DeepROSATHRI Observations of the NGC 1399/NGC 1404 Region: Morphology and Structure of the X‐Ray Halo

Abstract

We present the analysis of a deep (167 ks) ROSAT HRI observation of the cD galaxy NGC 1399 in the Fornax cluster, comparing it with previous work on this galaxy and with recent Chandra data. We find, in agreement with previous observations, an extended and asymmetric gaseous halo with a luminosity (in the 0.1-2.4 keV energy band) of L X = (5.50±0.04)×10 41 erg s −1 within 46 kpc (assuming a distance of D=19 Mpc). Using both HRI and, at larger radii, archival PSPC data, we find that the radial behavior of the X-ray surface brightness profile is not consistent with a simple Beta model and suggests instead three distinct components. We use a multi-component bidimensional model to study in detail these three components that we identify respectively with the cooling flow region, the galactic and the cluster halo. From these data we derive a binding mass distribution in agreement with that suggested by optical dynamical indicators, with an inner core dominated by luminous matter and an extended dark halo differently distributed on galactic and cluster scales. The HRI data and a preliminary analysis of Chandra public data, allow us to detect significant density fluctuations in the halo. We discuss possible non-equilibrium scenarios to explain the hot halo structure, including tidal interactions with neighboring galaxies, ram stripping from the intra-cluster medium and merging events.In the innermost region of NGC 1399, the comparison between the X-ray and radio emission suggests that the radio emitting plasma is displacing and producing shocks in the hot X-ray emitting gas. We do not detect the nuclear source in X-rays and we pose an upper limit of ∼ 4 × 10 39 erg s −1 (0.1-2.4 keV) to its X-ray luminosity.We found that the NGC 1404 halo is well represented by a single symmetric Beta model and follows the stellar light profile within the inner 8 kpc. The mass distribution is similar to the 'central' component of the NGC 1399 halo. At larger radii ram pressure stripping from the intra-cluster medium produces strong asymmetries in the gas distribution.Finally we discuss the properties of the point source population finding evidence of correlation between the source excess and NGC 1399.

THE FORNAX DEEP SURVEY WITH VST. I. THE EXTENDED AND DIFFUSE STELLAR HALO OF NGC 1399 OUT TO 192 kpc

“…The velocity dispersions of the PNe are consistent with the values derived by the star's kinematics and with those for the red GCs (Napolitano et al 2002;McNeil et al 2010). The X-ray data have shown the presence of a hot gaseous halo associated with NGC1399, which extends out to about 90 kpc (Paolillo et al 2002). It appears very asymmetric, consisting of three components that dominate at different scales: the central bright part, coincident with the peak of light; the "galactic" halo that extends out to ∼7 arcmin, which is offset toward the west with respect to the optical galaxy centroid; and, on larger scales, the "cluster" halo that is elongated in the opposite direction.…”

“…For R R break , the color map appears asymmetric with respect to the center, and it is more extended and redder on the SW side, where g − i∼1.0-1.2 mag. By comparing the X-ray density contours to the g − i color map, we found a surprising correspondence between the morphology and extension of the "galactic" X-ray halo (see Paolillo et al 2002) on the west side of the galaxy and the color asymmetries observed in the stellar light. In particular, the X-ray contours match the shape of the red tail observed in the inner regions of the galaxy on the west side.…”

“…The X-ray halo of NGC1399 extends out to about 90 kpc, and it is very asymmetric; Paolillo et al (2002) noted the presence of several components: the "galactic" halo extends out to ∼7 arcmin and is offset toward the west with respect to the optical galaxy centroid, while on larger scales the "cluster" halo is elongated in the opposite direction. The transition between the "galactic" and "cluster" X-ray density profile occurs at R∼7 arcmin, which is consistent with the value where the blue GC radial distribution starts to dominate over the red GC component, and it is a few arcminutes smaller than the break radius identified in the light distribution (see Figure 13).…”

“…Overview of the surface-density profiles of the blue and red GCs (blue and red circles), by Bassino et al (2006), and X-ray (magenta asterisks), by Paolillo et al (2002), compared with the surface brightness profile in the g band (black circles), as a function of the galactocentric radius for NGC1399. The surface-density profiles are arbitrarily shifted in the ordinate scale.…”

“…It appears very asymmetric, consisting of three components that dominate at different scales: the central bright part, coincident with the peak of light; the "galactic" halo that extends out to ∼7 arcmin, which is offset toward the west with respect to the optical galaxy centroid; and, on larger scales, the "cluster" halo that is elongated in the opposite direction. The galactic halo is very inhomogeneous, showing the presence of several cavities and arc-like features (see Figures3 and6 in Paolillo et al 2002). Different from the compact stellar systems and X-ray emission, the light distribution and colors in the faint outskirts of NGC1399 are still unexplored.…”