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Register a new userBuilding a cluster: shocks, cavities, and cooling filaments in the group-group merger NGC 6338
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We present deep Chandra, XMM-Newton, Giant Metrewave Radio Telescope and Halpha observations of the group-group merger NGC 6338. X-ray imaging and spectral mapping show that as well as trailing tails of cool, enriched gas, the two cool cores are embedded in an extensive region of shock heated gas with temperatures rising to ~5 keV. The velocity distribution of the member galaxies show that the merger is occurring primarily along the line of sight, and we estimate that the collision has produced shocks of Mach number M=2.3 or greater, making this one of the most violent mergers yet observed between galaxy groups. Both cool cores host potential AGN cavities and Halpha nebulae, indicating rapid radiative cooling. In the southern cool core around NGC 6338, we find that the X-ray filaments associated with the Halpha nebula have low entropies (<10 kev cm^2) and short cooling times (~200-300 Myr). In the northern core we identify an Halpha cloud associated with a bar of dense, cool X-ray gas offset from the dominant galaxy. We find no evidence of current jet activity in either core. We estimate the total mass of the system and find that the product of this group-group merger will likely be a galaxy cluster.
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Moderately strong shocks arise naturally when two subclusters merge. For instance, when a smaller subcluster falls into the gravitational potential of a more massive cluster, a bow shock is formed and moves together with the subcluster. After pericenter passage, however, the subcluster is decelerated by the gravity of the main cluster, while the shock continues moving away from the cluster center. These shocks are considered as promising candidates for powering radio relics found in many clusters. The aim of this paper is to explore the fate of such shocks when they travel to the cluster outskirts, far from the place where the shocks were initiated. In a uniform medium, such a runaway shock should weaken with distance. However, as shocks move to large radii in galaxy clusters, the shock is moving down a steep density gradient that helps the shock to maintain its strength over a large distance. Observations and numerical simulations show that, beyond $R_{500}$, gas density profiles are as steep as, or steeper than, $sim r^{-3}$, suggesting that there exists a Habitable zone for moderately strong shocks in cluster outskirts where the shock strength can be maintained or even amplified. A characteristic feature of runaway shocks is that the strong compression, relative to the initial state, is confined to a narrow region just behind the shock. Therefore, if such a shock runs over a region with a pre-existing population of relativistic particles, then the boost in radio emissivity, due to pure adiabatic compression, will also be confined to a narrow radial shell.
We present results from new Chandra, GMRT, and SOAR observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. The system shows three pairs of collinear cavities at 1 kpc, 8 kpc, and 20 kpc from the central source, from three distinct outbursts of the central AGN, which occurred 3x10^6, 2x10^7, and 9x10^7 yr ago. The H-alpha and X-ray observations reveal filaments of cool gas that has been uplifted by the X-ray cavities. The inner two cavity pairs are filled with radio emitting plasma, and each pair is associated with an elliptical surface brightness edge, which we unambiguously identify as shocks (with measured temperature jumps) with Mach numbers of M~1.7 and M~1.5 for the inner and outer shocks, respectively. Such clear signatures from three distinct AGN outbursts in an otherwise dynamically relaxed system provide a unique opportunity to study AGN feedback and outburst history. The mean power of the two most recent outbursts differs by a factor of six, from 1.5--10x10^42 erg/s, indicating that the mean jet power changes significantly over long (~10^7 yr) timescales. The total energy output of the most recent outburst is also more than an order of magnitude less than the total energy of the previous outburst (1.5x10^56 erg versus 4x10^57 erg), which may be a result of the lower mean power, or may indicate that the most recent outburst is ongoing. The outburst interval implied by both the shock and cavity ages (~10^7 yr) indicates that, in this system, shock heating alone is sufficient to balance radiative cooling close to the central AGN, which is the relevant region for regulating feedback between the ICM and the central SMBH.
The fate of cooling gas in the centers of galaxy clusters and groups is still not well understood, as is also the case for the complex process of triggering active galactic nucleus (AGN) outbursts in their central dominant galaxies, and the consequent re-heating of the gas by the AGN jets. With the largest known reservoir of cold molecular gas of any group-dominant galaxy and three epochs of AGN activity visible as cavities in its hot gas, NGC 5044 is an ideal system in which to study the cooling/AGN feedback cycle at the group scale. We present VLBA observations of NGC 5044 to ascertain the current state of the central AGN. We find a compact core and two small jets aligned almost in the plane of the sky, and in the orthogonal direction to the location of cavities. We construct the radio/sub-mm spectral energy distribution (SED) over more than three orders of magnitude. We find that below 5 GHz the spectrum is best fit by a self-absorbed continuous injection model representing emission coming from the jets, while the higher frequencies show clear signs of an advection dominated accretion flow. We derive a black hole mass and accretion rate consistent with independent measurements. We conclude that the age of the jets is much younger than the innermost cavities, marking the start of a new feedback cycle.
We study a merger of the NGC 4839 group with the Coma cluster using X-ray observations from the XMM-Newton and Chandra telescopes. X-ray data show two prominent features: (i) a long (~600 kpc in projection) and bent tail of cool gas trailing (towards south-west) the optical center of NGC 4839, and ii) a sheath region of enhanced X-ray surface brightness enveloping the group, which is due to hotter gas. While at first glance the X-ray images suggest that we are witnessing the first infall of NGC 4839 into the Coma cluster core, we argue that a post-merger scenario provides a better explanation of the observed features and illustrate this with a series of numerical simulations. In this scenario, the tail is formed when the group, initially moving to the south-west, reverses its radial velocity after crossing the apocenter, the ram pressure ceases and the ram-pressure-displaced gas falls back toward the center of the group and overshoots it. Shortly after the apocenter passage, the optical galaxy, dark matter and gaseous core move in a north-east direction, while the displaced gas continues moving to the south-west. The sheath is explained as being due to interaction of the re-infalling group with its own tail of stripped gas mixed with the Coma gas. In this scenario, the shock, driven by the group before reaching the apocenter, has already detached from the group and would be located close to the famous relic to the south-west of the Coma cluster.
We present results obtained from the analysis of a total of 110 ks Chandra observations of 3C 320 FR II radio galaxy, located at the centre of a cluster of galaxies at a redshift $z=0.342$. A pair of X-ray cavities have been detected at an average distance of $sim$38 kpc along the East and West directions with the cavity energy, age and total power equal to $sim$7.7$times$10$^{59}$ erg, $sim$7$times$10$^7$ yr and $sim$3.5$times$10$^{44}$ erg s$^{-1}$, respectively. The cooling luminosity within the cooling radius of $sim$100 kpc was found to be $L_{cool} sim8.5times10^{43}$ erg s$^{-1}$. Comparison of these two estimates implies that the cavity power is sufficiently high to balance the radiative loss. A pair of weak shocks have also been evidenced at distances of $sim$47 kpc and $sim$76 kpc surrounding the radio bubbles. Using the observed density jumps of $sim$1.8 and $sim$2.1 at shock locations along the East and West directions, we estimate the Mach numbers ($mathcal{M}$) to be $sim$1.6 and $sim$1.8, respectively. A sharp surface brightness edge was also detected at relatively larger radius ($sim$80 kpc) along the South direction. Density jump at this surface brightness edge was estimated to be $sim$1.6 and is probably due to the presence of a cold front in this cluster. The far-infrared luminosity yielded the star formation rate of 51 M$_{odot}$ yr$^{-1}$ and is 1/4$^{th}$ of the cooling rate ($dot{M}$ $sim$ 192 M$_{odot}$ yr$^{-1}$).
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