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We present spatially resolved integral field spectroscopic K-band data at a resolution of 0.13 (60pc) and interferometric CO(2-1) line observations of the prototypical merging system NGC6240. Despite the clear rotational signature, the stellar kinematics in the two nuclei are dominated by dispersion. We use Jeans modelling to derive the masses and the mass-to-light ratios of the nuclei. Combining the luminosities with the spatially resolved Br-gamma equivalent width shows that only 1/3 of the K-band continuum from the nuclei is associated with the most recent star forming episode; and that less than 30% of the systems bolometric luminosity and only 9% of its stellar mass is due to this starburst. The star formation properties, calculated from typical merger star formation histories, demonstrate the impact of different assumptions about the star formation history. The properties of the nuclei, and the existence of a prominent old stellar population, indicate that the nuclei are remnants of the progenitor galaxies bulges.
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Spatially resolved kinematics have been used to determine the dynamical status of star-forming galaxies with ambiguous morphologies, and constrain the importance of galaxy interactions during the assembly of galaxies. However, measuring the importance of interactions or galaxy merger rates requires knowledge of the systematics in kinematic diagnostics and the visible time with merger indicators. We analyze the dynamics of star-forming gas in a set of binary merger hydrodynamic simulations with stellar mass ratios of 1:1 and 1:4. We find that the evolution of kinematic asymmetries traced by star-forming gas mirrors morphological asymmetries derived from mock optical images, in which both merger indicators show the largest deviation from isolated disks during strong interaction phases. Based on a series of simulations with various initial disk orientations, orbital parameters, gas fractions, and mass ratios, we find that the merger signatures are visible for ~0.2-0.4 Gyr with kinematic merger indicators but can be approximately twice as long for equal-mass mergers of massive gas-rich disk galaxies designed to be analogs of z~2-3 submillimeter galaxies. Merger signatures are most apparent after the second passage and before the black holes coalescence, but in some cases they persist up to several hundred Myr after coalescence. About 20-60% of the simulated galaxies are not identified as mergers during the strong interaction phase, implying that galaxies undergoing violent merging process do not necessarily exhibit highly asymmetric kinematics in their star-forming gas. The lack of identifiable merger signatures in this population can lead to an underestimation of merger abundances in star-forming galaxies, and including them in samples of star-forming disks may bias the measurements of disk properties such as intrinsic velocity dispersion.
(abridged) Here we present HI line and 20-cm radio continuum data of the nearby galaxy pair NGC1512/1510 as obtained with the Australia Telescope Compact Array. These are complemented by GALEX UV-, SINGG Halpha- and Spitzer mid-infrared images, allowing us to compare the distribution and kinematics of the neutral atomic gas with the locations and ages of the stellar clusters within the system. For the barred, double-ring galaxy NGC1512 we find a very large HI disk, about 4x its optical diameter, with two pronounced spiral/tidal arms. Both its gas distribution and the distribution of the star-forming regions are affected by gravitational interaction with the neighbouring blue compact dwarf galaxy NGC1510. The two most distant HI clumps, at radii of about 80 kpc, show signs of star formation and are likely tidal dwarf galaxies. Star formation in the outer disk of NGC1512 is revealed by deep optical- and two-color ultraviolet images. Using the latter we determine the properties of about 200 stellar clusters and explore their correlation with dense HI clumps in the even larger 2XHI disk. The multi-wavelength analysis of the NGC1512/1510 system, which is probably in the first stages of a minor merger having started about 400 Myr ago, links stellar and gaseous galaxy properties on scales from one to 100 kpc.
We present a study of the HI gas content of a large K-band selected sample of 88 close major-merger pairs of galaxies (H-KPAIR) which were observed by $it Herschel$. We obtained the 21 cm HI fine-structure emission line data for a total of 70 pairs from this sample, by observing 58 pairs using the Green Bank Telescope (GBT) and retrieving the HI data for an addition 12 pairs from the literature. In this HI sample, 34 pairs are spiral-spiral (S+S) pairs, and 36 are spiral-elliptical (S+E). Based on these data, we studied the HI-to-stellar mass ratio, the HI gas fraction and the HI star formation efficiency (SFE$_{mathrm{HI}}$ = star formation rate/$M_{mathrm{HI}}$) and searched for differences between S+S and S+E pairs, as well as between pairs with and without signs for merger/interaction. Our results showed that the mean HI-to-stellar mass ratio of spirals in these pairs is $=7.6pm1.0 %$, consistent with the average HI gas fraction of spiral galaxies in general. The differences in the HI gas fraction between spirals in S+S and in S+E pairs, and between spirals in pairs with and without signs of merger/interaction are insignificant ($< 1 sigma$). On the other hand, the mean SFE$_{mathrm{HI}}$ of S+S pairs is $sim4.6times$ higher than that of S+E pairs. This difference is very significant ($sim 4sigma$) and is the main result of our study. There is no significant difference in the mean SFE$_{mathrm{HI}}$ between galaxies with and without signs of merger/interaction. The mean SFE$_{mathrm{HI}}$ of the whole pair sample is $10^{-9.55pm 0.09} mathrm{yr}^{-1}$, corresponding to a HI consumption time of $3.5pm0.7$ Gyrs.
We introduce a novel technique for empirically understanding galaxy evolution. We use empirically determined stellar evolution models to predict the past evolution of the Sloan Digital Sky Survey (SDSS-II) Luminous Red Galaxy (LRG) sample without any a-priori assumption about galaxy evolution. By carefully contrasting the evolution of the predicted and observed number and luminosity densities we test the passive evolution scenario for galaxies of different luminosity, and determine minimum merger rates. We find that the LRG population is not purely coeval, with some of galaxies targeted at z<0.23 and at z>0.34 showing different dynamical growth than galaxies targeted throughout the sample. Our results show that the LRG population is dynamically growing, and that this growth must be dominated by the faint end. For the most luminous galaxies, we find lower minimum merger rates than required by previous studies that assume passive stellar evolution, suggesting that some of the dynamical evolution measured previously was actually due to galaxies with non-passive stellar evolution being incorrectly modelled. Our methodology can be used to identify and match coeval populations of galaxies across cosmic times, over one or more surveys.
We present CO observations of 78 spiral galaxies in local merger pairs. These galaxies representa subsample of a Ks-band selected sample consisting of 88 close major-merger pairs (HKPAIRs), 44 spiral-spiral (S+S) pairs and 44 spiral-elliptical (S+E) pairs, with separation $<20 h^{-1}$ kpc and mass ratio <2.5. For all objects, the star formation rate (SFR) and dust mass were derived from HERSCHEL PACS and SPIRE data, and the atomic gas mass, MHI, from the Green Bank Telescope HI observations. The complete data set allows us to study the relation between the gas (atomic and molecular) mass, dust mass and SFR in merger galaxies. We derive the molecular gas fraction (MH2/M*), molecular-to-atomic gas mass ratio (MH2/MHI), gas-to-dust mass ratio and SFE (=SFR/MH2) and study their dependences on pair type (S+S compared to S+E), stellar mass and the presence of morphological interaction signs. We find an overall moderate enhancements (~2x) in both molecular gas fraction (MH2/M*), and molecular-to-atomic gas ratio (MH2/MHI) for star-forming galaxies in major-merger pairs compared to non-interacting comparison samples, whereas no enhancement was found for the SFE nor for the total gas mass fraction (MHI+MH2)/M*. When divided into S+S and S+E, low mass and high mass, and with and without interaction signs, there is a small difference in SFE, moderate difference in MH2/M*, and strong differences in MH2/MHI between subsamples. For MH2/MHI, the difference between S+S and S+E subsamples is 0.69+-0.16 dex and between pairs with and without interaction signs is 0.53+-0.18 dex. Together, our results suggest (1) star formation enhancement in close major-merger pairs occurs mainly in S+S pairs after the first close encounter (indicated by interaction signs) because the HI gas is compressed into star-forming molecular gas by the tidal torque; (2) this effect is much weakened in the S+E pairs.
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