A Comparison of Independent Star Formation Diagnostics for an Ultraviolet‐selected Sample of Nearby Galaxies

We present panoramic u′ and optical ground-based imaging observations of a complete sample of low-redshift (0 < z < 0.4) galaxies selected in the ultraviolet (UV) at 2000 A using the balloon-borne FOCA instrument of Milliard et al. This survey is highly sensitive to newly formed massive stars and hence to actively star-forming galaxies. We use the new data to investigate further the optical, stellar population and star formation properties of this unique sample, deriving accurate galaxy types and k-corrections based on the broad-band spectral energy distributions. When combined with our earlier spectroscopic surveys, these new data allow us to compare star formation measures derived from aperture-corrected Hα line fluxes, and UV(2000 A) and u′(3600 A) continuum fluxes on a galaxy-by-galaxy basis. As expected from our earlier studies, we find broad correlations over several decades in luminosity between the different dust-corrected star formation diagnostics, though the scatter is larger than that from observational errors, with significant offsets from trends expected according to simple models of the star formation histories (SFHs) of galaxies. Popular galaxy spectral synthesis models with varying metallicities and/or initial mass functions seem unable to explain the observed discrepancies. We investigate the star formation properties further by modelling the observed spectroscopic and photometric properties of the galaxies in our survey. We find that nearly half of the galaxies surveyed possess features that appear incompatible with simple constant or smoothly declining SFHs, favouring instead irregular or temporally varying SFHs. We demonstrate how this can reconcile the majority of our observations, enabling us to determine empirical corrections that can be used to calculate intrinsic star formation rates (as derived from Hα luminosities) from measures based on UV (or u′) continuum observations alone. We discuss the broader implications of our finding that a significant fraction of star-forming galaxies have complex SFHs, particularly in the context of recent determinations of the cosmic SFH.

Metallicity and age gradients of the stellar populations and dust extinction are studied for a sample of 32 non-active, seven type 1 Seyfert (S1) and 17 type 2 Seyfert (S2) spiral galaxies. The sample galaxies cover the whole range from face-on to edge-on view, and the variation of the optical and near-infrared colour gradients in the disc as a function of the inclination angle is investigated in order to separate colour changes caused by population gradients from those due to dust effects. The measurements show that the observed colour gradients in the discs of the non-active galaxies are significantly larger than those found for the S1 and S2 galaxies. In the near-infrared wavelengths, however, these differences disappear, and the colour gradients are the same for all three galaxy types. No systematic differences are found between the colour gradients of the discs of the S1 galaxies and those of the S2 galaxies. The data are compared to model images of dusty galaxies with a variety of age and metallicity gradients in the disc. The radial variations of the optical and near-infrared colours of the model galaxies are calculated from the radial changes of the ages and the metallicities of the stars, using broad-band colours of a single stellar population. The stellar content at a given position in the disc is determined by the average age, the metallicity and the star formation history. For the non-active galaxies, the observed colour gradients are represented best by a model with a metallicity gradient, with the inner regions of the stellar disc being more metal-rich than the outer regions. However, the presence of an age gradient, with the inner regions of the stellar disc being older than the outer regions, cannot be ruled out. For the S1 and S2 galaxies, the comparison between data and models indicates that the age and metallicity gradients in the stellar disc are small. As far as the internal dust extinction is concerned, the comparison between data and models indicates that both the non-active and the S2 galaxies show significant dust extinction, but they are not optically thick. Ke yw ords: dust, extinction ‐ galaxies: abundances ‐ galaxies: active ‐ galaxies: spiral ‐ galaxies: stellar content ‐ galaxies: structure.

We study the star formation rate and dust extinction properties of a sample of nearby star-forming galaxies as derived from Hα and UV (~2000 Å) observations and we compare them to those of a sample of starburst galaxies. The dust extinction in Hα is estimated from the Balmer decrement and the extinction in UV using the FIR to UV flux ratio or the attenuation law for starburst galaxies of Calzetti et al. ([CITE]). The Hα and UV emissions are strongly correlated with a very low scatter for the star-forming objects and with a much higher scatter for the starburst galaxies. The Hα to UV flux ratio is found to be larger by a factor ~2 for the starburst galaxies. We compare both samples with a purely UV selected sample of galaxies and we conclude that the mean Hα and UV properties of nearby star-forming galaxies are more representative of UV-selected galaxies than starburst galaxies. We emphasize that the Hα to UV flux ratio is strongly dependent on the dust extinction: the positive correlation found between and vanishes when the Hα and UV flux are corrected for dust extinction. The Hα to UV flux ratios converted into star formation rate and combined with the Balmer decrement measurements are tentatively used to estimate the dust extinction in UV.

We study the star formation rate and dust extinction properties of a sample of nearby star forming galaxies as derived from Halpha and UV (2000 A) observations and we compare them to those of a sample of starburst galaxies. The dust extinction in Halpha is estimated from the Balmer decrement and the extinction in UV using the FIR to UV flux ratio or the attenuation law for starburst galaxies of Calzetti et al. The Halpha and UV emissions are strongly correlated with a very low scatter for the star forming objects and with a much higher scatter for the starburst galaxies. The Halpha to UV flux ratio is found larger by a factor ~ 2 for the starburst galaxies. We compare both samples with a purely UV selected sample of galaxies and we conclude that the mean Halpha and UV properties of nearby star forming galaxies are more representative of UV selected galaxies than starburst galaxies. We emphasize that the Halpha to UV flux ratio is strongly dependent on the dust extinction: the positive correlation found between F{Halpha}/F{UV}$ and F{FIR}/F{UV} vanishes when the Halpha and UV flux are corrected for dust extinction. The Halpha to UV flux ratios converted into star formation rate and combined with the Balmer decrement measurements are tentatively used to estimate the dust extinction in UV.

A sample of local galaxies for which far infrared and uv fluxes are available is used to estimate the characteristic dust extinction in galaxies and to test whether standard dust properties are plausible. Assuming galaxies can be characterized by a single dust optical depth (certainly not valid for galaxies with a dominant starburst component), the infrared excess and ultraviolet colours of local galaxies are found to be consistent with normal Milky Way dust, with a mean value for E(B-V) of 0.16. A significant fraction of the dust heating is due to older, lower mass stars, and this fraction increases towards earlier galaxy types. Analysis of F_fir/F_uv versus uv colour diagrams for starburst galaxies in terms of a simple screen dust model does not support a Calzetti (1999) rather than a Milky Way extinction law, though the absence of the expected strong 2200 A feature in several galaxies with IUE spectra does show that more detailed radiative transfer models are needed, probably with non-spherical geometry. A simple treatment in which the 100/60 mu flux-ratio is used to subtract the optically thick starburst contribution to the far infrared radiation results in lower extinction estimates for the optically thin cirrus component, with a mean E(B-V) of 0.10 The uv luminosity density, corrected for dust extinction, is derived and a value for the local mean star-formation rate inferred. This is consistent with previous estimates from uv surveys and from H-alpha surveys.

Context. Radio used as a tracer of the star formation rate (SFR) presents enormous advantages because it is not affected by dust and radio sources that are located at the subarcsecond level. The interpretation of the low-frequency 1.4 GHz luminosity is hampered by the difficulty of modeling the paths of cosmic rays in the interstellar medium, however, and by their interactions with the magnetic field. Aims. We compare the SFR derived from radio observations and the SFRs derived from spectral energy distribution (SED) modeling. We aim at better understanding the behavior of the SFR radio tracer, with a specific emphasis on the link to star formation histories (SFHs). Methods. The analysis is based on a subsample of 1584 star-forming galaxies extracted from the Cosmic Evolution Survey (COSMOS) with observations of the Very large array project at 3 GHz. We used the SED modeling code investigating galaxy emission, CIGALE, with a nonparametric model for the SFH and fit the data over the wavelength range from the ultraviolet (UV) to the mid-infrared (mid-IR). We interpret the difference between radio and SED-based SFR tracers in the light of recent gradients in the derived SFH. To validate the robustness of the results, we searched for any remaining contribution of active galaxy nuclei and tested the impact of our SFH modeling approach. Results. Approximately 27% our galaxies present a radio SFR (SFRradio) that is at least ten times higher than the instantaneous SFR from SED fitting (SFRSED). This trend primarily affects the galaxies whose SFH activity decreased over the last 300 Myr. Both SFR indicators converge toward a consistent value when the SFHs are averaged over a period longer than 150 Myr to derive SFRSED. Conclusions. Although the radio at a low frequency of 1.4 GHz is a good tracer of the star formation activity of galaxies with a constant or increasing SFH, our results indicate that this is not the case for quenched galaxies. Our analysis suggests that the star formation time sensitivity of the low radio frequency might be longer than 150 Myr. Interestingly, the discrepancy between the SFRradio and SFRSED can be used as diagnostic to select post-starburst galaxies.

We study the ultraviolet (UV) continuum β slope of a sample of 166 clumps, individual star-forming regions observed in high-redshift galaxies. They are hosted by 67 galaxies with redshift between 2 and 6.2, strongly lensed by the Hubble Frontier Fields cluster of galaxies MACS J0416.1 − 2403. The β slope is sensitive to a variety of physical properties, such as the metallicity, the age of the stellar population, the dust attenuation throughout the galaxy, the stellar initial mass function (IMF), and the star formation history (SFH). The aim of this study is to compare the β-values of individual clumps with those measured on the entire galaxy, to investigate possible physical differences between these regions and their hosts. We found a median value of β ∼ −2.4, lower than that of integrated galaxies. This result confirms that clumps are sites of intense star formation, populated by young, massive stars, whose spectrum strongly emits in the UV. This is also consistent with the assumption that the dust extinction at the location of the clumps is lower than the average extinction of the galaxy, or that clumps have a different IMF or SFH. We made use of the correlations, discovered for high-redshift galaxies, of the β-value with those of redshift and UV magnitude, MUV, finding that clumps follow the same relations, extended to much fainter magnitudes (MUV &amp;lt; −13). We also find evidence of eight clumps with extremely blue (β ≲ −2.7) slopes, which could be the signpost of low-metallicity stars and constrain the emissivity of ionizing photons at high redshift.

In this study we investigate the relation between stellar mass, dust extinction and star formation rate (SFR) using ~150,000 star-forming galaxies from the SDSS DR7. We show that the relation between dust extinction and SFR changes with stellar mass. For galaxies at the same stellar mass dust extinction is anti-correlated with the SFR at stellar masses <10^10 M_solar. There is a sharp transition in the relation at a stellar mass of 10^10 M_solar. At larger stellar masses dust extinction is positively correlated with the SFR for galaxies at the same stellar mass. The observed relation between stellar mass, dust extinction and SFR presented in this study helps to confirm similar trends observed in the relation between stellar mass, metallicity and SFR. The relation reported in this study provides important new constraints on the physical processes governing the chemical evolution of galaxies. The correlation between SFR and dust extinction for galaxies with stellar masses >10^10 M_solar is shown to extend to the population of quiescent galaxies suggesting that the physical processes responsible for the observed relation between stellar mass, dust extinction and SFR may be related to the processes leading to the shut down of star formation in galaxies.

We present the burst ages for young stellar populations in a sample of six nearby (<10 Mpc) spiral galaxies using a differential pixel-based analysis of the ionized gas emission. We explore this as an alternative approach for connecting large-scale dynamical mechanisms with star formation processes in disc galaxies, based on burst ages derived from the Hα to far-UV (FUV) flux ratio. Images of each galaxy in Hα were taken with Taurus Tunable Filter and matched to FUV imaging from GALEX. The resulting flux ratio provides a robust measure of relative age across the disc which we discuss in terms of the large-scale dynamical motions. Systematic effects such as a variable initial mass function, non-solar metallicities, variable star formation histories (SFHs) and dust attenuation have been used to derive estimates of the systematic uncertainty. The resulting age maps show a wide range of patterns outside of those galaxies with the strongest spiral structure, confirming the idea that star formation is driven one by several processes, largely determined by the individual circumstances of the galaxy. Generally, grand design spirals such as M74, M100 and M51 exhibit age gradients across the main spiral arms, with the youngest star formation regions along the central and inner edges. Likewise, in the dominant star-forming complex of IC 2574 or the ring of M94, the most recent star formation is centrally confined to the regions of star formation activity. In M63 and M74 galaxy-wide trends emerge, contrary to the spiral structure in these galaxies, suggesting that spiral density waves are not the dominant driver in some cases. We argue that despite appearances, galaxy morphology is not an absolute discriminator of the SFH of an individual galaxy, nor of the processes triggering it. We conclude that Hα-to-FUV flux ratios are a relatively direct way to probe burst ages across galaxies and infer something of their dynamical histories, provided that sources of systematics are properly taken into account.

The <i>Orion</i> MIDEX mission is a 1.2m UV-visual observatory orbiting at L2 that will conduct the first-ever high spatial resolution survey of a statistically significant sample of visible star-forming environments in the Solar neighborhood in emission lines and continuum. This survey will be used to characterize the star and planet forming environments within 2.5 kpc of the Sun, infer global properties and star formation histories in these regions, understand how environment influences the process of star and planet formation, and develop a classification scheme for star forming regions. Based on these findings a similar survey will be conducted of large portions of the Magellanic Clouds, extending the classification scheme to new types of regions common in external galaxies, allowing the characterization of low mass star forming environments in the Magellanic Clouds, study of the spatial distribution of star forming environments and tracing of star formation history. Finally the mission will image a sample of external galaxies out to ~5 Mpc. The distribution of star forming region type will be mapped as a function of galactic environment to infer the distribution and history of low-mass star formation over galactic scales, and characterize the stellar content and star formation history of galaxies. We present in this paper an update on the development of the mission and the hardware necessary to deliver its required performance.

I summarize the method commonly used to estimate the current star formation rate per unit volume as a function of redshift from various star formation diagnostics, as well as the often conflicting results from the various multi-wavelength datasets collected in recent years. Combined together, and with the help of theoretical and empirical models, these results allow us to address the issues of excitation, reddening, metallicity, and star formation history in individual galaxies at low redshift. They seem to point to the fact, that a lot of the star formation in the local universe, and potentially at all redshift, occurs in burst mode rather than continuously.

The Star Formation Rate (SFR) is a crucial ingredient to understand the star formation history of the galaxies at all redshifts. This SFR is currently derived from the Hα line and the far-UV continuum luminosities since both are directly linked to the young stars but the main difficulty is the uncertainty on the dust extinction and both tracers must be corrected before any quantitative study.Keywordsstarburststar formationdust extinction

We present a study of the evolution of star-forming galaxies within what is known as the Wall structure at z∼0.73 in the field of the COSMOS survey. We use a sample of star-forming galaxies from a comprehensive range of environments and across a wide stellar mass range. We discuss the correlation between the environment and the galaxy's internal properties, including its metallicity from the present-day gas-phase value and its past evolution as imprinted in its stellar populations. We measured emission-line fluxes from the stacked spectra of galaxies selected within small stellar mass bins and in different environments. These fluxes were then converted to gas-phase metallicities. In addition, we built a simple yet comprehensive galaxy chemical evolution model, which is constrained by the gas-phase metallicities, stacked spectra, and photometry of galaxies to reach a full description of the galaxies' past star formation and chemical evolution histories in different environments. Parameters derived from best-fit models provide insights into the physical process behind the evolution. We reproduce the downsizing formation of galaxies in their star formation histories and in their chemical evolution histories at z∼0.73 so that more massive galaxies tend to grow their stellar mass and become enriched in metals earlier than less massive ones. In addition, the current gas-phase metallicity of a galaxy and its past evolution correlate with the environment it inhabits. Galaxies in groups, especially massive groups that have X-ray counterparts, tend to have higher gas-phase metallicities and are enriched in metals earlier than field galaxies of similar stellar mass. Galaxies in the highest stellar mass bin and located in X-ray groups exhibit a more complex and varied chemical composition. The evolution of a galaxy, including its star formation history and chemical enrichment history, exhibits a notable dependence on the environment where the galaxy is located. This dependence is revealed in our sample of star-forming galaxies in the Wall region at a redshift of z∼0.73. Strangulation due to interactions with the group environment, leading to an early cessation of gas supply, may have driven the faster mass growth and chemical enrichment observed in group galaxies. Additionally, the removal of metal-enriched gas could play a key role in the evolution of the most massive galaxies. Alternative mechanisms other than environmental processes are also discussed.

We develop a dust efflux model of radiation pressure acting on dust grains which successfully reproduces the relation between stellar mass, dust opacity and star formation rate observed in local star-forming galaxies. The dust content of local star-forming galaxies is set by the competition between the physical processes of dust production and dust loss in our model. The dust loss rate is proportional to the dust opacity and star formation rate. Observations of the relation between stellar mass and star formation rate at several epochs imply that the majority of local star-forming galaxies are best characterized as having continuous star formation histories. Dust loss is a consequence of sustained interaction of dust with the radiation field generated by continuous star formation. Dust efflux driven by radiation pressure rather than dust destruction offers a more consistent physical interpretation of the dust loss mechanism. By comparing our model results with the observed relation between stellar mass, dust extinction and star formation rate in local star-forming galaxies we are able to constrain the timescale and magnitude of dust loss. The timescale of dust loss is long and therefore dust is effluxed in a "Slow Flow". Dust loss is modest in low mass galaxies but massive galaxies may lose up to 70~80% of their dust over their lifetime. Our Slow Flow model shows that mass loss driven by dust opacity and star formation may be an important physical process for understanding normal star-forming galaxy evolution.

Physical characteristics of a large sample of compact galaxies with active star formation from the SDSS DR14 are derived. The sample includes approximately 30 000 compact isolated galaxies with angular diameters <6″. The emission lines Hβ with equivalent widths EW(Hβ) ≥ 1 nm are observed in the spectra of selected galaxies. The stellar masses of compact galaxies are distributed in a wide range from 105M⊙ to 1011M⊙ with a maximum at ~109M⊙. The oxygen abundances for the bulk of compact galaxies are distributed in the range 7.8…8.2 with a maximum at ~8.05. Compact galaxies are characterized with high specific star-formation rates of 10…100 Gyr–1. The SDSS spectroscopic data were supplemented by photometric data in the far- and near-ultraviolet ranges from the GALEX and in the mid-infrared range at 22 μm from the WISE all-sky surveys. The star-formation rate, concisely named “composite” one, was determined using combinations of two out of five observed luminosities: luminosity L(Hα) in the emission line Hα, monochromatic luminosities in the ultraviolet continuum L(FUV), and L(NUV) and in the mid-infrared continuum L(22 μm) as well as the total luminosities in the infrared range L(TIR). “Composite” star formation rates in compact galaxies with active star formation are compared with those determined from the extinction- and spectral aperture-corrected luminosities of galaxies in the hydrogen emission line Hβ. Relations for “composite” star formation rates with different combinations of indicators were obtained, which are mutually consistent and correspond to star-formation rates SFR(Hβ) derived from the luminosities of galaxies in the hydrogen emission line Hβ, corrected for extinction and spectral aperture.