arXiv subjects

M. E. Cluver

Publications and source records attributed to M. E. Cluver.

At least 55 records · Page 3Linked to original sources

arXiv2015

Galaxy And Mass Assembly (GAMA): end of survey report and data release 2

The Galaxy And Mass Assembly (GAMA) survey is one of the largest contemporary spectroscopic surveys of low-redshift galaxies. Covering an area of ~286 deg^2 (split among five survey regions) down to a limiting magnitude of r < 19.8 mag, we have collected spectra and reliable redshifts for 238,000 objects using the AAOmega spectrograph on the Anglo-Australian Telescope. In addition, we have assembled imaging data from a number of independent surveys in order to generate photometry spanning the wavelength range 1 nm - 1 m. Here we report on the recently completed spectroscopic survey and present a series of diagnostics to assess its final state and the quality of the redshift data. We also describe a number of survey aspects and procedures, or updates thereof, including changes to the input catalogue, redshifting and re-redshifting, and the derivation of ultraviolet, optical and near-infrared photometry. Finally, we present the second public release of GAMA data. In this release we provide input catalogue and targeting information, spectra, redshifts, ultraviolet, optical and near-infrared photometry, single-component Sérsic fits, stellar masses, H$α$-derived star formation rates, environment information, and group properties for all galaxies with r < 19.0 mag in two of our survey regions, and for all galaxies with r < 19.4 mag in a third region (72,225 objects in total). The database serving these data is available at http://www.gama-survey.org/.

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arXiv2015

Galaxy And Mass Assembly (GAMA) Blended Spectra Catalog: Strong Galaxy-Galaxy Lens and Occulting Galaxy Pair Candidates

We present the catalogue of blended galaxy spectra from the Galaxy And Mass Assembly (GAMA) survey. These are cases where light from two galaxies are significantly detected in a single GAMA fibre. Galaxy pairs identified from their blended spectrum fall into two principal classes: they are either strong lenses, a passive galaxy lensing an emission-line galaxy; or occulting galaxies, serendipitous overlaps of two galaxies, of any type. Blended spectra can thus be used to reliably identify strong lenses for follow-up observations (high resolution imaging) and occulting pairs, especially those that are a late-type partly obscuring an early-type galaxy which are of interest for the study of dust content of spiral and irregular galaxies. The GAMA survey setup and its autoz automated redshift determination were used to identify candidate blended galaxy spectra from the cross-correlation peaks. We identify 280 blended spectra with a minimum velocity separation of 600 km/s, of which 104 are lens pair candidates, 71 emission-line-passive pairs, 78 are pairs of emission-line galaxies and and 27 are pairs of galaxies with passive spectra. We have visually inspected the candidates in the Sloan Digital Sky Survey (SDSS) and Kilo Degree Survey (KiDS) images. Many blended objects are ellipticals with blue fuzz (Ef in our classification). These latter "Ef" classifications are candidates for possible strong lenses, massive ellipticals with an emission-line galaxy in one or more lensed images. The GAMA lens and occulting galaxy candidate samples are similar in size to those identified in the entire SDSS. This blended spectrum sample stands as a testament of the power of this highly complete, second-largest spectroscopic survey in existence and offers the possibility to expand e.g., strong gravitational lens surveys.

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arXiv2014

Galaxy And Mass Assembly (GAMA): The galaxy luminosity function within the cosmic web

We investigate the dependence of the galaxy luminosity function on geometric environment within the Galaxy And Mass Assembly (GAMA) survey. The tidal tensor prescription, based on the Hessian of the pseudo-gravitational potential, is used to classify the cosmic web and define the geometric environments: for a given smoothing scale, we classify every position of the surveyed region, $0.04<{z}<0.26$, as either a void, a sheet, a filament or a knot. We consider how to choose appropriate thresholds in the eigenvalues of the Hessian in order to partition the galaxies approximately evenly between environments. We find a significant variation in the luminosity function of galaxies between different geometric environments; the normalisation, characterised by $ϕ^{*}$ in a Schechter function fit, increases by an order of magnitude from voids to knots. The turnover magnitude, characterised by $M^*$, brightens by approximately $0.5$ mag from voids to knots. However, we show that the observed modulation can be entirely attributed to the indirect local-density dependence. We therefore find no evidence of a direct influence of the cosmic web on the galaxy luminosity function.

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arXiv2014

Recalibrating the Wide-field Infrared Survey Explorer (WISE) W4 Filter

We present a revised effective wavelength and photometric calibration for the Wide-field Infrared Survey Explorer (WISE) W4 band, including tests of empirically motivated modifications to its pre-launch laboratory-measured relative system response curve. We derived these by comparing measured W4 photometry with photometry synthesised from spectra of galaxies and planetary nebulae. The difference between measured and synthesised photometry using the pre-launch laboratory-measured W4 relative system response can be as large as 0.3 mag for galaxies and 1 mag for planetary nebulae. We find the W4 effective wavelength should be revised upward by 3.3%, from 22.1 micron to 22.8 micron, and the W4 AB magnitude of Vega should be revised from m = 6.59 to m = 6.66. In an attempt to reproduce the observed W4 photometry, we tested three modifications to the pre-launch laboratory-measured W4 relative system response curve, all of which have an effective wavelength of 22.8 micron. Of the three relative system response curve models tested, a model that matches the laboratory-measured relative system response curve, but has the wavelengths increased by 3.3% (or 0.73 micron) achieves reasonable agreement between the measured and synthesised photometry.

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arXiv2014

CO in Hickson Compact Group galaxies with enhanced warm \htwo\ emission: Evidence for galaxy evolution?

Galaxies in Hickson Compact Groups (HCGs) are believed to experience morphological transformations from blue, star-forming galaxies to red, early-type galaxies. Galaxies with a high ratio between the luminosities of the warm H2 to the 7.7mu PAH emission ("Molecular Hydrogen Emission Galaxies", MOHEGs) are predominantly in an intermediate phase, the green valley. Their enhanced H2 emission suggests that the molecular gas is affected in the transition. We study the properties of the molecular gas traced by CO in galaxies in HCGs with measured warm H2 emission in order to look for evidence of the perturbations affecting the warm H2 in the kinematics, morphology and mass of the molecular gas. We analyzed the molecular gas mass derived from CO(1-0), MH2, and its kinematics, and then compared it to the mass of the warm molecular gas, the stellar mass and star formation rate (SFR). Our results are the following. (i) The mass ratio between the CO-derived and the warm H2 molecular gas is in the same range as for field galaxies. (ii) Some galaxies, mostly MOHEGs, have very broad CO linewidths of up to 1000 kms. The line shapes are irregular and show various components. (iii) The mapped objects show asymmetric distributions of the cold molecular gas. (iv) The star formation efficiency (= SFR/MH2) of galaxies in HCGs is similar to isolated galaxies. No significant difference between MOHEGs and non-MOHEGs or between early-types and spirals has been found. (v) The molecular gas masses, MH2, and MH2/LK are lower in MOHEGs (predominantly early-types) than in non-MOHEGs (predominantly spirals). This trend remains when comparing MOHEGs and non-MOHEGs of the same morphological type. The differences in the molecular gas properties of MOHEGs support the view that they are suffering perturbations of the molecular gas, as well as a decrease in the molecular gas content and associated SFR.

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arXiv2014

Galaxy And Mass Assembly (GAMA): autoz spectral redshift measurements, confidence and errors

The Galaxy And Mass Assembly (GAMA) survey has obtained spectra of over 230000 targets using the Anglo-Australian Telescope. To homogenise the redshift measurements and improve the reliability, a fully automatic redshift code was developed (autoz). The measurements were made using a cross-correlation method for both absorption-line and emission-line spectra. Large deviations in the high-pass filtered spectra are partially clipped in order to be robust against uncorrected artefacts and to reduce the weight given to single-line matches. A single figure of merit (FOM) was developed that puts all template matches onto a similar confidence scale. The redshift confidence as a function of the FOM was fitted with a tanh function using a maximum likelihood method applied to repeat observations of targets. The method could be adapted to provide robust automatic redshifts for other large galaxy redshift surveys. For the GAMA survey, there was a substantial improvement in the reliability of assigned redshifts and in the lowering of redshift uncertainties with a median velocity uncertainty of 33 km/s.

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arXiv2014

Galaxy And Mass Assembly (GAMA): Mid-Infrared Properties and Empirical Relations from $WISE$

The Galaxy And Mass Assembly (GAMA) survey furnishes a deep redshift catalog that, when combined with the Wide-field Infrared Explorer ($WISE$), allows us to explore for the first time the mid-infrared properties of $> 110, 000$ galaxies over 120 deg$^2$ to $z\simeq 0.5$. In this paper we detail the procedure for producing the matched GAMA-$WISE$ catalog for the G12 and G15 fields, in particular characterising and measuring resolved sources; the complete catalogs for all three GAMA equatorial fields will be made available through the GAMA public releases. The wealth of multiwavelength photometry and optical spectroscopy allows us to explore empirical relations between optically determined stellar mass (derived from synthetic stellar population models) and 3.4micron and 4.6micron WISE measurements. Similarly dust-corrected Halpha-derived star formation rates can be compared to 12micron and 22micron luminosities to quantify correlations that can be applied to large samples to $z<0.5$. To illustrate the applications of these relations, we use the 12micron star formation prescription to investigate the behavior of specific star formation within the GAMA-WISE sample and underscore the ability of WISE to detect star-forming systems at $z\sim0.5$. Within galaxy groups (determined by a sophisticated friends-of-friends scheme), results suggest that galaxies with a neighbor within 100$\,h^{-1} $kpc have, on average, lower specific star formation rates than typical GAMA galaxies with the same stellar mass.

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arXiv2013

Shocks and Star Formation in Stephan's Quintet. I. Gemini Spectroscopy of Hα-bright knots

We present a Gemini-GMOS spectroscopic study of HST-selected Hα-emitting regions in Stephan's Quintet (HCG 92), a nearby compact galaxy group, with the aim of disentangling the processes of shock-induced heating and star formation in its intra-group medium. The $\approx$40 sources are distributed across the system, but most densely concentrated in the $\sim$kpc-long shock region. Their spectra neatly divide them into narrow- and and broad-line emitters, and we decompose the latter into three or more emission peaks corresponding to spatial elements discernible in HST imaging. The emission line ratios of the two populations of Hα-emitters confirm their nature as H II regions (90% of the sample) or molecular gas heated by a shock-front propagating at $\lesssim$300 km/s. Their redshift distribution reveals interesting three-dimensional structure with respect to gas-phase baryons, with no H II regions associated with shocked gas, no shocked regions in the intruder galaxy NGC 7318B, and a sharp boundary between shocks and star formation. We conclude that star formation is inhibited substantially, if not entirely, in the shock region. Attributing those H II regions projected against the shock to the intruder, we find a lopsided distribution of star formation in this galaxy, reminiscent of pile-up regions in models of interacting galaxies. The Hα luminosities imply mass outputs, star formation rates, and efficiencies similar to nearby star-forming regions. Two large knots are an exception to this, being comparable in stellar output to the prolific 30 Doradus region. We also examine Stephan's Quintet in the context of compact galaxy group evolution, as a paradigm for intermittent star formation histories in the presence of a rich, X-ray emitting intra-group medium.

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arXiv2013

Shock-Enhanced C+ Emission and the Detection of H2O from Stephan's Quintet's Group-Wide Shock using Herschel

We present the first Herschel spectroscopic detections of the [OI]63 and [CII]158 micron fine-structure transitions, and a single para-H2O line from the 35 x 15 kpc^2 shocked intergalactic filament in Stephan's 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]63micron emission. SPIRE FTS observations reveal water emission from the p-H2O (111-000) transition at several positions in the filament, but no other molecular lines. The H2O 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 region 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 H2 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. Low-velocity magnetic shocks can help explain both the [CII]/[OI] ratio, and the relatively high [CII]/H2 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.

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arXiv2013

Galaxy and Mass Assembly (GAMA): Witnessing the assembly of the cluster Abell 1882

We present a combined optical and X-ray analysis of the rich cluster Abell 1882 with the aim of identifying merging substructure and understanding the recent assembly history of this system. Our optical data consist of spectra drawn from the Galaxy and Mass Assembly (GAMA) survey, which lends itself to this kind of detailed study thanks to its depth and high spectroscopic completeness. We use 283 spectroscopically confirmed cluster members to detect and characterize substructure. We complement the optical data with X-ray data taken with both Chandra and XMM. Our analysis reveals that A1882 harbors two main components, A1882A and A1882B, which have a projected separation of 2Mpc and a line of sight velocity difference of v_{los}=-428km/s. The primary system, A1882A, has velocity dispersion sigma_v=500km/s and Chandra (XMM) temperature kT=3.57keV (3.31keV) while the secondary, A1882B, has sigma_v=457km/s and Chandra (XMM) temperature kT=2.39keV (2.12keV). The optical and X-ray estimates for the masses of the two systems are consistent within the uncertainties and indicate that there is twice as much mass in A1882A (M_{500}=1.5-1.9x10^{14}Msolar) when compared with A1882B (M_{500}=0.8-1.0x10^{14}Msolar). We interpret the A1882A/A1882B system as being observed prior to a core passage. Supporting this interpretation is the large projected separation of A1882A and A1882B and the dearth of evidence for a recent (<2Gyr) major interaction in the X-ray data. Two-body analyses indicate that A1882A and A1882B form a bound system with bound incoming solutions strongly favored. We compute blue fractions of f_b=0.28 and 0.18 for the spectroscopically confirmed member galaxies within r_{500} of the centers of A1882A and A1882B, respectively. These blue fractions do not differ significantly from the blue fraction measured from an ensemble of 20 clusters with similar mass and redshift.

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arXiv2013

Galaxy And Mass Assembly (GAMA): Linking Star Formation Histories and Stellar Mass Growth

We present evidence for stochastic star formation histories in low-mass (M* < 10^10 Msun) galaxies from observations within the Galaxy And Mass Assembly (GAMA) survey. For ~73,000 galaxies between 0.05<z<0.32, we calculate star formation rates (SFR) and specific star formation rates (SSFR = SFR/M*) from spectroscopic Halpha measurements and apply dust corrections derived from Balmer decrements. We find a dependence of SSFR on stellar mass, such that SSFRs decrease with increasing stellar mass for star-forming galaxies, and for the full sample, SSFRs decrease as a stronger function of stellar mass. We use simple parametrizations of exponentially declining star formation histories to investigate the dependence on stellar mass of the star formation timescale and the formation redshift. We find that parametrizations previously fit to samples of z~1 galaxies cannot recover the distributions of SSFRs and stellar masses observed in the GAMA sample between 0.05<z<0.32. In particular, a large number of low-mass (M* < 10^10 Msun) galaxies are observed to have much higher SSFRs than can be explained by these simple models over the redshift range of 0.05<z<0.32, even when invoking mass-dependent staged evolution. For such a large number of galaxies to maintain low stellar masses, yet harbour such high SSFRs, requires the late onset of a weak underlying exponentially declining SFH with stochastic bursts of star formation superimposed.

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arXiv2013

Galaxy And Mass Assembly: Evolution of the Halpha luminosity function and star formation rate density up to z<0.35

Measurements of the low-z Halpha luminosity function have a large dispersion in the local number density of sources, and correspondingly in the SFR density. The possible causes for these discrepancies include limited volume sampling, biases arising from survey sample selection, different methods of correcting for dust obscuration and AGN contamination. The Galaxy And Mass Assembly (GAMA) survey and Sloan Digital Sky Survey (SDSS) provide deep spectroscopic observations over a wide sky area enabling detection of a large sample of star-forming galaxies spanning 0.001<SFR(Halpha)<100 with which to robustly measure the evolution of the SFR density in the low-z universe. The large number of high SFR galaxies present in our sample allow an improved measurement of the bright end of the luminosity function, indicating that the decrease in number density of sources at bright luminosities is best described by a Saunders functional form rather than the traditional Schechter function. This result is consistent with other published luminosity functions in the FIR and radio. For GAMA and SDSS we find the r-band apparent magnitude limit, combined with the subsequent requirement for Halpha detection leads to an incompleteness due to missing bright Halpha sources with faint r-band magnitudes.

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arXiv2013

Galaxy And Mass Assembly (GAMA): Spectroscopic analysis

The Galaxy And Mass Assembly (GAMA) survey is a multiwavelength photometric and spectroscopic survey, using the AAOmega spectrograph on the Anglo-Australian Telescope to obtain spectra for up to ~300000 galaxies over 280 square degrees, to a limiting magnitude of r_pet < 19.8 mag. The target galaxies are distributed over 0<z<0.5 with a median redshift of z~0.2, although the redshift distribution includes a small number of systems, primarily quasars, at higher redshifts, up to and beyond z=1. The redshift accuracy ranges from sigma_v~50km/s to sigma_v~100km/s depending on the signal-to-noise of the spectrum. Here we describe the GAMA spectroscopic reduction and analysis pipeline. We present the steps involved in taking the raw two-dimensional spectroscopic images through to flux-calibrated one-dimensional spectra. The resulting GAMA spectra cover an observed wavelength range of 3750<lambda<8850 A at a resolution of R~1300. The final flux calibration is typically accurate to 10-20%, although the reliability is worse at the extreme wavelength ends, and poorer in the blue than the red. We present details of the measurement of emission and absorption features in the GAMA spectra. These measurements are characterised through a variety of quality control analyses detailing the robustness and reliability of the measurements. We illustrate the quality of the measurements with a brief exploration of elementary emission line properties of the galaxies in the GAMA sample. We demonstrate the luminosity dependence of the Balmer decrement, consistent with previously published results, and explore further how Balmer decrement varies with galaxy mass and redshift. We also investigate the mass and redshift dependencies of the [NII]/Halpha vs [OIII]/Hbeta spectral diagnostic diagram, commonly used to discriminate between star forming and nuclear activity in galaxies.

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arXiv2013

Enhanced Warm H2 Emission in the Compact Group Mid-Infrared "Green Valley"

We present results from a Spitzer, mid-infrared spectroscopy study of a sample of 74 galaxies located in 23 Hickson Compact Groups, chosen to be at a dynamically-active stage of HI 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(7.7micron PAH), 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 AGN) 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.

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arXiv2012

Detection of Powerful Mid-IR H2 Emission in the Bridge between the Taffy Galaxies

We report the detection of strong, resolved emission from warm H2 in the Taffy galaxies and bridge. Relative to the continuum and faint PAH emission, the H2 emission is the strongest in the connecting bridge, approaching L(H2)/L(PAH8μm) = 0.1 between the two galaxies, where the purely rotational lines of H2 dominate the mid-infrared spectrum in a way very reminiscent of the group-wide shock in the interacting group Stephan's Quintet. The surface brightness in the 0-0 S(0) and S(1) H2 lines in the bridge is more than twice that observed at the center of the Stephan's Quintet shock. We observe a warm H2 mass of 4.2 \times 108 M\odot in the bridge, but taking into account the unobserved bridge area, the total warm mass is likely to be twice this value. We use excitation diagrams to characterize the warm molecular gas, finding an average surface mass of 5 \times 106 M\odot kpc-2 and typical excitation temperatures of 150-175 K. H2 emission is also seen in the galaxy disks, although there the emission is more consistent with normal star forming galaxies. We investigate several possible heating mechanisms for the bridge gas, but favor the conversion of kinetic energy from the head-on collision via turbulence and shocks as the main heating source. Since the cooling time for the warm H2 is short (5000 yr), shocks must be permeating the molecular gas in bridge region in order to continue heating the H2.

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arXiv2010

Active Disk Building in a local HI-Massive LIRG: The Synergy between Gas, Dust, and Star Formation

HIZOA J0836-43 is the most HI-massive (M_HI = 7.5x10^10 Msun) galaxy detected in the HIPASS volume and lies optically hidden behind the Milky Way. Markedly different from other extreme HI disks in the local universe, it is a luminous infrared galaxy (LIRG) with an actively star forming disk (>50 kpc), central to its ~ 130 kpc gas disk, with a total star formation rate (SFR) of ~20.5 Msun yr^{-1}. Spitzer spectroscopy reveals an unusual combination of powerful polycyclic aromatic hydrocarbon (PAH) emission coupled to a relatively weak warm dust continuum, suggesting photodissociation region (PDR)-dominated emission. Compared to a typical LIRG with similar total infrared luminosity (L_TIR=10^11 Lsun), the PAHs in HIZOA J0836-43 are more than twice as strong, whereas the warm dust continuum (lambda > 20micron) is best fit by a star forming galaxy with L_TIR=10^10 Lsun. Mopra CO observations suggest an extended molecular gas component (H_2 + He > 3.7x10^9 Msun) and a lower limit of ~ 64% for the gas mass fraction; this is above average compared to local disk systems, but similar to that of z~1.5 BzK galaxies (~57%). However, the star formation efficiency (SFE = L_IR/L'_CO) for HIZOA J0836-43 of 140 Lsun (K km s^{-1} pc^2)^{-1} is similar to that of local spirals and other disk galaxies at high redshift, in strong contrast to the increased SFE seen in merging and strongly interacting systems. HIZOA J0836-43 is actively forming stars and building a massive stellar disk. Its evolutionary phase of star formation (M_stellar, SFR, gas fraction) compared to more distant systems suggests that it would be considered typical at redshift z~1. This galaxy provides a rare opportunity in the nearby universe for studying (at z~0.036) how disks were building and galaxies evolving at z~1, when similarly large gas fractions were likely more common.

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arXiv2010

Observations and modeling of the dust emission from the H2-bright galaxy-wide shock in Stephan's Quintet

Spitzer Space Telescope observations revealed powerful mid-infrared (mid-IR) H2 rotational line emission from the Stephan's Quintet (SQ) X-ray emitting large scale shock associated with a collision between two galaxies. Because H2 forms on dust grains, the presence of H2 is physically linked to the survival of dust, and we expect some dust emission to come from the molecular gas. To test this interpretation, IR observations and dust modeling are used to identify and characterize the thermal dust emission from the shocked molecular gas. The spatial distribution of the IR emission allows us to isolate the faint PAH and dust continuum emission associated with the molecular gas in the SQ shock. We model the spectral energy distribution (SED) of this emission, and fit it to Spitzer observations. Faint PAH and dust continuum emission are detected in the SQ shock, outside star-forming regions. The 12/24um flux ratio in the shock is remarkably close to that of the diffuse Galactic interstellar medium, leading to a Galactic PAH/VSG abundance ratio. However, the properties of the PAH emission spectrum in the shock differ from that of the Galaxy, which may suggest an enhanced fraction of large and neutrals PAHs. The IR SED is consistent with the expected emission from dust associated with the warm (>150K) H2 gas, heated by a UV radiation field of intensity comparable to that of the solar neighborhood, in agreement with GALEX UV observations. The presence of PAHs and dust grains in the high-speed (1000km/s) galaxy collision suggests that dust survives. We propose that the dust that survived destruction was in pre-shock gas at densites larger than a few 0.1cm-3, which was not shocked at velocities larger than 200km/s. [abridged]

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arXiv2009

Powerful H$_2$ Line-cooling in Stephan's Quintet : I - Mapping the Significant Cooling Pathways in Group-wide Shocks

We present results from the mid-infrared spectral mapping of Stephan's Quintet using the Spitzer Space Telescope. A 1000 km/s collision 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 $\times10^{8}$ M$_{\odot}$ of warm H$_2$ spread over $\sim$ 480 kpc$^2$ and additionally report the discovery of a second major shock-excited H$_2$ feature. This brings the total H$_2$ line luminosity of the group in excess of 10$^42$ erg/s. In the main shock, the H$_2$ line luminosity exceeds, by a factor of three, the X-ray luminosity from the hot shocked gas, confirming that the H$_2$-cooling pathway dominates over the X-ray. [Si II]34.82$μ$m emission, detected at a luminosity of 1/10th of that of the H$_2$, appears to trace the group-wide shock closely and in addition, we detect weak [FeII]25.99$μ$m 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 < $V_{s}$ < 300 km/s) experiencing depletion of iron and silicon onto dust grains. Star formation in the shock (as traced via ionic lines, PAH 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 H$_{2}$ power is driven by turbulent energy transfer from motions in a post-shocked layer. 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.

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