12 research outputs found
Stripped elliptical galaxies as probes of ICM physics: II. Stirred, but mixed? Viscous and inviscid gas stripping of the Virgo elliptical M89
Elliptical galaxies moving through the intra-cluster medium (ICM) are
progressively stripped of their gaseous atmospheres. X-ray observations reveal
the structure of galactic tails, wakes, and the interface between the galactic
gas and the ICM. This fine-structure depends on dynamic conditions (galaxy
potential, initial gas contents, orbit in the host cluster), orbital stage
(early infall, pre-/post-pericenter passage), as well as on the still
ill-constrained ICM plasma properties (thermal conductivity, viscosity,
magnetic field structure). Paper I describes flow patterns and stages of
inviscid gas stripping. Here we study the effect of a Spitzer-like temperature
dependent viscosity corresponding to Reynolds numbers, Re, of 50 to 5000 with
respect to the ICM flow around the remnant atmosphere. Global flow patterns are
independent of viscosity in this Reynolds number range. Viscosity influences
two aspects: In inviscid stripping, Kelvin-Helmholtz instabilities (KHIs) at
the sides of the remnant atmosphere lead to observable horns or wings.
Increasing viscosity suppresses KHIs of increasing length scale, and thus
observable horns and wings. Furthermore, in inviscid stripping, stripped
galactic gas can mix with the ambient ICM in the galaxy's wake. This mixing is
suppressed increasingly with increasing viscosity, such that viscously stripped
galaxies have long X-ray bright, cool wakes. We provide mock X-ray images for
different stripping stages and conditions. While these qualitative results are
generic, we tailor our simulations to the Virgo galaxy M89 (NGC 4552), where
Re~ 50 corresponds to a viscosity of 10% of the Spitzer level. Paper III
compares new deep Chandra and archival XMM-Newton data to our simulations.Comment: ApJ in press. 16 pages, 16 figures. Text clarified, conclusions
unchange
Stripped elliptical galaxies as probes of ICM physics: I. Tails, wakes, and flow patterns in and around stripped ellipticals
Elliptical cluster galaxies are progressively stripped of their atmospheres
due to their motion through the intra-cluster medium (ICM). Deep X-ray
observations reveal the fine-structure of the galaxy's remnant atmosphere and
its gas tail and wake. This fine-structure depends on dynamic conditions
(galaxy potential, initial gas contents, orbit through the host cluster),
orbital stage (early infall, pre-/post-pericenter passage), and ICM plasma
properties (thermal conductivity, viscosity, magnetic field structure). We aim
to disentangle dynamic and plasma effects in order to use stripped ellipticals
as probes of ICM plasma properties. This first paper of a series investigates
the hydrodynamics of progressive gas stripping by means of inviscid
hydrodynamical simulations. We distinguish a long-lasting initial relaxation
phase and a quasi-steady stripping phase. During quasi-steady stripping, the
ICM flow around the remnant atmosphere resembles the flow around solid bodies,
including a `deadwater' region in the near wake. Gas is stripped from the
remnant atmosphere predominantly at its sides via Kelvin-Helmholtz
instabilities. The downstream atmosphere is largely shielded from the ICM wind
and thus shaped into a tail. Observationally, both, this `remnant tail' and the
stripped gas in the wake can appear as a `tail', but only in the wake can
galactic gas mix with the ambient ICM. While the qualitative results are
generic, the simulations presented here are tailored to the Virgo elliptical
galaxy M89 (NGC 4552) for the most direct comparison to observations. Papers II
and III of this series describe the effect of viscosity and compare to Chandra
and XMM-Newton observations, respectively.Comment: ApJ, in press. 19 pages, 13 figures. Clarifications added, text
restructured. Conclusions unchange
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 33â 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
The narrow X-ray tail and double H-alpha tails of ESO 137-002 in Abell 3627
We present the analysis of a deep Chandra observation of a ~2L_* late-type
galaxy, ESO 137-002, in the closest rich cluster A3627. The Chandra data reveal
a long (>40 kpc) and narrow tail with a nearly constant width (~3 kpc) to the
southeast of the galaxy, and a leading edge ~1.5 kpc from the galaxy center on
the upstream side of the tail. The tail is most likely caused by the nearly
edge-on stripping of ESO 137-002's ISM by ram pressure, compared to the nearly
face-on stripping of ESO 137-001 discussed in our previous work. Spectral
analysis of individual regions along the tail shows that the gas throughout it
has a rather constant temperature, ~1 keV, very close to the temperature of the
tails of ESO 137-001, if the same atomic database is used. The derived gas
abundance is low (~0.2 solar with the single-kT model), an indication of the
multiphase nature of the gas in the tail. The mass of the X-ray tail is only a
small fraction (<5%) of the initial ISM mass of the galaxy, suggesting that the
stripping is most likely at an early stage. However, with any of the single-kT,
double-kT and multi-kT models we tried, the tail is always "over-pressured"
relative to the surrounding ICM, which could be due to the uncertainties in the
abundance, thermal vs. non-thermal X-ray emission, or magnetic support in the
ICM. The H-alpha data from SOAR show a ~21 kpc tail spatially coincident with
the X-ray tail, as well as a secondary tail (~12 kpc long) to the east of the
main tail diverging at an angle of ~23 degrees and starting at a distance of
~7.5 kpc from the nucleus. At the position of the secondary H-alpha tail, the
X-ray emission is also enhanced at the ~2 sigma level. We compare the tails of
ESO 137-001 and ESO 137-002, and also compare the tails to simulations. Both
the similarities and differences of the tails pose challenges to the
simulations. Several implications are briefly discussed.Comment: 15 pages, 6 figures, accepted for publication in Ap
Stripped elliptical galaxies as probes of ICM physics : III. Deep Chandra observation of NGC 4552 - Measuring the viscosity of the intracluster medium
We present results from a deep (200 ks) Chandra observation of the early-type galaxy NGC 4552 (M89) which is falling into the Virgo cluster. Previous shallower X-ray observations of this galaxy showed a remnant gas core, a tail to the South of the galaxy, and twin `horns' attached to the northern edge of the gas core [machacek05a]. In our deeper data, we detect a diffuse, low surface brightness extension to the previously known tail, and measure the temperature structure within the tail. We combine the deep Chandra data with archival XMM-Newton observations to put a strong upper limit on the diffuse emission of the tail out to a large distance (10Ăthe radius of the remnant core) from the galaxy center. In our two previous papers [roediger15a,roediger15b], we presented the results of hydrodynamical simulations of ram pressure stripping specifically for M89 falling into the Virgo cluster and investigated the effect of ICM viscosity. In this paper, we compare our deep data with our specifically tailored simulations and conclude that the observed morphology of the stripped tail in NGC 4552 is most similar to the inviscid models. We conclude that, to the extent the transport processes can be simply modeled as a hydrodynamic viscosity, the ICM viscosity is negligible. More generally, any micro-scale description of the transport processes in the high-ÎČ plasma of the cluster ICM must be consistent with the efficient mixing observed in the stripped tail on macroscopic scales
The discovery of lensed radio and x-ray sources behind the frontier fields cluster MACS J0717.5+3745 with the JVLA and Chandra
© 2016. The American Astronomical Society. All rights reserved.. We report on high-resolution JVLA and Chandra observations of the Hubble Space Telescope (HST) Frontier Cluster MACS J0717.5+3745. MACS J0717.5+3745 offers the largest contiguous magnified area of any known cluster, making it a promising target to search for lensed radio and X-ray sources. With the high-resolution 1.0-6.5 GHz JVLA imaging in A and B configuration, we detect a total of 51 compact radio sources within the area covered by the HST imaging. Within this sample, we find seven lensed sources with amplification factors larger than two. None of these sources are identified as multiply lensed. Based on the radio luminosities, the majority of these sources are likely star-forming galaxies with star-formation rates (SFRs) of 10-50 yr -1 located at . Two of the lensed radio sources are also detected in the Chandra image of the cluster. These two sources are likely active galactic nuclei, given their 2-10 keV X-ray luminosities of âŒ10 43-44 erg s -1 . From the derived radio luminosity function, we find evidence for an increase in the number density of radio sources at , compared to a sample. Our observations indicate that deep radio imaging of lensing clusters can be used to study star-forming galaxies, with SFRs as low as âŒ10 M o yr -1 , at the peak of cosmic star formation history
Gas sloshing regulates and records the evolution of the Fornax Cluster
We present results of a joint Chandra and XMM-Newton analysis of the Fornax Cluster, the nearest galaxy cluster in the southern sky. Signatures of merger-induced gas sloshing can be seen in the X-ray image. We identify four sloshing cold fronts in the intracluster medium, residing at radii of 3 kpc (west), 10 kpc (northeast), 30 kpc (southwest), and 200 kpc (east). Despite spanning over two orders of magnitude in radius, all four cold fronts fall onto the same spiral pattern that wraps around the BCG NGC 1399, likely all initiated by the infall of NGC 1404. The most evident front is to the northeast, 10 kpc from the cluster center, which separates low-entropy high-metallicity gas and high-entropy low-metallicity gas. The metallicity map suggests that gas sloshing, rather than an AGN outburst, is the driving force behind the redistribution of the enriched gas in this cluster. The innermost cold front resides within the radius of the strong cool core. The sloshing timescale within the cooling radius, calculated from the BruntâVĂ€sĂ€lĂ€ frequency, is an order of magnitude shorter than the cooling time. It is plausible that gas sloshing is contributing to the heating of the cool core, provided that gas of different entropies can be mixed effectively via KelvinâHelmholtz instability. The estimated age of the outermost front suggests that this is not the first infall of NGC 1404
The recent growth history of the Fornax Cluster derived from simultaneous sloshing and gas stripping: Simulating the infall of NGC 1404
We derive the recent growth history of the Fornax Cluster, in particular the recent infall of the giant elliptical galaxy NGC 1404. We show, using a simple cluster minor merger simulation tailored to Fornax and NGC 1404, that a second or more likely third encounter between the two reproduces all the main merger features observed in both objects; we firmly exclude a first infall scenario. Our simulations reveal a consistent picture: NGC 1404 passed by NGC 1399 about 1.1â1.3 Gyr ago from the northeast to the southwest and is now almost at the point of its next encounter from the south. This scenario explains the sloshing patterns observed in Fornaxâa prominent northern cold front and an inner southern cold front. This scenario also explains the truncated atmosphere, the gas-stripping radius of NGC 1404, and its faint gas tail. Independent of the exact history, we can make a number of predictions. A detached bow shock south of NGC 1404 should exist, which is a remnant of the galaxy's previous infall at a distance from NGC 1404 between 450 and 750 kpc with an estimated Mach number between 1.3 and 1.5. The wake of NGC 1404 also lies south of the galaxy with enhanced turbulence and a slight enhancement in metallicity compared to the undisturbed regions of the cluster. Southwest of NGC 1404, there is likely evidence of old turbulence originating from the previous infall. No scenario predicts enhanced turbulence outside of the cold front northwest of the cluster center
Revealing a Highly Dynamic Cluster Core in Abell 1664 with Chandra
We present new, deep (245 ks) Chandra observations of the galaxy cluster Abell 1664 (z = 0.1283). These images reveal rich structure, including elongation and accompanying compressions of the X-ray isophotes in the NEâSW direction, suggesting that the hot gas is sloshing in the gravitational potential. This sloshing has resulted in cold fronts, at distances of 50, 110, and 325 kpc from the cluster center. Our results indicate that the core of A1664 is highly disturbed, as the global metallicity and cooling time flatten at small radii, implying mixing on a range of scales. The central active galactic nucleus (AGN) appears to have recently undergone a mechanical outburst, as evidenced by our detection of cavities. These cavities are the X-ray manifestations of radio bubbles inflated by the AGN and may explain the motion of cold molecular CO clouds previously observed with the Atacama Large Millimeter Array (ALMA). The estimated mechanical power of the AGN, using the minimum energy required to inflate the cavities as a proxy, is erg sâ1, which may be enough to drive the molecular gas flows, and offset the cooling luminosity of the intracluster medium, at erg sâ1. This mechanical power is orders of magnitude higher than the measured upper limit on the X-ray luminosity of the central AGN, suggesting that its black hole may be extremely massive and/or radiatively inefficient. We map temperature variations on the same spatial scale as the molecular gas and find that the most rapidly cooling gas is mostly coincident with the molecular gas reservoir centered on the brightest cluster galaxy's systemic velocity observed with ALMA and may be fueling cold accretion onto the central black hole
The Cocoon Shocks of Cygnus A: Pressures and Their Implications for the Jets and Lobes
We use 2.0 Msec of Chandra observations to investigate the cocoon shocks of Cygnus A and some implications for its lobes and jet. Measured shock Mach numbers vary in the range 1.18â1.66 around the cocoon. We estimate a total outburst energy of , with an age of . The average postshock pressure is found to be , which agrees with the average pressure of the thin rim of compressed gas between the radio lobes and shocks, as determined from X-ray spectra. However, average rim pressures are found to be lower in the western lobe than in the eastern lobe by sime20%. Pressure estimates for hotspots A and D from synchrotron self-Compton models imply that each jet exerts a ram pressure gsim3 times its static pressure, consistent with the positions of the hotspots moving about on the cocoon shock over time. A steady, one-dimensional flow model is used to estimate jet properties, finding mildly relativistic flow speeds within the allowed parameter range. Models in which the jet carries a negligible flux of rest mass are consistent with the observed properties of the jets and hotspots. This favors the jets being light, implying that the kinetic power and momentum flux are carried primarily by the internal energy of the jet plasma rather than by its rest mass