}}$ could be observed as active galactic nuclei with $\log _{10} L_{\rm 2\!-\!10 \, keV}=41.5\!-\!42.5 \, \rm erg\, s^{-1}$. These results pave the way to future work with larger next-generation hydro-simulations, aiming to confirm our findings and prepare the application on data from upcoming large surveys such as Prime Focus Spectrograph, Euclid, and Wide Field Infrared Survey Telescope.

BreakBRD Galaxies. I. Global Properties of Spiral Galaxies with Central Star Formation in Red Disks

Abstract We introduce a collection of primarily centrally star-forming galaxies that are selected by disk color to have truncated disk star formation. We show that common explanations for centrally concentrated star formation—low stellar mass, bars, and high-density environments—do not universally apply to this sample. To gain insight into our sample, we compare these galaxies to a parent sample of strongly star-forming galaxies and to a parent sample of galaxies with low specific star formation rates. We find that in star formation and color space from ultraviolet to the infrared these galaxies either fall between the two samples or agree more closely with galaxies with high specific star formation rates. Their morphological characteristics also lie between high and low specific star formation rate galaxies, although their Petrosian radii agree well with that of the low specific star formation rate parent sample. We discuss whether this sample is likely to be quenching or showing an unusual star formation distribution while continuing to grow through star formation. Future detailed studies of these galaxies will give us insights into how the local conditions within a galaxy balance environmental influence to govern the distribution of star formation. In this first paper in a series, we describe the global properties that identify this sample as separate from more average spiral galaxies, and we identify paths forward to explore the underlying causes of their differences.

The maximum accretion rate of hot gas in dark matter haloes

ABSTRACT We revisit the question of ‘hot mode’ versus ‘cold mode’ accretion on to galaxies using steady-state cooling flow solutions and idealized 3D hydrodynamic simulations. We demonstrate that for the hot accretion mode to exist, the cooling time is required to be longer than the free-fall time near the radius where the gas is rotationally supported, Rcirc, i.e. the existence of the hot mode depends on physical conditions at the galaxy scale rather than on physical conditions at the halo scale. When allowing for the depletion of the halo baryon fraction relative to the cosmic mean, the longer cooling times imply that a virialized gaseous halo may form in halo masses below the threshold of $\sim 10^{12}\, {\rm M_{\odot }}$ derived for baryon-complete haloes. We show that for any halo mass there is a maximum accretion rate for which the gas is virialized throughout the halo and can accrete via the hot mode of ${\dot{M}}_{\rm crit}\approx 0.7(v_{\rm c}/100\, \rm km\ s^{-1})^{5.4}(R_{\rm circ}/10\, {\rm kpc})(Z/\, {\rm Z_{\odot }})^{-0.9}\, {\rm M_{\odot }}\, {\rm yr}^{-1}$, where Z and vc are the metallicity and circular velocity measured at Rcirc. For accretion rates $\gtrsim {\dot{M}}_{\rm crit}$ the volume-filling gas phase can in principle be ‘transonic’ – virialized in the outer halo but cool and free-falling near the galaxy. We compare ${\dot{M}}_{\rm crit}$ to the average star formation rate (SFR) in haloes at 0 < z < 10 implied by the stellar-mass–halo-mass relation. For a plausible metallicity evolution with redshift, we find that ${\rm SFR}\lesssim {\dot{M}}_{\rm crit}$ at most masses and redshifts, suggesting that the SFR of galaxies could be primarily sustained by the hot mode in halo masses well below the classic threshold of $\sim 10^{12}\, {\rm M_{\odot }}$.

The Impact of Far-Infrared/ Sub-Millimeter Data on the Star Formation Rates of Massive Dusty Galaxies at Cosmic Noon

We explore how the star formation rate (SFR), stellar mass, and other properties of massive dusty galaxies at cosmic noon are impacted when far-infrared (FIR)/sub-millimeter data are added to datasets containing only ultraviolet (UV) to near-infrared (NIR) data. For a sample of 92 massive (stellar mass > 4 × 10^10 MSun) dusty galaxies at z ∼ 1.5 to 3.0 (corresponding to ∼25% of cosmic history), we fit the spectral energy distributions (SEDs) based on DECam UV-to-optical data, VICS82, NEWFIRM, and Spitzer-IRAC NIR data, and Herschel-SPIRE FIR/sub-millimeter data using the Bayesian Analysis of Galaxies for Physical Inference and Parameter Estimation (BAGPIPES) SED-fitting code. We assume a delayed tau star formation history with a log10 prior on tau and derive the posterior distributions of stellar mass, SFR, extinction, and specific SFR. We find that adding FIR/sub-millimeter data leads to SFR estimates that can be both significantly higher or lower (typically by up to a factor of 10) than estimates based on UV-to-NIR data alone, depending on the type of galaxies involved. We find that the changes in SFR scale with changes in extinction. These results highlight the importance of including FIR/sub-millimeter data in order to accurately derive the SFRs of massive dusty galaxies at z ∼ 2.

Gamma-Ray Burst Host Galaxies: Specific Star Formation Rate versus Metallicity

Abstract The observed properties of long gamma-ray burst (GRB) host galaxies show them to often be of a rather low metallicity and/or of high specific star formation rate (SFR). It is not clear which of these properties is a dominant factor in determining if a galaxy will host a GRB or not. In fact there are indications, at least in the local universe, that the two may be anticorrelated and that the metallicity is the deciding parameter. Here, we consider GRB production models dependent on both quantities and show that when compared to the best available data, the respective star formation fractions appear indistinguishable out to a redshift of z ∼ 4. However, the fraction of galaxies hosting a GRB, as determined by the specific SFR, is less at tension with the observed host galaxy fraction than the corresponding metallicity determined fraction, but this conclusion is model dependent. Well established galaxy stellar mass and star formation rate functions at high redshift are crucial in breaking the apparent degeneracy between the specific rate and metallicity in GRB production probability.

A Submillimeter Perspective on the GOODS Fields (SUPER GOODS). IV. The Submillimeter Properties of X-Ray Sources in the CDF-S

Abstract The Chandra Deep Field (CDF)-S is the deepest X-ray image available and will remain so for the near future. We provide a spectroscopic (64.5%; 64% with spectral classifications) and photometric redshift catalog for the full 7 Ms sample, but much of our analysis focuses on the central (off-axis angles <5.′7) region, which contains a large, faint ALMA sample of 75 > 4.5σ 850 μm sources. We measure the 850 μm fluxes at the X-ray positions using the ALMA images, where available, or an ultradeep SCUBA-2 map. We find that the full X-ray sample produces ∼10% of the 850 μm extragalactic background light. We separate the submillimeter-detected X-ray sources into star-forming galaxies and active galactic nuclei (AGNs) using a star formation rate (SFR) versus X-ray luminosity calibration for high-SFR galaxies. We confirm this separation using the X-ray photon indices. We measure the X-ray fluxes at the accurate positions of the 75 ALMA sources and detect 70% at >3σ in either the 0.5–2 or 2–7 keV bands. However, many of these may produce both their X-ray and submillimeter emission by star formation. Indeed, we find that only 20% of the ALMA sources have intermediate X-ray luminosities (rest-frame 8–28 keV luminosities of 1042.5–1044 erg s−1), and none has a high X-ray luminosity (>1044 erg s−1). Conversely, after combining the CDF-S with the CDF-N, we find extreme star formation (SFR > 300 M ⊙ yr−1) in some intermediate X-ray luminosity sources but not in any high X-ray luminosity sources. We argue that the quenching of star formation in the most luminous AGNs may be a consequence of the clearing of gas in these sources.

Discovery of a galaxy overdensity around a powerful, heavily obscured FRII radio galaxy atz= 1.7: star formation promoted by large-scale AGN feedback?

We report the discovery of a galaxy overdensity around a Compton-thick Fanaroff–Riley type II (FRII) radio galaxy atz = 1.7 in the deep multiband survey around thez = 6.3 quasi-stellar object (QSO) SDSS J1030+0524. Based on a 6 h VLT/MUSE and on a 4 h LBT/LUCI observation, we identify at least eight galaxy members in this structure with spectroscopic redshiftz = 1.687 − 1.699, including the FRII galaxy atz = 1.699. Most members are distributed within 400 kpc from the FRII core. Nonetheless, the whole structure is likely much more extended, as one of the members was serendipitously found at ∼800 kpc projected separation. The classic radio structure of the FRII itself extends for ∼600 kpc across the sky. Most of the identified overdensity members are blue, compact galaxies that are actively forming stars at rates of ∼8–60M⊙yr−1. For the brightest of them, a half-light radius of 2.2 ± 0.8 kpc at 8000 Å rest-frame was determined based on adaptive optics-assisted observations with LBT/SOUL in the Ks band. We do not observe any strong galaxy morphological segregation or concentration around the FRII core. This suggests that the structure is far from being virialized and likely constitutes the progenitor of a local massive galaxy group or cluster caught in its main assembly phase. Based on a 500 ksChandraACIS-I observation, we found that the FRII nucleus hosts a luminous QSO (L2 − 10 keV = 1.3 × 1044erg s−1, intrinsic and rest-frame) that is obscured by Compton-thick absorption (NH = 1.5 ± 0.6 × 1024cm−2). Under standard bolometric corrections, the total measured radiative power (Lrad ∼ 4 × 1045erg s−1) is similar to the jet kinetic power that we estimated from radio observations at 150 MHz (Pkin = 6.3 × 1045erg s−1), in agreement with what is observed in powerful jetted AGN. OurChandraobservation is the deepest so far for a distant FRII within a galaxy overdensity. It revealed significant diffuse X-ray emission within the region that is covered by the overdensity. In particular, X-ray emission extending for ∼240 kpc is found around the eastern lobe of the FRII. Four out of the six MUSE star-forming galaxies in the overdensity are distributed in an arc-like shape at the edge of this diffuse X-ray emission. These objects are concentrated within 200 kpc in the plane of the sky and within 450 kpc in radial separation. Three of them are even more concentrated and fall within 60 kpc in both transverse and radial distance. The probability of observing four out of the sixz = 1.7 sources by chance at the edge of the diffuse emission is negligible. In addition, these four galaxies have the highest specific star formation rates of the MUSE galaxies in the overdensity and lie above the main sequence of field galaxies of equal stellar mass atz = 1.7. We propose that the diffuse X-rays originate from an expanding bubble of gas that is shock heated by the FRII jet, and that star formation is promoted by the compression of the cold interstellar medium of the galaxies around the bubble, which may be remarkable evidence of positive AGN feedback on cosmological scales. We emphasize that our conclusions about the feedback are robust because even assuming that the diffuse X-ray emission arises from inverse Compton scattering of photons of the cosmic microwave background by the relativistic electrons in the radio lobe, star formation may be promoted by the nonthermal pressure of the expanding lobe.

How feedback shapes galaxies: an analytic model

ABSTRACT We introduce a simple analytic model of galaxy formation that links the growth of dark matter haloes in a cosmological background to the build-up of stellar mass within them. The model aims to identify the physical processes that drive the galaxy-halo co-evolution through cosmic time. The model restricts the role of baryonic astrophysics to setting the relation between galaxies and their haloes. Using this approach, galaxy properties can be directly predicted from the growth of their host dark matter haloes. We explore models in which the effective star formation efficiency within haloes is a function of mass (or virial temperature) and independent of time. Despite its simplicity, the model reproduces self-consistently the shape and evolution of the cosmic star formation rate density, the specific star formation rate of galaxies, and the galaxy stellar mass function, both at the present time and at high redshifts. By systematically varying the effective star formation efficiency in the model, we explore the emergence of the characteristic shape of the galaxy stellar mass function. The origin of the observed double Schechter function at low redshifts is naturally explained by two efficiency regimes in the stellar to halo mass relation, namely, a stellar feedback regulated stage, and a supermassive black hole regulated stage. By providing a set of analytic differential equations, the model can be easily extended and inverted, allowing the roles and impact of astrophysics and cosmology to be explored and understood.

Mass and star formation rate of the host galaxies of compact binary mergers across cosmic time

ABSTRACT We investigate the properties of the host galaxies of compact binary mergers across cosmic time, by means of population-synthesis simulations combined with galaxy catalogues from the eagle suite. We analyse the merger rate per galaxy of binary neutron stars (BNSs), black hole–neutron star binaries (BHNSs), and binary black holes (BBHs) from redshift zero up to six. The binary merger rate per galaxy strongly correlates with the stellar mass of the host galaxy at any redshift considered here. This correlation is significantly steeper for BNSs than for both BHNSs and BBHs. Moreover, we find that the merger rate per galaxy depends also on host galaxy’s star formation rate (SFR) and metallicity. We derive a robust fitting formula that relates the merger rate per galaxy with galaxy’s SFR, stellar mass, and metallicity at different redshifts. The typical masses of the host galaxies increase significantly as redshift decreases, as a consequence of the interplay between delay time distribution of compact binaries and cosmic assembly of galaxies. Finally, we study the evolution of the merger rate density with redshift. At low redshift (z ≤ 0.1) early-type galaxies give a larger contribution to the merger rate density than late-type galaxies. This trend reverts at z ≥ 1.

Fast Outflows Identified in Early Star-forming Galaxies at z = 5–6

Abstract We present velocities of galactic outflows in seven star-forming galaxies at z = 5–6 with stellar masses of M * ∼ 1010.1 . Although it is challenging to observationally determine the outflow velocities, we overcome this by using ALMA [C ii] 158 μm emission lines for systemic velocities and deep Keck spectra with metal absorption lines for velocity profiles available to date. We construct a composite Keck spectrum of the galaxies at z = 5–6 with the [C ii]-systemic velocities, and fit outflow-line profiles to the Si ii λ1260, C ii λ1335, and Si iv λλ1394,1403 absorption lines in the composite spectrum. We measure the maximum (90%) and central outflow velocities to be and on average, respectively, showing no significant differences between the outflow velocities derived with the low- to high-ionization absorption lines. For M * ∼ 1010.1 , we find that the value of our z = 5–6 galaxies is 3 times higher than those of z ∼ 0 galaxies and comparable to z ∼ 2 galaxies. Estimating the halo circular velocity from the stellar masses and the abundance matching results, we investigate a – relation. Interestingly, for galaxies with M * = 1010.0–10.8 shows a clear positive correlation with and/or the galaxy star formation rate over z = 0–6 with a small scatter of dex, which is in good agreement with theoretical predictions. This positive correlation suggests that the outflow velocity is physically related to the halo circular velocity, and that the redshift evolution of at fixed M * is explained by the increase in toward high redshift.

Stellar populations of galaxies in the ALHAMBRA survey up to z ∼ 1

Aims.Our aim is to determine the distribution of stellar population parameters (extinction, age, metallicity, and star formation rates) of quiescent galaxies within the rest-frame stellar mass–colour diagrams andUVJcolour–colour diagrams corrected for extinction up toz ∼ 1. These novel diagrams reduce the contamination in samples of quiescent galaxies owing to dust-reddened galaxies, and they provide useful constraints on stellar population parameters only using rest-frame colours and/or stellar mass.Methods.We set constraints on the stellar population parameters of quiescent galaxies combining the ALHAMBRA multi-filter photo-spectra with our fitting code for spectral energy distribution, MUlti-Filter FITting (MUFFIT), making use of composite stellar population models based on two independent sets of simple stellar population (SSP) models. The extinction obtained by MUFFIT allowed us to remove dusty star-forming (DSF) galaxies from the sample of redUVJgalaxies. The distributions of stellar population parameters across these rest-frame diagrams are revealed after the dust correction and are fitted by LOESS, a bi-dimensional and locally weighted regression method, to reduce uncertainty effects.Results.Quiescent galaxy samples defined via classicalUVJdiagrams are typically contaminated by a ∼20% fraction of DSF galaxies. A significant part of the galaxies in the green valley are actually obscured star-forming galaxies (∼30–65%). Consequently, the transition of galaxies from the blue cloud to the red sequence, and hence the related mechanisms for quenching, seems to be much more efficient and faster than previously reported. The rest-frame stellar mass–colour andUVJcolour–colour diagrams are useful for constraining the age, metallicity, extinction, and star formation rate of quiescent galaxies by only their redshift, rest-frame colours, and/or stellar mass. Dust correction plays an important role in understanding how quiescent galaxies are distributed in these diagrams and is key to performing a pure selection of quiescent galaxies via intrinsic colours.

Do bulges stop stars forming?

ABSTRACT In this paper, we use the Herschel Reference Survey to make a direct test of the hypothesis that the growth of a stellar bulge leads to a reduction in the star formation efficiency of a galaxy (or conversely a growth in the gas-depletion time-scale) as a result of the stabilization of the gaseous disc by the gravitational field of the bulge. We find a strong correlation between star formation efficiency and specific star formation rate in galaxies without prominent bulges and in galaxies of the same morphological type, showing that there must be some other process besides the growth of a bulge that reduces the star formation efficiency in galaxies. However, we also find that galaxies with more prominent bulges (Hubble types E to Sab) do have significantly lower star formation efficiencies than galaxies with later morphological types, which is at least consistent with the hypothesis that the growth of a bulge leads to the reduction in the star formation efficiency. The answer to the question in the title is therefore yes and no: bulges may reduce the star formation efficiency in galaxies but there must also be some other process at work. We also find that there is a significant but small difference in the star formation efficiencies of galaxies with and without bars, in the sense that galaxies with bars have slightly higher star formation efficiencies.

Effect of galaxy mergers on star-formation rates

Context. Galaxy mergers and interactions are an integral part of our basic understanding of how galaxies grow and evolve over time. However, the effect that galaxy mergers have on star-formation rates (SFRs) is contested, with observations of galaxy mergers showing reduced, enhanced, and highly enhanced star formation. Aims. We aim to determine the effect of galaxy mergers on the SFR of galaxies using statistically large samples of galaxies, totalling over 200 000, which is over a large redshift range from 0.0 to 4.0. Methods. We trained and used convolutional neural networks to create binary merger identifications (merger or non-merger) in the SDSS, KiDS, and CANDELS imaging surveys. We then compared the SFR, with the galaxy main sequence subtracted, of the merging and non-merging galaxies to determine what effect, if any, a galaxy merger has on SFR. Results. We find that the SFR of merging galaxies are not significantly different from the SFR of non-merging systems. The changes in the average SFR seen in the star-forming population when a galaxy is merging are small, of the order of a factor of 1.2. However, the higher the SFR is above the galaxy main sequence, the higher the fraction is for galaxy mergers. Conclusions. Galaxy mergers have little effect on the SFR of the majority of merging galaxies compared to the non-merging galaxies. The typical change in SFR is less than 0.1 dex in either direction. Larger changes in SFR can be seen but are less common. The increase in merger fraction as the distance above the galaxy main sequence increases demonstrates that galaxy mergers can induce starbursts.

A New Calibration of Star Formation Rate in Galaxies Based on Polycyclic Aromatic Hydrocarbon Emission

Abstract Polycyclic aromatic hydrocarbon (PAH) emission has long been proposed to be a potential star formation rate indicator, as it arises from the photodissociation region bordering the Strömgren sphere of young, massive stars. We apply a recently developed technique of mid-infrared spectral decomposition to obtain a uniform set of PAH measurements from Spitzer low-resolution spectra of a large sample of star-forming galaxies spanning a wide range in stellar mass (M ⋆ ≈ 106–1011.4 M ⊙) and star formation rate (∼0.1–2000 M ⊙ yr−1). High-resolution spectra are also analyzed to measure [Ne ii] 12.8 μm and [Ne iii] 15.6 μm, which effectively trace the Lyman continuum. We present a new relation between PAH luminosity and star formation rate based on the [Ne ii] and [Ne iii] lines. Calibrations are given for the integrated 5–15 μm PAH emission, the individual features at 6.2, 7.7, 8.6, and 11.3 μm, as well as several mid-infrared bandpasses sensitive to PAH. We confirm that PAH emission is suppressed in low-mass dwarf galaxies, and we discuss the possible physical origin of this effect.

Narrow-band Hα imaging of nearby Wolf–Rayet galaxies

ABSTRACT We present narrow-band Hα imaging of nearby Wolf–Rayet (WR) galaxies known as a subset of starburst galaxies. The Hα images have been used to show morphology of star-forming regions in galaxies, which leads to speculate that the studied galaxies have most likely experienced merger or interaction with low luminous dwarf galaxies or H i clouds. We further derive the Hα-based star formation rates (SFRs) in galaxies using our Hα observations. These SFRs are well correlated with SFRs derived using other indicators at far-ultraviolet, far-infrared, and 1.4-GHz radio wavebands. It is noticed that the infrared excess (IRX) method gives the best SFR estimates, consistent with different models predication. These models also predict that the sample galaxies have probably gone through a continuous star formation at least for 1 Gyr over which the recent (<10 Myr) star formation has taken place in WR phase. This study presents main-sequence (MS) relation for nearby WR galaxies for the first time. This derived MS relation is found to be similar to previously known MS relation for normal nearby star-forming galaxies, suggesting that WR systems evolve in a similar fashion as normal star-forming galaxies evolve.

Metal Abundances and Star Formation Rates of Emission-line Galaxies in and around the Boötes Void

Abstract We explore the possible dependencies of galaxy metal abundance and star formation rate (SFR) on local environment, focusing on the volume of space in and around the Boötes Void. Our sample of star-forming galaxies comes from the second catalog of the Hα-selected KPNO International Spectroscopic Survey (KISS), which overlaps the void. This sample represents a statistically complete, line-flux-limited ensemble of 820 star-forming galaxies, all of which possess metallicity and SFR estimates. We carry out two distinct analyses of the KISS galaxies: one that probes the properties of the entire sample as a function of local density, and a second that details the properties of 33 KISS star-forming galaxies located within the Boötes Void. In both cases, we find no evidence that either the metallicity of the KISS galaxies or their SFRs depend on the environments within which the galaxies are located. Our global analysis does show weak trends for decreasing stellar mass, decreasing metallicity, and decreasing SFRs with decreasing local densities. However, we argue that the metallicity and SFR trends are artifacts of the stellar mass—local density trend. In particular, the change in metallicity with density is precisely what one would predict from the mass–metallicity relation, given the observed drop in stellar mass with decreasing metallicity. Likewise, the SFR trend with density disappears when one instead considers the mass-normalized specific SFR. The KISS galaxies dwelling in the Boötes Void are found to have metallicity and SFR properties nearly identical to those of a matched comparison sample, despite the fact that the former are located in density environments that are, on average, more than 16 times lower.

Horizon-AGN virtual observatory – 2. Template-free estimates of galaxy properties from colours

ABSTRACT Using the horizon-AGN hydrodynamical simulation and self-organizing maps (SOMs), we show how to compress the complex, high-dimensional data structure of a simulation into a 2D grid, which greatly facilitates the analysis of how galaxy observables are connected to intrinsic properties. We first verify the tight correlation between the observed 0.3–5 μm broad-band colours of horizon-AGN galaxies and their high-resolution spectra. The correlation is found to extend to physical properties such as redshift, stellar mass, and star formation rate (SFR). This direct mapping from colour to physical parameter space still works after including photometric uncertainties that mimic the COSMOS survey. We then label the SOM grid with a simulated calibration sample to estimate redshift and SFR for COSMOS-like galaxies up to z ∼ 3. In comparison to state-of-the-art techniques based on synthetic templates, our method is comparable in performance but less biased at estimating redshifts, and significantly better at predicting SFRs. In particular, our ‘data-driven’ approach, in contrast to model libraries, intrinsically allows for the complexity of galaxy formation and can handle sample biases. We advocate that observations to calibrate this method should be one of the goals of next-generation galaxy surveys.