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Register a new userDust emission and star formation in Stephans Quintet
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We analyse a comprehensive set of MIR/FIR observations of Stephans Quintet (SQ), taken with the Spitzer Space Observatory. Our study reveals the presence of a luminous (L_{IR}approx 4.6x10^43 erg/s) 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 PAH 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(sun)/yr, 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}approx 10^42 erg/s 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 IGM and account for the overall correspondence between FIR and X-ray emission.
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We investigated the star formation efficiency for all the dust emitting sources in Stephans Quintet (SQ). We inferred star formation rates using Spitzer MIR/FIR and GALEX FUV data and combined them with gas column density measurements by various authors, in order to position each source in a Kennicutt-Schmidt diagram. Our results show that the bright IGM star formation regions in SQ present star formation efficiencies consistent with those observed within local galaxies. On the other hand, star formation in the intergalactic shock region seems to be rather inhibited.
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.
Context. The Spitzer Space Telescope has detected a powerful (L(H2)~10^41 erg s-1) mid-infrared H2 emission towards the galaxy-wide collision in the Stephans Quintet (SQ) galaxy group. This discovery was followed by the detection of more distant H2-luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations set molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H2 gas coexisting with a hot (~ 5 10^6 K), X-ray emitting plasma. The surface brightness of H2 lines exceeds that of the X-rays and the 0-0 S(1) H2 linewidth is ~ 900 km s-1, of the same order of the collision velocity. These observations raise three questions we propose to answer: (i) Why H2 is present in the postshock gas ? (ii) How can we account for the H2 excitation ? (iii) Why H2 is a dominant coolant ? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H2 formation and excitation in the postshock medium. Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume filling intercloud gas. This produces a ~ 5 10^6 K, dust-free, X-ray emitting plasma. The shock velocity is smaller in clouds. We show that gas heated to temperatures less than 10^6 K cools, keeps its dust content and becomes H2 within the SQ collision age (~ 5 10^6 years). (ii) Since the bulk kinetic energy of the H2 gas is the dominant energy reservoir, we consider that the H2 emission is powered by the dissipation of kinetic turbulent energy. (Abridged)
The Stephans Quintet (SQ, HCG92) was observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared (IR) bands at 65, 90, 140, and 160 um. The AKARI four-band images of the SQ show far-IR emission in the intergalactic medium (IGM) of the SQ. In particular, the 160 um band image shows single peak emission in addition to the structure extending in the North-South direction along the shock ridge as seen in the 140 um band, H2 emission and X-ray emission. Whereas most of the far-IR emission in the shocked region comes from the cold dust component, shock-powered [CII]158um emission can significantly contribute to the emission in the 160 um band that shows a single peak at the shocked region. In the shocked region, the observed gas-to-dust mass ratio is in agreement with the Galactic one. The color temperature of the cold dust component (~20 K) is lower than that in surrounding galaxies (~30 K). We discuss a possible origin of the intergalactic dust emission.
We use smoothed particle hydrodynamics (SPH) models to study the large-scale morphology and dynamical evolution of the intergalactic gas in Stephans Quintet, and compare to multiwavelength observations. Specifically, we model the formation of the hot X-ray gas, the large-scale shock, and emission line gas as the result of NGC 7318b colliding with the group. We also reproduce the N-body model of Renaud and Appleton for the tidal structures in the group.
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