[en] We select a close 'major-merger candidate' galaxy pair sample in order to calculate the K_s luminosity function (LF) and pair fraction representative of the merger/interaction component of galaxy evolution in the local universe. The pair sample (projected separation 5 h ^-1 kpc ≤ r≤ 20 h ^-1 kpc, K_s -band magnitude difference ΔK_s ≤ 1 mag) is selected by combining the Two Micron All Sky Survey (2MASS) with the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). The resulting data set contains 340 galaxies covering 5800 deg². A stellar mass function is also translated from the LF. A differential pair fraction displays nearly constant fraction of galaxy pairs as a function of galaxy mass from 10⁹ to 10^11.5 M _sun. The differential pair fraction is less subject to absolute magnitude bias due to survey limitations than the standard total pair fraction. These results suggest that major-merger candidate pairs in the 0< z<0.1 universe are developed from ∼1.6% of the galaxy population without dependence on galaxy mass for pair components below 10¹¹ M _sun. The derived LF combined with merger model timescales give local merger rates per unit volume which decrease with masses greater than 10¹¹ M _sun.

[en] Previous studies have shown significant differences in the enhancement of the star formation rate (SFR) and star formation efficiency (SFE = SFR/M _mol) between spiral–spiral and spiral–elliptical mergers. In order to shed light on the physical mechanism of these differences, we present NOEMA observations of the molecular gas distribution and kinematics (linear resolutions of ∼2 kpc) in two representative close major-merger star-forming pairs: the spiral–elliptical pair Arp 142 and the spiral–spiral pair Arp 238. The CO in Arp 142 is widely distributed over a highly distorted disk without any nuclear concentration, and an off-center ringlike structure is discovered in channel maps. The SFE varies significantly within Arp 142, with a starburst region (region 1) near the eastern tip of the distorted disk showing an SFE ∼ 0.3 dex above the mean of the control sample of isolated galaxies and the SFE of the main disk (region 4) 0.43 dex lower than the mean of the control sample. In contrast, the CO emission in Arp 238 is detected only in two compact sources at the galactic centers. Compared to the control sample, Arp 238-E shows an SFE enhancement of more than 1 dex, whereas Arp 238-W has an enhancement of ∼0.7 dex. We suggest that the extended CO distribution and large SFE variation in Arp 142 are due to an expanding large-scale ring triggered by a recent high-speed head-on collision between the spiral galaxy and the elliptical galaxy, and the compact CO sources with high SFEs in Arp 238 are associated with nuclear starbursts induced by gravitational tidal torques in a low-speed coplanar interaction.

[en] The Stephan's Quintet (hereafter SQ) is a template source to study the impact of galaxies interaction on the physical state and energetics of their gas. We report on IRAM single-dish CO observations of the SQ compact group of galaxies. These observations follow up the Spitzer discovery of bright mid-IR H₂ rotational line emission (L(H₂) ≈ 10³⁵ W) from warm (10^2–3 K) molecular gas, associated with a 30 kpc long shock between a galaxy, NGC 7318b, and NGC 7319's tidal arm. We detect CO(1-0), (2-1) and (3-2) line emission in the inter-galactic medium (IGM) with complex profiles, spanning a velocity range of ≈1000 km s^–1. The spectra exhibit the pre-shock recession velocities of the two colliding gas systems (5700 and 6700 km s^–1), but also intermediate velocities. This shows that much of the molecular gas has formed out of diffuse gas accelerated by the galaxy-tidal arm collision. CO emission is also detected in a bridge feature that connects the shock to the Seyfert member of the group, NGC 7319, and in the northern star forming region, SQ-A, where a new velocity component is identified at 6900 km s^–1, in addition to the two velocity components already known. Assuming a Galactic CO(1-0) emission to H₂ mass conversion factor, a total H₂ mass of ≈5 × 10⁹ M_☉ is detected in the shock. The ratio between the warm H₂ mass derived from Spitzer spectroscopy, and the H₂ mass derived from CO fluxes is ≈0.3 in the IGM of SQ, which is 10--100 times higher than in star-forming galaxies. The molecular gas carries a large fraction of the gas kinetic energy involved in the collision, meaning that this energy has not been thermalized yet. The kinetic energy of the H₂ gas derived from CO observations is comparable to that of the warm H₂ gas from Spitzer spectroscopy, and a factor ≈5 greater than the thermal energy of the hot plasma heated by the collision. In the shock and bridge regions, the ratio of the PAH-to-CO surface luminosities, commonly used to measure the star formation efficiency of the H₂ gas, is lower (up to a factor 75) than the observed values in star-forming galaxies. We suggest that turbulence fed by the galaxy-tidal arm collision maintains a high heating rate within the H₂ gas. This interpretation implies that the velocity dispersion on the scale of giant molecular clouds in SQ is one order of magnitude larger than the Galactic value. The high amplitude of turbulence may explain why this gas is not forming stars efficiently.

[en] In this paper we address the question of whether star formation (SF) is driven by local processes or the large-scale environment. To do so, we investigate SF in collisional debris where the gravitational potential well and velocity gradients are shallower and compare our results with previous work on SF in noninteracting spiral and dwarf galaxies. We have performed multiwavelength spectroscopic and imaging observations (from the far-ultraviolet to the mid-infrared) of six interacting systems, identifying a total of 60 star-forming regions in their collision debris. Our analysis indicates that in these regions (1) the emission of the dust is at the expected level for their luminosity and metallicity, (2) the usual tracers of SFR display the typical trend and scatter found in classical star-forming regions, and (3) the extinction and metallicity are not the main parameters governing the scatter in the properties of intergalactic star-forming regions; age effects and variations in the number of stellar populations seem to play an important role. Our work suggests that local properties such as column density and dust content, rather than the large-scale environment seem to drive SF. This means that intergalactic star-forming regions can be used as a reliable tool to study SF.

[en] We present results from the mid-infrared spectral mapping of Stephan's Quintet using the Spitzer Space Telescope. A 1000 km s^-1 collision (t_col = 5 x 10⁶ yr) has produced a group-wide shock, and for the first time the large-scale distribution of warm molecular hydrogen emission is revealed, as well as its close association with known shock structures. In the main shock region alone we find 5.0 x 10⁸ M_sun of warm H₂ spread over ∼480 kpc² and additionally report the discovery of a second major shock-excited H₂ feature, likely a remnant of previous tidal interactions. This brings the total H₂ line luminosity of the group in excess of 10⁴² erg s^-1. In the main shock, the H₂ line luminosity exceeds, by a factor of 3, the X-ray luminosity from the hot shocked gas, confirming that the H₂-cooling pathway dominates over the X-ray. [Si II]34.82 μm emission, detected at a luminosity of 1/10th of that of the H₂, appears to trace the group-wide shock closely, and in addition, we detect weak [Fe II]25.99 μm emission from the most X-ray luminous part of the shock. Comparison with shock models reveals that this emission is consistent with regions of fast shocks (100 km s^-1 < V_s < 300 km s^-1) experiencing depletion of iron and silicon onto dust grains. Star formation in the shock (as traced via ionic lines, polycyclic aromatic hydrocarbon and dust emission) appears in the intruder galaxy, but most strikingly at either end of the radio shock. The shock ridge itself shows little star formation, consistent with a model in which the tremendous H₂ power is driven by turbulent energy transfer from motions in a post-shocked layer which suppresses star formation. The significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy-scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies.

[en] We present Atacama Large Millimeter Array (ALMA) Cycle-0 observations of the CO (6-5) line emission (rest-frame frequency = 691.473 GHz) and of the 435 μm dust continuum emission in the nuclear region of NGC 34, a local luminous infrared galaxy at a distance of 84 Mpc (1'' = 407 pc) which contains a Seyfert 2 active galactic nucleus (AGN) and a nuclear starburst. The CO emission is well resolved by the ALMA beam (0.''26 × 0.''23), with an integrated flux of f _CO(6-5) = 1004 (± 151) Jy km s^–1. Both the morphology and kinematics of the CO (6-5) emission are rather regular, consistent with a compact rotating disk with a size of 200 pc. A significant emission feature is detected on the redshifted wing of the line profile at the frequency of the H¹³CN (8-7) line, with an integrated flux of 17.7 ± 2.1(random) ± 2.7(systematic) Jy km s^–1. However, it cannot be ruled out that the feature is due to an outflow of warm dense gas with a mean velocity of 400 km s^–1. The continuum is resolved into an elongated configuration, and the observed flux corresponds to a dust mass of M _dust = 10^6.97±0.13 M _☉. An unresolved central core (radius ≅ 50 pc) contributes 28% of the continuum flux and 19% of the CO (6-5) flux, consistent with insignificant contributions of the AGN to both emissions. Both the CO (6-5) and continuum spatial distributions suggest a very high gas column density (≳ 10⁴ M _☉ pc^–2) in the nuclear region at radius ≲ 100 pc.

[en] We map for the first time the two-dimensional H₂ excitation of warm intergalactic gas in Stephan's Quintet on group-wide (50 × 35 kpc²) scales to quantify the temperature, mass, and warm H₂ mass fraction as a function of position using Spitzer . Molecular gas temperatures are seen to rise (to T > 700 K) and the slope of the power-law density–temperature relation flattens along the main ridge of the filament, defining the region of maximum heating. We also performed MHD modeling of the excitation properties of the warm gas, to map the velocity structure and energy deposition rate of slow and fast molecular shocks. Slow magnetic shocks were required to explain the power radiated from the lowest-lying rotational states of H₂, and strongly support the idea that energy cascades down to small scales and low velocities from the fast collision of NGC 7318b with group-wide gas. The highest levels of heating of the warm H₂ are strongly correlated with the large-scale stirring of the medium as measured by [C ii] spectroscopy with Herschel . H₂ is also seen associated with a separate bridge that extends toward the Seyfert nucleus in NGC 7319, from both Spitzer and CARMA CO observations. This opens up the possibility that both galaxy collisions and outflows from active galactic nuclei can turbulently heat gas on large scales in compact groups. The observations provide a laboratory for studying the effects of turbulent energy dissipation on group-wide scales, which may provide clues about the heating and cooling of gas at high z in early galaxy and protogalaxy formation.

[en] The infrared (IR) emission of 'M _* galaxies' (10^10.4 ≤ M _star ≤ 10^11.0 M _☉) in galaxy pairs, derived using data obtained in Herschel (PEP/HerMES) and Spitzer (S-COSMOS) surveys, is compared to that of single-disk galaxies in well-matched control samples to study the cosmic evolution of the star formation enhancement induced by galaxy-galaxy interaction. Both the mean IR spectral energy distribution and mean IR luminosity of star-forming galaxies (SFGs) in SFG+SFG (S+S) pairs in the redshift bin of 0.6 < z < 1 are consistent with no star formation enhancement. SFGs in S+S pairs in a lower redshift bin of 0.2 < z < 0.6 show marginal evidence for a weak star formation enhancement. Together with the significant and strong sSFR enhancement shown by SFGs in a local sample of S+S pairs (obtained using previously published Spitzer observations), our results reveal a trend for the star formation enhancement in S+S pairs to decrease with increasing redshift. Between z = 0 and z = 1, this decline of interaction-induced star formation enhancement occurs in parallel with the dramatic increase (by a factor of ∼10) of the sSFR of single SFGs, both of which can be explained by the higher gas fraction in higher-z disks. SFGs in mixed pairs (S+E pairs) do not show any significant star formation enhancement at any redshift. The difference between SFGs in S+S pairs and in S+E pairs suggests a modulation of the sSFR by the intergalactic medium (IGM) in the dark matter halos hosting these pairs.

[en] We present results from a Spitzer mid-infrared spectroscopy study of a sample of 74 galaxies located in 23 Hickson Compact Groups (HCGs), chosen to be at a dynamically active stage of H I depletion. We find evidence for enhanced warm H₂ emission (i.e., above that associated with UV excitation in star-forming regions) in 14 galaxies (∼20%), with 8 galaxies having extreme values of L(H₂ S(0)-S(3))/L(7.7 μm polycyclic aromatic hydrocarbon), in excess of 0.07. Such emission has been seen previously in the compact group HCG 92 (Stephan's Quintet), and was shown to be associated with the dissipation of mechanical energy associated with a large-scale shock caused when one group member collided, at high velocity, with tidal debris in the intragroup medium. Similarly, shock excitation or turbulent heating is likely responsible for the enhanced H₂ emission in the compact group galaxies, since other sources of heating (UV or X-ray excitation from star formation or active galactic nuclei) are insufficient to account for the observed emission. The group galaxies fall predominantly in a region of mid-infrared color-color space identified by previous studies as being connected to rapid transformations in HCG galaxy evolution. Furthermore, the majority of H₂-enhanced galaxies lie in the optical ''green valley'' between the blue cloud and red sequence, and are primarily early-type disk systems. We suggest that H₂-enhanced systems may represent a specific phase in the evolution of galaxies in dense environments and provide new insight into mechanisms which transform galaxies onto the optical red sequence.

[en] We present our initial results on the CO rotational spectral line energy distribution (SLED) of the J to J–1 transitions from J = 4 up to 13 from Herschel SPIRE spectroscopic observations of 65 luminous infrared galaxies (LIRGs) in the Great Observatories All-Sky LIRG Survey. The observed SLEDs change on average from one peaking at J ≤ 4 to a broad distribution peaking around J ∼ 6 to 7 as the IRAS 60-to-100 μm color, C(60/100), increases. However, the ratios of a CO line luminosity to the total infrared luminosity, L _IR, show the smallest variation for J around 6 or 7. This suggests that, for most LIRGs, ongoing star formation (SF) is also responsible for a warm gas component that emits CO lines primarily in the mid-J regime (5 ≲ J ≲ 10). As a result, the logarithmic ratios of the CO line luminosity summed over CO (5–4), (6–5), (7–6), (8–7) and (10–9) transitions to L _IR, log R _midCO, remain largely independent of C(60/100), and show a mean value of –4.13 (≡log R_midCO^SF) and a sample standard deviation of only 0.10 for the SF-dominated galaxies. Including additional galaxies from the literature, we show, albeit with a small number of cases, the possibility that galaxies, which bear powerful interstellar shocks unrelated to the current SF, and galaxies, in which an energetic active galactic nucleus contributes significantly to the bolometric luminosity, have their R _midCO higher and lower than R_midCO^SF, respectively