Globularl Clusters in M87!

[Steve Gottlieb]

While observing M87 last month in Jimi's 48-inch scope, we also tracked down two M87 globular cluster candidates listed in NED with primary designations SDSS J123057.38+122544.5 and SDSS J123152.89+121559. The exact classification of these objects is in dispute -- they are perhaps at the upper end of classical globulars or perhaps the nucleus of a former dwarf elliptical galaxy.

SDSS J123057.38+122544.5 was catalogued as globular #547 by Strom et al in a 1981 study "The halo globular clusters of the giant elliptical galaxy Messier 87" (ApJ, 245, 416-453). In a later 1987 study "The M87 globular cluster system. I - Dynamics" by Huchra and Brodie, this object was included as a confirmed globular of M87. SDSS J123057.38+122544.5 is located just 3.0' NE of the center of M87, directly opposite the two small galaxies at the southwest edge of the halo of M87. A mag 14.5 star lies just 20" SSW and is a perfect reference to focus on the cluster.

At 488x it was highly suspected as an 18-18.5 magnitude "star", but I didn't feel 100% confident of the observation. At 813x, though, it was confirmed as an extremely faint, stellar object, glimpsed several times at the same position.

The second object, SDSS J123152.89+121559, was viewed at 620x and appeared extremely faint (V = 17.7) and small, but definitely non-stellar, perhaps 6" diameter. Often this object hovered just at the edge of visibility but sometimes it clearly popped as a distinct, low surface brightness (non-stellar) glow. This globular candidate is situated 17' SE of the center of M87, nearly at the midpoint of two mag 10.8/11.5 stars 6.7' apart at the edge of the field.

The 2006 paper "Discovery of Ultracompact Dwarf Galaxies in the Virgo Cluster" by Jones et al (http://arxiv.org/abs/astro-ph/0509120) includes both object in a new classification -- Ultra-Compact Dwarf galaxies (Virgo UCD 3 and Virgo UCD 7). Formation scenarios for UCDs include the remnant nucleus of a stripped dwarf galaxy or a rare surviving relic of a dwarf galaxy formed in the early Universe . Star cluster origins include the merger of several smaller star clusters or an extension of the globular cluster sequence to higher masses. Multiple origins are also possible.

A more recent study "Structural Properties of Ultra-Compact Dwarf Galaxies in the Fornax and Virgo Clusters" in AJ, 136, 461 (2008) concludes "The positive color gradients found in the bright UCDs are consistent with them being either bright GCs or threshed dE galaxies (except VUCD3). However, the spectroscopic ages, metallicities, and alpha-abundances for Virgo UCDs, obtained in our previous work (Evstigneeva et al. 2007), are not consistent with the formation of UCDs by the simple removal of the envelope from the nuclei of dE galaxies. Since spectroscopic ages and metallicities are more powerful tools than colors, we have to reject the threshing hypothesis for the Virgo UCD origin. Hence, the Virgo UCDs are more consistent with being bright GCs."

Whatever astronomers eventually decide is the classification, it was very cool to glimpse these objects!!

Virgo UCD 3.jpg
Virgo UCD 7.jpg

[Howard B]

We had an engaging discussion of what these objects should or might be classified. Although the professional literature was somewhat divided, the papers Steve found mostly favored them being ultra compact dwarfs. It seems a little silly to me because they look just like big globulars regardless of how they formed. For instance, different galaxy types have extremely wide ranging shapes and form in different ways but they're all galaxies. Why not the same for globulars? They're all globulars but can be divided into subgroups that define their size and formation history.

Regardless of what they end up being called, the two objects Steve described above were a real thrill and highlight of our four nights observing with Jimi and his 48.

[Marko]

16' from the core. That one has significant apparent separation from where we stand. It's like the first time I sought out G1 in M31 I was stunned to find that it was so greatly removed from the galaxy unlike the many other globs I had tracked down that we within the more obvious glow for the most part.

[Tony B]

Steve - if I recall correctly you have also observed globulars in the LMC.
Tony B

[Jimi Lowrey]

I have been doing a lot of research on UCD's (Ultra Compact Dwarfs). This is what some astronomers think. The four main hypotheses for UCD formation are: 1) extra luminous GCs (Mieske et al. 2002) 2) the products of the merger of super star clusters (Fellhauer & Kroupa 2002); 3) the stripped nuclei of dwarf ellipticals (Bekki et al. 2003) and 4) primordial dwarf galaxies (Drinkwater et al. 2004). The recent work of Forbes et al. (2008) and references therein indicate that UCDs share many of the properties of massive star clusters. So what is a UCD? That remains to be determined . One thing that they agree on is they are massive groups of old stars. What ever they are or how they formed UCD's near other galaxies are one COOL objects!