Galactic Long Bar

Summary

One outstanding puzzle of the inner Milky Way is the nature of the “long bar”, a thin stellar overdensity extending from the central boxy bar/bulge and distinct from both that bar and the proposed “nuclear bar” (e.g., Benjamin et al. 2005; Lopez-Corredoira et al. 2007). As the first large spectroscopic study of this feature, this ancillary program targeted several hundred candidate red clump stars in the long bar, to measure velocities and basic chemistry.

Finding Targets

An object whose APOGEE_TARGET2 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

After the long bar’s discovery in the early 1990s in mid-IR surface brightness maps, several groups attempting to constrain its structural parameters have found conflicting results on its shape and orientation. Our understanding of its stellar kinematics and chemistry is also very sparse — because of the high extinction in the inner Galaxy, spectroscopic investigations of this feature have been minimal. These kinds of parameters can be obtained comparatively easily in external galaxies, but we have not yet been able to place the Milky Way’s central bar complex fully in the sequence of galactic bars.

Taking advantage of APOGEE’s IR coverage, this program selected several hundred candidate red clump giants in already-planned APOGEE fields towards the long bar. The goal was to measure the mean kinematics and metallicity for these stars as a function of position along the bar. N-body models suggest that bars may be quite effective at driving stellar radial migration, which has a strong impact on the chemical and age gradients in galactic disks.

Primary contacts

Other contacts

Target Selection Details

Approximately 600 stars in 11 fields (with longitude between 8 and 27 degrees) were selected with 2MASS, Spitzer+IRAC, and WISE photometry meeting the standard APOGEE data quality criteria. A color bin of 0.5 ≤ (J–K_s)₀ ≤ 0.8 was used to isolate red clump stars from RGB giants. Stars were selected to have Spitzer or WISE [4.5μ] magnitudes within 0.4 mag of the mean red clump star count peak for each field’s longitude (Zasowski et al. 2012), or H ≤ 12.75 (13.94), whichever is brighter, in one (three) visit fields. That magnitude limit was based on early ASPCAP performance evaluation at low S/N, to ensure that even though these stars are fainter than the nominal magnitude limit for their fields, the S/N will be sufficient to measure velocities and basic chemistries.

REFERENCES

Benjamin, R. A., et al. 2005, ApJL, 630, L149
Lopez-Corredoira, M., et al. 2007, AJ, 133, 154
Zasowski, G., et al. 2012, EPJ Web of Conferences, 19, 6006