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

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° 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 data release 3. 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 subhalos, or interactions with passing globular clusters.