WFPC2 Images of the Central Regions of Early-Type Galaxies. I. The Data

Abstract

We present high-resolution R-band images of the central regions of 67 early-type galaxies obtained with the Wide Field and Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). This homogeneously selected sample roughly doubles the number of early-type galaxies that have now been imaged at HST resolution and complements similar data on the central regions of radio galaxies and the bulges of spiral galaxies. Our sample strikingly confirms the complex morphologies of the central regions of early-type galaxies which have become apparent from previous studies with HST. In particular, we detect dust, either in the form of nuclear disks or with a filamentary distribution, in 43% of all galaxies, in good agreement with previous estimates. In addition, we find evidence for embedded stellar disks in a remarkably large fraction of 51%. In 14 of those galaxies the disklike structures are misaligned with the main galaxy, suggesting that they correspond to stellar bars in S0 galaxies. We analyze the luminosity profiles of the galaxies in our sample and classify galaxies according to their central cusp slope. To a large extent we confirm the results from previous HST surveys in that early-type galaxies reveal a clear dichotomy: the bright ellipticals (MB ≲ -20.5) are generally boxy and have luminosity profiles that break from steep outer power laws to shallow inner cusps (referred to as "core" galaxies). The fainter ellipticals, on the other hand, typically have disky isophotes and luminosity profiles that lack a clear break and have a steep central cusp (referred to as "power-law" galaxies). The advantages and shortcomings of classification schemes utilizing the extrapolated central cusp slope γ are discussed, and it is shown that γ might be an inadequate representation for galaxies whose luminosity profile slope changes smoothly with radius rather than resembling a broken power law. Thus, we introduce a new, alternative parameter and show how this affects the classification. In fact, we find evidence for an "intermediate" class of galaxies that cannot unambiguously be classified as either core or power-law galaxies and that have central cusp slopes and absolute magnitudes intermediate between those of core and power-law galaxies. It is unclear at present, however, whether these galaxies make up a physically distinct class or whether distance and/or resolution effects cause them to lose their distinct core or power-law characteristics.