HST Observations of NGC 7252
Session 17 -- Normal Galaxies
Oral presentation, Monday, 10:30-12:00, Wheeler Room

## [17.07] HST Observations of NGC 7252

Brad Whitmore (STScI), Francois Schweizer (DTM), Claus Leitherer, Kirk Borne, Carmelle Robert (STScI)

A population of about 40 blue pointlike objects has been discovered in NGC 7252 using the Planetary Camera on board of the Hubble Space Telescope. NGC 7252 (sometimes referred to as the Atoms-for-Peace'' galaxy) is one of the prototypical examples of a merger between two disk galaxies. Schweizer (1982: ApJ, 252, 455) has argued that the remnant will eventually become an elliptical galaxy. The luminosities, V$-$I colors, spatial distribution, and sizes are all compatible with the hypothesis that these objects formed $\leq$ 1 Gyr ago during the original merger, and that they are the progenitors of globular clusters similar to those we see around galaxies today. It therefore appears that the number of globular clusters is not a conserved quantity during the merger of two spiral galaxies, but increases instead. This weakens van den Bergh's objection against ellipticals being formed through disk mergers, based mainly on the fact that disk galaxies have fewer globular clusters per unit luminosity than ellipticals galaxies do.

The objects found in NGC 7252 are very similar to the pointlike sources recently discovered in NGC 1275 by Holtzman et al. (1992: AJ, 103, 691). However, NGC 1275 is a peculiar galaxy in the center of the Perseus cluster. While Holtzman et al. argue that the objects in NGC 1275 may be the progenitors of globular clusters, Richer et al. (1993: AJ, 105, 877) suggest that these objects may instead be related to the strong cooling flow in the cluster. Our discovery of a population of bright blue pointlike objects in NGC 7252, a prototypical merger, makes a much stronger connection between the formation of globular clusters and the merger history of a galaxy. Other findings are: (1) NGC 7252 has a single, semi-stellar nucleus; (2) spiral arms are seen within 3.5$''$ (1.6 kpc) of the center, presumably formed through the continued infall of gas into a disk around the center of the galaxy; (3) dust lanes and very weak spiral structure are seen out to about 9.2$''$ (4.3 kpc), primarily on the NE side; and (4) a ripple is found on the west side, 5.0$''$ from the center.