Spectral energy distribution fitting of the bulge and disk components of interacting galaxies




Palmer, Michael J. D.

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We perform spectral energy distribution fitting to the total integrated light, bulge and disk components of ≈ 650,000 spectroscopically selected galaxies from the sloan digital sky survey data release 7. Using 4-band photometry (u, g, r, i) we derive physical properties for these components with particular emphasis placed on the star formation rates (SFR) and stellar masses. Using the total integrated fits as an indicator of the goodness of fit, we show that reliable estimates of the SFR can be recovered using a specific SFR (sSFR) cut of log(sSFR /yr) ≥ −10.45. We construct a close pairs sample and match isolated controls based on stellar mass, z and local density for galaxies that pass the sSFR cut. We develop a method to cross correlate the pair galaxies’ star formation rate posterior probability distribution functions (SFR PDFs) with the control SFR PDFs as a function of the pair galaxies projected separation, rp. We show that the SFR of the close pair galaxies is enhanced relative to the control sample. The SFR enhancement is at a level of ≈ 0.25 dex above that of the control at the closest separations and declines to a plateau at ≈ 0.15 dex for separations of 30 < rp < 60 kpc/h. Between 60 < rp < 80 kpc/h there appears to be a slight increase in the enhancement to a level ≈ 0.25 dex above the control. It is suggested that we observe this increase, where other studies have failed to, based on the updated photometry provided by Simard et al. (2011). From our total pair sample we also select a subsample of galaxies that are classified as active galactic nuclei (AGN). We note that at close separations the pair AGN galaxies have enhanced SFRs relative to their matched controls. The SFR enhancement is largest at the smallest separations, reaching a level of ≈ 0.3 dex above the control. The SFR enhancement for the AGN pairs becomes consistent with their controls at projected separations of 20 < rp < 80 kpc/h. We construct a bulge and disk pair sample that are required to pass the sSFR cut and match control bulges and disks, respectively, that also pass the sSFR cut. We cross correlate the bulge and disk pairs with their respective controls. We measure significant SFR enhancement in the bulge component of the interacting pairs. The SFR enhancement is highest at small separations, ≈ 0.4 dex, and steadily declines to ≈ 0.1 dex before turning around beyond rp > 50 kpc/h to again reach a level ≈ 0.4 dex above the control bulges. The disk SFR enhancement is relatively flat beyond rp > 30 kpc/h to a level ≈ 0.1 dex above the control and largely consistent with the control at close separations. The bulge and disk results suggest that the majority of induced star formation during an interaction is occurring in the bulge component, but that there is still slight SFR enhancement in the disk. We suggest that the upturn in the total and bulge SFR enhancement could potentially be caused by a delay between the interaction of the galaxy pairs and the onset of induced star formation.



bulge and disk, galaxy evolution, galaxy interactions, mergers, star formation enhancement