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Register a new userFar-infrared emission from intergalactic medium in Stephans Quintet revealed by AKARI
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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.
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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 Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV line spectroscopy and integral-field unit observations of the intergalactic medium (IGM) in the Stephans Quintet (SQ) galaxy group. SQ hosts a 30 kpc long shocked ridge triggered by a galaxy collision at a relative velocity of 1000 km/s, where large amounts of cold (10-100 K) and warm (100-5000 K) molecular gas coexist with a hot plasma. COS spectroscopy along five lines-of-sight, probing 1 kpc-diameter regions in the IGM, reveals very broad (~2000 km/s) and powerful Ly$alpha$ line emission with complex line shapes. These Lyman-alpha line profiles are often similar to, or sometimes much broader than line profiles obtained in H$beta$, [CII], and CO (1-0) emission along the same lines-of-sight. In these cases, the breadth of the Ly$alpha$ emission, compared with H$beta$, implies resonance scattering. Line ratios of Ly$alpha$/H$beta$ for the two COS pointings closest to the center of the shocked ridge are close to the Case B recombination value, suggesting that at these positions Ly$alpha$ photons escape through scattering in a low density medium free of dust. Some Ly$alpha$ spectra show suppressed velocity components compared with [CII] and H$beta$, implying that some of the Ly$alpha$ photons are absorbed. Scattering indicates that the neutral gas of the IGM is clumpy, with multiple clumps along a given line of sight. Remarkably, over more than four orders of magnitude in temperature, the powers radiated by the multi-phase IGM in X-rays, Ly$alpha$, H$_2$, [CII] are comparable within a factor of a few. We suggest that both shocks and mixing layers co-exist and contribute to the energy dissipation associated with a turbulent energy cascade. This may be important for the cooling of gas at higher redshifts, where the metal content is lower than in this local system, and a high amplitude of turbulence more common.
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 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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