NGC 4365 is one of the ten brightest elliptical galaxies in the nearby Virgo Cluster.  Its globular clusters has been well studied and several attempts have been made to measure their ages, but with conflicting results!  Some studies indicate that these globulars are of young to intermediate ages, such as 2-8 Gyrs old. 

One reason this is particularly odd is that photometry of the rest of the galaxy’s stellar populations suggests that its stars are old.  So, did this galaxy somehow form the bulk of its stars and its globular clusters at different epochs?  Or, is there more to the story on the ages of its globular clusters? 

I’m working on spectra of these clusters taken with the FORS2 instrument on the VLT.  This work is being done in collaboration with Soeren Larsen (University of Utrecht) and ??.  We can measure different line indices and compare these with stellar population synthesis models to calculate ages.   Our goal is to measure the ages of these clusters and to explore the effects on the ages of using models for comparison.

NGC 3311 is the cD (centrally dominated) giant elliptical galaxy that sits at the center of the Hydra Cluster.  At a distance of 54 Mpc, Hydra is our third closest cluster, and NGC 3311 our third closest BCG (brightest cluster galaxy).  BCGs are unusual animals -- they tend to have extremely high numbers of globular clusters, relative to their regular starlight.  Most likely BCGs accrete globular clusters that are stripped from other galaxies as they interact in the cluster potential well. 

NGC 3311 has a particularly impressive globular cluster system -- possibly the largest one in our local universe.  While it’s nearby companion, NGC 3309, just below and to the right, appears to also contribute globular clusters, we find that it actually contributes very few globular clusters to

the field.  In fact, it contributes less than 2 percent of all the star-like objects in the field.  NGC 3311’s globular cluster system, on the other hand, is extremely rich, containing almost 16,000 globulars!  But how do we know how many belong to each galaxy? 

First of all, we define a grid over the entire field (left). 

Then, we need to model how many star clusters there are around each galaxy.  We use a generalized Sersic profile to estimate how many globular clusters we may find at any given radius.  Then, using these models, we can calculate how many clusters we should find in each box and compare it to how many we actually measure.  The best fit of the model parameters occurs when we get the closets fit of calculated and actual clusters in each box! 

Using these models, we find that less than 2.5% of the objects in our field belong to NGC 3309

Another interesting feature about globular cluster systems is their color bimodality.  Most galaxies contain some combination of blue, metal-poor and red, metal rich globulars.  Why is this?  The more metal-poor globulars most likely formed first, early on in the universe before galaxies had much chance to enrich their gas with heavier metals such as Fe, C, O, Ne, etc.  It’s possible that these metal-poor globular clusters formed in small dwarf-galaxy like clumps of mass, whose potential wells were too small to hold onto heavier metals for enriching their gas.  In this scenario, these small mass clumps began to merge into larger galaxies, their potential wells grew to the point they could retain ejected, metal rich material.  Globular clusters that formed after this time are, consequently, more metal rich.  NGC 3311 shows signs of bi-modality:

The image on the left shows the number of globular clusters versus the color (in g’-i’).  Each plot is for a different slice in magnitude.  The brightest clusters are at the top left and the faintest at the bottom right.  The red lines show the best fitting line for each of the metal poor and metal rich populations.  The green line shos the sum of those two functions, and is the best fit to the plot overall.  If you look closely, you can also see that the peaks of the two red functions get closer together with increasing brightness.

We can see this even more easily in the color magnitude diagram by taking the average of the red and blue sides of the globular cluster population.  You can see that on the blue side (on the left, smaller values of g’-i’) there is a slope, leading to redder colors at brighter magnitudes.

This is known as the Mass-Metallicity Relation (MMR) or the blue-tilt.  It was first detected in brightest-cluster galaxies but has since been seen in other systems. 

For more information about this, see Wehner et al. (2007) when it comes out!  (volume info to follow)

Seyferts Sextet actually contains only four galaxies.  The nice spiral toward the middle is a background galaxy and the fuzzy bit on the right was mistaken for a galaxy in early low-resolution images. 

Compact groups of galaxies are fascinating environments for galaxy interactions.  Compact groups are so compact that some contain four galaxies in the same volume that is occupied  by the Milky Way.  At first glance, it seems like galaxies this close (and moving so slowly relative to each other) should merge quickly -- and thus we shouldn’t see very many!  But, we do see them in substantial numbers, so they must have a way to survive.  New models suggest dark matter halos may play a role in their survival. 

Our group, consisting of Kyle Westfall ,Emily Freeland (both at the University of Wisconsin) and myself are exploring Seyferts Sextet in multi-wavelength space.  We have optical imaging and narrowband H-alpha data, and are currently processing data we recently obtained using the GMRT in Pune, India.  We find evidence of correlation between faint H-alpha detections and some small clumps in HI -- dwarf galaxies in formation?