Galaxy pairs in the Sloan Digital Sky Survey – V. Tracing changes in star formation rate and metallicity out to separations of 80 kpc

By J. Scudder, Sara L. Ellison, P. Torrey, David R. Patton, J. Trevor Mendel. 2012

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We present a sample of 1899 galaxies with a close companion taken from the Sloan Digital Sky Survey Data Release 7. The galaxy pairs are selected to have velocity differences Δv < 300 km s−1, projected separations (rp) < 80 kpc, mass ratios between 0.1 and 10, and robust measurements of star formation rates and gas-phase metallicities. We match the galaxies in total stellar mass, redshift and local density to a set of 10 control galaxies per pair galaxy. For each pair galaxy, we can therefore calculate the statistical change in star formation rate (SFR) and metallicity associated with the interaction process. Relative to the control sample, we find that galaxies in pairs show typical SFR enhancements that are, on average, 60 per cent higher than the control sample at rp < 30 kpc. It is at these small separations that the strongest enhancements in SFR (by up to a factor of ∼10) are measured, although such starbursts are rare, even amongst the closest pairs. In addition, the pairs demonstrate more modest SFR enhancements of ∼30 per cent out to at least 80 kpc (the widest separations in our sample). This is the first time that enhanced SFRs have been robustly detected out to such large projected separations. Galaxies in both major and minor mergers show significant SFR enhancements at all rp, although the strongest starbursts (with SFR enhancements of a factor of ∼10) appear to be found only in the major mergers. We also find evidence that SFR enhancements are synchronized in an interacting pair, such that a higher SFR in one galaxy is accompanied by an increased SFR in its companion. For the first time, we are also able to trace the metallicity changes in galaxy pairs as a function of projected separation. The metallicity is generally diluted in galaxy pairs by ∼0.02 dex, with an average metallicity decrement of −0.03 dex at the smallest separations, a trend that mirrors the SFR enhancements as a function of rp. The SFR and metallicity trends with projected separation are interpreted through a comparison with theoretical models. These simulations indicate that the peak in SFR enhancements at small separations is due to systems near the end of the merger process. The extended plateau in SFR enhancements out to at least 80 kpc is dominated by galaxies that have made a pericentric passage and are now experiencing triggered star formation on their trajectory towards apogalacticon, or on a subsequent close approach.

Subfields: Astronomy and Astrophysics, Instrumentation, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/b58f8eedf4958c983f3f02463679634189179b75

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Citations in the corpus

Galaxy pairs in the Sloan Digital Sky Survey – VIII. The observational properties of post-merger galaxies (2013, methodology): “11 of Scudder et al. 2012b).”
Galaxy pairs in the Sloan Digital Sky Survey – VI. The orbital extent of enhanced star formation in interacting galaxies (2013, methodology): “These enhancements are accompanied by bluer central colours (Patton et al. 2011), diluted metallicities (Scudder et al. 2012), and an increased incidence of active galactic nuclei(AGNs; Ellison et al. 2011) and luminous infrared galaxies (Ellison et al. 2013).”
The star formation main sequence and stellar mass assembly of galaxies in the Illustris simulation (2014, background): “Furthermore, the resulting SFR elevations are sufficient to match the observed interaction induced SFRs of close-pair galaxies in SDSS (Scudder et al. 2012; Patton et al. 2013).”
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.…”
Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution, and implications for the IMF (2012, result): “Only for nuclear separation 10 kpc9 do the discs have SFRs that are significantly elevated by the mutual tidal interactions (H12); this result is consistent with observed SFR elevations in mergers (e.g. Scudder et al. 2012).”
Galaxy pairs in the Sloan Digital Sky Survey – IX. Merger-induced AGN activity as traced by the Wide-field Infrared Survey Explorer (2014, background): “…Woods & Geller 2007; Di Matteo et al. 2007, 2008; Ellison et al. 2008; Cox et al. 2008; Smith & Struck 2010; Patton et al. 2011; Liu et al. 2012; Scudder et al. 2012; Patton et al. 2013), and the formation of spheroids (e.g., Toomre 1977; Lake & Dressler 1986; Shier & Fischer 1998; Rothberg &…”
REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, unknown): “One attempt is the recent determination of the distribution of SFR enhancements in SDSS galaxy pairs by Scudder et al. (2012) which we plot in blue in Figure 5.”
THEORETICAL EVOLUTION OF OPTICAL STRONG LINES ACROSS COSMIC TIME (2013, background): “For example, interacting or merging galaxies typically have lower central metallicities due to large-scale gas infall (Kewley et al. 2006a, 2010; Rupke et al. 2010b; Ellison et al. 2010; Rich et al. 2012; Scudder et al. 2012).”