A&A 419, 109–126 (2004) Astronomy DOI: 10.1051/0004-6361:20034049 & c ESO 2004 Astrophysics UV and FIR selected samples of galaxies in the local Universe Dust extinction and star formation rates J. Iglesias-P´aramo, V. Buat, J. Donas, A. Boselli, and B. Milliard Laboratoire d’Astrophysique de Marseille, BP 8, 13376 Marseille Cedex 12, France e-mail: [veronique.buat;jose.donas;alessandro.boselli;bruno.milliard]@oamp.fr Received 4 July 2003 / Accepted 11 February 2004 Abstract. We have built two samples of galaxies selected at 0.2 µm (hereafter UV) and 60 µm (hereafter FIR) covering a 2 sky area of 35.36 deg . The UV selected sample contains 25 galaxies brighter than AB0.2 = 17. All of them, but one el- liptical, are detected at 60 µm with a flux density larger or equal to 0.2 Jy. The UV counts are significantly lower than the Euclidean extrapolation towards brighter fluxes of previous determinations. The FIR selected sample contains 42 galaxies brighter than f60 = 0.6 Jy. Except four galaxies, all of them have a UV counterpart at the limiting magnitude AB0.2 = 20.3mag. The mean extinction derived from the analysis of the FIR to UV flux ratio is ∼1 mag for the UV selected sample and ∼2mag for the FIR selected one. For each sample we compare several indicators of the recent star formation rate (SFR) based on the FIR and/or the UV emissions. We find linear relationships with slopes close to unity between the different SFR indicator, which means that, over the whole converting offset. Various absolute calibrations for both samples are discussed in this paper. A pos- itive correlation between extinction and SFR is found when both samples are considered together although with a considerable scatter. A similar result is obtained when using the SFR normalized to the optical surface of the galaxies. Key words. ISM: dust, extinction – ultraviolet: galaxies – infrared: galaxies 1. Introduction Bell & Kennicutt 2001), has been extensively studied (e.g. Milliard et al. 1992; Donas et al. 1995; Treyer et al. 1998). This Tracing the star formation activity in galaxies at all redshifts situation should change dramatically in the near future once the is a fundamental step towards the understanding of the forma- GALEX data will be available. tion and evolution of the Universe. The star formation activity The first step towards the determination of statistical prop- is commonly quantified by the Star Formation Rate (SFR)de- erties of UV and FIR selected samples of galaxies is the deter- fined as the stellar mass formed per unit of time. In order to effi- mination of the galaxy counts and the luminosity function (LF). ciently constrain the models of galaxy formation and evolution Accurate determinations of these observables are important it is important to measure a SFR as current as possible in order for constraining models of galaxy formation and evolution. not to integrate on too large lookback times. Quite naturally, Nevertheless very few models predict the luminosity distribu- the light emitted by young stars can be used to measure this tion and spectral energy distribution of galaxies over a large SFR. The most commonly used tracers of the SFR in galaxies, range of wavelength from UV to FIR (Totani & Takeuchi 2002, at least in the nearby Universe, are the UV, FIR and Hα emis- and references therein; Xu et al. 1998). This is due, at least in sions (e.g. Kennicutt 1998). In the present study we focus our part, to our poor knowledge of the dust extinction in the uni- attention on the UV and FIR emissions. verse (amount of dust, dust emission, mechanism of stellar ab- Concerning the FIR window, the IRAS mission provided sorption on large scales). The reconstruction of the whole SED us with several now well studied samples such as the Point of galaxies from UV to FIR is difficult even in the well sam- Source Catalog (PSC, Joint IRAS Science 1994), the Faint pled nearby Universe because of the lack of multiwavelength Source Catalog (FSC, Moshir et al. 1990) and the redshift sur- data on large and homogeneous samples of galaxies (Boselli vey of the Point Source Catalog (PSCz, Saunders et al. 2000). et al. 2003; Flores et al. 1999; Rigopoulou et al. 2000; Cardiel The UV window, less explored mostly because limited obser- et al. 2003). This situation should evolve dramatically in the vations have been available until now (e.g. Donas et al. 1987; future with the planned observations of large fields with tele- Deharveng et al. 1994; Milliard et al. 1992; Kinney et al. 1993; scopes working at very different wavelengths (SIRTF, GALEX, Send offprint requests to:J.Iglesias-P´aramo, VIMOS, ASTRO-F). e-mail: [email protected] The FIR to UV flux ratio has been proved to be the best Tables 3, 4 and Fig. 1 are only available in electronic form at indicator of the dust extinction in normal galaxies (e.g. Buat http://www.edpsciences.org & Xu 1996; Meurer et al. 1999), becoming a fundamental Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20034049 110 J. Iglesias-P´aramo et al.: Extinction and SFR in FIR and UV selected samples Table 1. Basic properties of the selected FOCA fields. m028 (Coma), m067 (Abell 1367) and m050 (Virgo), and two of them point to the external parts of the Coma clus- lim ter (m030, m031). The Cancer cluster is however in Hubble FOCA field Id. FOCA RA (J2000) Dec (J2000) AB0.2 flow (Gavazzi et al. 1991), thus its members are similar to field m015 NGC 1023 02:36:41.8 +38:43:11 19.36 galaxies. The fractions of cluster galaxies in our UV selected m010 Cancer 08:20:24.6 +20:45:07 20.36 and FIR selected (hereafter UVsel and FIRsel respectively) m018 NGC 2715 08:58:30.4 +78:09:29 20.06 samples are 64% and 50% respectively. The possible contribu- m067 Abell 1367 11:45:24.5 +19:11:39 20.26 tion of the cluster environment to the properties of the UVsel and FIRsel galaxies are discussed in the text. + m033 NGC 4125 12:01:27.1 64:56:17 20.56 The detection and flux extraction of the UV objects in m050 NGC 4472 12:27:38.4 +08:35:57 19.86 the FOCA plates was carried out in an automatic way. Only m028 Coma 12:59:37.0 +28:06:29 21.06 UV sources with surface brightness (averaged over 15 × 15 arcsec2) brighter than 2.8 times the sky σ (the detection m030 SA 57a 13:06:15.4 +29:04:20 21.06 limit established by the automatic detection algorithm for each m031 SA 57b 13:11:50.1 +27:52:24 20.46 frame, given in Col. 5 of Table 1) were considered as detec- tions. The automatic determination of the UV flux is accurate for point like sources but uncertain for extended objects, so the parameter in the determination of their present SFR (e.g. Buat UV fluxes of the FIRsel galaxies were determined in two ways: et al. 1999, 2002; Hirashita et al. 2003, hereafter HBI). (1) for the galaxies which were not resolved by the automatic The aim of the present paper is to study the statistical prop- detection algorithm (i.e. only one UV source was detected) we erties of two samples extracted from the same area of the sky, used the UV flux provided by the detection algorithm1;(2)for selected according to UV and FIR criteria. The two selections the galaxies which were splitted in several sources (i.e. sev- (UV and FIR) appear very complementary since the sample of eral H regions were detected as individual sources) we per- UV selected galaxies will be biased towards active star forming formed aperture photometry by hand. The optical images were galaxies with low extinction, and on the contrary a FIR selec- used to set the size of the apertures in order to be sure that all tion is likely to favor galaxies with high extinction. An accurate the UV flux corresponding to the galaxies, even the most ex- knowledge of the statistical properties of UV and FIR selected ternal star forming regions, was included in the total UV flux. samples is crucial for the analysis of similarly selected samples In both cases the apertures used to measure the UV fluxes are at higher z, where the lack of complementary data prevents a close to the optical diameters of the galaxies. The average zero precise determinations of the dust extinction and SFR. point uncertainty of the FOCA data is about 0.2 mag. The spa- With these two samples in hand, we study the extinc- tial resolution of the FOCA frames is ≈20 arcsec FWHM and tion and SFR related properties and their dependence on the the 1σ astrometric uncertainty is ≈2.5 arcsec. selection method. More precisely we address the following Concerning the FIR data, the IRAS all sky survey ensures issues: (1) determine the mean extinction of purely UV or a total coverage of our selected FOCA fields. The total photo- FIR selected samples of galaxies, (2) observationally con- metric uncertainty of the FIR data of the IRAS mission is ≈10% strain models of galaxy formation and evolution, and (3) pro- at 60 and 100 µm. The positional uncertainty is variable and of vide the best recipes for the determination of the SFR, valid elliptical shape with major axis ≈40 arcsec. The spatial resolu- / for UV and or FIR selected samples of galaxies.