A New Metal-poor Globular Cluster and Resolved Stars in the Outer Disk of the Black Eye Galaxy M64: Implication for the Origin of the Type III Disk Break

Abstract

M64 is a nearby spiral galaxy with a Type III anti-truncation component. To trace the origin of the Type III component, we present Hubble Space Telescope/Advanced Camera for Surveys $F606W/F814W$ photometry of resolved stars in a field located in the outer disk ($2.5^{\prime }\lesssim r\lesssim 6.5^{\prime }$) of M64. At $r\approx 5.5^{\prime }$ (7 kpc) to the east, we discover a new metal-poor globular cluster ($R_{\mathrm{eff}}=5.73\pm 0.02$ pc and $M_V=-9.54\pm 0.09$ mag), M64-GC1. This is the first globular cluster found in M64. The color-magnitude diagram (CMD) of the resolved stars in M64-GC1 is well matched by 12 Gyr isochrones with [Fe/H] $=-1.5\pm 0.2$, showing that this cluster belongs to a halo. The CMD of the resolved stars in the entire ACS field shows two distinguishable red giant branches (RGBs): a curved metal-rich RGB and a vertical metal-poor RGB. The metal-rich RGB represents an old metal-rich ([Fe/H] $\approx -0.4$) disk population. In contrast, the CMD of the metal-poor RGB stars is very similar to the CMD of M64-GC1, showing that the metal-poor RGB represents a halo population. The radial number density profile of the metal-rich RGB stars is described by an exponential disk law, while the profile of the metal-poor RGB stars is described by a de Vaucouleurs's law. From these, we conclude that the origin of the Type III component in M64 is a halo which has a much lower metallicity than a disk or bulge population.

Accepted for publication in ApJ, 16 pages, 10 figures, 3 tables

References in corpus (7)

• The structure of galactic disks: Studying late-type spiral galaxies using SDSS
• The Outer Disks of Early-Type Galaxies. I. Surface-Brightness Profiles of Barred Galaxies
• The Tip of the Red Giant Branch Distances to Type Ia Supernova Host Galaxies. IV. Color Dependence and Zero-Point Calibration
• (Almost) Dark HI Sources in the ALFALFA Survey: The Intriguing Case of HI1232+20
• Formation of S0 galaxies through mergers. V. Antitruncated stellar discs resulting from major mergers
• Metallicity-corrected Tip of the Red Giant Branch Distance to NGC 4258
• The Enigmatic (Almost) Dark Galaxy Coma P: Distance Measurement and Stellar Populations from HST Imaging

Cited by in corpus (3)

• Distances to PHANGS Galaxies: New Tip of the Red Giant Branch Measurements and Adopted Distances
• Origins of the Evil Eye: M64's Stellar Halo Reveals the Recent Accretion of an SMC-mass Satellite
• A machine-learning photometric classifier for massive stars in nearby galaxies II. The catalog