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Register a new userA Chandra X-ray view of Stephans Quintet: Shocks and Star-formation
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We use a deep Chandra observation to examine the structure of the hot intra-group medium of the compact group of galaxies Stephans Quintet. The group is thought to be undergoing a strong dynamical interaction as an interloper, NGC 7318b, passes through the group core at ~850 km/s. A bright ridge of X-ray and radio continuum emission has been interpreted as the result of shock heating, with support from observations at other wavelengths. We find that gas in this ridge has a similar temperature (~0.6 keV) and abundance (~0.3 solar) to the surrounding diffuse emission, and that a hard emission component is consistent with that expected from high-mass X-ray binaries associated with star-formation in the ridge. The cooling rate of gas in the ridge is consistent with the current star formation rate, suggesting that radiative cooling is driving the observed star formation. The lack of a high-temperature gas component is used to place constraints on the nature of the interaction and shock, and we find that an oblique shock heating a pre-existing filament of HI may be the most likely explanation of the X-ray gas in the ridge. The mass of hot gas in the ridge is only ~2 per cent of the total mass of hot gas in the group, which is roughly equal to the deficit in observed HI mass compared to predictions. The hot gas component is too extended to have been heated by the current interaction, strongly suggesting that it must have been heated during previous dynamical encounters.
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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.
We present a Gemini-GMOS spectroscopic study of HST-selected H{alpha}-emitting regions in Stephans 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 H{alpha}-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 H{alpha} 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 Stephans 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.
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.
We present MERLIN L-band images of the compact galaxy group, Stephans Quintet. The Seyfert 2 galaxy, NGC 7319, the brightest member of the compact group, is seen to have a triple radio structure typical of many extra-galactic radio sources which have a flat spectrum core and two steep spectrum lobes with hot spots. The two lobes are asymmetrically distributed on opposite sides of the core along the minor axis of the galaxy. Ultraviolet emission revealed in a high resolution HRC/ACS HST image is strongly aligned with the radio plasma and we interpret the intense star formation in the core and north lobe as an event induced by the collision of the north radio jet with over-dense ambient material. In addition, a re-mapping of archive VLA L-band observations reveals more extended emission along the major axis of the galaxy which is aligned with the optical axis. Images formed from the combined MERLIN and archive VLA data reveal more detailed structure of the two lobes and hot spots.
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