NGC 4550: A Laboratory for Testing Galaxy Formation
Abstract
We present long-slit spectroscopy of the E7/S0 galaxy NGC 4550, recently discovered by Rubin et al. to have cospatial, counterrotating components. A detailed analysis of the line-of-sight velocity distribution (LOSVD) along the major axis shows striking bimodality. This bimodal LOSVD indicates the presence of two disk components, photometrically inseparable, but counterstreaming at projected velocities of - 110 km s^-1^ and + 150 km s^-1^; they have projected velocity dispersions of ~60 km s^-1^ and ~45 km s^-1^, respectively. Both components have exponential brightness profiles with nearly identical luminosities and scale lengths. Each component by itself resembles kinematically "normal" S0 disks at one scale length. The bulge of NGC 4550 is found to contain only ~15% of the light. We discuss possible formation scenarios for the two disks and argue against a dissipationless formation through a merger. A formation of the second disk component through later addition of gas is conceivable, but must occur adiabatically. A halo, contributing 55% of the mass at the half-light radius, is required to reduce the adiabatic compression of the preexisting disk, which would move the galaxy off its normal locus in the Tully-Fisher relation. The present structure of NGC 4550 provides manifest proof that galaxy disks can absorb an amount of external material comparable to their own mass, without being heated dramatically. A combination of factors make NGC 4550 ideal for detecting counterstreaming disks, and the possibility of unnoticed stellar counterrotation in other, previously studied S0 disks is discussed.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1992
- DOI:
- 10.1086/186635
- Bibcode:
- 1992ApJ...400L...5R
- Keywords:
-
- Galactic Evolution;
- Galactic Structure;
- Astronomical Photometry;
- Brightness Distribution;
- Galactic Bulge;
- Galactic Halos;
- Stellar Rotation;
- Velocity Distribution;
- Astrophysics;
- GALAXIES: EVOLUTION;
- GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4550;
- GALAXIES: FORMATION;
- GALAXIES: KINEMATICS AND DYNAMICS