Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Luminous X-Ray Halos of Two Compact Elliptical Galaxies
256
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
There is mounting evidence that compact elliptical galaxies (CEGs) are local analogs of the high-redshift red nuggets thought to represent progenitors of todays early-type galaxies (ETGs). We report the discovery of extended X-ray emission from a hot interstellar / intragroup medium in two CEGs, Mrk 1216 and PGC 032873, using shallow archival Chandra observations. We find that PGC 032873 has an average gas temperature $k_BT=0.67pm 0.06$ keV within a radius of 15 kpc, and a luminosity $L_{rm x} = (1.8pm 0.2)times 10^{41}$ erg s$^{-1}$ within a radius of 100kpc. For Mrk 1216, which is closer and more luminous $[L_{rm x}(rm <100~kpc) = (12.1pm 1.9)times 10^{41}$ erg s$^{-1}]$, we performed a spatially resolved spectral analysis in 7 annuli out to a radius of 73 kpc. Using an entropy-based hydrostatic equilibrium (HE) procedure, we obtain a good constraint on the $H$-band stellar mass-to-light ratio, $M_{rm stars}/L_H=1.33pm 0.21$ solar, in good agreement with stellar dynamical (SD) studies, which supports the HE approximation. We obtain a density slope $2.22pm 0.08$ within $R_e$ consistent with other CEGs and normal local ETGs, while the dark matter (DM) fraction within $R_e$, $f_{rm DM}=0.20pm 0.07$, is similar to local ETGs. We place a constraint on the SMBH mass, $M_{rm BH} = (5pm 4)times 10^{9}, M_{odot}$, with a 90% upper limit of $M_{rm BH} = 1.4times 10^{10}, M_{odot}$, consistent with a recent SD measurement. We obtain a halo concentration $(c_{200}=17.5pm 6.7)$ and mass [$M_{200} = (9.6pm 3.7)times 10^{12}, M_{odot}$], where $c_{200}$ exceeds the mean $Lambda$CDM value ($approx 7$), consistent with a system that formed earlier than the general halo population. We suggest that these galaxies, which reside in group-scale halos, should be classified as fossil groups. (Abridged)
rate research
Read More
Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics, and stellar population properties, in the extended, low surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers, velocity dispersion and rotation profiles have been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the outer edge at ~150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles, and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicates that non-thermal components to the pressure may be important in the inner ~10 kpc, and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances indicate longer star formation histories pointing towards late accretion of the halo. This is consistent with independent evidence for on-going accretion, and suggests a connection to the observed size evolution of elliptical galaxies with redshift.
Without the interference of a number of events, galaxies may suffer in crowded environments (e.g., stripping, harassment, strangulation); isolated elliptical galaxies provide a control sample for the study of galaxy formation. We present the study of a sample of isolated ellipticals using imaging from a variety of telescopes, focusing on their globular cluster systems as tracers of their stellar halos. Our main findings are: (a) GC color bimodality is common even in the most isolated systems; (b) the specific frequency of GCs is fairly constant with galaxy mass, without showing an increase towards high-mass systems like in the case of cluster ellipticals; (c) on the other hand, the red fraction of GCs follows the same inverted V shape trend with mass as seen in cluster ellipticals; and (d) the stellar halos show low Sersic indices which are consistent with a major merger origin.
Cosmological simulations predict that early-type galaxies (ETGs) are the results of extended mass accretion histories. The latter are characterized by different numbers of mergers, mergers mass ratios and gas fractions, and timing. Depending on the sequence and nature of these mergers that follow the first phase of the in-situ star formation, these accretion histories may lead to ETGs that have low or high mass halos, and that rotate fast or slow. Since the stellar halos maintain the fossil records of the events that led to their formation, a discontinuity may be in place between the inner regions of ETGs and their outer halos, because the time required for the halos stars to exchange their energies and momenta is very long compared with the age of these systems. Exquisite deep photometry and extended spectroscopy for significant samples of ETGs are then used to quantify the occurrence and significance of such a transition in the galaxies structural and kinematical parameters. Once this transition radius is measured, its dependency with the effective radius of the galaxies light distribution and total stellar masses can be investigated. Such correlations can then be compared with the predictions of accreted, i.e. ex-situ vs. in-situ components from cosmological simulations to validate such models.
In order to investigate the formation mechanisms of the rare compact elliptical galaxies (cE) we have compiled a sample of 25 cEs with good SDSS spectra, covering a range of stellar masses, sizes and environments. They have been visually classified according to the interaction with their host, representing different evolutionary stages. We have included clearly disrupted galaxies, galaxies that despite not showing signs of interaction are located close to a massive neighbor (thus are good candidates for a stripping process), and cEs with no host nearby. For the latter, tidal stripping is less likely to have happened and instead they could simply represent the very low-mass, faint end of the ellipticals. We study a set of properties (structural parameters, stellar populations, star formation histories and mass ratios) that can be used to discriminate between an intrinsic or stripped origin. We find that one diagnostic tool alone is inconclusive for the majority of objects. However, if we combine all the tools a clear picture emerges. The most plausible origin, as well as the evolutionary stage and progenitor type, can be then determined. Our results favor the stripping mechanism for those galaxies in groups and clusters that have a plausible host nearby, but favors an intrinsic origin for those rare cEs without a plausible host and that are located in looser environments.
Elliptical galaxies have hot coronae with X-ray luminosities and mean gas temperatures that span over wide ranges. This variation can be partially due to the energy budget of the hot gas, that depends on the host galaxy structure and internal kinematics. With the aid of realistic axisymmetric galaxy models, we performed a diagnostic study focussed on the effects of galaxy flattening and rotational support on the hot gas temperature.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
