A Comparison of Ultraviolet Imaging Telescope Far‐Ultraviolet and Hα Star Formation Rates

Using a complete sample of ∼300 star-forming galaxies within 11 Mpc of the Milky Way, we evaluate the consistency between star formation rates (SFRs) inferred from the far ultraviolet (FUV) non-ionizing continuum and Hα nebular emission, assuming standard conversion recipes in which the SFR scales linearly with luminosity at a given wavelength. Our analysis probes SFRs over 5 orders of magnitude, down to ultra-low activities on the order of ∼10−4 M☉ yr−1. The data are drawn from the 11 Mpc Hα and Ultraviolet Galaxy Survey (11HUGS), which has obtained Hα fluxes from ground-based narrowband imaging, and UV fluxes from imaging with GALEX. For normal spiral galaxies (SFR ∼ 1 M☉ yr−1), our results are consistent with previous work which has shown that FUV SFRs tend to be lower than Hα SFRs before accounting for internal dust attenuation, but that there is relative consistency between the two tracers after proper corrections are applied. However, a puzzle is encountered at the faint end of the luminosity function. As lower luminosity dwarf galaxies, roughly less active than the Small Magellanic Cloud, are examined, Hα tends to increasingly underpredict the total SFR relative to the FUV. The trend is evident prior to corrections for dust attenuation, which affects the FUV more than the nebular Hα emission, so this general conclusion is robust to the effects of dust. Although past studies have suggested similar trends, this is the first time this effect is probed with a statistical sample for galaxies with SFR ≲ 0.1 M☉ yr−1. By SFR ∼ 0.003 M☉ yr−1, the average Hα-to-FUV flux ratio is lower than expected by a factor of two, and at the lowest SFRs probed, the ratio exhibits an order of magnitude discrepancy for the handful of galaxies that remain in the sample. A range of standard explanations does not appear to be able to fully account for the magnitude of the systematic. Some recent work has argued for a stellar initial mass function which is deficient in high-mass stars in dwarf and low surface brightness galaxies, and we also consider this scenario. Under the assumption that the FUV traces the SFR in dwarf galaxies more robustly, the prescription relating Hα luminosity to SFR is re-calibrated for use in the low SFR regime when FUV data are not available.

Atacama Large Millimeter/submillimeter Array observations have shown that candidate “post-starburst” galaxies (PSBs) at z ∼ 0.6 can retain significant molecular gas reservoirs. These results would imply that—unlike many model predictions—galaxies can shut down their star formation before their cold gas reservoirs are depleted. However, these studies inferred star formation rates (SFRs) either from [O ii] line fluxes or from spectral energy distribution (SED) modeling and could have missed large dust-obscured contributions to the SFRs. In this study, we present Keck/NIRES observations of 13 massive (M * ≳ 1011 M ⊙) PSBs, which allow us to estimate Hα SFRs in these gas-rich PSBs. We confirm the previously inferred low SFRs for the majority of the sample: 11/13 targets show clear Hα absorption, with minimal infilling indicating dust-corrected SFRs of <4.1 M ⊙ yr−1. These SFRs are notably low given the large H2 reservoirs (∼(1–5) × 1010 M ⊙) present in 5/13 of these galaxies, placing them significantly offset from star-forming galaxies on the Kennicutt–Schmidt relation for star-forming galaxies. The [N ii]/Hα ratios of all 13 PSBs imply contributions from non-star-forming ionization mechanisms (e.g., active galactic nuclei, shocks, or hot evolved stars) to their Hα emission, suggesting that even these low ongoing SFRs may be overestimated. These low Hα SFRs, dust corrected using A v estimates from SED fitting, confirm that these galaxies are very likely quiescent and, thus, that galaxies can quench before their cold gas reservoirs are fully depleted.

Context. The tidal tails of post-merger galaxies exhibit ongoing star formation far from their disks. The study of such systems can be useful for our understanding of gas condensation in diverse environments. Aims. The ongoing star formation in the tidal tails of post-merger galaxies can be directly studied from ultraviolet (UV) imaging observations. Methods. The post merger galaxy NGC7252 (“Atoms-for-Peace” galaxy) is observed with the Astrosat UV imaging telescope (UVIT) in broadband NUV and FUV filters to isolate the star-forming regions in the tidal tails and study the spatial variation in star formation rates. Results. Based on ultraviolet imaging observations, we discuss star-forming regions of ages <200 Myr in the tidal tails. We measure star formation rates in these regions and in the main body of the galaxy. The integrated star formation rate (SFR) of NGC7252 (i.e., that in the galaxy and tidal tails combined) without correcting for extinction is found to be 0.81 ± 0.01 M⊙ yr−1. We show that the integrated SFR can change by an order of magnitude if the extinction correction used in SFR derived from other proxies are taken into consideration. The star formation rates in the associated tidal dwarf galaxies (NGC7252E, SFR = 0.02 M⊙ yr−1 and NGC7252NW, SFR = 0.03 M⊙ yr−1) are typical of dwarf galaxies in the local Universe. The spatial resolution of the UV images reveals a gradient in star formation within the tidal dwarf galaxy. The star formation rates show a dependence on the distance from the centre of the galaxy. This can be due to the different initial conditions responsible for the triggering of star formation in the gas reservoir that was expelled during the recent merger in NGC7252.

We present an analysis of the spectral energy distribution (SED) of the galaxy ESO 184-G82, the host of the closest known long gamma-ray burst (GRB) 980425 and its associated supernova (SN) 1998bw. We use our observations obtained at the Australia Telescope Compact Array (the third >3 sigma radio detection of a GRB host) as well as archival infrared and ultraviolet (UV) observations to estimate its star formation state. We find that ESO 184-G82 has a UV star formation rate (SFR) and stellar mass consistent with the population of cosmological GRB hosts and of local dwarf galaxies. However, it has a higher specific SFR (per unit stellar mass) than luminous spiral galaxies. The mass of ESO 184-G82 is dominated by an older stellar population in contrast to the majority of GRB hosts. The Wolf-Rayet region ~800 pc from the SN site experienced a starburst episode during which the majority of its stellar population was built up. Unlike that of the entire galaxy, its SED is similar to those of cosmological submillimeter/radio-bright GRB hosts with hot dust content. These findings add to the picture that in general, the environments of GRBs on 1-3 kpc scales are associated with high specific SFR and hot dust.

We present the variations in far-ultraviolet (FUV) and Hα star formation rates (SFR), SFRUV and SFRHα , respectively, at subkiloparsec scales in 11 galaxies as part of the Deciphering the Interplay between the Interstellar Medium, Stars, and the Circumgalactic medium survey. Using archival GALEX FUV imagery and Hα+[N ii] narrowband images obtained with the Vatican Advanced Technology Telescope, we detect a total of 1335 (FUV-selected) and 1474 (Hα-selected) regions of recent high-mass star formation, respectively. We find the Hα-to-FUV SFR ratios tend to be lower primarily for FUV-selected regions, where SFRHα generally underestimates the SFR by an average factor of 2–3, for SFR < 10−4 M ⊙ yr−1. In contrast, the SFRs are generally observed to be consistent for Hα-selected regions. This discrepancy arises from morphological differences between the two indicators. Extended FUV morphologies and larger areas covered by FUV-only regions, along with decreasing overlap between FUV clumps and compact H ii regions with R/R 25 suggest that stochastic sampling of the initial mass function may be more pronounced in the outer regions of galaxies. Our observed Hα-to-FUV SFR ratios are also consistent with stochastic star formation model predictions. However, using larger apertures that include diffuse FUV emission results in an offset of 1 dex between SFRHα and SFRUV, suggesting that the observed low Hα-to-FUV SFR ratios in galaxies are likely caused by diffuse FUV emission, which can contribute ∼60%–90% to the total FUV flux.

We present an analysis of the chemical abundance properties of ≈650 star-forming galaxies at z ≈ 0.6 – 1.8. Using integral-field observations from the K - band Multi-Object Spectrograph (KMOS), we quantify the [N ii]/Hα emission-line ratio, a proxy for the gas-phase Oxygen abundance within the interstellar medium. We define the stellar mass – metallicity relation at z ≈ 0.6 – 1.0 and z ≈ 1.2 – 1.8 and analyse the correlation between the scatter in the relation and fundamental galaxy properties (e.g. Hα star-formation rate, Hα specific star-formation rate, rotation dominance, stellar continuum half-light radius and Hubble-type morphology). We find that for a given stellar mass, more highly star-forming, larger and irregular galaxies have lower gas-phase metallicities, which may be attributable to their lower surface mass densities and the higher gas fractions of irregular systems. We measure the radial dependence of gas-phase metallicity in the galaxies, establishing a median, beam smearing-corrected, metallicity gradient of ΔZ/ΔR= 0.002 ± 0.004 dex kpc−1, indicating on average there is no significant dependence on radius. The metallicity gradient of a galaxy is independent of its rest-frame optical morphology, whilst correlating with its stellar mass and specific star-formation rate, in agreement with an inside-out model of galaxy evolution, as well as its rotation dominance. We quantify the evolution of metallicity gradients, comparing the distribution of ΔZ/ΔR in our sample with numerical simulations and observations at z ≈ 0 – 3. Galaxies in our sample exhibit flatter metallicity gradients than local star-forming galaxies, in agreement with numerical models in which stellar feedback plays a crucial role redistributing metals.

We present a near-infrared spectroscopic study of a stellar mass selected sample of galaxies at z~1 utilising the LIRIS multi-object spectrograph on the WHT. We detect continuum, and the H alpha line for our sample, which is one of the better direct tracers of star formation in external galaxies. We spectroscopically measure the H alpha emission from 41 massive (M_{*}>10^{10.5} Msol) galaxies taken from the POWIR Survey with spectroscopic redshifts 0.4<z_{spec}<1.4. We correct our H alpha fluxes for dust extinction by using multi-wavelength data, and investigate SFR trends with mass and colour. We find a drop in the fraction of massive galaxies with M_{*}>10^{11} Msol which are detected in H alpha emission at z<0.9. We furthermore find that the fraction of galaxies with H alpha emission drops steadily and significantly with redder (U-B) colours at z~1, and that the SSFR drops with increasing (U-B) colour for galaxies at all masses. By investigating the SFR-mass relation we find that the SFR is roughly constant with mass, in possible contrast to previous work, and that the specific star formation rate (SSFR) is lower in the most massive galaxies. The scatter in the SFR vs. mass relationship is very small for those systems with ongoing star formation which suggests that star formation in the most massive galaxies at z~1 shuts off rather abruptly over <1 Gyr, without an obvious gradual decline. We furthermore investigate the SFR as a function of (U-B) colour divided into different mass bins, revealing a tracer of the epoch of transition from star forming to passive, as a form of star formation "downsizing". This suggests that the shut off of star formation occurs before the change in a galaxy's colour. We find that galaxy stellar mass is the primary driving mechanisms behind the star formation history for these galaxies and discuss several possible mechanisms for regulating this process.

We have characterized 26 Spitzer/IRAC-selected sources from the SMUVS program that are undetected in the UltraVISTA DR5 H- and/or K s-band images, covering 94 arcmin2 of the COSMOS field, which have deep multiwavelength JWST photometry. We analyzed the JWST/NIRCam imaging from the PRIMER survey and ancillary HST data to reveal the properties of these galaxies from spectral energy distribution (SED) fitting. We find that the majority of these galaxies are detected by NIRCam at λ < 2 μm, with only four remaining as near-infrared dropouts in the deeper JWST images. Our results indicate that the UltraVISTA dropouts candidates are primarily located at z > 3 and are characterized by high dust extinctions, with a typical color excess E(B − V) = 0.5 ± 0.3 and stellar mass log(M*/M⊙)=9.5±1.0 . Remarkably, ∼75% of these sources show a flux enhancement between the observed photometry and modeled continuum SED that can be attributed to Hα emission in the corresponding NIRCam bands. The derived (Hα+ N [ii] + S [ii]) rest-frame equivalent widths and Hα star formation rates (SFRs) span values ∼100–2200 Å and ∼5–375 M ⊙ yr−1, respectively. The locations of these sources on the SFR–M * plane indicate that 35% of them are starbursts, 40% are main-sequence galaxies, and the remaining 25% are located in the star formation valley. Our sample includes one active galactic nucleus and six submillimeter sources, as revealed from ancillary X-ray and submillimeter photometry. The high dust extinctions combined with the flux boosting from Hα emission explain why these sources are relatively bright Spitzer galaxies and yet unidentified in the ultradeep UltraVISTA near-infrared images.

We present the first results on the spatial distribution of dust attenuation at 1.0 < z < 2.4 traced by the Balmer decrement, Hα/Hβ, in emission-line galaxies using deep JWST NIRISS slitless spectroscopy from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). Hα and Hβ emission-line maps of emission-line galaxies are extracted and stacked in bins of stellar mass for two grism redshift bins, 1.0 < z grism < 1.7 and 1.7 < z grism < 2.4. Surface brightness profiles for the Balmer decrement are measured and radial profiles of the dust attenuation toward Hα, A Hα , are derived. In both redshift bins, the integrated Balmer decrement increases with stellar mass. Lower-mass (7.6 ≤ Log(M */M ⊙) < 10.0) galaxies have centrally concentrated, negative dust attenuation profiles whereas higher-mass galaxies (10.0 ≤ Log(M */M ⊙) < 11.1) have flat dust attenuation profiles. The total dust obscuration is mild, with on average 0.07 ± 0.07 and 0.14 ± 0.07 mag in the low- and high-redshift bins respectively. We model the typical light profiles of star-forming galaxies at these redshifts and stellar masses with GALFIT and apply both uniform and radially varying dust attenuation corrections based on our integrated Balmer decrements and radial dust attenuation profiles. If the Hα star formation rates (SFRs) of these galaxies were measured after slit-loss corrections assuming uniform dust attenuation with typical JWST NIRSpec slit spectroscopy (0.″2 × 0.″5 shutters), the total SFR will be overestimated by 6% ± 21% and 26% ± 9% at 1.0 ≤ z < 1.7 and 1.7 ≤ z < 2.4 respectively.

We present the ultraviolet (UV) luminosity function of galaxies from the GALEX Medium Imaging Survey with measured spectroscopic redshifts from the first data release of the WiggleZ Dark Energy Survey. Our sample consists of 39 996 NUV &lt; 22.8 emission

We present multi-wavelength global star formation rate (SFR) estimates for 326 galaxies from the Star Formation Reference Survey (SFRS) in order to determine the mutual scatter and range of validity of different indicators. The widely used empirical SFR recipes based on 1.4 GHz continuum, 8.0 $\mu$m polycyclic aromatic hydrocarbons (PAH), and a combination of far-infrared (FIR) plus ultraviolet (UV) emission are mutually consistent with scatter of $\raise{-0.8ex}\stackrel{\textstyle <}{\sim }$0.3 dex. The scatter is even smaller, $\raise{-0.8ex}\stackrel{\textstyle <}{\sim }$0.24 dex, in the intermediate luminosity range 9.3<log(L(60 $\mu$m/L$_\odot$)<10.7. The data prefer a non-linear relation between 1.4 GHz luminosity and other SFR measures. PAH luminosity underestimates SFR for galaxies with strong UV emission. A bolometric extinction correction to far-ultraviolet luminosity yields SFR within 0.2 dex of the total SFR estimate, but extinction corrections based on UV spectral slope or nuclear Balmer decrement give SFRs that may differ from the total SFR by up to 2 dex. However, for the minority of galaxies with UV luminosity ${>}5\times10^9$ L$_{\odot}$ or with implied far-UV extinction <1 mag, the UV spectral slope gives extinction corrections with 0.22~dex uncertainty.

Context. The correlations between star formation rate (SFR), stellar mass (M⋆), and gas-phase metallicity for star-forming (SF) galaxies, known as global scaling relations or fundamental relations, have been studied during the past decades to understand the evolution of galaxies. However, the origin of these correlations and their scatter, which may also be related to their morphology or environment, is still a subject of debate. Aims. In this work, we establish fundamental relations, for the first time, in isolated systems in the local Universe (with 0.005 ≤ z ≤ 0.080), which can give insight into the underlying physics of star formation. We used a sample of isolated galaxies to explore whether star formation is regulated by smooth secular processes. In addition, galaxies in physically bound isolated pairs and isolated triplets may also interact with each other, where interaction itself may enhance or regulate star formation and the distribution of gas and metals within galaxies. Methods. We made use of published emission line flux information from the Sloan Digital Sky Survey (SDSS) to identify SF galaxies in the SDSS-based catalogue of isolated galaxies (SIGs), isolated pairs (SIPs), and isolated triplets (SITs). We also used these data to derive their aperture-corrected SFR (considering two different methods) and oxygen abundance, 12 + log(O/H), using bright line calibrations. Stellar masses for SIG, SIP, and SIT galaxies were estimated by fitting their spectral energy distribution on the five SDSS bands. Results. The SFR results found using both methods seem to be consistent. We compared our results with a sample of SF galaxies in the SDSS. We found that, on average, at a fixed stellar mass, the SIG SF galaxies have lower SFR values than Main Sequence (MS) SF galaxies in the SDSS and central galaxies in the SIP and SIT. On average, SIG galaxies have higher 12 + log(O/H) values than galaxies in the SIP, SIT, and comparison sample. When distinguishing between central and satellite galaxies in the SIP and SIT, centrals and SIG galaxies present similar values (∼8.55), while satellites have values close to M33 (∼8.4). Using the Dn(4000) parameter as a proxy of the age of the stellar populations, we found that, on average, SIG and central galaxies have higher Dn(4000) values than satellites and comparison galaxies. Conclusions. In general SIG galaxies do not present stellar starbursts produced by interactions with other galaxies, and therefore their gas is consumed more slowly and at a regular pace. On the contrary, SIP and SIT galaxies present higher SFR values at fixed mass (both in central and satellite galaxies). Therefore, the effect of adding one or two companion galaxies is noticeable and produces a similar effect as the typical average environment around galaxies in the local Universe. The successive interactions between the galaxies that form these pairs and triplets enhance the star formation. Based on our results for SIGs, we propose a ground level ‘nurture-free’ SFR–M⋆ and gas metallicity-SFR–M⋆ relations for SF galaxies in the local Universe.

&lt;p&gt;This thesis examines the evolution of massive disc galaxies as a function of cosmic time and environment by analysing a sample of luminous disc galaxies, located in the field and rich clusters at intermediate redshifts. &nbsp;The data utilised for this study are two-dimensional optical spectra obtained with the FORS2 instrument on the VLT, along with imaging from a variety of sources.&lt;/p&gt; &lt;p&gt;We investigate evolution in the field using the Tully-Fisher relation (TFR) and interstellar gas properties of these galaxies. &nbsp;A mild overall evolution is found, which appears to be slower than that derived bay studies of the overall field galaxy population. This suggests that the rapid evolution of the SFR density of the universe observed since z~1 is not in general driven by the evolution of the SFR in individual bright spiral galaxies.&lt;/p&gt; &lt;p&gt;The interstellar gas properties cover similar ranges to those observed across a large sample of local galaxies. &nbsp;However, a fraction have oxygen abundances significantly lower than local galaxies with similar high luminosities. &nbsp;The galaxies in this luminous, metal-poor subsample exhibit physical conditions similar to those of local faint and metal-poor star-forming galaxies. These galaxies must subsequently experience substantial evolution in luminosity and star formation rate. &nbsp;The mild overall field evolution thus appears to be due to a combination of some galaxies undergoing substantial evolution, with the remainder changing little.&lt;/p&gt; &lt;p&gt;We find no evidence for a change in TFR slope with redshift, although this is not well constrained. &nbsp;However, previous studies have used an observed correlation between TFR residuals and rotation velocity to argue that low mass galaxies have evolved significantly more than those with higher mass. &nbsp;We demonstrate that such a correlation does not necessarily indicate a physical difference in the evolution of galaxies with different rotation velocity.&lt;/p&gt; &lt;p&gt;Matched samples of intermediate-redshift field and cluster galaxies are compared using the TFR and the properties of their interstellar gas. &nbsp;While the distributions of basic properties are comparable for the two samples, we find evidence that the cluster galaxies are significantly brighter than those in the coeval field. &nbsp;The cluster galaxies have specific star formation rates that are, on average, significantly lower than for the field galaxies. &nbsp;However, a contrasting fraction appear to have much higher star formation activity, comparable to the highest seen in the field. &nbsp;This implies a bimodality in the star formation activity of distant cluster galaxies, which is not present for our field sample. &nbsp;In addition, star formation appears to be more centrally concentrated in the cluster galaxies. &nbsp;We find no substantial differences in the long term star formation histories of these cluster and field galaxies, as indicated by their gas-phase metallicities.&lt;/p&gt; &lt;p&gt;The most likely explanation for these results is that spiral galaxies entering intermediate-redshift clusters generally experience a short-lived enhancement of their star formation rate, followed by a decline.&lt;/p&gt;

We study a magnitude-limited sample of 10 gamma-ray burst (GRB) host galaxies with known spectroscopic redshifts (0.43 < z < 2.04). From an analysis of the spectral energy distributions (SEDs), based on published broad-band optical and near-infrared photometry, we derive photometric redshifts, galaxy types, ages of the dominant stellar populations, internal extinctions, and ultraviolet (UV) star-formation rates (SFRs) of the host galaxies. The photometric redshifts are quite accurate despite the heterogeneous nature of the sample: The r.m.s. errors are sigma(z) = 0.21 and sigma(Delta z/(1+z_spec)) = 0.16 with no significant systematic offsets. All the host galaxies have SEDs similar to young starburst galaxies with moderate to low extinction. A comparison of specific SFRs with those of high-redshift galaxies in the Hubble Deep Fields shows that GRB hosts are most likely similar to the field galaxies with the largest specific SFRs. On the other hand, GRB hosts are not significantly younger than starburst field galaxies at similar redshifts, but are found to be younger than a sample of all types of field galaxies.

ABSTRA C T The observational determination of the behaviour of the star formation rate (SFR) with lookback time or redshift has two main weaknesses: (i) the large uncertainty of the dust/extinction corrections, and (ii) that systematic errors may be introduced by the fact that the SFR is estimated using different methods at different redshifts. Most frequently, the luminosity of the Ha emission line, that of the forbidden line [O II] l3727 and that of the far-ultraviolet continuum are used with low-, intermediate- and high-redshift galaxies, respectively. To assess the possible systematic differences among the different SFR estimators and the role of dust, we have compared SFR estimates using Ha ,[ OII] l3727 A ˚ , ultraviolet (UV) and far-infrared (FIR) luminosities [SFR(Ha), SFR(O II), SFR(UV) and SFR(FIR), respectively] of a sample comprising the 31 nearby star-forming galaxies that have high-quality photometric data in the UV, optical and FIR. We review the different ‘standard’ methods for the estimation of the SFR and find that while the standard method provides good agreement between SFR(Ha) and SFR(FIR), both SFR(O II) and SFR(UV) are systematically higher than SFR(FIR), irrespective of the extinction law. We show that the excess in the SFR(O II) and SFR(UV) is mainly due to an overestimation of the extinction resulting from the effect of underlying stellar Balmer absorptions in the measured emission line fluxes. Taking this effect into consideration in the determination of the extinction brings the SFR(O II) and SFR(UV) in line with the SFR(FIR), and simultaneously reduces the internal scatter of the SFR estimations. Based on these results, we have derived ‘unbiased’ SFR expressions for the SFR(UV), SFR(O II) and SFR(Ha). We have used these estimators to recompute the SFR history of the Universe using the results of published surveys. The main results are that the use of the unbiased SFR estimators brings into agreement the results of all surveys. Particularly important is the agreement achieved for the SFR derived from the FIR/millimetre and optical/UV surveys. The ‘unbiased’ star formation history of the Universe shows a steep rise in the SFR from za 0t oza 1 with SFR/O1a zU 4:5 , followed by a decline for z . 2 where SFR/O1a zU 21:5 . Galaxy formation models tend to have a much flatter slope from za 0t o za 1.