Hα Star Formation Rate Research Articles

A High Incidence of Dusty Hα Emitters at z > 3 Among UltraVISTA Dropout Galaxies in COSMOS Revealed by JWST

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.

A First Look at Spatially Resolved Balmer Decrements at 1.0 < z < 2.4 from JWST NIRISS Slitless Spectroscopy

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.

The Evolution of Gas-Phase Metallicity and Resolved Abundances in Star-forming Galaxies at z ≈ 0.6 – 1.8

Abstract 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.

A star-forming dwarf galaxy candidate in the halo of NGC 4634

Context. The halos of disk galaxies form a crucial connection between the galaxy disk and the intergalactic medium. Massive stars, H II regions, or dwarf galaxies located in the halos of galaxies are potential tracers of recent accretion and/or outflows of gas, and are additional contributors to the photon field and the gas phase metallicity. Aims. We investigate the nature and origin of a star-forming dwarf galaxy candidate located in the halo of the edge-on Virgo galaxy NGC 4634 with a projected distance of 1.4 kpc and a Hα star formation rate of ∼4.7 × 10−3 M⊙ yr−1 in order to increase our understanding of these disk-halo processes. Methods. With optical long-slit spectra we measured fluxes of optical nebula emission lines to derive the oxygen abundance 12 + log(O/H) of an H II region in the disk of NGC 4634 and in the star-forming dwarf galaxy candidate. Abundances derived from optical long-slit data and from Hubble Space Telescope (HST) r-band data, Hα data, Giant Metrewave Radio Telescope (GMRT) H I data, and photometry of SDSS and GALEX data were used for further analysis. With additional probes of the luminosity–metallicity relation in the B-band from the Hα-luminosity, the H I map, and the relative velocities, we are able to constrain a possible origin of the dwarf galaxy candidate. Results. The high oxygen abundance (12 + log(O/H) ≈ 8.72) of the dwarf galaxy candidate leads to the conclusion that it was formed from pre-enriched material. Analysis of auxiliary data shows that the dwarf galaxy candidate is composed of material originating from NGC 4634. We cannot determine whether this material has been ejected tidally or through other processes, which makes the system highly interesting for follow up observations.

SDSS IV MaNGA: Dependence of Global and Spatially Resolved SFR–M∗ Relations on Galaxy Properties

Abstract The galaxy integrated Hα star formation rate–stellar mass relation, or SFR(global)–M *(global) relation, is crucial for understanding star formation history and evolution of galaxies. However, many studies have dealt with SFR using unresolved measurements, which makes it difficult to separate out the contamination from other ionizing sources, such as active galactic nuclei and evolved stars. Using the integral field spectroscopic observations from SDSS-IV MaNGA, we spatially disentangle the contribution from different Hα powering sources for ∼1000 galaxies. We find that, when including regions dominated by all ionizing sources in galaxies, the spatially resolved relation between Hα surface density (ΣHα (all)) and stellar mass surface density (Σ*(all)) progressively turns over at the high Σ*(all) end for increasing M *(global) and/or bulge dominance (bulge-to-total light ratio, B/T). This in turn leads to the flattening of the integrated Hα(global)–M *(global) relation in the literature. By contrast, there is no noticeable flattening in both integrated Hα(H ii)–M *(H ii) and spatially resolved ΣHα (H ii)–Σ*(H ii) relations when only regions where star formation dominates the ionization are considered. In other words, the flattening can be attributed to the increasing regions powered by non-star-formation sources, which generally have lower ionizing ability than star formation. An analysis of the fractional contribution of non-star-formation sources to total Hα luminosity of a galaxy suggests a decreasing role of star formation as an ionizing source toward high-mass, high-B/T galaxies and bulge regions. This result indicates that the appearance of the galaxy integrated SFR–M * relation critically depends on their global properties (M *(global) and B/T) and relative abundances of various ionizing sources within the galaxies.

Star Formation in the Local Universe from the CALIFA Sample. II. Activation and Quenching Mechanisms in Bulges, Bars, and Disks

Abstract We estimate the current extinction-corrected Hα star formation rate (SFR) of the different morphological components that shape galaxies (bulges, bars, and disks). We use a multicomponent photometric decomposition based on Sloan Digital Sky Survey imaging to Calar Alto Legacy Integral Field Area Integral Field Spectroscopy (IFS) datacubes for a sample of 219 galaxies. This analysis reveals an enhancement of the central SFR and specific SFR (sSFR = SFR/M ⋆) in barred galaxies. Along the main sequence, we find that more massive galaxies in total have undergone efficient suppression (quenching) of their star formation, in agreement with many studies. We discover that more massive disks have had their star formation quenched as well. We evaluate which mechanisms might be responsible for this quenching process. The presence of type 2 AGNs plays a role at damping the sSFR in bulges and less efficiently in disks. Also, the decrease in the sSFR of the disk component becomes more noticeable for stellar masses around for bulges, it is already present at . The analysis of the line-of-sight stellar velocity dispersions (σ) for the bulge component and of the corresponding Faber–Jackson relation shows that AGNs tend to have slightly higher σ values than star-forming galaxies for the same mass. Finally, the impact of environment is evaluated by means of the projected galaxy density, Σ5. We find that the SFR of both bulges and disks decreases in intermediate- to high-density environments. This work reflects the potential of combining IFS data with 2D multicomponent decompositions to shed light on the processes that regulate the SFR.

KMOS3D Reveals Low-level Star Formation Activity in Massive Quiescent Galaxies at 0.7 &lt; z &lt; 2.7∗

Abstract We explore the Hα emission in the massive quiescent galaxies observed by the KMOS3D survey at 0.7 &lt; z &lt; 2.7. The Hα line is robustly detected in 20 out of 120 UVJ-selected quiescent galaxies, and we classify the emission mechanism using the Hα line width and the [N ii]/Hα line ratio. We find that AGNs are likely to be responsible for the line emission in more than half of the cases. We also find robust evidence for star formation activity in nine quiescent galaxies, which we explore in detail. The Hα kinematics reveal rotating disks in five of the nine galaxies. The dust-corrected Hα star formation rates are low (0.2–7 M ⊙ yr−1), and place these systems significantly below the main sequence. The 24 μm-based, infrared luminosities, instead, overestimate the star formation rates. These galaxies present a lower gas-phase metallicity compared to star-forming objects with similar stellar mass, and many of them have close companions. We therefore conclude that the low-level star formation activity in these nine quiescent galaxies is likely to be fueled by inflowing gas or minor mergers, and could be a sign of rejuvenation events.

ZFIRE: SIMILAR STELLAR GROWTH IN Hα-EMITTING CLUSTER AND FIELD GALAXIES AT z ∼ 2

ABSTRACT We compare galaxy scaling relations as a function of environment at with our ZFIRE survey12 where we have measured Hα fluxes for 90 star-forming galaxies selected from a mass-limited ( ) sample based on ZFOURGE.13 The cluster galaxies (37) are part of a confirmed system at z = 2.095 and the field galaxies (53) are at all are in the COSMOS legacy field. There is no statistical difference between Hα-emitting cluster and field populations when comparing their star formation rate (SFR), stellar mass ( ), galaxy size ( ), SFR surface density (Σ( )), and stellar age distributions. The only difference is that at fixed stellar mass, the Hα-emitting cluster galaxies are ( ) ∼ 0.1 larger than in the field. Approximately 19% of the Hα emitters in the cluster and 26% in the field are IR-luminous ( &gt; 2 × 1011 ). Because the luminous IR galaxies in our combined sample are ∼5 times more massive than the low-IR galaxies, their radii are ∼70% larger. To track stellar growth, we separate galaxies into those that lie above, on, or below the Hα star-forming main sequence (SFMS) using ΔSFR( ) = ±0.2 dex. Galaxies above the SFMS (starbursts) tend to have higher Hα SFR surface densities and younger light-weighted stellar ages than galaxies below the SFMS. Our results indicate that starbursts (+SFMS) in the cluster and field at are growing their stellar cores. Lastly, we compare to the (SFR– ) relation from Rhapsody-G cluster simulations and find that the predicted slope is nominally consistent with the observations. However, the predicted cluster SFRs tend to be too low by a factor of ∼2, which seems to be a common problem for simulations across environment.

ZFIRE: GALAXY CLUSTER KINEMATICS, HαSTAR FORMATION RATES, AND GAS PHASE METALLICITIES OF XMM-LSS J02182-05102 AT ${z}_{\mathrm{cl}}=1.6233$

We spectroscopically survey the galaxy cluster XMM-LSS J02182-05102 (hereafter IRC 0218) using LRIS (optical) and MOSFIRE (near-infrared) on Keck I as part of the ZFIRE survey. IRC 0218 has a narrow redshift range of defined by 33 members of which 20 are at R Mpc. The cluster redshift and velocity dispersion are and km s−1. We reach NIR line sensitivities of ∼0.3 × 10−17 erg s−1 cm−2 that, combined with multi-wavelength photometry, provide extinction-corrected Hα star formation rates (SFR), gas phase metallicities from [N ii]/Hα, and stellar masses. We measure an integrated Hα SFR of ∼325 (26 members; R Mpc) and show that the elevated star formation in the cluster core (R Mpc) is driven by the concentration of star-forming members, but the average SFR per Hα-detected galaxy is half that of members at R ∼ 1 Mpc. However, we do not detect any environmental imprint when comparing attenuation and gas phase metallicities: the cluster galaxies show similar trends with as to the field, e.g., more massive galaxies have larger stellar attenuation. IRC 0218's gas phase metallicity– relation (MZR) is offset to lower metallicities relative to and has a slope of 0.13 ± 0.10. Comparing the MZR in IRC 0218 to the COSMOS cluster at z = 2.1 shows no evolution ( Gyr): the MZR for both galaxy clusters are remarkably consistent with each other and virtually identical to several field surveys at .

Environmental Effects on the ISM and Star Formation Properties of Nearby Spiral Galaxies

AbstractWe studied molecular gas properties in a sample of 98 Hi - flux selected spiral galaxies within ~ 25 Mpc using the CO J = 3 − 2 line, observed with the JCMT, and subdivided into isolated, group, and Virgo subsamples. We find a larger mean H2 mass in the Virgo galaxies compared to group galaxies, despite their lower mean Hi mass. Combining our data with complementary Hα star formation rate measurements, Virgo galaxies have a longer molecular gas depletion times compared to group galaxies, perhaps due to heating processes in the cluster environment or differences in the turbulent pressure.

Hα star formation rates in massive galaxies at z∼ 1

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.

COMPARISON OF Hα AND UV STAR FORMATION RATES IN THE LOCAL VOLUME: SYSTEMATIC DISCREPANCIES FOR DWARF GALAXIES

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.

Minor Galaxy Interactions: Star Formation Rates and Galaxy Properties

We study star formation in a sample of 1204 galaxies in minor ( ≥ 2) pair and compact groups, drawn from the Sloan Digital Sky Survey Data Release 5. We analyze an analogous sample of 2409 galaxies in major ( < 2) pair and compact groups to ensure that our selection reproduces previous results, and we use a field sample of 65,570 galaxies for comparison. Our major and minor pair samples include only galaxies in spectroscopically confirmed pair, where the recessional velocity separation ΔV < 500 km s-1 and the projected spatial separation ΔD < 50 kpc h-1. The relative magnitude (a proxy for the mass ratio) of the pair is an important parameter in the effectiveness of the tidally triggered star formation in minor interactions. As expected, the secondary galaxies in minor pair show evidence for tidally triggered star formation, whereas the primary galaxies in the minor pair do not. The galaxy color is also an important parameter in the effectiveness of triggered star formation in the major galaxy pair. In the major pair sample, there is a correlation between the specific Hα star formation rate (SSFR) and ΔD in the blue primary and blue secondary galaxies; for the red primary and red secondary galaxies, there is none. Galaxies in pair have a higher mean SSFR at every absolute magnitude compared to matched sets of field galaxies, and the relative increase in mean SSFR becomes larger with decreasing intrinsic luminosity. We also detect a significantly increased AGN fraction in the pair galaxies compared to matched sets of field galaxies.

The Rest‐Frame Optical Spectra of SCUBA Galaxies

We present near-infrared spectroscopy and narrowband imaging at the wavelength of redshifted Hα for a sample of 30 high-redshift, far-infrared luminous galaxies. This sample is selected from surveys in the submillimeter, millimeter, and radio wave bands and has complete redshift coverage with a median redshift of z ~ 2.4. We use our data to measure the Hα properties of these systems and to gauge the prevalence of active galactic nuclei (AGNs) in these galaxies through their [N II]/Hα ratios and Hα line widths. Removing obvious AGNs, we find that the predicted Hα star formation rates in this diverse population are suppressed (by a factor of ~10) compared to those derived from their far-infrared luminosities. Using the AGN indicators provided by our near-infrared spectra, we estimate that AGNs are present in at least 40% of the galaxies in our sample. To further investigate this, we construct a composite rest-frame spectrum for both the entire sample and those galaxies that individually show no signs of nuclear activity. We find [N II]/Hα ratios for both composite spectra that suggest that the energy output of the galaxies is star formation rather than AGN dominated. However, we also find that the Hα line in the composite non-AGN spectrum is best fitted with an underlying broad-line component with a narrow/broad flux ratio of 0.45 ± 0.20. The median Hα line width for our sample (removing obvious AGNs) is 400 ± 70 km s-1 (FWHM), and the typical spatial extent of the Hα emission in our narrowband observations is ≲4-8 kpc, which indicates a dynamical mass of (1-2) × 1011 M☉ with corresponding dynamical times of 10-20 Myr. Using both high-resolution imaging and spectroscopically identified velocity offsets, we find that seven of the far-infrared luminous galaxies have companions, suggesting that they are undergoing interactions/mergers, and from their relative velocities we can determine a dynamical mass of (1.5 ± 0.9) × 1011 M☉. These measurements are comparable to millimeter CO estimates for the dynamical masses of these systems on similar scales and larger than recent estimates of the dynamical masses of UV-selected galaxies at similar redshifts derived in an identical manner. Using the [N II]/Hα index to predict abundances, we investigate the luminosity-metallicity relation for these galaxies and find that many have metallicities consistent with UV-selected high-redshift galaxies and slightly lower than local luminous infrared and elliptical galaxies (although we caution that our metallicity estimates have possible systematic uncertainties). We also compared our Hα and far-infrared luminosities with deep Chandra observations of a subset of our survey fields and use these data to further assess their AGN content. We conclude that these high-redshift, far-infrared luminous galaxies represent a population of massive, metal-rich, merging systems with high instantaneous star formation rates, strong dust obscuration, and actively fueled AGNs that are likely to be the progenitors of massive local elliptical galaxies.

Oii] as a Star Formation Rate Indicator

We investigate the [O II] emission line as a star formation rate (SFR) indicator using integrated spectra of 97 galaxies from the Nearby Field Galaxies Survey (NFGS). The sample includes all Hubble types and contains SFRs ranging from 0.01 to 100 M⊙ yr-1. We compare the Kennicutt [O II] and Hα SFR calibrations and show that there are two significant effects that produce disagreement between SFR([O II]) and SFR(Hα): reddening and metallicity. Differences in the ionization state of the interstellar medium do not contribute significantly to the observed difference between SFR([O II]) and SFR(Hα) for the NFGS galaxies with metallicities log (O/H) + 12 ≳ 8.5. The Kennicutt [O II]-SFR relation assumes a typical reddening for nearby galaxies; in practice, the reddening differs significantly from sample to sample. We derive a new SFR([O II]) calibration that does not contain a reddening assumption. Our new SFR([O II]) calibration also provides an optional correction for metallicity. Our SFRs derived from [O II] agree with those derived from Hα to within 0.03–0.05 dex. We show that the reddening, E(B-V), increases with intrinsic (i.e., reddening-corrected) [O II] luminosity for the NFGS sample. We apply our SFR([O II]) calibration with metallicity correction to two samples: high-redshift 0.8 < z < 1.6 galaxies from the NICMOS Hα survey and 0.5 < z < 1.1 galaxies from the Canada-France Redshift Survey. The SFR([O II]) and SFR(Hα) for these samples agree to within the scatter observed for the NFGS sample, indicating that our SFR([O II]) relation can be applied to both local and high-z galaxies. Finally, we apply our SFR([O II]) to estimates of the cosmic star formation history. After reddening and metallicity corrections, the star formation rate densities derived from [O II] and Hα agree to within ∼30%.

Star formation rates of distant luminous infrared galaxies derived from Hα and IR luminosities

We present a study of the star formation rate (SFR ) for a sample of 16 distant galaxies detected by ISOCAM at 15 μ m in the CFRS0300+00 and CFRS1400+52 fields. Their high quality and intermediate resolution VLT/FORS spectra have allowed a proper correction of the Balmer emission lines from the underlying absorption. Extinction estimates using the Hβ /Hγ and the Hα /Hβ Balmer decrement are in excellent agreement, providing a robust measurement of the instantaneous SFR based on the extinction-corrected Hα luminosity. Star formation has also been estimated exploiting the correlations between IR luminosity and those at MIR and radio wavelengths. Our study shows that the relationship between the two SFR estimates follow two distinct regimes: (1) for galaxies with SFR IR below ~100 /yr, the SFR deduced from Hα measurements is a good approximation of the global SFR and (2) for galaxies near ULIRG regime. The corrected Hα SFR understimated the SFR by a factor of 1.5 to 2. Our analyses suggest that heavily extincted regions completely hidden in optical bands (such as those found in Arp 220) contribute less than 20% of the global budget of star formation up to .

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

We have used archival ultraviolet (UV) imaging of 50 nearby star-forming galaxies obtained with the Ultraviolet Imaging Telescope (UIT) to derive integrated near-UV and far-UV magnitudes, and have combined these data with Hα, far-infrared, and thermal radio continuum measurements to explore the consistency of UV and Hα star formation rates (SFRs). In agreement with previous studies, we find that the UV and Hα SFRs are qualitatively consistent, even before corrections for extinction are applied. The uncorrected UV SFRs are systematically lower by a factor of 1.5 (with a factor of 2 scatter) among luminous galaxies with SFR ≳ 1 M☉ yr-1, indicating a higher effective attenuation of the far-UV radiation. Among less luminous galaxies there is no significant offset between the Hα and far-UV SFR scales. This behavior is consistent with that of higher redshift samples observed by Sullivan et al., Glazebrook et al., and Yan et al. for comparable ranges of galaxy luminosities and absolute SFRs. Far-infrared and thermal radio continuum data available for a subset of our sample allow us to estimate the attenuation in the UV and at Hα independently. The UV and Hα attenuations appear to be correlated, and confirm systematically higher attenuations in the UV. Although the galaxies in our sample show modest levels of attenuation (with median values of 0.9 mag at Hα and 1.4 mag at 1550 Å), the range across the sample is large, ~4 mag for Hα and ≳5 mag in the far-UV (1550 Å). This indicates that the application of a single characteristic extinction correction to Hα or UV SFRs is only realistic for large, well-defined and well-studied galaxy samples, and that extinction bias may be important for UV or emission-line-selected samples of star-forming galaxies.

The Hα SFR of the Universe at z = 0.24 and z = 0.4

The evolution of the Star Formation Rate (SFR) density of the Universe has been a major topic of study during the last years. The most recent picture considers a drop of an order of magnitude from z = 1 to z = 0, but this conclusion is the result of combining several samples with different statistics, selection effects and SFR tracers (Ferguson et al., 2000). Our group has recently determined the local Ha luminosity function (i.e. the Star Formation Rate function) and the SFR density of the local Universe (Gallego et al.,1995). This measurement is based on the Ha luminosity of a complete sample of 178 galaxies detected by their Ha in emission. We have extended now this study to z = 0.24 and z = 0.4 by studying a complete sample of star-forming galaxies. The objects were selected by their Ha emission in narrow-band filters centered at Hα redshifted to z = 0.24 (8200A) and z = 0.4 (9200A), where the sky emission has a minimum. The extinction and contamination by [NII]λ6584 were corrected using average values for the local sample (Gallego et al., 1997).

The Hα Luminosity Function and Star Formation Rate atz∼ 0.2

We have measured the Hα + [N II] fluxes of the I-selected Canada-France Redshift Survey (CFRS) galaxies lying at a redshift z below 0.3 and hence derived the Hα luminosity function. The magnitude limits of the CFRS mean that only the galaxies with MB -21 mag were observed at these redshifts. We obtained a total Hα luminosity density of at least 1039.44±0.04 ergs s-1 Mpc-3 at a mean z = 0.2 for galaxies with rest-frame EW(Hα + [N II]) 10 A. This is twice the value found in the local universe by Gallego et al. Our Hα star formation rate, derived from Madau, is higher than the UV observations at the same z, implying a UV dust extinction of ~1 mag. We found a strong correlation between the Hα luminosity and the absolute magnitude in the B band: M(BAB) = 46.7 - 1.6 log L(Hα). This work will serve as a basis of future studies of Hα luminosity distributions measured from optically selected spectroscopic surveys of the distant universe, and it will provide a better understanding of the physical processes responsible for the observed galaxy evolution.