* Astronomy

Title: The complex X-ray morphology of NGC 7618: A major group-group merger in the local Universe?
Authors: R. P. Kraft, C. Jones, P. E. J. Nulsen, M. J. Hardcastle


Researchers present results from a short Chandra/ACIS-S observation of NGC 7618, the dominant central galaxy of a nearby (z=0.017309, d=74.1 Mpc) group. They detect a sharp surface brightness discontinuity 14.4 kpc North of the nucleus subtending an angle of 130° with an X-ray tail extending about 70 kpc in the opposite direction. The temperature of the gas inside and outside the discontinuity is 0.79±0.03 and 0.81±0.07 keV, respectively.
There is marginal evidence for a discontinuous change in the elemental abundance (Zinner=0.65±0.25, Zouter =0.17±0.21 at 90% confidence), suggesting that this may be an `abundance' front.
Fitting a two-temperature model to the ASCA/GIS spectrum of the NGC7618/UGC 12491 pair shows the presence of a second, much hotter (T= about 2.3 keV) component.
They consider several scenarios for the origin of the edge and the tail including a radio lobe/IGM interaction, non-hydrostatic `sloshing', equal-mass merger and collision, and ram-pressure stripping. In the last case, we consider the possibility that NGC 7618 is falling into UGC 12491, or that both groups are falling into a gas poor cluster potential.
There are significant problems with the first two models, however, and they conclude that the discontinuity and tail are most likely the result of ram pressure stripping of the NGC 7618 group as it falls into a larger dark matter potential.


Position(2000): RA = 23 19 47.40 Dec = +42 51 09.4
Elliptic galaxy NGC7618 in the constellation of Andromeda
Size 14'1 x 14'1
Magnitude 13


Adaptively smoothed ASCA/GIS image of NGC 7618 and UGC 12491. The
minimum (violet) and maximum (white) surface brightnesses in this image are about 38.4 and134.2 cts arcmin-2, respectively. The background, estimated from an off-source region at the edge of the field of view is 23.5±1.1 cts arcmin-2. The violet regions in this image are therefore significant at more than 10s.



Contours from previous image overlaid onto optical/DSS image of NGC 7618. The active nucleus is co-incident with the peak of the optical isophotes. The contours correspond to a surface brightness of 3, 5, 9, 16, 28, 50, 88, 155, 274, and 484 ×10-5 cts arcsec-2 s-1


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