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-Chandra image of the 0.5 − 2 keV diffuse emission in NGC 4552 in the Virgo Cluster. Point sources have been removed and the image has been background subtracted, exposure corrected and smoothed with a Gaussian kernel with σ = 1 ′′. Contours denote X-ray surface brightness levels of 0.4, 1, 2 × 10 −8 photons s −1 cm −2 arcsec −2 , respectively. The X-ray surface brightness exhibits a sharp leading edge 40 ′′ (3.1 kpc) to the north of the galaxy center, horns of emission extending southeast and southwest of the leading edge, and a ∼ 2 ′ (∼ 10 kpc) tail of emission to the south-southeast. The bright inner rings of emission ∼ 17 ′′ (1.3 kpc) from the galaxy's nucleus are discussed in Paper II. North is up and east is to the left.
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We use a 54.4 ks Chandra observation to study ram-pressure stripping in NGC4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandra images in the 0.5-2 keV band show a sharp leading edge in the surface brightness 3.1 kpc north of the galaxy center, a cool (kT =0.51^{+0.09}_{-0.06} keV) tail with mean density n_e ~5.4 +/- 1.7 x 10^{-3} cm^{-3}...
Contexts in source publication
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... Figure 1 we present the 0.5 − 2 keV Chandra image of the diffuse emission in NGC 4552 overlaid with X-ray surface brightness contours showing the X-ray features of interest for our analysis. Point sources were removed from the image and the point source regions filled with the local average emission level using CIAO tool dmfilth. ...
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... two narrow band (0.5 − 1 and 1 − 2 keV) images were background subtracted and exposure corrected using monoenergetic instrument maps of 0.9 and 1.5 keV, respectively. The fluxed narrow band images were then summed and smoothed with a 1 ′′ Gaussian kernel to produce the image in Figure 1. We chose conservative Gaussian smoothing for our images to minimize potential numerical artifacts from the smoothing algorithm. ...
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... Fig. 2 we show projections across the horns and tail in a 0.3 − 1.5 keV image of the diffuse emission (binned to pixel size 2 ′′ × 2 ′′ but without background subtraction), demonstrating that these features are highly significant. We see in Figure 1 an enhanced region of X-ray emission in the southwest quadrant of the galaxy, near the beginning of the tail, that we denote as the southern lump SL. In Figure 3 we show the 0.3 − 1.5 keV emission in the galaxy for radii 24 ′′ ≤ r ≤ 48 ′′ in 16 equal sectors numbered counterclockwise from 1. ...
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... third possibility is that AGN outbursts could be responsible for the distorted X-ray mor- phology seen at large radii in NGC 4552. The nucleus of NGC 4552 does harbor a supermassive black hole ( Filho et al. 2004) and does undergo outbursts, as evidenced by the ring structures seen in Figure 1 and analyzed in detail in Paper II. However, the X-ray signatures of such outbursts are residual bright-rimmed cavities and buoyant bubbles, as found in M87 (e.g., Forman et al. 2005), and/or X-ray edges corresponding to shocks driven into the ambient medium by the outburst (e.g., Fabian et al. 2003 for NGC 1275 in Perseus and Nulsen et al. 2005 for Hydra A). ...
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... the X-ray signatures of such outbursts are residual bright-rimmed cavities and buoyant bubbles, as found in M87 (e.g., Forman et al. 2005), and/or X-ray edges corresponding to shocks driven into the ambient medium by the outburst (e.g., Fabian et al. 2003 for NGC 1275 in Perseus and Nulsen et al. 2005 for Hydra A). This is quite different from the 'flattened leading edge -trailing tail' morphology seen in Figure 1. ...
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... contrast, the qualitative correspondence between the main X-ray features shown in Figure 1, i.e. the leading edge, horns, and tail, and those found in simulations (e.g. see Stevens et al. 1999;Toniazzo & Schindler 2001;Acreman et al. 2003) of ram-pressure stripping of elliptical galaxies moving through surrounding cluster gas is striking. ...
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... NGC 4552 in Virgo, only a qualitative, not quantitative, comparison of these simulations to our observation should be made. For supersonic stripping, the galaxy ISM is initially pushed back and fanned out ( Stevens et al. 1999 Figure 2c, t = 1.6 Gyr slice) causing the edge between the galaxy gas and the ICM to flatten, as is seen for NGC 4552 ( Fig. 1). This sharp surface brightness discontinuity, 3.1 kpc north of the galaxy's center, coupled with a gas tail in the opposite direction, fixes the galaxy's direction of motion in the plane of the sky. Irregular filaments of stripped material also are seen in simulation images extending back from the leading edge, similar to the 'horns' ...
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... filaments signal the onset of Kelvin-Helmholtz instabilities, where stripping is occurring at the boundary between galaxy and ICM gas. In the simulation images, galaxy gas, once stripped and decelerated, forms a distinctive tail of emission behind the galaxy, again similar to what we see in Figure 1. The three-dimensional simulations of Toniazzo & Schindler (2001) demonstrate that the tail is in general not axisymmetric with respect to the direction of motion, but may appear angular or curved, as is also found in our image. ...
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... three-dimensional simulations of Toniazzo & Schindler (2001) demonstrate that the tail is in general not axisymmetric with respect to the direction of motion, but may appear angular or curved, as is also found in our image. Thus the most likely explanation for the origin of the features shown in Figure 1 is ram- pressure stripping of galaxy gas due to the motion of NGC 4552 through the Virgo Cluster ICM. Table 4. ...
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... find that the temperature of the ISM on the galaxy side of the (leading) northern edge (0.43 keV) is a factor ∼ 5 lower than the temperature of the gas in the ambient Virgo ICM (∼ 2.2 keV). As shown in Figure 1, this temperature drop corresponds to a sharp increase in surface brightness (and thus gas density as quantified in §3.2). This is consistent with the interpretation of the northern edge as a cold front, the leading edge of the galaxy ISM as it undergoes ram pressure stripping, in broad agreement with expectations from simulations (e.g. ...
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... results are consistent with simulations of ram-pressure stripping (see, e.g. Stevens et al. 1999, Figure 2, model 1b), where irregular filaments due to the onset of hydrodynamical instabilities during stripping are cooler and denser than the surrounding ICM. toward the north across the galaxy's leading edge. ...
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... Figure 7 we compare the surface brightness in a rectangular region of width 5 kpc projected onto the tail's central axis in 1.5 kpc bins along the length of the tail (filled circles) to the projection of the surface brightness of the surrounding Virgo ICM in a parallel region of the same dimensions, orientation, and bin size (open squares). We define the beginning of the tail (and r = 0 of the projection in Figure 7) at the point ∼ 4 kpc south of NGC 4552's center, where the tail visually appears distinct from the galaxy's more spherical, hot gas halo (see Figure 1). The solid and long- dashed lines denote the best linear fits to the projected surface brightness of the tail and Virgo ICM, respectively. ...
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... the body being stripped has a well defined shoulder, as in A3667 ( Vikhlinin et al. 2001), the body sonic point is at the location of the shoulder. However, in NGC 4552 (see Figure 1), the 'horns' make the precise location of the shoulder difficult to discern. Instead we use Figure 9 of Moeckel (1949) to find the body sonic point for Mach 2.2 motion as that point on the body surface where the angle, θ d , of the tangent line from the symmetry axis to the body surface is θ d ≈ 43 • . ...
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... keV band ( Fig. 1 ) show two ringlike structures approximately ∼1.3 kpc away from the centre of the galaxy, which is associated with the LINER type AGN in its centre (Machacek et al. 2006b ). Moreo v er, it shows a ∼10 kpc Xray tail at its south, along the motion direction, associated with the ram-pressure stripping of the galaxy's hot atmosphere (Machacek et al. 2006a ). Notably, the α/Fe ratios in the stellar content of M89 are higher than the average ratios in ellipticals with similar mass . ...
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... Sabater et al. 2013). Indeed lines of support include: i) recent observations of Hα emission in NGC 4406 (Kenney et al. 2008), some of which is colocated in the region of its X-ray plume, suggest a collision with NGC 4438 as its origin; ii) NGC 4261 has a well defined disk of dust, 240 pc in diameter, that is aligned with its radio axis and perpendicular to the stellar component of the galaxy; iii) in NGC 4374 the dust lane major axis is strongly misaligned with the major axis of the stars and the kinematic of molecular gas is inconsistent with a stellar mass loss origin; iv) the giant elliptical NGC 4552 which is falling into the Virgo cluster, has central X-ray emission features (Machacek et al. 2006a) characteristic of a supersonic ram-pressure stripping of the gas, produced by its motion in the Virgo ICM. Overall, albeit suggestive, these remain qualitative features that require to be further tested with future targeted and deeper observations. ...
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