MNRAS 000, 1–33 (2018) Preprint 22 October 2018 Compiled using MNRAS LATEX style file v3.0 The Star Formation Reference Survey III: A Multi-wavelength View of Star Formation in Nearby Galaxies Smriti Mahajan1,2⋆, M. L. N. Ashby2, S. P. Willner2, P. Barmby3, G. G. Fazio2, A. Maragkoudakis3,4,5, S. Raychaudhury6, A. Zezas4,5 1Department of Physical Sciences, Indian Institute of Science Education and Research Mohali, Knowledge City, Sector 81, Manauli 140306, Punjab, India 2Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 USA 3Department of Physics and Astronomy, University of Western Ontario, London, Ontario, N6A 3K7 Canada 4University of Crete, Department of Physics, GR-71003 Heraklion, Greece 5Foundation for Research and Technology—Hellas (FORTH), Heraklion 71003, Greece 6Inter-University Centre for Astronomy & Astrophysics, Ganeshkhind, Pune, India Accepted 2018 September 11. Received 2018 August 31; in original form 2018 May 1 ABSTRACT We present multi-wavelength global star formation rate (SFR) estimates for 326 galax- ies 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 µm polycyclic aromatic hydrocarbons (PAH), and a combination of far-infrared (FIR) plus ultraviolet (UV) emission are mutually consistent with scatter of .0.3 dex. The scatter is even smaller, .0.24 dex, in the intermediate luminosity range 9.3 < log(L60 µm/L⊙) < 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, 9 for the minority of galaxies with UV luminosity >5 × 10 L⊙ or with implied far-UV extinction <1 mag, the UV spectral slope gives extinction corrections with 0.22 dex uncertainty. Key words: galaxies: star formation – infrared: galaxies – radio continuum: galaxies – ultraviolet: galaxies 1 INTRODUCTION tion rate density are therefore essential for constraining the arXiv:1810.01336v2 [astro-ph.GA] 19 Oct 2018 models of structure formation in the Universe. However, un- Star formation is critical for galaxy evolution. Stars have til a few years ago, accurate measurements of SFR even in created almost all the elements heavier than helium in the nearby galaxies were difficult owing to lack of knowledge of Universe and play a key role in recycling dust and metals in the effect of dust on different SFR tracers. galaxies. Hence the rate at which a galaxy forms stars is one of the most important drivers of its evolution. Understand- In addition to the standard optical spectral lines ing global trends in star formation rate (SFR henceforth) (e.g., Hα, [O ii]), the indicators most commonly used to among different galaxy populations is required for interpret- quantify star formation in a galaxy are the global radio ing the ‘Hubble sequence,’ which is a representation of not continuum, mid- and far-infrared (MIR and FIR), and ul- just the evolutionary trend in galaxy morphology but also traviolet (UV) emission. Different wavelengths trace stellar gas content, mass, bars, dynamical structure, and environ- populations at different stages of evolution as well as differ- ment, all of which influence the SFR (Kennicutt 1998, and ent galaxy components. For instance, stars more massive references therein). SFR measurements and the star forma- than 8 M⊙ produce the core-collapse supernovae whose ∼ remnants (SNRs) accelerate relativistic electrons, which have lifetimes .100 Myr (Condon 1992). The resulting non- ⋆ E-mail: [email protected] thermal radio synchrotron emission, which dominates a c 2018 The Authors 2 Mahajan et al. galaxy’s radio luminosity at low frequencies (.5 GHz), is The primary purpose of this work is to present GALEX therefore a measure of past formation of massive stars. ultraviolet photometry for SFRS galaxies. By combining Thermal radio emission, which dominates at high radio fre- GALEX photometry with photometry at other wavelengths, quencies (&10 GHz; e.g., Klein & Emerson 1981; Gioia et al. we test and mutually calibrate widely used empirical formu- 1982; Tabatabaei et al. 2017) is a measure of current produc- las to calculate global SFRs for galaxies using tracers span- tion of ionizing photons. The massive stars producing such ning all available wavelengths. In choosing among the many photons have lifetimes of order 10 Myr, and high-frequency calibrations available in the literature, we have preferred radio observations thus probe very recent SFR in star- those that give mutually consistent results. The wide ranges forming and normal galaxies.1 of morphologies, luminosities, sizes, SFRs, and stellar masses UV light is emitted predominantly by stars younger spanned by the SFRS galaxies, together with the sample’s than around 200 Myr and is therefore a good measure of well-defined selection criteria, makes it an ideal sample to SFR over time-scales of tens of Myr. But the interpretation quantify the relation between different SFR measures in of this indicator is hampered by the presence of dust clouds nearby galaxies and hence a benchmark for comparing the enshrouding young star-forming regions. Dust absorbs UV SFR measures of high redshift galaxies. Studies such as photons and reemits their energy at FIR wavelengths, mak- this one have been performed elsewhere (e.g., Hopkins et al. ing FIR luminosity a more reliable SFR indicator. For most 2001; Bell 2003; Schmitt et al. 2006; Johnson et al. 2007; star-forming galaxies, a combination of UV and FIR lumi- Zhu et al. 2008; Davies et al. 2016; Wang et al. 2016) but on nosity accounts for a major fraction of the galaxy’s bolomet- samples often small or chosen without well-defined criteria ric luminosity. or with narrow sample boundaries, where systematic devi- Generally the FIR emission from any galaxy has at least ations from the underlying correlations cannot be well ex- two components, one originating from the interstellar dust plored. A recent study by Brown et al. (2017) used GALEX heated by the diffuse radiation field and a second contribu- photometry as a SFR tracer, but their sample was restricted tion from star formation activity in and near the H ii re- to galaxies with strong emission lines, and they did not use gions (Soifer et al. 1987, and references therein). If the sec- FIR at all. ond component can be measured, its FIR luminosity can be This paper is organized as follows. 2 describes the § converted to an SFR measure. datasets used in this paper, and 3 describes and compares § Numerous attempts have been made to utilize the FIR– the star formation tracers. 4 investigates extinction indi- § UV energy budget to quantify dust attenuation in various cators and whether they can give useful measures of SFR. samples of galaxies selected at different wavelengths (e.g., These are followed by a discussion of our findings in the con- Xu & Buat 1995; Meurer et al. 1999; Buat et al. 2002, 2005; text of the existing literature in 5, followed by a summary § Cortese et al. 2006; da Cunha et al. 2010). One approach is of our results in 6. Throughout this paper, star formation 2 § to use the FIR/UV flux ratio (or the infrared excess IRX rates are based on a Salpeter IMF in the range 0.1–100 M⊙. as it is more popularly known, e.g., Meurer et al. 1999; Kong et al. 2004; Seibert et al. 2005). An alternative is to use reddening inferred from the Balmer decrement (e.g., Buat et al. 2002; Gilbank et al. 2010) or from UV colour 2 THE DATA (e.g., Meurer et al. 1999; Lee et al. 2009; Gilbank et al. 2.1 Sample selection 2010). The reddening measure is then combined with an assumed extinction curve to yield extinction at UV wave- The SFRS (Paper I) is a statistically robust, representa- lengths. The problem is understanding both random and tive sample of 367 star-forming galaxies in the local Uni- systematic errors for the derived extinction values for differ- verse. The sample selection criteria were defined objectively ent galaxy populations. to guarantee that the SFRS has known biases and selection In order to understand the relation between indicators weights, making it possible to relate conclusions from the of star formation and dust extinction, the different SFR and SFRS to magnitude-limited or volume-limited FIR-selected extinction indicators need to be quantified and compared for samples. Moreover, the SFRS spans the full ranges of prop- a statistical sample of galaxies covering a wide range in phys- erties exhibited by FIR-selected star-forming galaxies in the ical and intrinsic properties and having known biases. This nearby Universe. While much larger galaxy samples exist, idea motivated the Star Formation Reference Survey (SFRS; for huge samples it is difficult to obtain the complete data Ashby et al. 2011, Paper I henceforth). Which SFR indica- sets needed to explore multi-wavelength correlations. The tors can be used to estimate the global SFR of a galaxy? SFRS is therefore an ideal tool for understanding the global When are multi-wavelength data required? How closely does properties of nearby (z . 0.1) star-forming galaxies. any single SFR indicator measure a galaxy’s ‘total’ SFR? Is The SFRS was drawn from the PSCz catalog the relation between individual SFR indicators universal for (Saunders et al. 2000), an all-sky redshift survey of 15,000 all types of star-forming galaxies? What are the advantages galaxies observed by IRAS and brighter than 0.6 Jy at and disadvantages of different extinction indicators? 60 µm.