MNRAS 450, 3381–3409 (2015) doi:10.1093/mnras/stv703 Ionized gas in the XUV disc of the NGC 1512/1510 system A.´ R. Lopez-S´ anchez,´ 1,2‹ T. Westmeier,3 C. Esteban4,5 and B. S. Koribalski6 1Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia 2Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia 3International Centre for Radio Astronomy Research, The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia 4Instituto de Astrof´ısica de Canarias, E-38200 La Laguna, Tenerife, Spain 5Departamento de Astrof´ısica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain 6CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO BOX 76, Epping, NSW 1710, Australia Accepted 2015 March 27. Received 2015 March 26; in original form 2014 June 4 Downloaded from ABSTRACT We present deep, intermediate-resolution, optical spectroscopy of 136 genuine UV-bright regions located in both the inner and outer regions of NGC 1512. This galaxy is in close inter- action with the blue compact dwarf galaxy NGC 1510 and possesses two prominent H I arms where extended ultraviolet complexes are found. Our data were taken using 2dF/AAOmega at http://mnras.oxfordjournals.org/ the 3.9 m Anglo-Australian Telescope and are combined with the H I data from Local Volume H I Survey and Galaxy Evolution Explorer UV data. We detect ionized gas in 82 per cent of the complexes, many of them located between 1 and 6.6 R25. We found significant differences between regions along the Arm 1 – 8.25 12+log(O/H) 8.45 –, and knots located in the ex- ternal debris of Arm 2, –8.40 12+log(O/H) 8.60–. Considering a radial and an azimuthal gradient following the H I arms, we confirm that Arm 2 has experienced an enhancement in star formation because of the interaction with NGC 1510 and flattened the radial metallicity at large radii. Arm 1 appears to retain the original and poorly disturbed radial distribution. at Oxford Brookes University on June 4, 2015 We trace the kinematics of the system up to 78 kpc using the Hα emission, which matches well that provided by the H I. We estimate that the gas existing at large galactocentric radii had a metallicity of 12+log(O/H) ∼ 8.1 before the interaction started around 400 Myr ago. The metals within the H I gas are very likely not coming from the inner regions of NGC 1512 but probably from material accreted during minor mergers or outflow-enriched intergalactic medium gas during the life of the galaxy. Key words: galaxies: abundances – galaxies: dwarf – galaxies: evolution – galaxies: individ- ual: NGC 1510 – galaxies: individual: NGC 1512 – galaxies: kinematics and dynamics. & Jarrett 2012), being this material the fuel for present and future 1 INTRODUCTION star-forming events. The extended ultraviolet (XUV) emission dis- One of the most surprising discoveries obtained by the Galaxy covered by GALEX is located well beyond the Hα or B25 radius of Evolution Explorer (GALEX) satellite (Martin et al. 2005)wasthe galaxies, and it seems to exist in ∼20–30 per cent of the local disc finding of UV-bright complexes in the outskirts of nearby spiral galaxy population (Thilker et al. 2007; Zaritsky & Christlein 2007; galaxies (Gil de Paz et al. 2005, 2007a; Thilker et al. 2005). Diffuse Lemonias et al. 2011). XUV-discs have even been found around stellar tails and shells were already found surrounding nearby galax- E/S0 galaxies (Salim & Rich 2010; Thilker et al. 2010;Moffett ies using deep optical images (e.g. Malin & Carter 1983) but these et al. 2012). The origin of these UV-bright complexes seems to be faint features were thought to be mainly composed by old stars. H II young stellar clusters associated with a recent or still on-going star regions located up to 30 kpc from the main galaxy have been also formation activity (Gil de Paz et al. 2007b; Bresolin et al. 2009a; found recently (e.g. Ryan-Weber et al. 2004;Meureretal.2006). Bresolin, Kennicutt & Ryan-Weber 2012). XUV-discs should be Indeed, spiral galaxies typically possess a large H I disc that reaches embedded in larger H I envelopes – a 2X-H I disc as defined by well beyond their optical size (e.g. Freeman et al. 1977;Walter Koribalski & Lopez-S´ anchez´ (2009) – which are providing the fuel et al. 2008; Westmeier, Braun & Koribalski 2011; For, Koribalski for their star formation activity (e.g. Koribalski & Lopez-S´ anchez´ 2009; Bigiel et al. 2010b;Werketal.2010a). The study of star formation processes in galaxy outskirts gives E-mail: [email protected] key clues about the formation and evolution of galaxies, not only C 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 3382 A.´ R. Lopez-S´ anchez´ et al. because these regions may probe physical conditions similar to UV colours) and the most recent star formation event (from Hα those present of the early Universe, but also because they test the emission); (iii) what is the metallicity distribution throughout the inside–out scenario of galaxy disc formation. In the first case, the system, studying not only radial but azimuthal chemical gradients, analysis of XUV-complexes in the outer regions of galaxies com- and how this affects the metal redistribution in galaxies; (iv) what is plements the study of dwarf galaxies as building blocks of larger the nature of the XUV-complexes and search for tidal dwarf galaxy objects but also the nature of tidal dwarf galaxies (TDGs), as well (TDG) or external dwarf galaxies candidates; and (v) what are the as the viability of star formation in regions of low gas density. In relationships between the neutral gas, the stellar mass, the metallic- the second case, these UV-bright regions provide a powerful tool to ity, and the star formation in these regions and the comparison with investigate the mass and chemical enrichment of galaxies from their what is observed in dwarf galaxies. centre to their external regions via colours, gas-to-stars ratios, and Here, we present the results of our analysis of the XUV emission metallicity gradients. Following the inside–out scenario, the galac- in the impressive galaxy pair NGC 1512/1510. At the adopted dis- tic stellar disc grows gradually with time as it accretes gas from tance of 9.5 Mpc, 1 arcmin corresponds to 2.49 kpc, and 2 arcsec their gaseous, extended discs (e.g. White & Frenk 1991; Bouwens, (the diameter of a 2dF fibre) corresponds to 83 pc. This system Cayon & Silk 1997) producing a relatively quick self-chemical en- hosts hundreds of independent UV-bright regions associated with richment and an almost universal negative metallicity gradient once dense H I clouds in the outskirts, the blue compact dwarf galaxy this is normalized to the galaxy optical size (Zaritsky, Kennicutt & (BCDG) NGC 1510 (which is located at just 13.8 kpc from the Huchra 1994; Boissier & Prantzos 1999, 2000). Indeed, detailed centre of NGC 1512), a central star-forming ring and two TDG Downloaded from observations using integral-field spectroscopy (IFS) data from the candidates at very large distance (projected radius of 83 kpc) iden- CALIFA survey (Sanchez´ et al. 2012; Husemann et al. 2013)have tified by their H I emission. A review of the properties of the proven that a characteristic oxygen abundance gradient does exist NGC 1512/1510 system, as well as a very detailed UV and H I in galaxy discs (Rosales-Ortega et al. 2012;Sanchez´ et al. 2014). analysis, can be found in Koribalski & Lopez-S´ anchez´ (2009, here- However, it has been observationally found that the outskirts after KLS09). Recently, Bresolin et al. (2012) obtained deep FORS2 http://mnras.oxfordjournals.org/ of galaxies do not follow the same pattern as their main disc. In – 8 m Antu unit of the European Southern Observatory Very Large particular, a flattening of the metallicity gradient in the external Telescope on Cerro Paranal, Chile – optical spectroscopy of 62 UV- regions of spiral galaxies has been found (Bresolin et al. 2009a; bright complexes within NGC 1512, including star-forming regions Kewley et al. 2010; Rupke, Kewley & Barnes 2010;Werketal. in the optical disc and in XUV knots of NGC 1512, although the 2010a,b; Bresolin, Kennicutt & Ryan-Weber 2012), and it is also majority (all but 16) were located within the effective radius, Re detected in our Milky Way (e.g. Vilchez & Esteban 1996; Esteban (4.26 arcmin = 11.76 kpc), of the galaxy. In six of the observed et al. 2013). It has been suggested that the flat metallicity gradient knots they were able to determine the oxygen and nitrogen abun- in the outer discs is a consequence of galaxy interactions, however dances following the direct Te method, yielding to average values there are still some non-interacting galaxies where this behaviour is of 12+log(O/H) ∼ 8.17 ± 0.09 and log(N/O) ∼−1.32 ± 0.12. also observed (Werk et al. 2011;Sanchez´ et al. 2014). In fact, for the at Oxford Brookes University on June 4, 2015 Milky Way (Esteban et al. 2013) suggest that a levelling out of the 2 OBSERVATIONS AND DATA REDUCTION star formation efficiency beyond the isophotal radius can explain the flattening of the abundance gradients in the external Galactic The observations were carried out at the 3.9 m AAT at Siding Spring disc. In any case, the finding of a flat metallicity gradient in the Observatory (NSW, Australia) between 2008 Nov 29 and Dec 2.