Robin Ciardullo, John Feldmeier (Penn State), and George Jacoby (NOAO) used the KPNO 4-m telescope to find intracluster Planetary Nebula throughout the extent of the Virgo cluster of galaxies. The distribution of planetaries suggests that the cluster is significantly extended along the line of sight, and that galaxies apparently within its projected core cannot be regarded as being at the same distance. Overall, the Virgo cluster is poorly organized, and may still be accreting surrounding galaxies.
Measuring the distance to the nearby Virgo cluster of galaxies has long been considered by many to be a critical step along the path to measuring the Hubble constant. One nasty issue, however, is understanding how well organized and defined the cluster is itself. If Virgo is compact and "relaxed," then most of the galaxies in its core can be assumed to be at the same distance, while if it were highly-structured and poorly organized, it's problematic to relate distance measures to various galaxies within the cluster to each other. Being able to compare and average galaxy distances within the cluster is central to trying to measure the distance to the Virgo cluster overall.
One method that has been used to determine the distance to the Virgo cluster is by observing planetary nebulae (PN). Planetary nebulae are created as stars like our own sun near the end of their lives. As it happens, the various brightnesses that nebulae are seen to have appear to be drawn from a standard distribution, or luminosity function (LF), which Ciardullo and Jacoby have shown to be constant from galaxy to galaxy, in turn allowing them to use the PNLF as a distance indicator.
Ciardullo et al. used the 4-m Prime Focus CCD Camera to identify planetary nebulae (PN) in the outer regions of the giant elliptical galaxy M87 by their ionized oxygen emission. M87 has long been assumed to be at the center of the Virgo cluster; thus, measuring its distance has been particularly important. Ciardullo et al., however, found that the PNLF of M87 is poorly described by the PNLF found in other galaxies (Figure 1). In their new work, they find that this is because M87 is "contaminated" by a foreground population of planetary nebulae. This foreground population does not appear to be associated with any other galaxy, but rather is part of the cluster. In other words Ciardullo et al. have identified a population of stars that has been detached from their parent galaxies and are now adrift within the cluster potential.
Figure 1:
The [O III] 5007Å luminosity function of IPN found in front of M87
compared to two Virgo cluster models. The solid curve gives the PNLF
expected from an isothermal cluster with core radius 1.7º (Binggeli,
Tammann, Sandage 1987). A better model, displayed by the dotted line, is one
in which the intracluster PN are assumed to have a uniform density and
extend ~ 4Mpc from M87.
This hypothesis was confirmed by follow-up observations of six blank 14' × 14' fields in the Virgo cluster taken with the 4-m by John Feldmeier (Penn State graduate student), Ciardullo, and Jacoby (see Figure 2). So far, close to two hundred intracluster planetary nebulae (IPN), have been detected, and the survey is still ongoing. From these observations, it appears that the intracluster stars in Virgo are very numerous (over 20% of the total starlight in the cluster), are not centrally condensed, are not dynamically relaxed, and extend up to four megaparsecs in front of M87.
Figure 2: This is a 2.5º × 5.7º image of the Virgo cluster, with all the detections of intracluster stars found to date. The six PFCCD fields are indicated by the large squares of side ~ 14'. The number of intracluster planetaries varies significantly from field to field, generally dropping off with distance from the cluster center. The intracluster planetaries in subclump A of Virgo (Fields 1, 3, 4, 5) are a few Mpc less distant from us than those in subclump B (2 & 6).
Although the presence of intracluster stars complicates the distance
determination of the Virgo cluster, the same stars can serve as a powerful
probe of the structure and history of the cluster itself. In the future, two
new tools will be used to learn much more about the intracluster stars. The
Kitt Peak CCD Mosaic Imager will greatly increase the amount of the Virgo
cluster that can be surveyed for intracluster planetaries, and the Medium
Resolution multi-fiber Spectrograph on the Hobby-Eberly Telescope will
efficiently measure the velocities of large samples of IPN.