68 research outputs found
Capturing the 3D motion of an infalling galaxy via fluid dynamics
The Fornax Cluster is the nearest galaxy cluster in the southern sky. NGC 1404 is a bright elliptical galaxy falling through the intracluster medium of the Fornax Cluster. The sharp leading edge of NGC 1404 forms a classical "cold front" that separates 0.6 keV dense interstellar medium and 1.5 keV diffuse intracluster medium. We measure the angular pressure variation along the cold front using a very deep (670\,ksec) {\sl Chandra} X-ray observation. We are taking the classical approach -- using stagnation pressure to determine a substructure's speed -- to the next level by not only deriving a general speed but also directionality which yields the complete velocity field as well as the distance of the substructure directly from the pressure distribution. We find a hydrodynamic model consistent with the pressure jump along NGC 1404's atmosphere measured in multiple directions. The best-fit model gives an inclination of 33∘ and a Mach number of 1.3 for the infall of NGC 1404, in agreement with complementary measurements of the motion of NGC 1404. Our study demonstrates the successful treatment of a highly ionized ICM as ideal fluid flow, in support of the hypothesis that magnetic pressure is not dynamically important over most of the virial region of galaxy clusters
Abell 1201: a Minor merger at second core passage
We present an analysis of the structures and dynamics of the merging cluster
Abell~1201, which has two sloshing cold fronts around a cooling core, and an
offset gas core approximately 500kpc northwest of the center. New Chandra and
XMM-Newton data reveal a region of enhanced brightness east of the offset core,
with breaks in surface brightness along its boundary to the north and east.
This is interpreted as a tail of gas stripped from the offset core. Gas in the
offset core and the tail is distinguished from other gas at the same distance
from the cluster center chiefly by having higher density, hence lower entropy.
In addition, the offset core shows marginally lower temperature and metallicity
than the surrounding area. The metallicity in the cool core is high and there
is an abrupt drop in metallicity across the southern cold front. We interpret
the observed properties of the system, including the placement of the cold
fronts, the offset core and its tail in terms of a simple merger scenario. The
offset core is the remnant of a merging subcluster, which first passed
pericenter southeast of the center of the primary cluster and is now close to
its second pericenter passage, moving at ~1000 km/s. Sloshing excited by the
merger gave rise to the two cold fronts and the disposition of the cold fronts
reveals that we view the merger from close to the plane of the orbit of the
offset core.Comment: accepted by Ap
A Chandra study of the large-scale shock and cool filaments in Hydra A: Evidence for substantial gas dredge-up by the central outburst
We present the results of a Chandra study of the Hydra A galaxy cluster,
where a powerful AGN outburst created a large-scale cocoon shock. We
investigated possible azimuthal variations in shock strength and shape, finding
indications for a weak shock with a Mach number in the range ~1.2-1.3. We
measured the temperature change across the shock front. However, the detection
of a temperature rise in the regions immediately inside of the front is
complicated by the underlying temperature profile of the cluster atmosphere. We
measured the global temperature profile of the cluster up to 700 kpc, which
represents the farthest measurement obtained with Chandra for this cluster. A
"plateau" in the temperature profile in the range ~70-150 kpc indicates the
presence of cool gas, which is likely the result of uplift of material by the
AGN outburst. After masking the cool filaments visible in the hardness ratio
map, the plateau disappears and the temperature profile recovers a typical
shape with a peak around 190 kpc, just inside the shock front. However, it is
unlikely that such a temperature feature is produced by the shock as it is
consistent with the general shape of the temperature profiles observed for
relaxed galaxy clusters. We studied the spectral properties of the cool
filaments finding evidence that ~10^11 M_sun of low-entropy material has been
dredged up by the rising lobes from the central 30 kpc to the observed current
position of 75-150 kpc. The energy required to lift the cool gas is >~2.2 x
10^60 erg, which is comparable to the work required to inflate the cavities and
is ~25% of the total energy of the large-scale shock. Our results show that the
AGN feedback in Hydra A is acting not only by directly heating the gas, but
also by removing a substantial amount of potential fuel for the SMBH.Comment: 11 pages, 9 figures, accepted for publication in ApJ (version with
full resolution figures available at
http://www.bo.astro.it/~myriam/files/papers/gitti-hydra.pdf
Chandra observations of nuclear outflows in the elliptical galaxy NGC 4552 in the virgo cluster
We use a 54.4 ks Chandra observation to study nuclear outflow activity in NGC 4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandra images in the 0.5–2 keV band show two ringlike features ∼1.7 kpc in diameter in the core of NGC 4552, as reported previously by Filho et al. We use spherically symmetric point explosion shock models to argue that the shape of the surface brightness profile across the rims of the rings and the temperature of hot gas in the rings are consistent with a Mach 1.7 shock carrying mean mechanical power Lshock ∼ 3 × 1041 ergs s-1 produced by a ∼1.4 × 1055 ergs nuclear outburst ∼1–2 Myr ago. We find the gas temperature in the central ∼100 pc of the galaxy to be 1.0 ± 0.2 keV, hotter than elsewhere in the galaxy, suggesting that we may be directly observing the reheating of the galaxy ISM by the outburst
Dark Matter Subhalos and the X-ray Morphology of the Coma Cluster
Structure formation models predict that clusters of galaxies contain numerous
massive subhalos. The gravity of a subhalo in a cluster compresses the
surrounding intracluster gas and enhances its X-ray emission. We present a
simple model, which treats subhalos as slow moving and gasless, for computing
this effect. Recent weak lensing measurements by Okabe et al. have determined
masses of ~ 10^13 solar masses for three mass concentrations projected within
300 kpc of the center of the Coma Cluster, two of which are centered on the
giant elliptical galaxies NGC 4889 and NGC 4874. Adopting a smooth spheroidal
beta-model for the gas distribution in the unperturbed cluster, we model the
effect of these subhalos on the X-ray morphology of the Coma Cluster, comparing
our results to Chandra and XMM-Newton X-ray data. The agreement between the
models and the X-ray morphology of the central Coma Cluster is striking. With
subhalo parameters from the lensing measurements, the distances of the three
subhalos from the Coma Cluster midplane along our line of sight are all tightly
constrained. Using the model to fit the subhalo masses for NGC 4889 and NGC
4874 gives 9.1 x 10^12 and 7.6 x 10^12 solar masses, respectively, in good
agreement with the lensing masses. These results lend strong support to the
argument that NGC 4889 and NGC 4874 are each associated with a subhalo that
resides near the center of the Coma Cluster. In addition to constraining the
masses and 3-d location of subhalos, the X-ray data show promise as a means of
probing the structure of central subhalos.Comment: ApJ, in press. Matches the published versio
XMM-Newton Observation of an X-ray Trail Between the Spiral Galaxy NGC6872 and the Central Elliptical NGC6876 in the Pavo Group
We present XMM-Newton observations of a trail of enhanced X-rayemission
extending along the 8'.7 X 4' region between the spiral NGC6872 and the
dominant elliptical NGC6876 in the Pavo Group,the first known X-ray trail
associated with a spiral galaxy in a poor galaxy group and, with projected
length of 90 kpc, one of the longest X-ray trails observed in any system. The
X-ray surface brightness in the trail region is roughly constant beyond ~20 kpc
of NGC6876 in the direction of NGC6872. The trail is hotter (~ 1 keV) than the
undisturbed Pavo IGM (~0.5 keV) and has low metal abundances (0.2 Zsolar). The
0.5-2 keV luminosity of the trail, measured using a 67 X 90 kpc rectangular
region, is 6.6 X 10^{40} erg/s. We compare the properties of gas in the trail
to the spectral properties of gas in the spiral NGC6872 and in the elliptical
NGC6876 to constrain its origin. We suggest that the X-ray trail is either IGM
gas gravitationally focused into a Bondi-Hoyle wake, a thermal mixture of ~64%
Pavo IGM gas with ~36% galaxy gas that has been removed from the spiral NGC6872
by turbulent viscous stripping, or both, due to the spiral's supersonic motion
at angle xi ~ 40 degrees with respect to the plane of the sky, past the Pavo
group center (NGC6876) through the densest region of the Pavo IGM. Assuming xi
= 40 degrees and a filling factor eta in a cylindrical volume with radius 33
kpc and projected length 90 kpc, the mean electron density and total hot gas
mass in the trail is 9.5 X 10^{-4}*eta^{-1/2} cm^{-3} and 1.1 X
10^{10}*eta^{1/2} Msolar, respectively.Comment: typos corrected in Eq. 7 & 8, figures and discussion unchanged, 39
pages, 11 postscript figures, submitted to Ap
Buoyant AGN bubbles in the quasi-isothermal potential of NGC 1399
The Fornax Cluster is a low-mass cool-core galaxy cluster. We present a deep
{\sl Chandra} study of NGC 1399, the central dominant elliptical galaxy of
Fornax. The cluster center harbors two symmetric X-ray cavities coincident with
a pair of radio lobes fed by two collimated jets along a north-south axis. A
temperature map reveals that the AGN outburst has created a channel filled with
cooler gas out to a radius of 10 kpc. The cavities are surrounded by cool
bright rims and filaments that may have been lifted from smaller radii by the
buoyant bubbles. X-ray imaging suggests a potential ghost bubble of
5\,kpc diameter to the northwest. We find that the amount of gas lifted by AGN
bubbles is comparable to that which would otherwise cool, demonstrating that
AGN driven outflow is effective in offsetting cooling in low-mass clusters. The
cluster cooling time scale is times longer than the dynamical time scale,
which is consistent with the lack of cold molecular gas at the cluster center.
The X-ray hydrostatic mass is consistent within 10\% with the total mass
derived from the optical data. The observed entropy profile rises linearly,
following a steeper slope than that observed at the centers of massive
clusters; gas shed by stars in NGC 1399 may be incorporated in the hot phase.
However, it is far-fetched for supernova-driven outflow to produce and maintain
the thermal distribution in NGC 1399 and it is in tension with the metal
content in the hot gas.Comment: 11 pages, 6 figures, Matches the version published in Ap
A detached double X-ray tail in the merging galaxy cluster Z8338 with a large double tail
When subhalos infall into galaxy clusters, their gas content is ram pressure
stripped by the intracluster medium (ICM) and may turn into cometary tails. We
report the discovery of two spectacular X-ray double tails in a single galaxy
cluster, Z8338, revealed by 70 ks Chandra observations. The brighter one, with
an X-ray bolometric luminosity of , is a
detached tail stripped from the host halo and extended at least 250 kpc in
projection. The head of the detached tail is a cool core with the front tip of
the cold front 30 kpc away from the nucleus of its former host galaxy.
The cooling time of the detached cool core is Gyr. For the detached
gas, the gravity of the once-associated dark matter halo further enhances the
Rayleigh-Taylor (RT) instability. From its survival, we find that a magnetic
field of a few G is required to suppress the hydrodynamic instability. The
X-ray temperature in the tail increases from 0.9 keV at the front tip to 1.6
keV in the wake region, which suggests the turbulent mixing with the hotter
ICM. The fainter double X-ray tail, with a total X-ray luminosity of , appears to stem from the cool core of a
subcluster in Z8338, and likely was formed during the ongoing merger. This
example suggests that X-ray cool cores can be displaced and eventually
destroyed by mergers, while the displaced cool cores can survive for some
extended period of time.Comment: 11 pages, 8 figures, MNRAS accepte
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