NGC 2782: a merger remnant with young stars in its gaseous tidal tail*

Mon. Not. R. Astron. Soc. 421, 3612–3621 (2012) doi:10.1111/j.1365-2966.2012.20589.x NGC 2782: a merger remnant with young stars in its gaseous tidal tail S. Torres-Flores,1,2,3 † C. Mendes de Oliveira,1 D. F. de Mello,4,5 S. Scarano, Jr1 and F. Urrutia-Viscarra1 1 Departamento de Astronomia, Instituto de Astronomia, Geofı́sica e Ciências Atmosféricas da USP, Rua do Matão 1226, Cidade Universitária, 05508-090, São Paulo, Brazil 2 Laboratoire d’Astrophysique de Marseille, OAMP, Université de Provence & CNRS, 38 rue F. Joliot–Curie, 13388 Marseille, Cedex 13, France 3 Departamento de Fı́sica, Universidad de La Serena, Av. Cisternas 1200 Norte, La Serena, Chile 4 Observational Cosmology Laboratory, Code 665, Goddard Space Flight Center, Greenbelt, MD 20771, USA 5 Catholic University of America, Washington, DC 20064, USA Accepted 2012 January 17. Received 2012 January 17; in original form 2010 September 1 We have searched for young star-forming regions around the merger remnant NGC 2782. By using Galaxy Evolution Explorer far-ultraviolet and near-ultraviolet imaging and H I data we found seven ultraviolet sources, located at distances greater than 26 kpc from the centre of NGC 2782, and coinciding with its western H I tidal tail. These regions were resolved in several smaller systems when Gemini/Gemini multi-object spectrograph (GMOS) r-band images were used. We compared the observed colours to stellar population synthesis models and found that these objects have ages of ∼1 to 11 Myr and masses ranging from 103.9 to 104.6 M . By using Gemini/GMOS spectroscopic data we confirm memberships and derive high metallicities for three of the young regions in the tail (12+log(O/H) = 8.74 ± 0.20, 8.81 ± 0.20 and 8.78 ± 0.20). These metallicities are similar to the value presented by the nuclear region of NGC 2782 and also similar to the value presented for an object located close to the main body of NGC 2782. The high metallicities measured for the star-forming regions in the gaseous tidal tail of NGC 2782 could be explained if they were formed out of highly enriched gas which was once expelled from the centre of the merging galaxies when the system collided. An additional possibility is that the tail has been a nursery of a few generations of young stellar systems which ultimately polluted this medium with metals, further enriching the already pre-enriched gas ejected to the tail when the galaxies collided. Key words: galaxies: interactions – intergalactic medium – galaxies: star clusters: general. 1 I N T RO D U C T I O N The intergalactic medium (IGM) of interacting galaxies and the tidal tails of mergers have shown to commonly be the birthplace of a number of new stellar systems. These systems span the range from small clusters (Knierman et al. 2003; Tran et al. 2003) and intergalactic H II regions (IH II; e.g. Mendes de Oliveira et al. 2004)  Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnologı́a e Innovación Productiva (Argentina) – Observing run: GN-2009B-Q-113. †E-mail: to tidal dwarf galaxies (TDGs; Mirabel, Dottori & Lutz 1992; Duc & Mirabel 1998; Weilbacher et al. 2000; Weilbacher, Duc & Fritze-v. Alvensleben 2003; Bournaud et al. 2004; Mundell et al. 2004; Mendes de Oliveira et al. 2006). Since the total luminosity of these young stellar systems is mostly dominated by the ultraviolet (UV) ionizing radiation coming from massive stars, the use of the Galaxy Evolution Explorer (GALEX) satellite (Martin et al. 2005) has become an important tool to study the star formation processes and the stellar populations of these systems (Neff et al. 2005; Hancock et al. 2007, 2009; de Mello, Torres-Flores & Mendes de Oliveira 2008a; Smith et al. 2008, 2010; Boquien et al. 2009, 2010; Torres-Flores et al. 2009). Another crucial ingredient for the study of the stellar populations and formation processes of young stellar systems is the knowledge of their element abundances. This can, in fact, constrain the formation scenario of these stellar associations. For example, Weilbacher et al. (2003) studied the oxygen abundance of several knots in tidal features which had been classified as TDG candidates.  C 2012 The Authors C 2012 RAS Monthly Notices of the Royal Astronomical Society  ABSTRACT Star-forming regions in NGC 2782 2 DATA 2.1 Ultraviolet data The UV GALEX images of NGC 2782 have been published by Thilker et al. (2007). In our case, the UV data analysed in this work were taken from the public archival data of the GALEX satellite. From this archive, we used images in the near-ultraviolet (NUV, λeff = 2271 Å) and far-ultraviolet (FUV, λeff = 1528 Å) bands. In the NUV band, the exposure time was 2109 s, and in the FUV band, it was 2108 s. FUV and NUV fluxes were calculated using Morrissey et al. (2005) mλ = −2.5log [F λ /aλ ] + bλ , where aFUV = 1.4 × 10−15 erg s−1 cm−2 Å−1 , aNUV = 2.06 × 10−16 erg s−1 cm−2 Å−1 , bFUV = 18.82 and bNUV = 20.08 for FUV and NUV, respectively. The fluxes were multiplied by the effective filter bandpass (λFUV = 269 Å and λNUV = 616 Å) to give units of erg s−1 cm−2 . The GALEX fields of view are 1.◦ 28 and 1.◦ 24 in FUV and NUV, respectively, and the pixel scale is 1.5 arcsec pixel−1 . The images had a resolution (full width at half-maximum ) of 4.2 and 5.3 arcsec in FUV and NUV, respectively.  C 2012 The Authors, MNRAS 421, 3612–3621 C 2012 RAS Monthly Notices of the Royal Astronomical Society  2.2 r-band and spectroscopic data Observations of NGC 2782 were carried out with the Gemini multiobject spectrograph (GMOS) at the Gemini north observatory as part of the science programme GN-2009B-Q-113. r-band images for two fields in NGC 2782 were observed. Fields were centred on the western gaseous tidal tail (RA = 09h 13m 53.s 5, Dec. = 40◦ 09 31. 6, J2000) and on the object NGC 2782 (RA = 09h 14m 03.s 2, Dec. = 40◦ 06 59. 1, J2000). The exposure times for both r-band images were 15 min, under a seeing of 0.54 and 0.77 arcsec, respectively. Zero-point calibrations were taken from the Gemini website. We checked these values using a few bright stars (on the same field) from the Sloan Digital Sky Survey (SDSS) data base (Abazajian et al. 2009). Archival GMOS multislit spectroscopic exposures (3 × 1200 s) were available for five objects on the western tidal tail of NGC 2782, covering from 3400 to 6100 Å (grating B600). These Gemini data were originally acquired by Werk et al. (2011), who provide more details on the observations and an independent reduction and analysis (...truncated)