The isolated elliptical NGC 1132: Evidence for a merged group of galaxies?

Numerical simulations predict that some poor groups of galaxies have merged by the present epoch into giant ellipticals. To identify the possible remnants of such mergers, we have compiled a sample of nearby isolated ellipticals. ASCA observations of the first galaxy studied, NGC 1132, reveal an X-ray halo that extends out to at least ≈250 kpc h^-1₁₀₀. The temperature (∼1 keV), metallicity (∼0.25 solar), and luminosity (∼2.5 × 10⁴² h^-2100 ergs s^-1) of NGC 1132's X-ray halo are comparable with those of poor group halos. The total mass inferred from the X-ray emission, ∼1.9^+0.8_-0.6 × 10¹³ h^-1₁₀₀ M_⊙, is also like that of an X-ray detected group. Optical imaging uncovers a dwarf galaxy population clustered about NGC 1132 that is consistent in number density and projected radial distribution with that of an X-ray group. The similarities of NGC 1132 to poor groups in both the X-ray band and at the faint end of the galaxy luminosity function, combined with the deficit of luminous galaxies in the NGC 1132 field, are compatible with the merged group picture. Another possibility is that the NGC 1132 system is a " failed" group (i.e., a local overdensity in which other bright galaxies never formed).