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Register a new userInfrared Emission from the Nearby Cool Core Cluster Abell 2597
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We observed the brightest central galaxy (BCG) in the nearby (z=0.0821) cool core galaxy cluster Abell 2597 with the IRAC and MIPS instruments on board the Spitzer Space Telescope. The BCG was clearly detected in all Spitzer bandpasses, including the 70 and 160 micron wavebands. We report aperture photometry of the BCG. The spectral energy distribution exhibits a clear excess in the FIR over a Rayleigh-Jeans stellar tail, indicating a star formation rate of ~4-5 solar masses per year, consistent with the estimates from the UV and its H-alpha luminosity. This large FIR luminosity is consistent with that of a starburst or a Luminous Infrared Galaxy (LIRG), but together with a very massive and old population of stars that dominate the energy output of the galaxy. If the dust is at one temperature, the ratio of 70 to 160 micron fluxes indicate that the dust emitting mid-IR in this source is somewhat hotter than the dust emitting mid-IR in two BCGs at higher-redshift (z~0.2-0.3) and higher FIR luminosities observed earlier by Spitzer, in clusters Abell 1835 and Zwicky 3146.
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We present a detailed X-ray study of the intracluster medium (ICM) of the nearby, cool-core galaxy cluster Abell 478, with Chandra and XMM observations. Using a wavelet smoothing hardness analysis, we derive detailed temperature maps of A478, revealing a surprising amount of temperature structure. The broad band Chandra spectral fits yield temperatures which are significantly hotter than those from XMM, but the Fe ionization temperature shows good agreement. We show that the temperature discrepancy is slightly reduced when comparing spectra from regions selected to enclose nearly isothermal gas. However, by simulating multi-temperature spectra and fitting them with a single temperature model, we find no significant difference between Chandra and XMM, indicating that non-isothermality cannot fully explain the discrepancy. We have discovered 4 hot spots located between 30--50 kpc from the cluster center, where the gas temperature is roughly a factor of 2 higher than in the surrounding material. We estimate the combined excess thermal energy present in these hot spots to be (3+/-1)x10^59 erg. The location of and amount of excess energy present in the hot spots are suggestive of a common origin within the cluster core, which hosts an active galactic nucleus. This cluster also possesses a pair of X-ray cavities coincident with weak radio lobes, as reported in a previous analysis, with an associated energy <10% of the thermal excess in the hot spots. The presence of these hot spots could indicate strong-shock heating of the ICM from the central radio source -- one of the first such detections in a cool core cluster. We also probe the mass distribution in the core and find it to be characterized by a logarithmic slope of -0.35+/-0.22, which is significantly flatter than an NFW cusp of -1. (abridged)
The nature of the interaction between low-excitation gas filaments at ~10^4 K, seen in optical line emission, and diffuse X-ray emitting coronal gas at ~10^7 K in the centers of galaxy clusters remains a puzzle. The presence of a strong, empirical correlation between the two gas phases is indicative of a fundamental relationship between them, though as yet of undetermined cause. The cooler filaments, originally thought to have condensed from the hot gas, could also arise from a merger or the disturbance of cool circumnuclear gas by nuclear activity. Here, we have searched for intrinsic line emission polarization in cool core galaxy clusters as a diagnostic of fundamental transport processes. Drawing on developments in solar astrophysics, direct energetic particle impact induced polarization holds the promise to definitively determine the role of collisional processes such as thermal conduction in the ISM physics of galaxy clusters, while providing insight into other highly anisotropic excitation mechanisms such as shocks, intense radiation fields and suprathermal particles. Under certain physical conditions, theoretical calculations predict of order ten percent polarization. Our observations of the filaments in four nearby cool core clusters place stringent upper limits (<0.1%) on the presence of emission line polarization, requiring that if thermal conduction is operative, the thermal gradients are not in the saturated regime. This limit is consistent with theoretical models of the thermal structure of filament interfaces.
Abell~1142 is a low-mass galaxy cluster at low redshift containing two comparable Brightest Cluster Galaxies (BCG) resembling a scaled-down version of the Coma Cluster. Our Chandra analysis reveals an X-ray emission peak, roughly 100 kpc away from either BCG, which we identify as the cluster center. The emission center manifests itself as a second beta-model surface brightness component distinct from that of the cluster on larger scales. The center is also substantially cooler and more metal rich than the surrounding intracluster medium (ICM), which makes Abell 1142 appear to be a cool core cluster. The redshift distribution of its member galaxies indicates that Abell 1142 may contain two subclusters with each containing one BCG. The BCGs are merging at a relative velocity of ~1200 km/s. This ongoing merger may have shock-heated the ICM from ~ 2 keV to above 3 keV, which would explain the anomalous L_X--T_X scaling relation for this system. This merger may have displaced the metal-enriched cool core of either of the subclusters from the BCG. The southern BCG consists of three individual galaxies residing within a radius of 5 kpc in projection. These galaxies should rapidly sink into the subcluster center due to the dynamical friction of a cuspy cold dark matter halo.
We present multi-wavelength observations of the centre of RXCJ1504.1-0248 - the galaxy cluster with the most luminous and relatively nearby cool core at z~0.2. Although there are several galaxies within 100 kpc of the cluster core, only the brightest cluster galaxy (BCG), which lies at the peak of the X-ray emission, has blue colours and strong line-emission. Approximately 80 Msun/yr of intracluster gas is cooling below X-ray emitting temperatures, similar to the observed UV star formation rate of ~140 Msun/yr. Most star formation occurs in the core of the BCG and in a 42 kpc long filament of blue continuum, line emission, and X-ray emission, that extends southwest of the galaxy. The surrounding filamentary nebula is the most luminous around any observed BCG. The number of ionizing stars in the BCG is barely sufficient to ionize and heat the nebula, and the line ratios indicate an additional heat source is needed. This heat source can contribute to the Halpha-deduced star formation rates (SFRs) in BCGs and therefore the derived SFRs should only be considered upper limits. AGN feedback can slow down the cooling flow to the observed mass deposition rate if the black hole accretion rate is of the order of 0.5 Msun/yr at 10% energy output efficiency. The average turbulent velocity of the nebula is vturb ~325 km/s which, if shared by the hot gas, limits the ratio of turbulent to thermal energy of the intracluster medium to less than 6%.
We present the properties of intracluster medium (ICM) in the cool core of the massive cluster of galaxies Abell 1835 obtained with the data by $ Chandra$ $X$-$ray$ $Observatory$. We find distinctive spiral patterns with the radius of 70 kpc (or 18 arcsec) as a whole in the residual image of X-ray surface brightness after the 2-dimensional ellipse model of surface brightness is subtracted. The size is smaller by a factor of 2 -- 4 than that of other clusters known to have a similar pattern. The spiral patterns consist of two arms. One of them appears as positive, and the other does as negative excesses in the residual image. Their X-ray spectra show that the ICM temperatures in the positive- and negative-excess regions are $5.09^{+0.12}_{-0.13}$ keV and $6.52^{+0.18}_{-0.15}$ keV, respectively. In contrast, no significant difference is found in the abundance or pressure, the latter of which suggests that the ICM in the two regions of the spiral patterns is in pressure equilibrium or close. The spatially-resolved X-ray spectroscopy of the central region ($r<40$ arcsec) divided into 92 sub-regions indicates that Abell 1835 is a typical cool core cluster. We also find that the spiral patterns extend from the cool core out to the hotter surrounding ICM. The residual image reveals some lumpy sub-structure in the cool core. The line-of-sight component of the disturbance velocity responsible for the sub-structures is estimated to be lower than 600 km/s. Abell 1835 may be now experiencing an off-axis minor merger.
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