Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Unusually High Halo Concentration of the Fossil Group NGC 6482: Evidence for Weak Adiabatic Contraction
103
0
0.0
(
0
)
Authors
David A. Buote
Ask ChatGPT about the research
No Arabic abstract
We revisit the massive isolated elliptical galaxy / fossil group NGC 6482 for which previous X-ray studies of a modest Chandra observation obtained a very uncertain, but also possibly very high, halo concentration. We present new measurements of the hot gas surface brightness, temperature, and iron abundance using the modest Chandra observation and a previously unpublished Suzaku observation, the latter of which allows measurements of the gas properties to be extended out to ~r_2500. By constructing hydrostatic equilibrium models of the gas with separate components for the gas, BCG stellar mass, and the dark matter (DM), we measure c_200 = 32.2 +/- 7.1 and M_200 = (4.5 +/- 0.6 x 10^12 M_sun using an NFW DM profile. For a halo of this mass, c_200 exceeds the mean value (7.1) expected for relaxed LCDM halos by $3.5 sigma$ in terms of the observational error, and by $6 sigma$ considering the intrinsic scatter in the LCDM c-M relation, which situates NGC 6482 as the most extreme outlier known for a fossil system. We explored several variants of adiabatic contraction (AC) models and, while the AC models provide fits of the same quality as the un-contracted models, they do have the following advantages: (1) smaller c_200 that is less of an outlier in the LCDM c-M relation, and (2) baryon fractions that agree better with the mean cosmic value. While the standard AC prescriptions yield a BCG stellar mass that is uncomfortably small compared to results from stellar population synthesis (SPS) models, a weaker AC variant that artificially shuts off cooling and star formation at z=2 yields the same stellar mass as the un-contracted models. For these reasons, we believe our X-ray analysis prefers this weaker AC variant applied to either an NFW or Einasto DM halo. Finally, the BCG stellar mass strongly favors SPS models with a Chabrier or Kroupa IMF over a Salpeter IMF. (Abridged)
rate research
Read More
Fossil groups are considered the end product in a galaxy groups evolution -- a massive central galaxy that dominates the luminosity budget of the group, as the outcome of efficient merging between intermediate-luminosity members. Little is however known about the faint satellite systems of fossil groups. Here we present a SUBARU/Suprime-Cam wide-field, deep imaging study in the B- and R-band of the nearest fossil group NGC 6482 (M_{tot}sim4times10^{12}M_{sun}), covering the virial radius out to 310 kpc. We perform detailed completeness estimations and select group member candidates by a combination of automated object detection and visual inspection. A fiducial sample of 48 member candidates down to M_R -10.5 mag is detected, making this study the deepest of a fossil group up to now. We investigate the photometric scaling relations, the colour-magnitude relation, and the luminosity function of our galaxy sample. We find evidence of recent and ongoing merger events among bright group galaxies. The colour-magnitude relation is comparable to that of nearby galaxy clusters, and exhibits significant scatter at the faintest luminosities. The completeness-corrected luminosity function is dominated by early-type dwarfs and is characterized by a faint end slope alpha=-1.32pm0.05. We conclude that the NGC 6482 fossil group shows photometric properties consistent with those of regular galaxy clusters and groups, including a normal abundance of faint satellites.
We have analyzed the Chandra archival data of NGC 1132, a well-known fossil group, i.e. a system expected to be old and relaxed long after the giant elliptical galaxy assembly. Instead, the Chandra data reveal that the hot gas morphology is disturbed and asymmetrical, with a cold front following a possible bow shock. We discuss possible origins of the disturbed hot halo, including sloshing by a nearby object, merger, ram pressure by external hotter gas and nuclear outburst. We consider that the first two mechanisms are likely explanations for the disturbed hot halo, with a slight preference for a minor merger with a low impact parameter because of the match with simulations and previous optical observations. In this case, NGC 1132 may be a rare example of unusual late mergers seen in recent simulations. Regardless of the origin of the disturbed hot halo, the paradigm of the fossil system needs to be reconsidered.
We studied the temperature and metal abundance distributions of the intra-cluster medium (ICM) in a group of galaxies NGC 1550 observed with Suzaku. The NGC 1550 is classified as a fossil group, which have few bright member galaxies except for the central galaxy. Thus, such a type of galaxy is important to investigate how the metals are enriched to the ICM. With the Suzaku XIS instruments, we directly measured not only Si, S, and Fe lines but also O and Mg lines and obtained those abundances to an outer region of ~0.5 r_180 for the first time, and confirmed that the metals in the ICM of such a fossil group are indeed extending to a large radius. We found steeper gradients for Mg, Si, S, and Fe abundances, while O showed almost flat abundance distribution. Abundance ratios of alpha-elements to Fe were similar to those of the other groups and poor clusters. We calculated the number ratio of type II to type Ia supernovae for the ICM enrichment to be 2.9 +- 0.5 within 0.1 r_180, and the value was consistent with those for the other groups and poor clusters observed with Suzaku. We also calculated metal mass-to-light ratios (MLRs) for Fe, O and Mg with B-band and K-band luminosities of the member galaxies of NGC 1550. The derived MLRs were comparable to those of NGC 5044 group in the r<0.1 r_180 region, while those of NGC 1550 are slightly higher than those of NGC 5044 in the outer region.
Groups are the most common association of galaxies in the Universe, found in different configuration states such as loose, compact and fossil groups. We have studied the galaxy group MKW 4s, dominated by the giant early-type galaxy NGC 4104 at z=0.0282. Our aim was to understand the evolutionary stage of this group and to place it within the framework of the standard LambdaCDM cosmological scenario. We have obtained deep optical data with CFHT/Megacam (g and r bands) and we have applied both the galfit 2D image fitting program and the IRAF/ellipse 1D radial method to model the brightest group galaxy (BGG) and its extended stellar envelope. We have also analysed publicly available XMM-Newton and Chandra X-ray data. From N-body simulations of dry-mergers with different mass ratios of the infalling galaxy, we could constrain the dynamical stage of this system. Our results show a stellar shell system feature in NGC 4104 and an extended envelope that was reproduced by our numerical simulations of a collision with a satellite galaxy about 4--6 Gyr ago. The initial pair of galaxies had a mass ratio of at least 1:3. Taking into account the stellar envelope contribution to the total r band magnitude and the X-ray luminosity, MKW 4s falls into the category of a fossil group. Our results show that we are witnessing a rare case of a shell elliptical galaxy in a forming fossil group.
We present a new insight on NGC 6034 and UGC 842, two groups of galaxies previously reported in the literature as being fossil groups. The study is based on optical photometry and spectroscopy obtained with the CTIO Blanco telescope and Sloan Digital Sky Survey archival data. We find that NGC 6034 is embedded in a large structure, dominated by three rich clusters and other small groups. Its first and next four ranked galaxies have magnitude differences in the r band and projected distances which violate the optical criteria to classify it as a fossil group. We confirm that the UGC 842 group is a fossil group, but with about half the velocity dispersion that is reported in previous works. The velocity distribution of its galaxies reveals the existence of two structures in its line of sight, one with sigmaV ~ 223 km/s and another with sigmaV ~ 235 km/s, with a difference in velocity of ~820 km/s. The main structure is dominated by passive galaxies, while these represent ~60% of the second structure. The X-ray temperature for the intragroup medium of a group with such a velocity dispersion is expected to be kT ~0.5-1 keV, against the observed value of kT ~1.9 keV reported in the literature. This result makes UGC 842 a special case among fossil groups because (1) it represents more likely the interaction between two small groups, which warms the intragroup medium and/or (2) it could constitute evidence that member galaxies lost energy in the process of spiraling toward the group center, and decreased the velocity dispersion of the system. As far as we know, UGC 842 is the first low-mass fossil group studied in detail.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
