Draft version May 20, 2020 Typeset using LATEX preprint2 style in AASTeX63 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 Jisu Kang,1 Yoo Jung Kim,1 Myung Gyoon Lee,1 and In Sung Jang2 1Astronomy Program, Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea 2Leibniz-Institut f¨ur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany (Accepted May 18, 2020) Submitted to ApJ 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 F 606W/F 814W photometry of resolved stars in a field located in the outer disk (2.′5 . r . 6.′5) of M64. At r ≈ 5.′5 (7 kpc) to the east, we discover a new metal-poor globular cluster (Reff = 5.73 ± 0.02 pc and MV = −9.54 ± 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 ± 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] ≈ −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. Keywords: galaxies: halos — galaxies: individual (M64) — galaxies: spiral — galaxies: arXiv:2005.09633v1 [astro-ph.GA] 19 May 2020 star clusters: individual (M64-GC1) — galaxies: stellar content 1. INTRODUCTION Galaxy due to its prominent dust lane around M64 (NGC 4826) is an early type spiral galaxy the bright bulge. The basic properties of M64 ((R)SA(rs)ab) with nicknames of Black Eye are listed in Table 1. Galaxy, Evil Eye Galaxy, and Sleeping Beauty M64 shows several interesting features. First, it is an isolated galaxy located in the low den- sity environment. It used to be known to Corresponding author: Myung Gyoon Lee [email protected],[email protected] have only one known companion dwarf galaxy, T NGC 4789A (MB ≈ −14 mag) (Jacobs et al. 2 Kang et al. Table 1. Basic Parameters of M64 HI distribution is much more extended (out to ≈ ′′ Parameter Value Ref. r 600 ) (Watkins et al. 2016). Another interesting feature of M64 is that R.A.(J2000) 12h 56m 43s.64 1 its disk is composed of three substructures. Decl.(J2000) +21◦ 40′ 58′′. 7 1 From the recent deep BV surface photome- Type (R)SA(rs)ab 2 try, Watkins et al. (2016) found that the sur- A A A B, V , I 0.150, 0.113, 0.062 3 face brightness profiles of M64 can be divided A A F 606W , F 814W 0.102, 0.063 3 into three parts: 1) an inner disk, 2) an outer E B V ( − ) 0.037 3 disk, and 3) a Type III anti-truncation (up- (m − M)0 28.18 ± 0.03(ran) ± 0.08(sys) 4 bending) component. In the inner disk region Distance 4.33 ± 0.18 Mpc 4 at r < 200′′, spiral arms and evidence of re- −1 Image scale 21.0 pc arcsec 4 cent star formation are seen. We can notice 1.26 kpc arcmin−1 4 dust lanes, young stars, strong UV emission, 75.6 kpc degree−1 4 and a high density of HI gas from the multi- BT , (B − V )T 9.36 ± 0.10, 0.84 ± 0.01 2 band images of M64 (see Watkins et al. 2016, T T B0 , (B − V )0 8.82 ± 0.10, 0.71 ± 0.01 2 Figure 3). The recent star formation in the M T M T . B , V −19 36 ± 0 13, −20 07 ± 0 13 2,4 inner disk is believed to be due to a merger Position angle 115 deg (B), 110 deg (Ks) 2 with a gas-rich dwarf galaxy (Braun et al. 1994; ′′ ′′ D25(B) 600. 00 × 324. 00 2 Watkins et al. 2016). In the outer disk region ′′ ′′ ′′ ′′ Dtot(Ks) 617. 60 × 352. 03 5 at 200 <r < 400 , no spiral arms or evi- −1 vhelio 408 ± 4 km s 2 dence of recent star formation are seen. Its color is red, indicating that the disk stars may be References—(1) NED (2) de Vaucouleurs et al. (1991) old and metal-rich (Watkins et al. 2016). Out- (3) Schlafly & Finkbeiner (2011) (4) This study (5) ′′ ′′ Jarrett et al. (2003) side of the outer disk at 400 <r< 900 , the Type III anti-truncation component is located. It is an upbending structure following a break at r ≈ 400′′ in the surface brightness profiles of 2009). Although several dwarf galaxies includ- M64. Its surface brightness is as low as µB > 27 T −2 ing Coma P (MB ≈ −10 mag) were recently mag arcsec . suggested to be another companions of M64 While the origins of the inner disk and the (Brunker et al. 2019; Carlsten et al. 2020), it outer disk are quite clear, the origin of the can be still considered as an isolated galaxy. Type III break is still mysterious. According Second, it hosts two counter-rotating gas rings: to Watkins et al. (2016), the origin of the Type the inner gas ring (r < 1′) follows the galaxy ro- III component in M64 can be explained by three tation, while the outer gas ring rotates in the op- scenarios. First, it can be originated from disk posite direction (Braun et al. 1992). The outer stars that migrated from the inner region. This ring is considered to be a remnant of a recent possibility is supported by the (B − V ) color of merger with a gas-rich satellite (Braun et al. the Type III component stars which is as red as 1992, 1994; Rubin 1994; Walterbos et al. 1994; that of the outer disk stars. However, the un- ′′ Rix et al. 1995; Watkins et al. 2016). Third, certainty of the color measurement for r > 400 recent star formation is concentrated only in is significant because of its low surface bright- the inner disk region at r < 200′′, while the ness. Moreover, it is difficult for disk stars to migrate such a large distance (several disk scale M64-GC1 and RGB stars 3 lengths beyond the spiral features). Second, the ′× ′ Type III component can be originated from star 12 12 SDSS color image of M64 formation in extended gas induced by counter rotating kinematics of M64. If so, the (B − V ) 44 color of the Type III component stars may be M64 much bluer (younger) than previously known. Third, it can be originated from halo stars. If Decl. so, the stars in this component must be old and 40 metal-poor. ′ Photometry of resolved stars located in the a = a25,B = 5 low surface brightness region of a galaxy is an HST/ACS excellent tool to study the nature of diffuse 21° 36′ 1′ galaxy light in terms of stellar populations and M64-GC1 to trace the origin of the stellar populations. In 12h 57m 00s 56m 40s 20s this study, we explore the origin of the Type III R.A. break using the photometry of resolved stars in an outer disk field of M64 based on the Hub- ′ × ′ ble Space Telescope (HST)/Advanced Camera Figure 1. The color map of the 12 12 SDSS image of M64. North is up and east to the left. for Surveys (ACS) images. To date, little is The image scale bar is shown on the bottom right. known about the resolved stars or star clus- ′ The large ellipse represents a25,B = 5.0 where a25,B ters in M64. We adopt the distance to M64, is the projected galactocentric distance at which 4.33 Mpc as determined later in this study. the B-band surface brightness is 25 mag arcsec−2. The foreground reddening toward M64 is as low The location of the HST/ACS field is marked in as E(B − V )= 0.037 (AF 606W = 0.102 and rectangles. The small circle on the HST/ACS field AF 814W =0.063) (Schlafly & Finkbeiner 2011). marks the new globular cluster (M64-GC1) found This paper is organized as follows. Section 2 in this study (see Figure 3). describes data used in this study and how we ′′ reduce and analyze the data. In Section 3, we s for F 814W . The final image scale is 0.05 per present main results: a discovery of a new glob- pixel. ular cluster, color-magnitude diagrams (CMDs) In Figure 1, the location of the HST/ACS of the resolved field stars, estimation of the tip field is marked on the SDSS color image of M64. ′ ′ of the red giant branch (TRGB) distance, and The ACS field is positioned at r = 2.3 to 6.2 ′ ′ radial number density profiles of the resolved (a = 2.4 to 6.6) along the eastern major axis stars.