Identifying the Globular Cluster Systems of the Sculptor Group
Abstract
We present results from a study to identify the globular cluster systems of six of the brightest galaxies of the Sculptor Group: NGC 45, NGC 55, NGC 247, NGC 253, NGC 300, and NGC 7793.
Our observations were made with the CTIO 4-m telescope, Mosaic II camera, and CMR filter set.
The 30 arcmin field of Mosaic II captures the entire expected extent of the globular cluster systems of these galaxies in a single pointing of the telescope.
We identify the cluster candidates through their morphology and C-R and M-R colors.
Selecting objects by isophotal area, isophotal flux, ellipticity, C-R and M-R, we find 100-150 objects per galaxy with properties similar to known globular clusters in NGC 253.
We estimate that half of these objects are true clusters, while the other half are contaminating background galaxies.
With spectroscopic follow-up, these cluster candidates will provide important clues to understanding the formation of globular clusters in small galaxies like the LMC and M33.
Motivation
Did halos form before disks in late-type galaxies?
Some evidence FOR:
- Milky Way's halo globular clusters are among oldest objects in the Universe, with ages 12-15 Gyr
- Milky Way disk appears to be ~10 Gyr old
Some evidence AGAINST:
- Halo formation is a continuing process. Example: infall of Sagittarius dwarf into Milky Way (Ibata et al. 1994), which is contributing ~4 globular clusters to the halo (Da Costa & Armandroff 1995)
- The LMC's oldest globular clusters are as old as the Milky Way's oldest clusters (Mighell et al. 1996, Olsen et al. 1998, Johnson et al. 1999), yet they have disk kinematics (Schommer et al. 1992)
- M33's clusters have halo kinematics (Schommer et al. 1991), but many have exclusively red horizontal branches, suggesting intermediate age (Sarajedini et al. 1998)
What is the dominant formation site of globular clusters in late-type galaxies?
Method
The identification of clusters in late-type galaxies is difficult because the number of clusters is small compared to the number of contaminating foreground stars and background galaxies. We circumvent this problem by:
- Observing galaxies which are nearby enough to partially resolve the clusters yet far enough away to capture the bulk of a cluster system in a single image with a large-format CCD camera
- Using the automated program SExtractor (Bertin & Arnouts 1996) to create a database from which we select likely candidate clusters
Our observations were taken with the CTIO 4-m telescope and Mosaic II camera on the nights of Nov 11-13, 1999, two of which were photometric.
We used three filters, Kron-Cousins R and Washington C and M, which combined give improved age-metallicity discrimination over BVR.
Exposure times were chosen to achieve S/N of 100 in R for clusters as faint as Mv = -6.
The Mosaic II camera delivers excellent image quality over a 30'x30' field of view; combined with good seeing ( 1 arcsec), Mosaic II lets us discriminate candidate clusters from stars and background galaxies over a 20x20 square-kpc area at a distance of 2 Mpc, typical of the nearest Sculptor Group galaxies.
The processed and stacked images were analyzed with SExtractor:
- Detection and measurement limits were set by the limiting surface brightness, 3-sigma above the summed noise
- Likely cluster candidates selected by isophotal area, isophotal flux, ellipticity, C-R and M-R colors
Beasley & Sharples (2000) have spectroscopically identified 14 globular clusters in NGC 253. These known clusters were used to test and tune our selection criteria, described in Fig. 1.
References:
Beasley, M. A., Sharples, R. M. 2000, MNRAS, 311, 673
Bertin, E., Arnouts, S. 1996, A&A, 117, 393
Da Costa, G. S., Armandroff, T. E. 1995, AJ, 109, 2533
Ibata, R. A., Gilmore, G., Irwin, M. J. 1994, Nature, 370, 194
Johnson, J. A., Bolte, M., Stetson, P. B., Hesser, J. E., Somerville, R. S. 1999, ApJ, 527, 199
Mighell, K. J., Rich, R. M., Shara, M., Fall, S. M., 1996, AJ, 111, 2314
Olsen, K. A., Hodge, P. W., Mateo, M., Olszewski, E. W., Schommer, R. A., Suntzeff, N. B., Walker, A. R. 1998, MNRAS, 300, 665
Sarajedini, A., Geisler, D., Harding, P., Schommer, R. 1998, ApJ, 508, 37
Schommer, R. A., Christian, C. A., Caldwell, N., Bothum, G. D., Huchra, J. 1991, AJ, 101, 873
Schommer, R. A., Suntzeff, N. B., Olszewski, E. W., Harris, H. C. 1992, AJ, 103, 447
Figures:
Figure 1: The area within the isophote having surface brightness 3-sigma above the background (per square-arcsec) is plotted vs. the flux within the isophote, for objects measured in a stacked R image of NGC 253 (1 arcsec seeing) using the program SExtractor.
Only those objects with e < 0.4, 0.25 < M-R < 0.8, and 0 < C-R < 2.4 are shown.
Objects with spectra from Beasley & Sharples (2000) are marked with filled circles for globular clusters, diamonds for background galaxies, and asterisks for stars.
A number of the Beasley & Sharples objects fall outside our selection criteria or were masked as lying too close to the galaxy disk, and so are not plotted here.
The line shows the locus defined by a scaled PSF measured from a single bright star.
The majority of the globular clusters lie above this line but below the sequence defined by the galaxies.
The box encloses 100 objects of unknown nature, 50% of which we expect are globular clusters in NGC 253, as estimated from the number of clusters and galaxies within the box.
JPG (34 kb) or Postscript (59 kb)
Figure 2: Color-color diagram of globular clusters (filled circles), background galaxies (diamonds), stars (asterisks), and candidate clusters picked from the box of Fig. 1 (small open circles) in NGC 253.
A typical error bar, representing both random photometric error and uncertainty in the calibration, is shown in the lower left-hand corner.
The lines represent three composite stellar populations simulated from the models of Schaerer et al. (1993), with ages of 1 Gyr (solid line), 4 Gyr (dotted line), and 12 Gyr (dashed line).
The lengths of the lines correspond to the range of metallicities -1.7 < [Fe/H] < 0.0.
Although the combination of photometric error and age-metallicity degeneracy prevents us from deriving precise ages and abundances, the bulk of the identified globular clusters appear to be of old or intermediate age, as already noted by Beasley & Sharples (2000).
The bluest appears young, perhaps analogous to the blue globular clusters of the Magellanic
Clouds.
Our candidate clusters, if confirmed through spectra, appear to contain a balanced distribution of young and older clusters.
JPG (20 kb) or Postscript (52 kb)
Figure 3: The area within the isophote having surface brightness 3 above the background (per arcsec2) is plotted vs. the flux within the isophote, for objects measured in a stacked R image of NGC 45, NGC 55, NGC 247, NGC 300, and NGC 7793 using the program SExtractor.
Only those objects with e<0.4 are shown.
The box edges are as defined in Fig. 2, after scaling for differences in distance and exposure time.
Each box encloses 100-150 candidate clusters.
JPG (108 kb) or Postscript (742 kb)
Figure 4: A GLOBULAR CLUSTER IN NGC 253: This closeup of a globular cluster spectroscopically identified by Beasley & Sharples (2000) shows the object to be round yet noticeably broad compared to the profile of a nearby star.
Our analysis of this image has turned up 100 objects with properties similar to previously identified globular clusters.
JPG (74 kb) or Gzipped Postscript (884 kb)
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