Cold, warm, and hot gas in the late-stage merger NGC 7252

Abstract

We present the first observations of the neutral hydrogen distribution and x-ray emission in the prototypical merger remnant NGC 7252, the "Atoms-for-Peace" galaxy. These data are supplemented by accurate B and R surface photometry, reaching a limit of μ_B_ = 26.5 mag arcsec^-2^, and images taken through a narrow-band Hα filter. We find all of the 2 X 10^9^ h^-2^ M_sun_ of atomic gas to be restricted to the outer, tidal regions of this system (H_0_ = 100 h km s^-1^ Mpc^-1^). By contrast, the molecular gas traced by the ^12^CO(1-0) map of Wang et al. [ApJ, 396,510(1992)] is confined to an inner rotating disk of radius 7" and has an Hα counterpart. The gap between the atomic and molecular gas distributions is filled in by diffuse Hα emission and perhaps by x-ray emission. The velocity field of the atomic gas in the tidal tails indicates that they are swinging through space in the same sense as the rotation of the inner gas disk. The H I at the apparent base of the northwestern tail seems to be falling back toward the main body of the galaxy, yet there is no H I associated with this main stellar body: This suggests ongoing efficient conversion of the atomic gas into other phases in this region. The Hα velocity anomalies previously found in the remnant body may be produced in part by the combination of tail-related, noncircular motions and the inner gas-disk rotation. Both tidal tails have bluer B-R colors than the main body of the remnant, with the bluest regions coinciding with peaks in the gas column density. Each tail contains one giant H II region near the end of its optical light distribution. These H II regions are associated with large concentrations of gas and stars that approach the sizes and gas contents of dwarf galaxies. The H I extends beyond the end of the optical tails and reaches projected distances of 62 h^-1^ kpc east and 120 h^-1^ kpc northwest from the center. We discuss the possible relevance of these data to: (1) the transformation of merged spirals into ellipticals; (2) the generation of ripples by returning tidal material; and (3) the formation of bound stellar systems from tidally torn material.

Cold, warm, and hot gas in the late-stage merger NGC 7252

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