Globular Clusters in M31

Summary

Spectra of the integrated stellar light of globular clusters in M31

Finding Targets

An object whose APOGEE_TARGET1 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Description

By studying the chemical composition and internal kinematics of M31 clusters observed in integrated light with the APOGEE spectrograph (i.e., each cluster observed with a single fiber), this program aims to determine the abundance pattern of M31’s old halo and bulge, provide insights into the star formation timescales in the halo and bulge, and constrain the initial mass function of their first stellar generations. These data will greatly expand upon the set of elemental cluster abundances obtained in optical studies (e.g., Colucci et al. 2009; Schiavon et al. 2012) by enabling the determination of abundances of elements such as O (the most abundant metal and a key indicator of the timescales for star formation) which lack lines at optical wavelengths. Other key elements accessible by APOGEE include C, N, and Na, whose abundances based on optical spectra are uncertain or unavailable altogether. Further, these data will allow for the derivation of internal velocity dispersions for the target sample’s massive clusters.

Target Selection Details

From the initial list of more than 350 M31 globular clusters (Caldwell et al. 2009), about 250 objects brighter than H = 15.0 mag (Vega) were targeted, along with the M31 core, M32, and M110. To isolate the integrated cluster spectra from that of the background (unresolved) M31 stellar populations, each observation of a cluster in the vicinity of the M31 bulge was accompanied by one of a very nearby “non-cluster” background region, ideally ≤ 10 arcsec offset from the cluster. As this distance is significantly smaller than the fiber collision radius of APOGEE fibers, simultaneous observations of the cluster and background positions could not be made. We adopted a scheme whereby two designs were made, each containing a mixture of cluster targets and background regions for clusters on the other design. Globular clusters at large M31-centric distances, against a faint stellar background, do not require background region counterparts and were instead targeted on both plates.

Primary Contact

Other Contacts

Charli Sakari, Carlos Allende Prieto, Dmitry Bizyaev, Robert O’Connell, Matthew Shetrone

REFERENCES

Caldwell, N. et al. 2009, AJ, 137, 94
Colucci, J.E. et al. 2009, ApJ, 704, 835
Sakari, C.M., Shetrone, M., Venn, K., McWilliam, A., & Dotter, A. 2013, MNRAS, 434, 358
Sakari, C.M., Venn, K., Shetrone, M., Dotter, A., & Mackey, D. 2014, MNRAS, 443, 2285
Sakari, C.M., Venn, K.A., Mackey, D., et al. 2015, MNRAS, 448, 1314
Schiavon, R. P., Caldwell, N., Morrison, H., et al. 2012, AJ, 143, 14