Tracing Red Giant Members of the Globular Cluster Palomar 5 with APOGEE and Gaia

TL;DR

This study combines APOGEE and Gaia data to identify and analyze red giant members of Palomar 5, revealing density variations in its tidal streams and providing insights into the Milky Way's gravitational influences.

Contribution

It presents a combined spectroscopic and astrometric approach to identify Palomar 5 members and investigates stream density variations with implications for galactic structure.

Findings

Identified 8 core and stream members of Palomar 5.

Most candidate members deviate from known trends, indicating possible non-members.

Supports presence of density variations in Pal 5's tidal streams.

Abstract

The globular cluster Palomar 5 (Pal 5) is in the process of being tidally shredded as it orbits the Milky Way. Its core is currently at a heliocentric distance of ~21 kpc, near apogalacticon (~18 kpc), and it reaches ~5-7 kpc at perigalacticon. Pal 5's leading and trailing arms stretch over 20 degrees on the sky, making them sensitive probes of the Milky Way's mass distribution. In this work, we search for red giant members of Pal 5 using spectroscopic data from APOGEE DR17 and photometric and astrometric data from Gaia DR3. Based on position and proper motion, we identify eight members of Pal 5: six in the core and two in the stream. The clustering algorithm HDBSCAN finds these same eight. We then use chemical tagging with APOGEE abundances to search for additional members across five APOGEE fields overlapping Pal 5. While several dozen candidates are identified, most deviate (some significantly) from known kinematic and color-magnitude trends, suggesting that they are less likely to be true members. We estimate the expected number of giants in the APOGEE pointings based on the area and stellar mass of the streams. Given APOGEE's limiting magnitude, we find that few, if any, new giants are expected, especially if the stream is more diffuse at these locations. Our results support the presence of density variations in Pal 5's tidal streams, consistent with earlier studies attributing such features to baryonic perturbers in the Milky Way, dark matter subhaloes, or interactions with passing globular clusters.