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Register a new userMerging Galaxies in the SDSS EDR
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S. S. Allam
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We present a new catalog of merging galaxies obtained through an automated systematic search routine. The 1479 new pairs of merging galaxies were found in approximately 462 sq deg of the Sloan Digital Sky Survey Early Data Release (SDSS EDR; Stoughton et al. 2002) photometric data, and the pair catalog is complete for galaxies in the magnitude range 16.0 <= g* <= 20. The selection algorithm, implementing a variation on the original Karachentsev (1972) criteria, proved to be very efficient and fast. Merging galaxies were selected such that the inter-galaxy separations were less than the sum of the component galaxies radii. We discuss the characteristics of the sample in terms of completeness, pair separation, and the Holmberg effect. We also present an online atlas of images for the SDSS EDR pairs obtained using the corrected frames from the SDSS EDR database. The atlas images also include the relevant data for each pair member. This catalog will be useful for conducting studies of the general characteristics of merging galaxies, their environments, and their component galaxies. The redshifts for a subset of the interacting and merging galaxies and the distribution of angular sizes for these systems indicate the SDSS provides a much deeper sample than almost any other wide-area catalog to date.
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We investigate the incidence of major mergers creating >10e11 Msun galaxies in present-day groups and clusters more massive than 2.5e13 Msun. We identify 38 pairs of massive galaxies with mutual tidal interaction signatures selected from >5000 galaxies with >5e10 Msun that reside in 845 such groups. We fit the images of each galaxy pair as the line-of-sight projection of symmetric models and identify mergers by the presence of residual asymmetries around each progenitor, such as off-center isophotes, broad tidal tails, and dynamical friction wakes. At the resolution and sensitivity of the SDSS, such mergers are found in 16% of high-mass, galaxy-galaxy pairs with magnitude differences of <1.5 and <30 kpc projected separations. We find that 90% of these mergers have nearly equal-mass progenitors with red-sequence colors and centrally-concentrated morphologies, the hallmarks of dissipationless merger simulations. Mergers at group centers are more common than between 2 satellites, but both are morphologically indistinguishable and we tentatively conclude that the latter are likely located at the dynamical centers of recently accreted subhalos. The frequency of central and satellite merging diminishes with group mass consistent with dynamical friction expectations. Based on reasonable assumptions, the centers of these massive halos are growing in stellar mass by 1-9% per Gyr, on average. Compared to all LRG-LRG mergers, we find a 2-9 times higher rate for their merging when restricted to these dense environments. Our results imply that the massive end of the galaxy population continues to evolve hierarchically at a measurable level, and that the centers of massive groups are the preferred environment for merger-driven galaxy assembly. (abridged)
The Millennium Galaxy Catalogue (MGC) is a deep ($mu_{rm B,lim}=26$ mag arcsec$^{-2}$), wide field CCD imaging survey, covering 37.5deg$^2$ and is completely contained within the 2dFGRS and SDSS-EDR. We compare the photometry and completeness of the 2dFGRS and the SDSS-EDR with the MGC. We have also undertaken a photometric comparison to SCOS and SDSS-DR1 data. We find that $B_{MGC}-B_{2dF}=0.035$ mag with an uncertainty of 0.142 mag per galaxy, $B_{MGC}-B_{SCOS}=0.032$ mag with an uncertainty of 0.108 mag, $B_{MGC}-B_{SDSS-EDR}=0.032$ mag with an uncertainty of 0.094 mag, and $B_{MGC}-B_{SDSS-DR1}=0.039$ mag with an uncertainty of 0.086 mag. We find that high surface brightness 2dFGRS galaxies are systematically too faint. In the SDSS there is a weak non-linear scale error, which is negligible for faint galaxies. LSBGs in the SDSS are systematically fainter. We find that the 2dFGRS catalogue has 5.2% stellar contamination, 7.0% of objects are resolved into 2 or more by the MGC and is 8.7% incomplete compared to the MGC. From our all object spectroscopic survey we find that the MGC is itself misclassifying 5.6% of galaxies as stars, hence the 2dFGRS misses 14.3% of the galaxies. The SDSS-EDR galaxy catalogue has 1.3% stellar contamination and 5.3% of galaxies misclassified as stars, and is 1.8% incomplete compared to the MGC. Altogether 7.1% of the total galaxy population are missing from the SDSS-EDR catalogue from incompleteness or misclassification.
Galaxy mergers are key events in galaxy evolution, often causing massive starbursts and fueling active galactic nuclei (AGN). In these highly dynamic systems, it is not yet precisely known how much starbursts and AGN respectively contribute to the total luminosity, at what interaction stages they occur, and how long they persist. Here we estimate the fraction of the bolometric infrared (IR) luminosity that can be attributed to AGN by measuring and modeling the full ultraviolet to far-infrared spectral energy distributions (SEDs) in up to 33 broad bands for 24 merging galaxies with the Code for Investigating Galaxy Emission. In addition to a sample of 12 confirmed AGN in late-stage mergers, found in the $Infrared$ $Array$ $Satellite$ Revised Bright Galaxy Sample or Faint Source Catalog, our sample includes a comparison sample of 12 galaxy mergers from the $Spitzer$ Interacting Galaxies Survey, mostly early-stage. We perform identical SED modeling of simulated mergers to validate our methods, and we supplement the SED data with mid-IR spectra of diagnostic lines obtained with $Spitzer$ InfraRed Spectrograph. The estimated AGN contributions to the IR luminosities vary from system to system from 0% up to 91% but are significantly greater in the later-stage, more luminous mergers, consistent with what is known about galaxy evolution and AGN triggering.
We present the first luminous, spatially resolved binary quasar that clearly inhabits an ongoing galaxy merger. SDSS J125455.09+084653.9 and SDSS J125454.87+084652.1 (SDSS J1254+0846 hereafter) are two luminous z=0.44 radio quiet quasars, with a radial velocity difference of just 215 km/s, separated on the sky by 21 kpc in a disturbed host galaxy merger showing obvious tidal tails. The pair was targeted as part of a complete sample of binary quasar candidates with small transverse separations drawn from SDSS DR6 photometry. We present follow-up optical imaging which shows broad, symmetrical tidal arm features spanning some 75 kpc at the quasars redshift. Numerical modeling suggests that the system consists of two massive disk galaxies prograde to their mutual orbit, caught during the first passage of an active merger. This demonstrates rapid black hole growth during the early stages of a merger between galaxies with pre-existing bulges. Neither of the two luminous nuclei show significant instrinsic absorption by gas or dust in our optical or X-ray observations, illustrating that not all merging quasars will be in an obscured, ultraluminous phase. We find that the Eddington ratio for the fainter component B is rather normal, while for the A component L/LEdd is quite (>3sigma) high compared to quasars of similar luminosity and redshift, possibly evidence for strong merger-triggered accretion. More such mergers should be identifiable at higher redshifts using binary quasars as tracers.
Gravitational-wave (GW) recoil of merging supermassive black holes (SMBHs) may influence the co-evolution of SMBHs and their host galaxies. We examine this possibility using SPH/N-body simulations of gaseous galaxy mergers in which the merged BH receives a recoil kick. With our suite of over 200 merger simulations, we identify systematic trends in the behavior of recoiling BHs. Our main results are as follows. (1) While BHs kicked at nearly the central escape speed (vesc) are essentially lost to the galaxy, in gas rich mergers, BHs kicked with up to about 0.7 vesc may be confined to the central few kpc of the galaxy. (2) The inflow of cold gas during a gas-rich major merger may cause a rapid increase in central escape speed; in such cases recoil trajectories will depend on the timing of the BH merger relative to the change in vesc. (3) Recoil events generally reduce the lifetimes of bright active galactic nuclei (AGN) but may actually extend AGN lifetimes at lower luminosities. (4) Recoiling AGN may be observable via kinematic offsets (v > 500 km s^-1) or spatial offsets (R > 1 kpc) for lifetimes of up to about 10 - 100 Myr. (5) Rapidly-recoiling BHs may be up to about 5 times less massive than their stationary counterparts. These mass deficits lower the normalization of the M - sigma relation and contribute to both intrinsic and overall scatter. (6) Finally, the displacement of AGN feedback by a recoil event causes higher central star formation rates in the merger remnant, thereby extending the starburst phase of the merger and creating a denser, more massive stellar cusp.
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