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NGC 6845: Metallicity Gradients and Star Formation in a Complex Compact Group ?

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NGC 6845: Metallicity Gradients and Star Formation in a Complex Compact Group ? Mon. Not. R. Astron. Soc. 000, 1{?? (0000) Printed 4 October 2018 (MN LATEX style file v2.2) NGC 6845: metallicity gradients and star formation in a complex compact group ? D. Olave-Rojas1y, S. Torres-Flores1, E. R. Carrasco2, C. Mendes de Oliveira3, D. F. de Mello4 & S. Scarano Jr5, 1Departamento de F´ısica y Astronom´ıa,Universidad de La Serena, Av. Cisternas 1200, La Serena, Chile 2Gemini Observatory/AURA, Southern Operations Center, Casilla 603, La Serena, Chile 3 Instituto de Astronomia, Geof´ısica e Ci^enciasAtmosf´ericas da Universidade de S~aoPaulo, Cidade Universit´aria,CEP:05508-900, S~aoPaulo, SP, Brazil 4 Catholic University of America, Washington, DC 20064, USA 5 Departamento de F´ısica - CCET, Universidade Federal de Sergipe, Rod. Marechal Rondon s/n, 49.100-000, Jardim Rosa Elze, S~aoCristov~ao,SE, Brazil 4 October 2018 ABSTRACT We have obtained Gemini/GMOS spectra of 28 regions located across the interacting group NGC 6845, spanning from the inner regions of the four major galaxies (NGC 6845A, B, C, D) to the tidal tails of NGC 6845A. All regions in the tails are star- forming objects with ages younger than 10 Myr. We derived the gas-phase metallicity gradients across NGC 6845A and its two tails and we find that these are shallower than those for isolated galaxies. NGC 6845A has a gas-phase oxygen central metal- licity of 12+log(O/H)∼8.5 and a flat gas-phase metallicity gradient (β=0.002±0.004 −1 dex kpc ) out to ∼4 × R25 (to the end of the longest tidal tail). Considering the mass-metallicity relation, the central region of NGC 6845A displays a lower oxygen abundance than the expected for its mass. Taking into account this fact and consider- ing the flat oxygen distribution measured along the eastern tidal tail, we suggest that an interaction event has produced a dilution in the central metallicity of this galaxy and the observed flattening in its metal distribution. We found that the star forma- tion process along the eastern tidal structure has not been efficient enough to increase the oxygen abundances in this place, suggesting that this structure was formed from enriched material. Key words: galaxies: abundances galaxies: interactions intergalactic medium galax- ies: star clusters: general galaxies: star formation. arXiv:1508.05070v1 [astro-ph.GA] 20 Aug 2015 1 INTRODUCTION ies (Moran et al. 2012) showed that metallicity gradients within the optical radii of galaxies are flat for massive galax- Local interacting/merging systems provide us with ideal lab- ies while metallicities decline steadily with radius for galax- oratories to study the effect of tidal forces in the kinematic, ies with low stellar mass (log(M ) < 10.2). Other studies morphology and chemical evolution of galaxies. (Toomre & ∗ have derived metallicity gradients of interacting, warped, Toomre 1972, Schweizer 1978). In particular, several gas-rich minor mergers or paired galaxies (Kewley et al. 2010, Werk interacting galaxies present lower nuclear metallicities and et al. 2011, Rich et al. 2012) and found shallower profiles shallower metallicity gradients than non-interacting galaxies than generally found in galaxies of similar mass. Recently, of similar masses, suggesting that large scale gas flows may Rosa et al. (2014) using Gemini data found that oxygen be linked to the chemical evolution of these systems (Kewley gradients are flatter for pairs of galaxies than for isolated et al. 2010, Rupke et al. 2010,b, Werk et al. 2011, Bresolin spiral galaxies. These works were focused on the study of et al. 2012). metallicity gradients of interacting galaxies up to twice the A recent systematic survey of gas-phase metallicity gra- optical radii of the galaxies but none of them dealt with dients of a large sample of nearby non-interacting disk galax- galaxies with tidal tails. In an attempt to probe metallicities at increasingly larger radii, our group has focused on deriv- ing metallicity gradients for galaxies with long tidal tails in ? Observing run: GS-2011B-Q-36 y : [email protected] c 0000 RAS 2 D. Olave-Rojas et al. the optical and Hi as is the case of the galaxies NGC 92 that NGC 6845A has a long tidal arm projected onto NGC (Torres-Flores et al. 2014) and NGC 2782 (Werk et al. 2011, 6845B. They were able to confirm that NGC 6845A and Torres-Flores et al. 2012). The presence of tidal tails allows NGC 6845B are part of the system. Also, they found one probing metallicities not only to much outer radii than in exceptionally large knot in NGC 6845A - they named it as cases of galaxies with no tails, but also in the intragroup \knot a". Later, NGC 6845C and NGC 6845D were con- medium, where intergalactic Hii regions are found (as it is firmed as members of the group by Rose & Graham (1979). the case for Stephan's quintet, Trancho et al. 2012). Our Deep images by Rose & Graham (1979) showed the bright previous studies analyzed NGC 92 and NGC 2782, systems eastern arm of NGC 6845A extending towards NGC 6845B which display tidal tails with flat metallicity distributions. and a tidal bridge bluer than the inner disk of NGC 6845A. Metallicities were obtained by using the nebular spectra of Rodrigues et al. (1999) found that the tidal bridge be- several young star forming regions located along the tidal tween NGC 6845A and NGC 6845B has a (B-I) colour bluer tails. These regions are metal rich, which suggests that they than that of the inner disk of NGC 6845A. These authors were born from material expelled from the galaxies involved found eight strong condensations, identified as Hii regions in the interaction. with ages between 3-8Myrs. They also studied the blue knot The study of systems in extreme phases, such as when \a", but they named it \7". collisions happen and form tidal tails, may help elucidate Gordon et al. (2003) obtained Hi (21-cm) observations processes which are rare in the nearby Universe but may of NGC 6845 by using the Australia Telescope Compact Ar- have been common at high-z. Formation of new systems due ray (ATCA), during 1997 and 1998. The beam size was 43 × to strong interactions, and subsequent evolution of several 36 arcsec2, with a channel width of 20 km s−1. Using these generations of these systems may enhance the metallicity of data, Gordon et al. (2003) derived integrated Hi intensity the outskirts of galaxies and intragroup medium and may be maps, mean velocity field and Hi velocity dispersion maps. an important mechanism for driving metals from the centers In addition, 20-cm radio continuum maps were derived from of the large galaxies outward and to the intragroup medium. these observations. Gordon et al. (2003) found that the One extreme case of recently formed objects due to interac- Hi emission is associated with the galaxies NGC 6845A tions is extranuclear Hα-emitting complexes such as those and NGC 6845B and that it has a typical column density 20 −2 found in the ultraluminous infrared galaxies (ULIRGS) sam- of NHI ∼18.3×10 cm . The far-ultraviolet (FUV) Galaxy ple studied by Miralles-Caballero et al. (2012). These may Evolution Explorer (GALEX ) image of this group shows be precursors of the so called tidal dwarf galaxies. They may that the galaxies NGC 6845A and NGC 6845B have strong form independent new systems or fall back onto the parent UV emission and galaxies NGC 6845C and NGC 6845D have galaxies. very weak or null UV emission (see top panel in Fig. 1). In order to study these environmental effects on galaxy The optical and morphological properties of the galax- formation and evolution we have an observational program ies in the NGC 6845 quartet are summarized in Table 1. to obtain deep imaging and spectroscopy of a sample of Total magnitudes in the B-band and total (B-V) colors for galaxies with tidal tails. Two groups have already been stud- all members of the group were obtained from the Hyper- ied, NGC 92 and NGC 2782. In the current paper, we an- Leda data base (Makarov et al. 2014). Sizes, Hi masses and alyze the group NGC 6845, formed by two spirals and two inclinations for the main members of NGC 6845 were taken lenticular galaxies, NGC 6845A, B, C and D (also known as from literature (see Table 1). Klemola 30 by Klemola 1969, and ESO284-IG008), where one of the galaxies (NGC 6845A) has long tidal tails. Here we present deep Gemini imaging and spectroscopy of the 2.2 Data system revealing star-forming regions and yielding a map of the metal distribution across a projected distance to the cen- Images and spectra for NGC 6845 were taken with the Gem- ter of the galaxy of 140 kpc. This group has a mean optical ini MultiObject Spectrograph (GMOS Hook et al. 2004) at recession velocity of 6701 km s−1, which has been corrected the Gemini South telescope under the science program GS- by the Virgo, Great Attractor and Shapley infall (Mould 2011B-Q-36 (PI: S. Torres-Flores). −1 −1 We used these data to determine the oxygen abundances et al. 2000). Assuming H0=73 km s Mpc this velocity implies a distance to NGC 6845 of 91.8 Mpc. We note that of the objects located in the tidal tails of NGC 6845A and 1 arcsec ' 0.445 kpc at this distance. between NGC 6845A and NGC 6845B. The paper is organized as follows. In Section 2 we de- scribe the system and the data, imaging and spectroscopy, taken with the Gemini South telescope.
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