The Faint Galaxy Content of Groups

Even though loose groups are the most common environments of galaxies in the nearby universe, we know surprisingly little about the galaxies that comprise them. This is because group studies are typically hampered by small number statistics: a typical group contains only a few bright galaxies. The advent of multi-object spectroscopy now makes it possible to study the fainter group members in greater detail.

To determine the group membership at fainter magnitudes, Ann Zabludoff (University of Arizona) and I have been using the fiber spectrograph at Las Campanas Observatory to obtain redshifts for all galaxies brighter than V=19 magnitude in a sample of about 17 groups. We find that the groups with detectable X-ray emission, previously known as groupings of three or four bright galaxies, have at least 30-60 members down to our magnitude limit. The large number of group members indicates that the X-ray detected groups are real systems and not chance superpositions of unbound galaxies along the line-of-sight. With 30-60 redshift measurments per group, we are able to calculate reliable velocity for these systems for the first time. We find that the velocity dispersions of the groups do not decrease from the center to the virial radius, implying that significant mass must lie outside the galaxies in the form of a group halo.

The non-X-ray detected groups typically have fewer than 10 members down to out magnitude limit. These groups also tend to have lower velocity dispersions and early-type fractions than the X-ray detected groups. As a consequence, we are unable to determine whether these groups are bound systems or simply superpositions of galaxies along the line-of-sight. However, it is important to note that the Local Group, a system apparently collapsing for the first time, would appear to have the same optical and X-ray properties as these non-X-ray detected groups if it were moved to their distances. As the majority of groups in the nearby universe are non-X-ray detected, our observations argue that most groups may be in the process of forming today.

In the near future we will extend our spectroscopy program to fainter galaxies with the goal of determining the luminosity function for groups.

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