GALAXY PAIRS IN THE SLOAN DIGITAL SKY SURVEY. I. STAR FORMATION, ACTIVE GALACTIC NUCLEUS FRACTION, AND THE LUMINOSITY/MASS–METALLICITY RELATION

By Sara L. Ellison, David R. Patton, L. Simard, A. McConnachie. 2008

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We present a sample of 1716 galaxies with companions within Δv <500 km s−1, rp< 80 h−170 kpc and stellar mass ratio 0.1 < M1/M2 < 10 from the Sloan Digital Sky Survey Data Release 4. The galaxy pairs are selected from the Main Galaxy Sample using stringent and well-understood criteria for redshift, spectral quality, available stellar masses, and metallicities. In agreement with previous studies, we find an enhancement in the star-formation rate (SFR) of galaxy pairs at projected separations <30–40 h−170 kpc. In addition, we find that this enhancement is highest (and extends to the greatest separations) for galaxies of approximately equal mass, the so-called major pairs. However, SFR enhancement can still be detected for a sample of galaxy pairs whose masses are within a factor of 10 of each other. Based on these results, we define a sample of close pairs (Δv <500 km s−1, rp < 30 h−170 kpc, and 0.1 < M1/M2 < 10) which we use to investigate interaction-induced effects in the luminosity–metallicity (LZ) relation. In agreement with the one previous study of the LZ relation in paired galaxies, we find an offset to lower metallicities (by ∼0.1 dex) for a given luminosity for galaxies in pairs compared to the control sample. We also present the first mass–metallicity (MZ) relation comparison between paired galaxies and the field and again find an offset to lower metallicities (by ∼0.05 dex) for a given mass. The smaller offset in the MZ relation indicates that both higher luminosities and lower metallicities may contribute to the shift of pairs relative to the control in the LZ relation. We show that the offset in the LZ relation depends on galaxy half-light radius, rh. Galaxies with rh≲ 3 h−170 kpc and with a close companion show a 0.05–0.1 dex downward offset in metallicity compared to control galaxies of the same size. Larger galaxies do not show this offset and have LZ and MZ relations consistent with the control sample. We investigate the physical impetus behind this empirical dependence on rh and consider the galaxy's dynamical time and bulge fractions as possible causes. We conclude that the former is unlikely to be a fundamental driver of the offset in the LZ relation for paired galaxies, but that bulge fraction may play a role. Finally, we study the active galactic nucleus (AGN) fraction in both the pair and control sample and find that whilst selecting galaxies in different cuts of color and asymmetry yields different AGN fractions, the fraction for pairs and the control sample are consistent for a given set of selection criteria. This indicates that if AGNs are ignited as a result of interactions, this activity begins later than the close pairs stage (i.e. once the merger is complete).

Subfields: Astronomy and Astrophysics, Ecology

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Citations in the corpus

The cosmic evolution of massive black holes in the Horizon-AGN simulation (2016, background): “Observations also find an enhancement of AGN activity in galaxies with companions (Ellison et al. 2008, 2011; Silverman et al. 2011; Koss et al. 2012; Lackner et al. 2014).”
Galaxy pairs in the Sloan Digital Sky Survey – VI. The orbital extent of enhanced star formation in interacting galaxies (2013, methodology): “Ellison et al. (2008) dealt with this effect by randomly culling 67.5 per cent of pairs with separations> 55 arcsec, using the results of the Patton & Atfield (2008) census of SDSS pairs.”
The impact of gas inflows on star formation rates and metallicities in barred galaxies (2011, background): “As shown by Ellison et al. (2008a), when galaxies exhibit enhanced star formation (as we believe the higher mass barred galaxies do) the luminosity–metallicity relation can be affected by changes in both metallicity and luminosity.”
Galaxy pairs in the Sloan Digital Sky Survey – IX. Merger-induced AGN activity as traced by the Wide-field Infrared Survey Explorer (2014, background): “Culling of wide separation pairs accounts for fibre collisions (Ellison et al. 2008; Patton & Atfield 2008).”
Galaxy Zoo: the properties of merging galaxies in the nearby Universe – local environments, colours, masses, star formation rates and AGN activity (2009, background): “The recent study by Ellison et al. (2008) also found little evidence for increased AGN activity in their close-pairs sample and concluded that, if AGN are induced by mergers, then they must occur at stages later than close-pairs typically examine.”
DIRECT OXYGEN ABUNDANCES FOR LOW-LUMINOSITY LVL GALAXIES (2012, background): “…scatter include variations in the star formation history (e.g., recent starbursts, Contini et al. 2002), variations in stellar surface mass density (Ellison et al. 2008), inflow of metal poor gas, perhaps triggered by interactions (Lee et al. 2004), and variations in local galaxy density (e.g.,…”
Galaxy Evolution in Cosmological Simulations with Outflows II: Metallicities and Gas Fractions (2011, background): “Moreover, departures from the mean relation are strongly correlated with other galaxy properties (Ellison et al. 2008a; Cooper et al. 2008; Peeples, Pogge, & Stanek 2009; Lara-López et al. 2010; Mannucci et al. 2010).”
Bulge mass is king: the dominant role of the bulge in determining the fraction of passive galaxies in the Sloan Digital Sky Survey (2014, background): “Closer (interacting) pairs are found to provoke enhanced star formation (Ellison et al. 2008,2010, Scudder et al. 2012, Patton et al. 2013), and the final coalescence of galaxies at the end of the merger sequence leads initially to the highest enhancements in star formation (e.g. Ellison et al.…”
METALLICITY GRADIENTS AND GAS FLOWS IN GALAXY PAIRS (2010, background): “Similarly, Ellison et al. (2008) and Michel-Dansac et al. (2008) showed that galaxy pairs in the Sloan Digital Sky Survey have lower central metallicities for their stellar mass than isolated galaxies.”
The Sydney-AAO Multi-object Integral field spectrograph (2011, background): “Direct galaxy-galaxy interactions are also expected to play a critical role, with major galaxy mergers triggering star formation (e.g. Ellison et al. 2008) and transforming the morphology of galaxies, although the fraction of galaxies undergoing major mergers (i.e. those with mass ratios of 3:1 or…”
Galaxy pairs in the Sloan Digital Sky Survey – II. The effect of environment on interactions (2010, background): “Majorgalaxygalaxy interactions (where the ratio of galaxy masses or luminosities is close to unity) trigger the strongest star formation(Woods, Geller & Barton 2006; Ellison et al. 2008).”
Galaxy pairs in the Sloan Digital Sky Survey – VIII. The observational properties of post-merger galaxies (2013, background): “…Alonso et al. 2006; Ellison et al. 2008, 2013; Li et al 2008a; Scudder et al. 2012b; Patton et al. 2013), metallicity (Michel-Dansac et al. 2008; Ellison et al. 2008; Reichard et al. 2009; Peeples et al. 2009; Alonso, Michel-Dansac & Lambas 2010; Scudder et al. 2012b; Chung et al. 2013),…”
GALAXY MERGERS AND THE MASS–METALLICITY RELATION: EVIDENCE FOR NUCLEAR METAL DILUTION AND FLATTENED GRADIENTS FROM NUMERICAL SIMULATIONS (2010, background): “The finding that motivated this study was the observation that interacting galaxies have suppressed nuclear metallicities compared to isolated galaxies of similar luminosity or mass (Kewley et al. 2006; Ellison et al. 2008; Rupke et al. 2008; Michel-Dansac et al. 2008; Peeples et al. 2009).”
GAS-PHASE OXYGEN GRADIENTS IN STRONGLY INTERACTING GALAXIES. I. EARLY-STAGE INTERACTIONS (2010, background): “…part of the scatter in the relation is due to a nuclear underabundance in interacting galaxies, at both low (Lee et al. 2004; Ekta & Chengalur 2010) and high masses (Kewley et al. 2006a; Rupke et al. 2008; Ellison et al. 2008; Michel-Dansac et al. 2008; Peeples et al. 2009; Sol Alonso et al. 2010).”
Galaxy pairs in the Sloan Digital Sky Survey – V. Tracing changes in star formation rate and metallicity out to separations of 80 kpc (2012, methodology): “The SFRs and metallicities of our pairs are significantly offset from the control over significantly larger distance scales than has been previously seen (e.g., Lambas et al. 2003; Alonso et al. 2006; Woods & Geller 2007; Ellison et al. 2008b).”
THE METALLICITY EVOLUTION OF INTERACTING GALAXIES (2011, methodology): “The stellar masses for the close pair systems included in the Ellison et al. (2008) sample have had their stellar masses determined via optical and IR colors (Kauffmann et al. 2003; Tremonti et al. 2004) which have been shown to compare well with the spectrallydetermined stellar masses (Drory et…”
De re metallica: the cosmic chemical evolution of galaxies (2018, background): “Soon after, Hoopes et al. (2007) pointed out a dependence of the shape of the MZR on galaxy size, and Ellison et al. (2008b) was the first to explicit state the dependence of metallicity on SFR for a given mass. Mannucci et al. (2010) introduced a 3D relation between mass, metallicity and SFR,…”
The structures of distant galaxies – III. The merger history of over 20 000 massive galaxies at z < 1.2 (2008, background): “Observational attempts to determine the role of galaxy mergers in the local universe have been limited to studies of how star formation and black hole growth are induced by the merger process (e.g., Barton et al. 2000; Ellison et al. 2008).”
Galaxy Zoo: the fraction of merging galaxies in the SDSS and their morphologies (2009, background): “…et al. 2006; Conselice, Rajgor & Myers 2008; Lin et al. 2008; Lotz et al. 2008a; Hsieh. et al. 2008, Patton et al. 2008), with comparatively little carried out to examine their morphologies and internal properties (though see Li et al. 2008; Ellison et al. 2008) for reasons to be discussed (§1.1).”