An empirical calibration of star formation rate estimators

This work presents the main ultraviolet (UV) and far-infrared (FIR) properties of two samples of nearby galaxies selected from the GALEX (λ = 2315 A, hereafter NUV) and IRAS (λ = 60 μm) surveys, respectively. They are built in order to obtain detection at both wavelengths for most of the galaxies. Star formation rate (SFR) estimators based on the UV and FIR emissions are compared. Systematic differences are found between the SFR estimators for individual galaxies based on the NUV fluxes corrected for dust attenuation and on the total IR luminosity. A combined estimator based on NUV and IR luminosities seems to be the best proxy over the whole range of values of SFR. Although both samples present similar average values of the birthrate parameter b, their star-formation-related properties are substantially different: NUV-selected galaxies tend to show larger values of b for lower masses, SFRs, and dust attenuation, supporting previous scenarios of star formation history (SFH). Conversely, about 20% of the FIR-selected galaxies show high values of b, SFR, and NUV attenuation. These galaxies, most of them being LIRGs and ULIRGs, break down the downsizing picture of SFH; however, their relative contribution per unit volume is small in the local universe. Finally, the cosmic SFR density of the local universe is estimated in a consistent way from the NUV and IR luminosities.

Context. Galaxy evolution has been studied by interpreting the spectral energy distribution of galaxies using spectral synthesis codes. This method has been crucial in discovering different pillars of modern galaxy evolution theories. However, this analysis was mostly carried out using spectral synthesis codes that are purely stellar, that is, they assume that the nebular contribution to the total continuum is negligible. The code FADO is the first publicly available population spectral synthesis tool that treats the contribution from ionised gas to the observed emission self-consistently. This is expected to have a particularly strong effect in star-forming (SF) galaxies. Aims. We study the impact of the nebular contribution on the determination of the star formation rate (SFR), stellar mass, and consequent effect on the star-forming main sequence (SFMS) at low redshift. Methods. We applied FADO to the spectral database of the SDSS to derive the physical properties of galaxies. As a comparison, we used the data in the MPA-JHU catalogue, which contains the properties of SDSS galaxies derived without the nebular contribution. We selected a sample of SF galaxies with Hα and Hβ flux measurements, and we corrected the fluxes for the nebular extinction through the Balmer decrement. We then calculated the Hα luminosity to estimate the SFR. Then, by combining the stellar mass and SFR estimates from FADO and MPA-JHU, the SFMS was obtained. Results. The Hα flux estimates are similar between FADO and MPA-JHU. Because the Hα flux was used as tracer of the SFR, FADO and MPA-JHU agree in their SFR. The stellar mass estimates are slightly higher for FADO than for MPA-JHU on average. However, considering the uncertainties, the differences are negligible. With similar SFR and stellar mass estimates, the derived SFMS is also similar between FADO and MPA-JHU. Conclusions. Our results show that for SDSS normal SF galaxies, the additional modelling of the nebular contribution does not affect the retrieved fluxes and consequentially also does not influence SFR estimators based on the extinction-corrected Hα luminosity. For the stellar masses, the results point to the same conclusion. These results are a consequence of the fact that the vast majority of normal SF galaxies in the SDSS have a low nebular contribution. However, the obtained agreement might only hold for local SF galaxies, but higher-redshift galaxies might show different physical properties when FADO is used. This would then be an effect of the expected increased nebular contribution.

We present observational evidence that leakage of ionising photons from star-forming regions can affect the quantification of the star formation rate (SFR) in galaxies. This effect could partially explain the differences between the SFR estimates using the far ultraviolet (FUV) and the Halpha emission. We find that leakage could decrease the SFR(Ha)/SFR(FUV) ratio by up to a 25 per cent. The evidence is based on the observation that the SFR(Ha)/SFR(FUV) ratio is lower for objects showing a shell Halpha structure than for regions exhibiting a much more compact morphology. The study has been performed on three object samples: low luminosity dwarf galaxies from the Local Volume Legacy survey and star-forming regions in the Large Magellanic Cloud and the nearby Local Group galaxy M33. For the three samples we find differences (1.1-1.4sigma) between the SFR(Ha)/SFR(FUV) for compact and shell objects. Although leakage cannot entirely explain the observed trend of SFR(Ha)/SFR(FUV) ratios for systems with low SFR, we show the mechanism can lead to different SFR estimates when using Halpha and FUV luminosities. Therefore, further study is needed to constrain the contribution of leakage to the low SFR(Ha)/SFR(FUV) ratios observed in dwarf galaxies and its impact on the Halpha flux as a SFR indicator in such objects.

Using high-resolution (sub-kiloparsec scale) submillimeter data obtained by ALMA, we analyze the star formation rate (SFR), gas content and kinematics in SDP 81, a gravitationally-lensed star-forming galaxy at redshift 3. We estimate the SFR surface density ($\Sigma_{\mathrm{SFR}}$) in the brightest clump of this galaxy to be $357^{+135}_{-85}\,\mathrm{M_{\odot}\,yr^{-1}\,kpc^{-2}}$, over an area of $0.07\pm0.02\,\mathrm{kpc}^2$. Using the intensity-weighted velocity of CO$\,$(5-4), we measure the turbulent velocity dispersion in the plane-of-the-sky and find $\sigma_{\mathrm{v,turb}} = 37\pm5\,\mathrm{km\,s}^{-1}$ for the star-forming clump, in good agreement with previous estimates along the line of sight. Our measurements of gas surface density, freefall time and turbulent Mach number reveal that the role of turbulence is vital to explaining the observed SFR in this clump. While the Kennicutt Schmidt (KS) relation predicts a SFR surface density of $\Sigma_{\mathrm{SFR,KS}} = 52\pm17\,\mathrm{M_{\odot}\,yr^{-1}\,kpc^{-2}}$, the single-freefall model by Krumholz, Dekel and McKee (KDM) predicts $\Sigma_{\mathrm{SFR,KDM}} = 106\pm37\,\mathrm{M_{\odot}\,yr^{-1}\,kpc^{-2}}$. In contrast, the multi-freefall (turbulence) model by Salim, Federrath and Kewley (SFK) gives $\Sigma_{\mathrm{SFR,SFK}} = 491^{+139}_{-194}\,\mathrm{M_{\odot}\,yr^{-1}\,kpc^{-2}}$. Although the SFK relation overestimates the SFR in this clump (possibly due to the ignorance of magnetic field), it provides the best prediction among the available models. Finally, we compare the star formation and gas properties of this high-redshift galaxy to local star-forming regions and find that the SFK relation provides the best estimates of SFR in both local and high-redshift galaxies.

Using combinations of H\alpha, ultraviolet (UV), and infrared (IR) emission, we estimate the star formation rate (SFR) surface density, \Sigma_SFR, at 1 kpc resolution for 30 disk galaxies that are targets of the IRAM HERACLES CO survey. We present a new physically-motivated IR spectral energy distribution-based approach to account for possible contributions to 24\mum emission not associated with recent star formation. Considering a variety of "reference" SFRs from the literature, we revisit the calibration of the 24\mum term in hybrid (UV+IR or H\alpha+IR) tracers. We show that the overall calibration of this term remains uncertain at the factor of two level because of the lack of wide-field, robust reference SFR estimates. Within this uncertainty, published calibrations represent a reasonable starting point for 1 kpc-wide areas of star-forming disk galaxies but we re-derive and refine the calibration of the IR term in these tracers to match our resolution and approach to 24\mum emission. We compare a large suite of \Sigma_SFR estimates and find that above \Sigma_SFR \sim 10^-3 M_\odot yr^-1 kpc^-2 the systematic differences among tracers are less than a factor of two across two orders of magnitude dynamic range. We caution that methodology and data both become serious issues below this level. We note from simple model considerations that focusing on a part of a galaxy dominated by a single stellar population the intrinsic uncertainty in H\alpha and FUV-based SFRs are \sim 0.3 and \sim 0.5 dex.

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.

Different wavelength regimes and methods for estimating the space density of the star formation rate (SFR) result in discrepant values. While it is recognized that ultraviolet (UV) and Hα emission-line data must be corrected for the effects of extinction, the magnitude of the required correction is uncertain. Even when these corrections are made there remains a significant discrepancy between SFRs derived from UV and Hα measurements compared with those derived from far-infrared (FIR) and radio luminosities. Since the FIR-radio–derived SFRs are not affected by extinction and simple corrections to reconcile the UV and Hα measurement with these do not fully account for the discrepancies, a more sophisticated correction may be required. Recent results suggest that at least part of the solution may be a form of extinction that increases with increasing SFR (or luminosity, given the common assumption that SFR is proportional to luminosity). We present an analysis of the effects of a dust reddening dependent on star formation rate applied to estimators of SFR. We show (1) that the discrepancies between Hα and FIR-radio SFR estimates may be explained by such an effect and we present an iterative method for applying the correction and (2) that UV-based estimates of SFR are harder to reconcile with FIR-radio estimates using this method, although the extent of the remaining discrepancy is less than for a non-SFR–dependent correction. Particularly at high redshift, our understanding of extinction at UV wavelengths may require a still more complex explanation.

Star formation rates (SFRs) in galaxies offer a view of various physical processes across them, and are measured using various tracers, such as Hα and ultraviolet (UV). Different physical mechanisms can affect Hα and UV emission, resulting in a discrepancy in the corresponding SFR estimates (ΔSFR). We investigate the effects of ram pressure on the SFR measurements and ΔSFR across five galaxies from the GASP survey caught in the late stages of gas stripping due to ram pressure. We probe spatially resolved ΔSFR at pixel scales of 0.5 kpc, and compare disks to tails and regions dominated by the dense gas to diffuse ionized gas (DIG) regions. The regions dominated by dense gas show similar SFR values for UV and Hα tracers, while the regions dominated by the DIG show up to 0.5 dex higher SFR(UV). There is a large galaxy-by-galaxy variation in ΔSFR, with no difference between the disks and the tails. We discuss the potential causes of variations in ΔSFR between the dense gas and DIG areas. We conclude that the dominant cause of discrepancy are recent variations in star formation histories, where star formation recently dropped in the DIG-dominated regions leading to changes in ΔSFR. The areal coverage of the tracers shows areas with Hα and no UV emission; these areas have LINER-like emission (excess in [O i λ6300]/Hα line ratio), indicating that they are ionized by processes other than star formation.

The resolution of the cosmic far-infrared background at long wavelengths has uncovered a population of high-redshift, highly infrared-luminous galaxies that indicate that a significant amount of highly obscured star formation occurred in the early history of the Universe, which has not been taken into account in studies of the cosmic star formation and mass assembly histories. Since such studies are important tests of hierarchical galaxy formation models in a cold dark matter- dominated universe, it is vital that the contribution of these luminous submillimeter-selected galaxies to the buildup of stellar mass in the Universe be understood and included when testing galaxy formation models. Progress in understanding the nature of submillimeter-selected galaxies has been slow, however, due to the faintness of the population outside of the submillimeter bands and the coarse resolution of the single-dish submillimeter surveys in which the galaxies were discovered. The slow progress has at least two important consequences: (1) there is still much that is unknown about submillimeterselected galaxies at all wavelengths, so the properties of and evolutionary predictions for the population are often inferred by making analogies to local galaxies of similar luminosity and assuming the high-redshift galaxies represent a uniform population of galaxies; and (2) the relationships of the submillimeter-selected galaxies to other high-redshift galaxy populations selected at other wavelengths remain poorly understood. In this work we capitalize on recently-available observing resources of the Spitzer Space Telescope, at near- and mid-infrared wavelengths, and the Green Bank Telescope, at 1 cm, to improve the characterization of the largest representative sample of submillimeter-selected galaxies with spectroscopic redshifts. We combine our new data with data from the literature at a variety of wavelengths to place new constraints on the fundamental properties of gas mass, stellar mass, infrared luminosity, and spectral energy distribution, and use our new constraints to test a variety of assumptions which have been used in the past to predict and infer characteristics of submillimeter-selected galaxies. We also use our new data to compare different populations of high-redshift galaxies selected at different wavelengths in an effort to understand the relationships between the different types of galaxies. First, we use observations of CO rotational line emission with the Green Bank Telescope to constrain the cold gas mass and gas conditions in several submillimeter-selected galaxies. We obtain the first detection of CO(1→0) emission from a submillimeter-selected galaxy, finding that the CO(4→3)/CO(1→0) brightness temperature ratio of ~ 0.26 suggests n(H₂) > 3 − 10 × 10² cm⁻³ and the presence of sub-thermally excited gas. The integrated line flux implies a cold molecular gas mass 4 times larger than the mass predicted from the CO(4→3) line, assuming a brightness temperature ratio of 1.0, suggesting that extrapolating molecular gas masses from J upper ≥ 3 transitions of CO, which is the primary method of estimating molecular gas masses of high-z galaxies in the literature, leads to considerable uncertainties. Next, we use deep imaging with the Multiband Imaging Photometer for Spitzer (MIPS) on the Spitzer Space Telescope of the spectroscopic sample of radio-detected submillimeter-selected galaxies of Chapman et al. (2005) to derive new estimates of the infrared luminosity for these objects. Our Spitzer data constrain the Wien side of the infrared spectral energy distribution peak of high-redshift submillimeter-selected galaxies, and thus are extremely important to determine the contribution of hot dust emission to the total infrared luminosity. We find that most submillimeter-selected galaxies do not have dominant contributions from hot dust at rest-frame mid-infrared wavelengths. We also find that the spectral energy distribution of the nearest ultraluminous infrared galaxy, Arp 220, is significantly different on average from most high-z submillimeter-selected galaxies and is thus a poor template with which to predict properties of submillimeter-selected galaxies even though it has been often used in the past. Using our new infrared luminosity estimates constrained by multiple infrared and submillimeter data points, we show that submillimeter-selected galaxies display a relatively tight, almost linear correlation between total infrared luminosity and radio luminosity, which is not largely different from the far-infrared–radio correlation of local galaxies selected by the Infrared Astronomical Satellite (IRAS). We examine the rest-frame ultraviolet through near-infrared spectral energy distributions of the same sample of radio-detected submillimeter-selected galaxies with spectroscopic redshifts, obtained using measurements from imaging with the Infrared Array Camera (IRAC) on Spitzer in combination with observed-frame optical and near-infrared data from the literature. We find from these spectral energy distributions, which trace the stellar light from a galaxy in the absence of an active nucleus, that submillimeter-selected galaxies suffer significant extinction at rest-frame optical wavelengths, and that both stars and dust emission contribute to the near-infrared luminosity in many z > 2 submillimeter-selected galaxies. We estimate stellar masses for the individual galaxies in the sample using restframe H-band luminosities interpolated from the observed spectral energy distributions, obtaining a median stellar mass for the sample of 6 − 7 × 10¹⁰ Msun. By comparing our stellar mass estimates to molecular gas and dynamical mass estimates for 13 individual submillimeter-selected galaxies in our sample observed in CO emission lines, we determine that the molecular gas fraction in submillimeter-selected galaxies declines with increasing stellar mass, which is suggestive of an evolutionary trend. If the molecular gas masses for the 13 galaxies for which gas mass estimates are available are typical of the entire radiodetected submillimeter-selected galaxy population, then a typical lower limit to the total baryonic mass of submillimeter-selected galaxies is ~ 10¹¹ Msun, and these galaxies are unlikely to significantly increase their stellar mass in the current epoch of activity which is the source of their enormous infrared luminosity. Lastly, we compare the IRAC and MIPS properties of submillimeter-selected galaxies and their stellar masses to those of high-redshift ultraviolet- and optically-selected galaxies, 24 μm-selected galaxies, and powerful radio galaxies. In the IRAC bands, submillimeter-selected galaxies are brighter and redder than ultraviolet-selected galaxies, suggesting they have higher dust content, higher stellar mass, a higher contribution from an active nucleus, or some combination of these factors. The near-infrared colors of submillimeterselected galaxies are most similar to those of high-z radio galaxies, objects which are known to contain powerful, obscured active nuclei. However, submillimeter-selected galaxies are fainter in the MIPS 24 μm band than high-z radio galaxies, suggesting that the dust in the submillimeter-selected galaxies is not heated to such high temperatures as in the radio sources. We find that the typical stellar mass of submillimeter-selected galaxies is larger by a factor of 3–4 than that of high-redshift ultraviolet-selected galaxies, roughly similar to the typical stellar mass of optically-selected high-z galaxies, and lower than the typical stellar mass of powerful high-z radio galaxies. These comparisons suggest that submillimeter-selected galaxies are among the more massive galaxies of their epoch, but not necessarily the most massive, as has been suggested in the literature. However, systematic errors in the stellar masses of any of the high-redshift galaxy samples of a factor of a few, which are certainly possible, can alter this conclusion.

We address the problem of modeling the far-infrared (FIR) spectrum and deriving the star-formation rate (SFR) and the dust mass of spiral galaxies. We use the realistic physical model of Popescu et al. ([CITE]) to describe the overall ultra-violet (UV), optical and FIR spectral energy distribution (SED) of a spiral galaxy. The model takes into account the 3-dimensional old and young stellar distributions in the bulge and the disk of a galaxy, together with the dust geometry. The geometrical characteristics of the galaxy and the intrinsic optical and near-infrared spectra are determined by the galaxy's observed K-band photometry. The UV part of the spectrum is assumed to be proportional to the SFR through the use of population synthesis models. By solving the radiative transfer equation, we are able to determine the absorbed energy, the dust temperature and the resulting FIR spectrum. The model has only three free parameters: SFR, dust mass, and the fraction of the UV radiation which is absorbed locally by dense dust in the HII regions. Using this model, we are able to fit well the FIR spectra of 62 bright IRAS galaxies from the “SCUBA Local Universe Galaxy Survey" of Dunne et al. ([CITE]). As a result, we are able to determine, among others, their SFR and dust mass. We find that, on average, the SFR (in absolute units), the star-formation efficiency, the SFR surface density and the ratio of FIR luminosity over the total intrinsic luminosity, are larger than the respective values of typical spiral galaxies of the same morphological type. We also find that the mean gas-to-dust mass ratio is close to the Galactic value, while the average central face-on optical depth of these galaxies in the V band is 2.3. Finally, we find a strong correlation between SFR or dust mass and observed FIR quantities like total FIR luminosity or FIR luminosity at 100 and 850 . These correlations yield well-defined relations, which can be used to determine a spiral galaxy's SFR and dust-mass content from FIR observations.

The Halpha and optical broadband images of 25 nearby Wolf-Rayet (WR) galaxies are presented. The WR galaxies are known to have the presence of a recent ($\le$10 Myr) and massive star formation episode. The photometric Halpha fluxes are estimated, and corrected for extinction and line contamination in the filter pass-bands. The star formation rates (SFRs) are estimated using Halpha images and from the archival data in the far-ultraviolet (FUV), far-infrared (FIR) and 1.4 GHz radio continuum wave-bands. A comparison of SFRs estimated from different wavebands is made after including similar data available in literature for other WR galaxies. The Halpha based SFRs are found to be tightly correlated with SFRs estimated from the FUV data. The correlations also exist with SFRs estimates based on the radio and FIR data. The WR galaxies also follow the radio-FIR correlation known for normal star forming galaxies, although it is seen here that majority of dwarf WR galaxies have radio deficiency. An analysis using ratio of non-thermal to thermal radio continuum and ratio of FUV to Halpha SFR indicates that WR galaxies have lesser non-thermal radio emission compared to normal galaxies, most likely due to lack of supernova from the very young star formation episode in the WR galaxies. The morphologies of 16 galaxies in our sample are highly suggestive of an ongoing tidal interaction or a past merger in these galaxies. This survey strengthens the conclusions obtained from previous similar studies indicating the importance of tidal interactions in triggering star-formation in WR galaxies.

We present a comprehensive study of the physical properties of ∼ 105 galaxies with measurable star formation in the Sloan Digital Sky Survey (SDSS). By comparing physical information extracted from the emission lines with continuum properties, we build up a picture of the nature of star-forming galaxies at z &amp;lt; 0.2. We develop a method for aperture correction using resolved imaging and show that our method takes out essentially all aperture bias in the star formation rate (SFR) estimates, allowing an accurate estimate of the total SFRs in galaxies. We determine the SFR density to be 1.915+0.02−0.01 (random)+0.14−0.42 (systematic) h7010−2 M⊙ yr−1 Mpc−3 at z= 0.1 (for a Kroupa initial mass function) and we study the distribution of star formation as a function of various physical parameters. The majority of the star formation in the low-redshift Universe takes place in moderately massive galaxies (1010–1011 M⊙), typically in high surface brightness disc galaxies. Roughly 15 per cent of all star formation takes place in galaxies that show some sign of an active nucleus. About 20 per cent occurs in starburst galaxies. By focusing on the SFR per unit mass we show that the present to past average SFR, the Scalo b-parameter, is almost constant over almost three orders of magnitude in mass, declining only at M* &amp;gt; 1010 M⊙. The volume averaged b parameter is 0.408+0.005−0.002 (random)+0.029−0.090 (systematic)h−170. We use this value to constrain the star formation history of the Universe. For the concordance cosmology the present-day Universe is forming stars at at least 1/3 of its past average rate. For an exponentially declining cosmic star formation history this corresponds to a time-scale of 7+0.7−1.5 Gyr. In agreement with other work we find a correlation between b and morphological type, as well as a tight correlation between the 4000-Å break (D4000) and b. We discuss how D4000 can be used to estimate b parameters for high-redshift galaxies.

Context. The star formation rate (SFR) is a key ingredient for studying the formation and evolution of galaxies. Being able to obtain accurate estimations of the SFR, for a wide range of redshifts, is crucial for building and studying galaxy evolution paths over cosmic time. Aims. Based on a statistical sample of galaxies, the aim of this paper is to constrain a set of SFR calibrators that are able to work in a large redshift range, from z = 0 to z = 0.9. Those calibrators will help to homogenize SFR estimations of star-forming galaxies and to remove any possible biases from the study of galaxy evolution. Methods. Using the VIMOS Public Extragalactic Redshift Survey (VIPERS), we estimated a set of SFR based on photometric and spectroscopic data. We used, as estimators, photometric bands from ultraviolet (UV) to mid-infrared (mid-IR), and the spectral lines Hβ, [O II]λ3727, and [O III]λ5007. Assuming a reference SFR obtained from the spectral energy distribution reconstructed with Code Investigating GALaxy Emission (CIGALE), we estimated the reliability of each band as an SFR tracer. We used the GALEX-SDSS-WISE Legacy Catalog (GSWLC, z &lt; 0.3) to trace the dependence of these SFR calibrators with redshift. Results. The far and near UV (FUV and NUV, respectively), u-band and 24 μm bands, as well as LTIR, are found to be good SFR tracers up to z ∼ 0.9 with a strong dependence on the attenuation prescription used for the bluest bands (scatter of SFR of 0.26, 0.14, 0.15, 0.23, and 0.24 dex for VIPERS, and 0.25, 0.24, 0.09, 0.12, and 0.12 dex for GSWLC). The 8 μm band provides only a rough estimate of the SFR as it depends on metallicity and polycyclic aromatic hydrocarbon properties (scatter of 0.23 dex for VIPERS). We estimated the scatter of rest-frame luminosity estimations from CIGALE to be 0.26, 0.14, 0.12, 0.15, and 0.20 dex for FUV, NUV, ugriz, Ks, and 8–24 μm-LTIR. At intermediate redshift, the Hβ line is a reliable SFR tracer (scatter of 0.19 dex) and the [O II]λ3727 line gives an equally good estimation when the metallicity from the R23 parameter is taken into account (0.17 for VIPERS and 0.20 dex for GSWLC). A calibration based on [O III] retrieves the SFR only when additional information such as the metallicity or the ionization parameter of galaxies are used (0.26 for VIPERS and 0.20 dex for GSWLC), diminishing its usability as a direct SFR tracer. Based on rest-frame luminosities estimated with CIGALE, we propose our own set of calibrations from FUV, NUV, u-band, 8, 24 μm, LTIR, Hβ, [O II], and [O III].

We investigate the use of the [O ii] λ3727 emission line as a star formation rate (SFR) estimator using Sloan Digital Sky Spectra for nearly 100,000 star-forming galaxies and 5500 galaxies with narrow-line active galactic nuclei. Consistent with previous work, we find that the [O ii]/Hα ratio in star-forming galaxies depends strongly on gas-phase metallicity. Using metallicities derived from the [N ii] λ6584/[O ii] λ3727 method, we refine a metallicity-dependent SFR estimator based on [O ii] that is calibrated within a scatter of 0.056 dex against the more commonly used SFR indicator based on Hα emission. The scatter increases to only 0.12 dex if the metallicity is estimated using the stellar mass–metallicity relation. With the aim of extending the [O ii]-based SFR estimator to active galaxies, we calculate radiation pressure-dominated photoionization models to constrain the amount of [O ii] emission arising from the narrow-line region. We use the sample of active galaxies to demonstrate that the SFRs derived from [O ii], after accounting for nonstellar contamination, are consistent with independent SFR diagnostics estimated from the stellar continuum of the host galaxies.

The stellar population and star clusters around six regions in the Large Magellanic Cloud (LMC) are studied to understand the correlation between star formation and cluster formation rates. We used the stellar data base of the OGLE II LMC survey and the star cluster catalogues. The observed distributions of stellar density in the colour−magnitude diagrams (CMDs) were compared with synthetic ones generated from stellar evolutionary models. By minimising the reduced χ 2 values, the star formation history of the regions were obtained in terms of star formation rates (SFR). All the regions were found to show large SFRs between the ages 500−2 Gyr with lower values for younger and older ages. A correlated peak in the cluster and SFRs is found for ages ∼1 Gyr, and for ages less than 100 Myr. Five of the six regions show significant cluster formation in the age range of 100−300 Myr, when the SFRs were found to be very low. This indicates anti-correlation between star and cluster formation rates for the 100−300 Myr age range. A possible reason may be that the stars are predominantly formed in clusters, whether bound or unbound, as a result of star formation during the above age range. The enhanced cluster formation rate in the 100−300 Myr age range could be correlated with the encounter of the LMC with the Small Magellanic Cloud, while the enhanced star and cluster formation at ∼1 Gyr does not correspond to any interaction. This could indicate that the star formation induced by interactions is biased towards group or cluster formation of stars.