An island universe of star clusters in Andromeda

Title: PHAT Stellar Cluster Survey I: Year 1 Catalog and Integrated Photometry
Authors: L. Clifton Johnson and the PHAT Team
First Author’s Institution: Department of Astronomy, University of Washington

The Hubble Space Telescope is a uniquely capable machine that enables science which is simply not possible with any other observatory. For this reason, NASA devotes a large fraction of Hubble’s observing time to key projects that collect observations over hundreds of orbits over several years. Collectively, these observations constitute a legacy archive that will enable investigations for decades to come. In the past, these sorts of observing programs have enabled precise determinations of the Hubble constant, discovered the most distant galaxy currently known, produced a detailed mapping of the Orion Nebula Cluster, and a heaping portion of other science.

Today, I’ll be discussing new results from one of these large Hubble observing programs, the Panchromatic Hubble Andromeda Treasury (PHAT) survey. PHAT is an ongoing UV to near IR wide-field survey of the Andromeda galaxy that will produce a deep, extremely high resolution map of a large fraction of the disk of the Andromeda galaxy. For the first time astronomers will have uniform observations of a massive disk galaxy extending from the crowded central bulge to the fainter, more tenuous outskirts of the galaxy. These data can easily resolve individual main sequence stars in Andromeda. While there are many science goals for the PHAT dataset, today I’ll focus on the new PHAT star cluster catalog that is being built using the multi-wavelength imaging.

The footprint of the PHAT survey is shown in magenta outline on top of a GALEX far ultraviolet image in grayscale. Green boxes outline 'bricks' that make up the year 1 imaging data. Filled blue circles show the locations of stat clusters within the PHAT footprint.

Star clusters are particularly interesting because we expect that most stars are born in clusters. By this I mean that stars tend to form in bursts – a large number of stars form after the collapse of a single molecular cloud core. Since only a small fraction of the mass of a molecular cloud is converted into stars during a single episode of star formation, most star clusters are born unbound: the gas that made up most of the gravitating mass of the natal cloud is blown away by feedback processes, producing an unbound stellar system. Sometimes the star formation process is particularly efficient, and the star cluster remains bound for longer than a few million years, producing an open cluster. Over the following hundreds of millions of years, stellar evolution, gravitational interactions in the crowded center of the star cluster, and the perturbing influence of the gravitational potential of the galaxy as a whole can finally dissolve the star cluster, adding stars to the disk of the galaxy.

Theories for why some star clusters dissolve immediately, others dissolve more slowly, and yet others remain bound for billions of years can be tested using detailed observations of a statistically well-understood population of star clusters. Building such a sample is not easy. In the Milky Way, star cluster samples are incomplete beyond the solar neighborhood due to dust obscuration in the plane of the Galaxy. More complete catalogs exist for the Large and Small Magellanic Clouds, but these were built up over many years of heterogenous ground-based observations, so quantifying the statistical quality of these star cluster catalogs is quite difficult. More worryingly, the LMC and SMC are dwarf galaxies, deficient in heavy elements compared to the cosmic average. Drawing universal conclusions about star clusters based on LMC and SMC catalogs is thus quite difficult. Thankfully, the Local Group contains another massive disk galaxy, the relatively nearby (~750 kpc) Andromeda galaxy.

The number of clusters detected as a function of F475W magnitude. F475W is wide-band Hubble filter that roughly corresponds to SDSS g-band. The black histogram is for the PHAT dataset while the red histogram counts all previously identified clusters in the year 1 PHAT footprint.

The PHAT dataset will yield the largest sample of star clusters for any galaxy. More importantly, the instruments and observing strategy used to produce the catalog are well-documented, enabling sophisticated statistical analyses of the data to precisely infer the nature, origin, and fate of star clusters. Above, I’ve reproduced a figure from the paper that illustrates the footprint of the PHAT survey. Although at present PHAT only covers a small fraction of Andromeda, the star cluster catalog is already quite impressive. To the right, I’ve reproduced a histogram from the paper that illustrates the level of completeness in the PHAT dataset. Clearly, previous cluster surveys missed the majority of the faint star clusters that PHAT has detected. The new high-resolution, high sensitivity observations allow astronomers to pick out stellar groupings in crowded fields that would have been missed in shallow ground-based imaging. The new clusters consist mostly of low mass (~ 1000 solar masses) clusters, along with a smattering of older or heavily dust-obscured massive clusters.

However, that’s not the whole story: PHAT does much more than simply detect star clusters in a single optical passband. The entire PHAT footprint will be imaged in wavelengths from the near ultraviolet to the near infrared — covering the wavelength range where stars output the bulk of their luminosity. Using color-color diagrams and color-magnitude diagrams, the PHAT team can also infer the masses and ages of the clusters. While the final PHAT star cluster catalog will use sophisticated techniques to accurately pin down the ages and masses of these clusters, as a first pass the authors produce the three diagrams below to illustrate the range in star cluster properties represented in the PHAT dataset. This figure contains a lot of information, so I’ll step through it slowly.


Click through for a much larger version. Left panel: A color-color diagram for the cluster in the PHAT year 1 footprint. Red points represent relatively bright clusters while white points represent faint clusters. A blue line indicates the location of a solar metallicity stellar population model in this parameter space. Center panel: The same color-color diagram, but smoothed to show the density of star clusters in any given region of parameter space. The red rhombus shows the location of low-mass star clusters which are unusually bright due to the presence of a single evolved giant star, the green rhombus is the location of metal poor globular clusters, and the blue line is the location of low mass clusters with no evolved stars. Right panel: A color-magnitude diagram with three simple stellar population models for different cluster masses. The catalog contains a significant number of ~103 solar mass clusters.

In the leftmost panel, the authors present a color-color diagram. The colors of a cluster are determined by the age, metallicity, and dust reddening rather than the cluster’s absolute brightness, a proxy for mass. Since the photometric errors are so tiny, the PHAT team can use diagrams like this to accurately gauge the age of the cluster. For reference, the location of a somewhat reddened solar metallically stellar population model is overplotted as a blue line in all three panels. In the color-color diagrams, a young cluster lives at the top left hand (blue) corner while an old cluster would live on the bottom right hand (red) corner of the diagram. Intermediate mass clusters that only contain a thousand or so stars can stochastically appear to be much redder than one would expect due to the presence of a single, very bright evolved star. This produces the plume of clusters toward the top right hand corner of the diagram. The density of star clusters and the location of various families of star clusters are displayed in the center panel. Clearly, young, intermediate-age, and old clusters are all well-represented in the sample. Lastly, the righthand panel is a color-magnitude diagram for the cluster sample. The location of a cluster in this diagram depends on the absolute brightness and thus the mass of the cluster. However, at a fixed mass, older clusters will be dimmer due to the loss of massive main sequence stars. The three blue lines show the path a 103, 104, and 105 solar mass cluster takes through this parameter space as it ages.

Future studies using this dataset will be used to understand the physical processes that lead to cluster disruption, the relationship between young clusters and their natal molecular clouds, and the dependence of cluster properties on galactic environment. It’s certainly an exciting time for star cluster studies. I, for one, am looking forward to hearing more results from the PHAT survey!

About Nathan Goldbaum

Nathan is a third year graduate student in Astrophysics at UCSC. His interests include the local ISM, molecular clouds, and the role of star formation in galactic evolution.

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