451 research outputs found
Role of Electon Excitation and Nature of Molecular Gas in Cluster Central Elliptical Galaxies
We present observations in CO(3-2) that, combined with previous observations
in CO(2-1), constrain the physical properties of the filamentary molecular gas
in the central 6.5 kpc of NGC 1275, the central giant elliptical galaxy
of the Perseus cluster. We find this molecular gas to have a temperature
K and a density -, typically
warmer and denser than the bulk of Giant Molecular Clouds (GMCs) in the Galaxy.
Bathed in the harsh radiation and particle field of the surrounding
intracluster X-ray gas, the molecular gas likely has a much higher ionization
fraction than that of GMCs. For an ionization fraction of ,
similar to that of Galactic diffuse ()
partially-molecular clouds that emit in HCN(1-0) and HCO(1-0), we show that
the same gas traced in CO can produce the previously reported emissions in
HCN(3-2), HCO(3-2), and CN(2-1) from NGC 1275; the dominant source of
excitation for all the latter molecules is collisions with electrons. To
prevent collapse, as evidenced by the lack of star formation in the molecular
filaments, they must consist of thin strands that have cross-sectional radii
0.2-2 pc if supported solely by thermal gas pressure; larger radii
are permissible if turbulence or poloidal magnetic fields provide additional
pressure support. We point out that the conditions required to relate CO
luminosities to molecular gas masses in our Galaxy are unlikely to apply in
cluster central elliptical galaxies. Rather than being virialized structures
analogous to GMCs, we propose that the molecular gas in NGC 1275 comprises
pressure-confined structures created by turbulent flows.Comment: 41 pages, 1 table, 12 figures; accepted by Ap
The diverse hot gas content and dynamics of optically similar low-mass elliptical galaxies
The presence of hot X-ray emitting gas is ubiquitous in massive early-type
galaxies. However, much less is known about the content and physical status of
the hot X-ray gas in low-mass ellipticals. In the present paper we study the
X-ray gas content of four low-mass elliptical galaxies using archival Chandra
X-ray observations. The sample galaxies, NGC821, NGC3379, NGC4278, and NGC4697,
have approximately identical K-band luminosities, and hence stellar masses, yet
their X-ray appearance is strikingly different. We conclude that the unresolved
emission in NGC821 and NGC3379 is built up from a multitude of faint compact
objects, such as coronally active binaries and cataclysmic variables. Despite
the non-detection of X-ray gas, these galaxies may host low density, and hence
low luminosity, X-ray gas components, which undergo a Type Ia supernova (SN Ia)
driven outflow. We detect hot X-ray gas with a temperature of kT ~ 0.35 keV in
NGC4278, the component of which has a steeper surface brightness distribution
than the stellar light. Within the central 50 arcsec (~3.9 kpc) the estimated
gas mass is ~3 x 10^7 M_sun, implying a gas mass fraction of ~0.06%. We
demonstrate that the X-ray gas exhibits a bipolar morphology in the
northeast-southwest direction, indicating that it may be outflowing from the
galaxy. The mass and energy budget of the outflow can be maintained by evolved
stars and SNe Ia, respectively. The X-ray gas in NGC4697 has an average
temperature of kT ~ 0.3 keV, and a significantly broader distribution than the
stellar light. The total gas mass within 90 arcsec (~5.1 kpc) is ~2.1 x 10^8
M_sun, hence the gas mass fraction is ~0.4%. Based on the distribution and
physical parameters of the X-ray gas, we conclude that it is most likely in
hydrostatic equilibrium, although a subsonic outflow may be present.Comment: 14 pages, 8 figures, 3 tables, accepted for publication in Ap
Hot X-ray coronae around massive spiral galaxies: a unique probe of structure formation models
Luminous X-ray gas coronae in the dark matter halos of massive spiral
galaxies are a fundamental prediction of structure formation models, yet only a
few such coronae have been detected so far. In this paper, we study the hot
X-ray coronae beyond the optical disks of two normal massive spirals, NGC1961
and NGC6753. Based on XMM-Newton X-ray observations, hot gaseous emission is
detected to ~60 kpc - well beyond their optical radii. The hot gas has a
best-fit temperature of kT~0.6 keV and an abundance of ~0.1 Solar, and exhibits
a fairly uniform distribution, suggesting that the quasi-static gas resides in
hydrostatic equilibrium in the potential well of the galaxies. The bolometric
luminosity of the gas in the (0.05-0.15)r_200 region (r_200 is the virial
radius) is ~6e40 erg/s for both galaxies. The baryon mass fractions of NGC1961
and NGC6753 are f_b~0.1, which fall short of the cosmic baryon fraction. The
hot coronae around NGC1961 and NGC6753 offer an excellent basis to probe
structure formation simulations. To this end, the observations are confronted
with the moving mesh code Arepo and the smoothed particle hydrodynamics code
Gadget. Although neither model gives a perfect description, the observed
luminosities, gas masses, and abundances favor the Arepo code. Moreover, the
shape and the normalization of the observed density profiles are better
reproduced by Arepo within ~0.5r_200. However, neither model incorporates
efficient feedback from supermassive black holes or supernovae, which could
alter the simulated properties of the X-ray coronae. With the further advance
of numerical models, the present observations will be essential in constraining
the feedback effects in structure formation simulations.Comment: 19 pages, 13 figures, 6 tables, accepted for publication in Ap
Isotropic AGN Heating with Small Radio Quiet Bubbles in the NGC 5044 Group
(abridged) A Chandra observation of the X-ray bright group NGC 5044 shows
that the X-ray emitting gas has been strongly perturbed by recent outbursts
from the central AGN and also by motion of the central dominant galaxy relative
to the group gas. The NGC 5044 group hosts many small radio quiet cavities with
a nearly isotropic distribution, cool filaments, a semi-circular cold front and
a two-armed spiral shaped feature of cool gas. A GMRT observation of NGC 5044
at 610 MHz shows the presence of extended radio emission with a "torus-shaped"
morphology. The largest X-ray filament appears to thread the radio torus,
suggesting that the lower entropy gas within the filament is material being
uplifted from the center of the group. The radio emission at 235 MHz is much
more extended than the emission at 610 MHz, with little overlap between the two
frequencies. One component of the 235 MHz emission passes through the largest
X-ray cavity and is then deflected just behind the cold front. A second
detached radio lobe is also detected at 235 MHz beyond the cold front. All of
the smaller X-ray cavities in the center of NGC 5044 are undetected in the GMRT
observations. Since the smaller bubbles are probably no longer momentum driven
by the central AGN, their motion will be affected by the group "weather" as
they buoyantly rise outward. Hence, most of the enthalpy within the smaller
bubbles will likely be deposited near the group center and isotropized by the
group weather. The total mechanical power of the smaller radio quiet cavities
is erg s which is sufficient to suppress about
one-half of the total radiative cooling within the central 10 kpc. This is
consistent with the presence of H emission within this region which
shows that at least some of the gas is able to cool
AGN Driven Weather and Multiphase Gas in the Core of the NGC 5044 Galaxy Group
A deep Chandra observation of the X-ray bright group, NGC 5044, shows that
the central region of this group has been strongly perturbed by repeated AGN
outbursts. These recent AGN outbursts have produced many small X-ray cavities,
cool filaments and cold fronts. We find a correlation between the coolest X-ray
emitting gas and the morphology of the Ha filaments. The Ha filaments are
oriented in the direction of the X-ray cavities, suggesting that the warm gas
responsible for the Halpha emission originated near the center of NGC 5044 and
was dredged up behind the buoyant, AGN-inflated X-ray cavities. A detailed
spectroscopic analysis shows that the central region of NGC 5044 contains
spatially varying amounts of multiphase gas. The regions with the most
inhomogeneous gas temperature distribution tend to correlate with the extended
235 MHz and 610 MHz radio emission detected by the GMRT. This may result from
gas entrainment within the radio emitting plasma or mixing of different
temperature gas in the regions surrounding the radio emitting plasma by AGN
induced turbulence. Accounting for the effects of multiphase gas, we find that
the abundance of heavy elements is fairly uniform within the central 100 kpc,
with abundances of 60-80% solar for all elements except oxygen, which has a
significantly sub-solar abundance. In the absence of continued AGN outbursts,
the gas in the center of NGC 5044 should attain a more homogeneous distribution
of gas temperature through the dissipation of turbulent kinetic energy and heat
conduction in approximately 10e8 yr. The presence of multiphase gas in NGC 5044
indicates that the time between recent AGN outbursts has been less than
approximately 10e8 yr
Chandra X-ray Observations of the Hydra A Cluster: An Interaction Between the Radio Source and the X-Ray-Emitting Gas
We present Chandra X-ray Observations of the Hydra A cluster of galaxies, and
we report the discovery of structure in the central 80 kpc of the cluster's
X-ray-emitting gas. The most remarkable structures are depressions in the X-ray
surface brightness, kpc diameter, that are coincident with Hydra
A's radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and
there is no evidence for shock-heated gas surrounding the radio lobes. We
suggest the gas within the surface brightness depressions was displaced as the
radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The
gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of
the brightest cluster galaxy (BCG), and the cooling time of the gas is Myr in the inner 10 kpc. These properties are consistent with the presence
of a \sim 34 \msunyr cooling flow within a 70 kpc radius. Bright X-ray
emission is present in the BCG surrounding a recently-accreted disk of nebular
emission and young stars. The star formation rate is commensurate with the
cooling rate of the hot gas within the volume of the disk, although the sink
for the material cooling at larger radii remains elusive.Comment: 4 pages, 3 figures; submitted to ApJ Letter
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