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Register a new userGalaxy pairs in the SDSS - XIII. The connection between enhanced star formation and molecular gas properties in galaxy mergers
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We investigate the connection between star formation and molecular gas properties in galaxy mergers at low redshift (z$leq$0.06). The study we present is based on IRAM 30-m CO(1-0) observations of 11 galaxies with a close companion selected from the Sloan Digital Sky Survey (SDSS). The pairs have mass ratios $leq$4, projected separations r$_{mathrm{p}} leq$30 kpc and velocity separations $Delta$V$leq$300 km s$^{-1}$, and have been selected to exhibit enhanced specific star formation rates (sSFR). We calculate molecular gas (H$_{2}$) masses, assigning to each galaxy a physically motivated conversion factor $alpha_{mathrm{CO}}$, and we derive molecular gas fractions and depletion times. We compare these quantities with those of isolated galaxies from the extended CO Legacy Data base for the GALEX Arecibo SDSS Survey sample (xCOLDGASS, Saintonge et al. 2017) with gas quantities computed in an identical way. Ours is the first study which directly compares the gas properties of galaxy pairs and those of a control sample of normal galaxies with rigorous control procedures and for which SFR and H$_{2}$ masses have been estimated using the same method. We find that the galaxy pairs have shorter depletion times and an average molecular gas fraction enhancement of 0.4 dex compared to the mass matched control sample drawn from xCOLDGASS. However, the gas masses (and fractions) in galaxy pairs and their depletion times are consistent with those of non-mergers whose SFRs are similarly elevated. We conclude that both external interactions and internal processes may lead to molecular gas enhancement and decreased depletion times.
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We compare the radial profiles of the specific star formation rate (sSFR) in a sample of 169 star-forming galaxies in close pairs with those of mass-matched control galaxies in the SDSS-IV MaNGA survey. We find that the sSFR is centrally enhanced (within one effective radius) in interacting galaxies by ~0.3 dex and that there is a weak sSFR suppression in the outskirts of the galaxies of ~0.1 dex. We stack the differences profiles for galaxies in five stellar mass bins between log(M/Mstar) = 9.0-11.5 and find that the sSFR enhancement has no dependence on the stellar mass. The same result is obtained when the comparison galaxies are matched to each paired galaxy in both stellar mass and redshift. In addition, we find that that the sSFR enhancement is elevated in pairs with nearly equal masses and closer projected separations, in agreement with previous work based on single-fiber spectroscopy. We also find that the sSFR offsets in the outskirts of the paired galaxies are dependent on whether the galaxy is the more massive or less massive companion in the pair. The more massive companion experiences zero to a positive sSFR enhancement while the less massive companion experiences sSFR suppression in their outskirts. Our results illustrate the complex tidal effects on star formation in closely paired galaxies.
We present ~1 resolution (~2 kpc in the source plane) observations of the CO(1-0), CO(3-2), Halpha, and [N II] lines in the strongly-lensed z=2.26 star-forming galaxy SDSS J0901+1814. We use these observations to constrain the lensing potential of a foreground group of galaxies, and our source-plane reconstructions indicate that SDSS J0901+1814 is a nearly face-on (i~30 degrees) massive disk with r_{1/2}>~4 kpc for its molecular gas. Using our new magnification factors (mu_tot~30), we find that SDSS J0901+1814 has a star formation rate (SFR) of 268^{+63}_{-61} M_sun/yr, M_gas=(1.6^{+0.3}_{-0.2})x10^11x(alpha_CO/4.6) M_sun, and M_star=(9.5^{+3.8}_{-2.8})x10^10 M_sun, which places it on the star-forming galaxy main sequence. We use our matched high-angular resolution gas and SFR tracers (CO and Halpha, respectively) to perform a spatially resolved (pixel-by-pixel) analysis of SDSS J0901+1814 in terms of the Schmidt-Kennicutt relation. After correcting for the large fraction of obscured star formation (SFR_Halpha/SFR_TIR=0.054^{+0.015}_{-0.014}), we find SDSS J0901+1814 is offset from normal star-forming galaxies to higher star formation efficiencies independent of assumptions for the CO-to-H_2 conversion factor. Our mean best-fit index for the Schmidt-Kennicutt relation for SDSS J0901+1814, evaluated with different CO lines and smoothing levels, is n=1.54+/-0.13; however, the index may be affected by gravitational lensing, and we find n=1.24+/-0.02 when analyzing the source-plane reconstructions. While the Schmidt-Kennicutt index largely appears unaffected by which of the two CO transitions we use to trace the molecular gas, the source-plane reconstructions and dynamical modeling suggest that the CO(1-0) emission is more spatially extended than the CO(3-2) emission.
A key task of observational extragalactic astronomy is to determine where -- within galaxies of diverse masses and morphologies -- stellar mass growth occurs, how it depends on galaxy properties and what processes regulate star formation. Using spectroscopic indices derived from the stellar continuum at $sim 4000$AA, we determine the spatially resolved star-formation histories of 980000 spaxels in 2404 galaxies in the SDSS-IV MaNGA IFU survey. We examine the spatial distribution of star-forming, quiescent, green valley, starburst and post-starburst spaxels as a function of stellar mass and morphology to see where and in what types of galaxy star formation is occurring. The spatial distribution of star-formation is dependent primarily on stellar mass, with a noticeable change in the distribution at mstar$>10^{10}$msun. Galaxies above this mass have an increasing fraction of regions that are forming stars with increasing radius, whereas lower mass galaxies have a constant fraction of star forming regions with radius. Our findings support a picture of inside-out growth and quenching at high masses. We find that morphology (measured via concentration) correlates with the fraction of star-forming spaxels, but not with their radial distribution. We find (post-)starburst regions are more common outside of the galaxy centre, are preferentially found in asymmetric galaxies, and have lower gas-phase metallicity than other regions, consistent with interactions triggering starbursts and driving low metallicity gas into regions at $<1.5R_e$.
We present a detailed study of the Circinus Galaxy, investigating its star formation, dust and gas properties both in the inner and outer disk. To achieve this, we obtained high-resolution Spitzer mid-infrared images with the IRAC (3.6, 5.8, 4.5, 8.0 micron) and MIPS (24 and 70 micron) instruments and sensitive HI data from the Australia Telescope Compact Array (ATCA) and the 64-m Parkes telescope. These were supplemented by CO maps from the Swedish-ESO Submillimetre Telescope (SEST). Because Circinus is hidden behind the Galactic Plane, we demonstrate the careful removal of foreground stars as well as large- and small-scale Galactic emission from the Spitzer images. We derive a visual extinction of Av = 2.1 mag from the Spectral Energy Distribution of the Circinus Galaxy and total stellar and gas masses of 9.5 x 10^{10} Msun and 9 x 10^9 Msun, respectively. Using various wavelength calibrations, we find obscured global star formation rates between 3 and 8 Msun yr^{-1}. Star forming regions in the inner spiral arms of Circinus, which are rich in HI, are beautifully unveiled in the Spitzer 8 micron image. The latter is dominated by polycyclic aromatic hydrocarbon (PAH) emission from heated interstellar dust. We find a good correlation between the 8 micron emission in the arms and regions of dense HI gas. The (PAH 8 micron) / 24 micron surface brightness ratio shows significant variations across the disk of Circinus.
Currently-proposed galaxy quenching mechanisms predict very different behaviours during major halo mergers, ranging from significant quenching enhancement (e.g., clump-induced gravitational heating models) to significant star formation enhancement (e.g., gas starvation models). To test real galaxies behaviour, we present an observational galaxy pair method for selecting galaxies whose host haloes are preferentially undergoing major mergers. Applying the method to central L* (10^10 Msun < M_* < 10^10.5 Msun) galaxies in the Sloan Digital Sky Survey (SDSS) at z<0.06, we find that major halo mergers can at most modestly reduce the star-forming fraction, from 59% to 47%. Consistent with past research, however, mergers accompany enhanced specific star formation rates for star-forming L* centrals: ~10% when a paired galaxy is within 200 kpc (approximately the host halos virial radius), climbing to ~70% when a paired galaxy is within 30 kpc. No evidence is seen for even extremely close pairs (<30 kpc separation) rejuvenating star formation in quenched galaxies. For galaxy formation models, our results suggest: (1) quenching in L* galaxies likely begins due to decoupling of the galaxy from existing hot and cold gas reservoirs, rather than a lack of available gas or gravitational heating from infalling clumps, (2) state-of-the-art semi-analytic models currently over-predict the effect of major halo mergers on quenching, and (3) major halo mergers can trigger enhanced star formation in non-quenched central galaxies.
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