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Register a new userMERLIN observations of Stephans Quintet
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Authors
E. Xanthopoulos
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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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We present a multiwavelength study of the highly evolved compact galaxy group known as Seyferts Sextet (HCG79: SS). We interpret SS as a 2-3 Gyr more evolved analog of Stephans Quintet (HCG92: SQ). We postulate that SS formed by sequential acquisition of 4-5 primarily late-type field galaxies. Four of the five galaxies show an early-type morphology which is likely the result of secular evolution driven by gas stripping. Stellar stripping has produced a massive/luminous halo and embedded galaxies that are overluminous for their size. These are interpreted as remnant bulges of the accreted spirals. H79d could be interpreted as the most recent intruder being the only galaxy with an intact ISM and uncertain evidence for tidal perturbation. In addition to stripping activity we find evidence for past accretion events. H79b (NGC6027) shows a strong counter-rotating emission line component interpreted as an accreted dwarf spiral. H79a shows evidence for an infalling component of gas representing feedback or possible cross fueling by H79d. The biggest challenge to this scenario involves the low gas fraction in the group. If SS formed from normal field spirals then much of the gas is missing. Finally, despite its advanced stage of evolution, we find no evidence for major mergers and infer that SS (and SQ) are telling us that such groups coalesce via slow dissolution.
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.
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 smoothed particle hydrodynamic models of the interactions in the compact galaxy group, Stephans Quintet. This work is extension of the earlier collisionless N-body simulations of Renaud et al. in which the large-scale stellar morphology of the group was modeled with a series of galaxy-galaxy interactions in the simulations. Including thermohydrodynamic effects in this work, we further investigate the dynamical interaction history and evolution of the intergalactic gas of Stephans Quintet. The major features of the group, such as the extended tidal features and the group-wide shock, enabled us to constrain the models reasonably well, while trying to reproduce multiple features of the system. We found that reconstructing the two long tails extending from NGC 7319 toward NGC 7320c one after the other in two separate encounters is very difficult and unlikely, because the second encounter usually destroys or distorts the already-generated tidal structure. Our models suggest the two long tails may be formed simultaneously from a single encounter between NGC 7319 and 7320c, resulting in a thinner and denser inner tail than the outer one. The tails then also run parallel to each other as observed. The model results support the ideas that the group-wide shock detected in multi-wavelength observations between NGC 7319 and 7318b and the starburst region north of NGC 7318b are triggered by the high-speed collision between NGC 7318b and the intergalactic gas. Our models show that a gas bridge is formed by the high-speed collision and clouds in the bridge continue to interact for some tens of millions of years after the impact. This produces many small shocks in that region, resulting a much longer cooling time than that of a single impact shock.
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.
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