The surface brightness distribution of Virgo cluster galaxies

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Mcdonald, Michael
Astronomy , Astrophysics , Extragalactic
We have acquired near-IR H and optical $g$,$r$,$i$,$z$ imaging for a sample of 299 Virgo cluster galaxies down to a limiting magnitude of B$_T$=16.0 mag. The low sensitivity of near-IR wavelengths to dust extinction is crucial for an unbiased geometric deprojection of fundamental galaxy parameters. We find, in agreement with Tully and Verheijen (1997), a clear dichotomy between high and low surface brightness galaxy disks. The difference between the low and high brightness peaks of Virgo galaxies is $\sim$2 H-mag arcsec$^{-2}$, significantly larger than any systematic errors. The high surface brightness galaxies have two distinct classes of bulge with high and low concentration, while low surface brightness galaxies have only low concentration bulges. The distribution of the effective surface brightnesses of our entire sample shows that early-type galaxies exhibit a similar structural bimodality though offset from that of spiral galaxies towards higher surface brightnesses. We find that the structural bimodality is uncorrelated with colour or any other structural parameter except, perhaps, the circular velocity. Simulations of realistic surface brightness profiles show that a bimodality in effective surface brightness is unexpected based on normal distributions of fundamental bulge and disk parameters. Rather, the structural bimodality is likely linked to the galaxy dynamics, namely the specific angular momentum of the galaxy; high surface brightness galaxies have low angular momentum and can collapse to form dynamically important disks, while low surface brightness galaxies are dominated by the dark halo everywhere. Finally, our bulge-disk decompositions of all the sample galaxy images using various fitting functions have revealed that galaxies of all morphologies, including flattened and spheroidal systems, exhibit a ``disk'' component which is best described by a generalized ``Sersic'' function. We also find that the majority of galaxy disks show significant deviations from a pure exponential. Further dynamical studies of both cluster and field galaxies are needed to properly explain the observed structural bimodality in both early- and late-type galaxies.
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