P.-A. Duc

SHOCK-ENHANCED C+ EMISSION AND THE DETECTION OF H2O FROM THE STEPHAN'S QUINTET GROUP-WIDE SHOCK USING HERSCHEL

We present the first Herschel spectroscopic detections of the [OI]63µm and [CII]158µm fine-structure transitions, and a single para-H_2O line from the 35 x 15 kpc^2 shocked intergalactic filament in Stephans Quintet. The filament is believed to have been formed when a high-speed intruder to the group collided with clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (> 1000 km s^(-1)) luminous [CII] line profiles, as well as fainter [OI]63µm emission. SPIRE FTS observations reveal water emission from the p-H_2O (1_(11)-0_(00)) transition at several positions in the filament, but no other molecular lines. The H_2O line is narrow, and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [CII]/PAH_(tot) and [CII]/FIR ratios are too large to be explained by normal photo-electric heating in PDRs. HII nregion excitation or X-ray/Cosmic Ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [CII], [OI] and warm H_2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the IGM is dissipated to small scales and low-velocities, via shocks and turbulent eddies. nLow-velocity magnetic shocks can help explain both the [CII]/[OI] ratio, and the relatively high [CII]/H_2 ratios observed. The discovery that [CII] emission can be enhanced, in large-scale turbulent regions in collisional environments has implications for the interpretation of [CII] emission in high-z galaxies.

STAR FORMATION IN COLLISION DEBRIS: INSIGHTS FROM THE MODELING OF THEIR SPECTRAL ENERGY DISTRIBUTION

During galaxy-galaxy interactions, massive gas clouds can be injected into the intergalactic medium which in turn become gravitationally bound, collapse, and form stars, star clusters, or even dwarf galaxies. The objects resulting from this process are both pristine, as they are forming their first generation of stars, and chemically evolved because the metallicity inherited from their parent galaxies is high. Such characteristics make them particularly interesting laboratories in which to study star formation. After having investigated their star-forming properties, we use photospheric, nebular, and dust modeling to analyze their spectral energy distribution (SED) from the far-ultraviolet to the mid-infrared regime for a sample of seven star-forming regions. Our analysis confirms that the intergalactic star-forming regions in Stephans Quintet, around Arp 105 and NGC 5291, appear devoid of stellar populations older than 10^9 years. We also find an excess of light in the near-infrared regime (from 2 μm to 4.5 μm) which cannot be attributed to stellar photospheric or nebular contributions. This excess is correlated with the star formation rate intensity suggesting that it is probably due to emission by very small grains fluctuating in temperature as well as the polycyclic aromatic hydrocarbons line at 3.3 μm. Comparing the attenuation via the Balmer decrement to the mid-infrared emission allows us to check the reliability of the attenuation estimate. It suggests the presence of embedded star-forming regions in NGC 5291 and NGC 7252. Overall the SED of star-forming regions in collision debris (and Tidal Dwarf Galaxies) resemble more that of dusty star-forming regions in galactic disks than to that of typical star-forming dwarf galaxies.

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS).II. Constraints on star formation in ram-pressure stripped gas

Context. Several galaxies in the Virgo cluster are known to have large Hu2009i gas tails related to a recent ram-pressure stripping event. The Virgo cluster has been extensively observed at 1539 A in the far-ultraviolet for the GALEX Ultraviolet Virgo Cluster Survey (GUViCS), and in the optical for the Next Generation Virgo Survey (NGVS), allowing a study of the stellar emission potentially associated with the gas tails of 8 cluster members. On the theoretical side, models of ram-pressure stripping events have started to include the physics of star formation. n nAims. We aim to provide quantitative constraints on the amount of star formation taking place in the ram-pressure stripped gas, mainly on the basis of the far-UV emission found in the GUViCS images in relation with the gas content of the tails. n nMethods. We have performed three comparisons of the young stars emission with the gas column density: visual, pixel-by-pixel, and global. We have compared our results to other observational and theoretical studies. n nResults. We find that the level of star formation taking place in the gas stripped from galaxies by ram-pressure is low with respect to the available amount of gas. Star formation is lower by at least a factor 10 compared to the predictions of the Schmidt Law as determined in regular spiral galaxy disks. It is also lower than measured in dwarfs galaxies and the outer regions of spirals, and than predicted by some numerical simulations. We provide constraints on the star formation efficiency in the ram-pressure stripped gas tails, and compare these with current models.

DUST EMISSION AND STAR FORMATION IN STEPHAN'S QUINTET

We analyze a comprehensive set of MIR/FIR observations of Stephans Quintet (SQ), taken with the Spitzer Space Telescope. Our study reveals the presence of a luminous (L_(IR) ≈ 4.6 × 10^(43) erg s^(-1)) and extended component of infrared dust emission, not connected with the main bodies of the galaxies, but roughly coincident with the X-ray halo of the group. We fitted the inferred dust emission spectral energy distribution of this extended source and the other main infrared emission components of SQ, including the intergalactic shock, to elucidate the mechanisms powering the dust and polycyclic aromatic hydrocarbon emission, taking into account collisional heating by the plasma and heating through UV and optical photons. Combining the inferred direct and dust-processed UV emission to estimate the star formation rate (SFR) for each source we obtain a total SFR for SQ of 7.5 M_☉ yr^(-1), similar to that expected for non-interacting galaxies with stellar mass comparable to the SQ galaxies. Although star formation in SQ is mainly occurring at, or external to the periphery of the galaxies, the relation of SFR per unit physical area to gas column density for the brightest sources is similar to that seen for star formation regions in galactic disks. We also show that available sources of dust in the group halo can provide enough dust to produce up to L_(IR) ≈ 10^(42) erg s^(-1) powered by collisional heating. Though a minority of the total infrared emission (which we infer to trace distributed star-formation), this is several times higher than the X-ray luminosity of the halo, so could indicate an important cooling mechanism for the hot intergalactic medium (IGM) and account for the overall correspondence between FIR and X-ray emission. We investigate two potential modes of star formation in SQ consistent with the data, fueled either by gas from a virialized hot IGM continuously accreting onto the group, whose cooling is enhanced by grains injected from an in situ population of intermediate mass stars, or by interstellar gas stripped from the galaxies. The former mode offers a natural explanation for the observed baryon deficiency in the IGM of SQ as well as for the steep L_(X)-T_(X) relation of groups such as SQ with lower velocity dispersions.

Accretion-inhibited star formation in the warm molecular disk of the green-valley elliptical galaxy NGC 3226?

We present archival Spitzer photometry and spectroscopy and Herschel photometry of the peculiar Green Valley elliptical galaxy NGC 3226. The galaxy, which contains a low-luminosity active galactic nucleus (AGN), forms a pair with NGC 3227 and is shown to lie in a complex web of stellar and Hi filaments. Imaging at 8 and 16 mu m reveals a curved plume structure 3 kpc in extent, embedded within the core of the galaxy and coincident with the termination of a 30 kpc long H I tail. In situ star formation associated with the infrared (IR) plume is identified from narrowband Hubble Space Telescope (HST) imaging. The end of the IR plume coincides with a warm molecular hydrogen disk and dusty ring containing 0.7-1.1 x 10(7) M-circle dot detected within the central kiloparsec. Sensitive upper limits to the detection of cold molecular gas may indicate that a large fraction of the H-2 is in a warm state. Photometry derived from the ultraviolet (UV) to the far-IR shows evidence for a low star-formation rate of similar to 0.04M(circle dot) yr(-1) averaged over the last 100 Myr. A mid-IR component to the spectral energy distribution (SED) contributes similar to 20% of the IR luminosity of the galaxy, and is consistent with emission associated with the AGN. The current measured star formation rate is insufficient to explain NGC 3226s global UV-optical green colors via the resurgence of star formation in a red and dead galaxy. This form of cold accretion from a tidal stream would appear to be an inefficient way to rejuvenate early-type galaxies and may actually inhibit star formation.

Ionization processes in a local analogue of distant clumpy galaxies: VLT MUSE IFU spectroscopy and FORS deep images of the TDG NGC 5291N

We present IFU observations with MUSE@VLT and deep imaging with FORS@VLT of a dwarf galaxy recently formed within the giant collisional HI ring surrounding NGC 5291. This TDG-like object has the characteristics of typical z=1-2 gas-rich spiral galaxies: a high gas fraction, a rather turbulent clumpy ISM, the absence of an old stellar population, a moderate metallicity and star formation efficiency. The MUSE spectra allow us to determine the physical conditions within the various complex substructures revealed by the deep optical images, and to scrutinize at unprecedented spatial resolution the ionization processes at play in this specific medium. Starburst age, extinction and metallicity maps of the TDG and surrounding regions were determined using the strong emission lines Hbeta, [OIII], [OI], [NII], Halpha and [SII] combined with empirical diagnostics. Discrimination between different ionization mechanisms was made using BPT--like diagrams and shock plus photoionization models. Globally, the physical conditions within the star--forming regions are homogeneous, with in particular an uniform half-solar oxygen abundance. At small scales, the derived extinction map shows narrow dust lanes. Regions with atypically strong [OI] emission line immediately surround the TDG. The [OI] / Halpha ratio cannot be easily accounted for by photoionization by young stars or shock models. At larger distances from the main star--forming clumps, a faint diffuse blue continuum emission is observed, both with the deep FORS images and MUSE data. It does not have a clear counterpart in the UV regime probed by GALEX. A stacked spectrum towards this region does not exhibit any emission line, excluding faint levels of star formation, nor stellar absorption lines that might have revealed the presence of old stars. Several hypotheses are discussed for the origin of these intriguing features.

ERRATUM: "ACCRETION-INHIBITED STAR FORMATION IN THE WARM MOLECULAR DISK OF THE GREEN-VALLEY ELLIPTICAL GALAXY NGC 3226?" (2014, APJ, 797, 117)

P-A. Duc, U. Lisenfeld, and P. Ogle 1 NASA Herschel Science Center, Infrared Processing and Analysis Center, Caltech, 770S Wilson Av., Pasadena, CA 91125; apple@ipac.caltech.edu 2 Astrophysics Research Institute, John Moores University, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK 3 Instituto de Astronomia, National Autonomous University of Mexico, P. O. 70-264, 04510 D. F., Mexico 4 NRAO, Charlottesville 5 Spitzer NASA Herschel Science Center, 1200 E. California Blvd., Caltech, Pasadena, CA 91125 6 Department of Physics, University of Crete, GR-71003, Heraklion, Greece 7 Institute for Astronomy, Astrophysics, Space Applications & Remote Sensing, National Observatory of Athens, GR-15236, Penteli, Greece 8 Chercheur Associe, Observatoire de Paris, F-75014, Paris, France 9 Service d’Astrophysique, Laboratoire AIM, CEA-Saclay, Orme des Merisiers, Bat 709, 91191 Gif sur Yvette, France 10 Dept. Fisica Teorica y del Cosmos, University of Granada, Edifica Mecenas, Granada, Spain 11 NASA Extragalactic Database, IPAC, Caltech, 1200 E. California Blvd, Caltech, Pasadena, CA 91125

The dust emission SED of X-ray emitting regions in Stephan's Quintet

We analysed the Spitzer maps of Stephans Quintet in order to investigate the nature of the dust emission associated with the X-ray emitting regions of the large scale intergalactic shock and of the group halo. This emission can in principle be powered by dust-gas particle collisions, thus providing efficient cooling of the hot gas. However the results of our analysis suggest that the dust emission from those regions is mostly powered by photons. Nonetheless dust collisional heating could be important in determining the cooling of the IGM gas and the large scale star formation morphology observed in SQ.

Dust Emission from Stephan’s Quintet

We present new infrared images of the prototype of galaxy compact groups, Stephan’s Quintet (SQ), taken with Spitzer/MIPS at 24, 70 and 160 μm, and compare these with existing images at X‐ray, UV/optical, radio wavelengths as well as with archival Spitzer/IRAC imaging at 8 μm. The new images reveal an extended (on scales of up to ca. 70 kpc) component of FIR emission which is roughly correlated with diffuse soft X‐ray emission arising from the so‐called SHOCK and HALO regions. A further interesting aspect of this study is the displacement of the main star formation sites from the central galaxy disks to the peripheries of the galaxies or to the intergalactic medium (IGM).

A Tidal Dwarf Galaxy in the Hercules Cluster

A candidate Tidal Dwarf Galaxy, ce-61, was identified in the merger system IC 1182 in the Hercules supercluster. The multi-wavelength data we obtained so far do not prove, however, that it is kinematically detached from the IC 1182 system and gravitationally bound.