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Distribution of the temperature and metal abundance in the hot intra-cluster medium of the Virgo cluster of galaxies

1999
195 pages
Supervisor:
  • H. Inoue
Thesis: PhD
  • Kanagawa U.
(1999)
Report number:
  • ISAS-RN-745
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Abstract:
Spatially resolved X-ray spectra of the Virgo cluster of galaxies have been extensively studied with the ASCA satellite. The cluster consists of a main region surrounding M87 with a radius of about 2.5° and an extended south region reaching M49, which have been almost completely mapped with a total exposure time ∼\thicksim 700 ksec covering an area ∼19deg2\thicksim 19 deg^2. The observations have provided the first comprehensive view of the Virgo cluster in the energy range 0.5 - 10 keV. Based on the imaging data from ASCA and ROSAT, we masked out a large number of contaminating sources in the observed regions and investigated the temperature and metal abundance distributions for the pure intra-cluster medium (ICM) component. The energy spectrum in the central region within ∼10′\thicksim 10^\prime from M87 is represented by two temperatures of about 1 keV and 2.5 keV. Because the cool component is confined around M87, this component is likely to be interstellar medium (ISM) associated with M87. In larger scales, the ICM temperatnre shows a gradual decrease with distance from M87 from the central ∼\thicksim 2.5 keV to ∼\thicksim 1.7 keV around the M49 region. An unusual hot spectrum (kTkT > 4 keV), recently discovered by Kikuchi et al. (2000), is confirmed in the linking region between M87 and M49. Within ∼\thicksim 1 ° of M87 the ICM spectra can be fitted with iso-thcrmal models, whereas the outer region indicates significantly uneven temperature distribution with an amplitude of about 1 keV. The abundance distribution of Fe around M87 shows a central peak of about 0.6 solar followed by an almost flat profile at ∼\thicksim 0.3 solar up to a radius of 2°. Abundances of Si and S are around 2 solar in the center and decrease rapidly with radius to about 0.2 solar in the outer region. This abundance feature can be partly explained by a radial variation in the mixture of the hot ICM (with a metallicity 0.2 - 0.3 solar) and the cool α\alpha-rich ISM. The metal mass density distribution in the duster scale is similar to the ICM profile and extends more widely than the galaxy distribution, which is considered to be the source of the metals. Since the origin of the cool ISM is mostly stellar mass loss, the observed high abundance of Si and S suggests that the interstellar matter was enriched with α\alpha elements in the star-formation period due to type-II supernovae. As for the hot ICM component, the high Si/Fe abundance ratio in outer regions suggests that the Virgo cluster has a fairly deep potential well. The concentration of Fe in the central region for both hot and cool components suggests an enrichment by type-Ia supernovae exploding in a gas-poor environment. Temperature distribution in the whole observed field has been examined using energy spectra and hardness ratios, after corrections for the effects of point-spread function and stray light. The temperature shows a significant variation in the outer region, which is characterized by a typical spatial scale of 200 - 300 kpc (35′−50′35^\prime - 50^\prime). This scale roughly corresponds to a size of typical groups of galaxies. Our result suggests that the gas forms such clumps before falling into the gravitational potential of the main cluster, which causes local heating of ICM. This feature is consistent with the bottom-up scenario for the structure formation. The observed temperature variation indicates that such local heatings of the ICM arc occurring in various regions in the Virgo cluster within the recent 11 x 10910^9 yr. Comparison of the ICM temperature with optical velocity dispersion shows that βspec∼1.0±0.5\beta_{spec} \thicksim 1.0 \pm 0.5, which is the ratio of galaxy velocity dispersion to the ICM temperature, for a large region in the duster. Peculiar values of βspec>2.0\beta_{spec} > 2.0 are seen in some regions including the MSG subcluster. This suggests that this region is not yet undergoing a direct gas collisoin with the main M87 cluster.
Note:
  • Ph.D. Thesis •
  • Hard copy at Fermilab (library@fnal.gov)
  • thesis
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