arXiv:2005.09633v1 [astro-ph.GA] 19 May 2020 lse ( cluster 6-C swl ace y1 y scrnswt F/]= [Fe/H] with isochrones of Gyr (CMD) 12 by diagram matched color-magnitude well The is M64. M64-GC1 in found cluster globular (2 lse eog oahl.TeCDo h eovdsasi h entire ([Fe/H] the metal-rich RGB old in metal-rich an stars represents curved RGB resolved a metal-rich the The RGB. (RGBs): of poor CMD branches The giant red halo. distinguishable a to belongs cluster rgno h yeIIcmoet epresent we component, III Surveys Type the of origin htteoii fteTp I opnn nM4i aowihhsamu a has which halo a population. is M64 bulge F in or component law. disk Vaucouleurs’s III a de Type dis than a the exponential by of described an origin the is by that stars described The RGB is metal-poor t stars population. the to RGB halo of similar metal-rich a very the represents is of RGB stars profile RGB metal-poor metal-poor the the that of showing CMD the contrast, In orsodn uhr yn yo Lee Gyoon Myung author: Corresponding Beauty Eye Sleeping and Black Galaxy, Eye of Evil Galaxy, nicknames with ((R)SA(rs)ab) [email protected],[email protected] 1 srnm rga,Dprmn fPyisadAtooy Se Astronomy, and Physics of Department Program, Astronomy 6 sanab prlglx ihaTp I nitucto compon anti-truncation III Type a with galaxy spiral nearby a is M64 6 NC42)i nerytp prlgalaxy spiral type early an is 4826) (NGC M64 yee sn L using 2020 Typeset 20, May version Draft . ′ 5 e ea-orGoua lse n eovdSasi h O the in Stars Resolved and Cluster Globular Metal-poor New A . lc y aayM4 mlcto o h rgno h yeI Type the of Origin the for Implication M64: Galaxy Eye Black Keywords: r 2 ebi-ntttfu srpyi osa AP,A e St der An (AIP), Potsdam f¨ur Astrophysik Leibniz-Institut R . F eff 606 6 . ′ 1. iuKang Jisu )o 6.At M64. of 5) 5 = W/F A T INTRODUCTION E . aais ao aais niiul(6)—glxe:sia galaxies — spiral galaxies: — (M64) individual galaxies: — halos galaxies: trcutr:idvda M4G1 aais tla content stellar galaxies: — (M64-GC1) individual clusters: star 73 X 814 preprint2 ± W 0 , 1 . 2p and pc 02 o ugKim Jung Yoo htmtyo eovdsasi edlctdi h ue disk outer the in located field a in stars resolved of photometry r ≈ tl nAASTeX63 in style 5 . ′ 7kc otees,w icvranwmtlpo globular metal-poor new a discover we east, the to kpc) (7 5 M eu 82,Rpbi fKorea of Republic 08826, Seoul Acpe a 8 2020) 18, May (Accepted V umte oApJ to Submitted = , 1 ABSTRACT yn yo Lee Gyoon Myung − 9 ubeSaeTelescope Space Hubble . 54 ± iyevrnet tue ob nw to known be ( galaxy, dwarf to 4789A companion NGC used known den- one It low only the have in located environment. galaxy sity isolated an is it h rgtble h ai rpriso M64 of properties in basic listed around The are lane bulge. dust bright prominent the its to due Galaxy 0 . 6 hw eea neetn etrs First, features. interesting several shows M64 9mg,M4G1 hsi h first the is This M64-GC1. mag), 09 u ainlUiest,1Gaa-o Gwanak-gu, Gwanak-ro, 1 University, National oul rwre1,142Ptdm Germany Potsdam, 14482 16, ernwarte , − 1 Table 1 and . 5 M − ≈ ± B T Avne aeafor Camera /Advanced a,wieteprofile the while law, k o hs,w conclude we these, rom eCDo M64-GC1, of CMD he nSn Jang Sung In ailnme density number radial 0 n etclmetal- vertical a and − ≈ C edsostwo shows field ACS . 1 0 h eovdsasin stars resolved the ,soigta this that showing 2, hlwrmetallicity lower ch . . )ds population. disk 4) n.T rc the trace To ent. 4mg ( mag) 14 trDs fthe of Disk uter IDs Break Disk II 2 aose al. et Jacobs : 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
1.0 aperture corrections are on average ±0.03 mag (Monachesi et al. 2016). 0.8 Artificial star tests were also carried out us- ing the DOLPHOT routine fakelist to gener- ate the star lists. We generated 1,000 artifi- 0.6 cial stars with a magnitude range of 23.5 < F 606W < 27.5 and a color range of 0.5 < ≤ (F 606W − F 814W ) < 3.0, and re-ran DOLPHOT 0.4 24.0 F814W < 24.4 24.4 ≤ F814W < 24.8 with FakeStar parameters (FakeMatch = 3, Recovery fraction FakePSF = 2, FakeStarPSF = 1, RandomFake 0.2 = 1, FakePad = 0). We repeated this pro- HST/ACS cedure 100 times to generate 100,000 artificial coverage stars in total. Figure 2 shows recovery fraction 0.0 0 2 4 6 8 of the bright stars along the radial bin according a [arcmin] to the two different magnitude ranges (24.0 ≤ F 814W < 24.4, 24.4 ≤ F 814W < 24.8). The recovery fraction decreases with decreasing pro- Figure 2. The recovery fraction of the bright stars jected galactocentric distance a, but it is higher along the radial bin according to the two different than 50% even at the innermost region. This re- magnitude range (24.0 ≤ F 814W < 24.4, 24.4 ≤ F 814W < 24.8). This result was derived from the sult was used to correct the radial number den- artificial star tests. The vertical dashed lines mark sity profile of the RGB stars in Section 3.4. the HST/ACS coverage and the horizontal dotted line marks the 50% completeness. 3. RESULTS 3.1. Discovery of a New Globular Cluster vidual flc images were used as input images. We used DOLPHOT input parameters listed in Ta- From the visual inspection of the ACS images, ble A2 of Monachesi et al. (2016) which were we discovered a new globular cluster, henceforth ′ used for the photometry of GHOSTS (Galaxy named M64-GC1, located at r ≈ 5.5 in the east. Halos, Outer disks, Substructure, Thick disks, This is the first globular cluster found in M64, and Star clusters) fields. We applied selec- and it is the only star cluster with resolved stars tion criteria for the stars similar to those used we could find in the images. The position of in the GHOSTS survey (Radburn-Smith et al. M64-GC1 is marked by a circle in Figure 1. This cluster is seen as a bluish point source in 2011): –0.06 < SHARPF 606W + SHARPF 814W the SDSS color image. Figure 3 is a pseudo < 1.30, CROWDF 606W + CROWDF 814W < 1.0, color map of the zoomed-in HST/ACS image for S/NF 606W,F 814W > 5, typeF 606W,F 814W = 1, M64-GC1. This new cluster shows a very round flagF 606W,F 814W ≤ 2. We increased the max- imum crowding parameter from 0.16 to 1.0 con- shape and its outer region is partially resolved sidering the crowding of the M64 field. We into individual stars, which verifies that it is use the Vega magnitude system following the indeed a star cluster. DOLPHOT output parameters. Aperture correc- We derived the radial number density profile tions were automatically applied by DOLPHOT of the resolved stars by counting the detected routine (Apcor = 1). The systematic errors of stars with F 814W < 26.5 mag in the star clus- ′′ ter region at RGC < 10 from the cluster cen- ter, as displayed in Figure 4. No stars are de- M64-GC1 and RGB stars 5
15 Cluster Background 8′′×8′′ HST color image of M64-GC1 region region
10 ] -2
′′ [arcsec 1.2 Σ 5
′′ RGC = 2.7 1′′
0 2 4 6 8 10 RGC [arcsec]
Figure 3. The color map of the 8′′ ×8′′ zoomed-in Figure 4. The radial number density profile of HST image of M64-GC1. North is up and east to the detected stars with F 814W < 26.5 mag around the left. The image scale bar is shown on the bot- M64-GC1. No stars are detected in the central re- ′′ ′′ tom right. Note that the outer region of M64-GC1 gion at RGC < 1. 2. Magenta lines at RGC = 1. 2 is partially resolved into individual stars. They are and 2′′. 7 show the boundary for the cluster region, ′′ ′′ mostly RGB stars of the cluster. Dashed lines rep- and pale blue lines at RGC = 4. 0 and 7. 0 denote ′′ ′′ resent the circles with radii of RGC = 1. 2 and 2. 7. the background region adopted in this study.
′′ tected in the central region at RGC < 1.2 due 0.0 with a step of 0.2 dex (green lines). These to severe crowding. However, the radial profile isochrones are shifted according to the distance ′′ ′′ clearly shows an excess at 1.2 < RGC < 2.7 ((m − M)0 = 28.2) and foreground reddening and a flat distribution in the outer region at (AF 606W = 0.102 and AF 814W = 0.063) of M64 ′′ RGC > 4 . This shows that most detected stars (Schlafly & Finkbeiner 2011). ′′ ′′ at 1.2 < RGC < 2.7 are the members of the In the CMD, most stars in the cluster re- cluster. gion are located along a vertical branch at In Figure 5(a) we display the CMD of the re- (F 606W − F 814W ) ≈ 1.0, which is consis- solved stars in M64-GC1. We plotted the stars tent with RGB isochrones with low metallic- ′′ ′′ at 1.2 < RGC < 2.7, most of which are consid- ity, [Fe/H] ≈ −1.5. In contrast, the stars in ered to be cluster members, by magenta circles. the background region are seen along curved Then we plotted the stars in the outer back- branches with much redder colors. Therefore, ′′ ′′ ground region at 4.0 < RGC < 7.0, which must the stars in the cluster region must be mostly be background sources outside the cluster, by the members of the new metal-poor globular pale blue circles. The area of the outer back- cluster. We estimate the mean metallicity of ground region is about six times larger than that the cluster, from the comparison of these RGB of the central cluster region. For comparison, stars with 12 Gyr isochrones, obtaining [Fe/H] we overlayed PARSEC isochrones (Bressan et al. = −1.5 ± 0.2. This shows that M64-GC1 is in- 2012) for ages of 12 Gyr and [Fe/H] = –2.2 to 6 Kang et al. 23.0 23.0 M64-GC1 (1.2′′< R < 2.7′′ ) Coma P ′′ GC ′′ 23.5 Control field (4.0 < RGC < 7.0 ) 23.5 Control field
24.0 [Fe/H] = −2.2 −0.6 24.0 −0.4 24.5 24.5 [Fe/H] = −2.2 −0.6 −0.2 −0.4
F814W 25.0 0.0 F814W 25.0 −0.2
25.5 25.5 0.0
26.0 DM = 28.2 26.0 DM = 28.7 AF606W = 0.102 AF606W = 0.077 A = 0.063 A = 0.048 26.5 F814W 26.5 F814W -1 0 1 2 3 4 -1 0 1 2 3 4 F606W - F814W F606W - F814W
Figure 5. (Left) CMD of the resolved stars in M64-GC1. Magenta circles mark the stars at 1′′. 2 < ′′ ′′ ′′ RGC < 2. 7, pale blue circles mark the background stars at 4. 0 < RGC < 7. 0, and grey dots mark the stars in the entire ACS field. Green lines denote isochrones for age = 12 Gyr and [Fe/H]= −2.2 to 0.0 with a step of 0.2 dex, shifted according to (m − M)0 = 28.2, AF 606W = 0.102, and AF 814W = 0.063. Blue and red boxes denote the boundaries for selecting bright metal-poor RGB stars and metal-rich RGB stars later in this study. (Right) CMD of the resolved stars in Coma P. Magenta circles for the Coma P stars, and pale blue circles for the background stars. Green lines denote the same isochrones shifted according to (m − M)0 = 28.7, AF 606W = 0.077, and AF 814W = 0.048. deed an old metal-poor globular cluster belong- of Coma P show a much broader color (metal- ing to the halo population. licity) distribution with a slightly higher mean For comparison, we display the CMD of the metallicity, [Fe/H] ≈ −0.9. Indeed, M64-GC1 resolved stars in Coma P, a new dwarf com- has a lower metallicity than Coma P. More- panion of M64, in Figure 5(b). Brunker et al. over, there are a dozen young stars located at (2019) presented F 606W/F 814W photometry (F 606W − F 814W ) ≈ 0 in the CMD of Coma of the resolve stars in Coma P. To obtain the P, which are not seen in the CMD of M64-GC1. CMD of the resolved stars in Coma P, we ap- We estimate the total integrated magnitude plied a similar reduction and photometry pro- and effective radius of M64-GC1 from surface cess for the HST/ACS F 606W/F 814W images photometry using IRAF/ELLIPSE task on the of Coma P (PI: Salzer, PID: 14108) as used for ACS images (Jedrzejewski 1987). Figures 6(a) M64 images. The CMD of Coma P we obtained and (b) display the surface brightness and color is consistent with the result of Brunker et al. profiles of M64-GC1. We fit the surface bright- (2019). The same isochrones in Figure 5(a) ness profiles of the cluster with a King profile are overlayed after a shift according to the dis- (King 1962), as shown by dotted lines in Figure ′′ ′′ tance ((m − M)0 = 28.7) and foreground red- 6(a). We derive core radius rc = 0.13 ± 0.01 ′′ ′′ dening (AF 606W = 0.077 and AF 814W = 0.048) (2.7 ± 0.2 pc) and tidal radius rt =2.59 ± 0.02 of Coma P given by Brunker et al. (2019). Un- (54.4 ± 0.4 pc) in F 814W images, and rc = ′′ ′′ ′′ ′′ like the RGB stars in M64-GC1, the RGB stars 0.12 ± 0.01 (2.5 ± 0.2 pc) and rt =2.64 ± 0.02 M64-GC1 and RGB stars 7 16 16 (a) (c) F814W = 17.8 18 18
] F606W = 18.5 -2 20 20 F814W ± rc = 0.13 0.01 22 ± 22 rt = 2.59 0.02 [mag arcsec
µ F606W 24 ± 24
rc = 0.12 0.01 Total magnitude [mag] ± rt = 2.64 0.02 26 26
1.0 (b) 1.0 (d)
color 0.5 color 0.5
0.01 0.10 1.00 0.01 0.10 1.00 RGC [arcsec] RGC [arcsec]
Figure 6. (a) Surface brightness profiles of M64-GC1 obtained using IRAF/ELLIPSE (blue triangles for F 606W , and red boxes for F 814W ). Dotted lines represent King profile fitting. (b) (F 606W −F 814W ) color ′′ profile of M64-GC1. Its color is almost constant with (F 606W − F 814W ) ≈ 0.7 to 0.8 at RGC < 1. 5. (c) Integrated magnitude profiles of M64-GC1. Total magnitudes are F 606W = 18.5 mag and F 814W = 17.8 mag (marked by dashed lines). (d) Integrated (F 606W − F 814W ) color profile of M64-GC1.
(55.4 ± 0.4 pc) in F 606W images. These val- MF 814W (GC1) = −10.44 ± 0.09 mag (corre- ues are obtained after correcting the PSF ef- sponding to the Johnson Cousins system mag- fect. The color profile is almost constant with nitudes, MV = −9.54 ± 0.09 mag and MI = ′′ (F 606W − F 814W ) ≈ 0.7 to 0.8 at RGC < 1.5. −10.46 ± 0.09 mag). In Table 2 we list the The integrated magnitude and color profiles basic information of M64-GC1 derived in this of the cluster are shown in Figures 6(c) and study. (d). From this we derive the total mag- nitude of the cluster, F 606W (GC1) = 18.5 3.2. CMDs of the Resolved Field Stars mag, and F 814W (GC1) = 17.8 mag. We For the following analysis we divided the en- also fit the S´ersic profile to the M64-GC1 im- tire ACS field into two regions: an inner region ′ age using GALFIT (Peng et al. 2010). From at a < a25,B =5.0 (6.3 kpc) and an outer region this we derive the total magnitude of the clus- at a ≥ a25,B. Here, a25,B is the projected galac- ter, F 606W (GC1) = 18.53 ± 0.01 mag, and tocentric distance at which the B-band surface −2 F 814W (GC1) = 17.80 ± 0.01 mag (correspond- brightness is 25 mag arcsec . a25,B is shown ing to the Johnson Cousins system magnitudes, by a large ellipse in Figure 1. In Figure 7 we V0 = 18.64 ± 0.03 mag and I0 = 17.72 ± 0.02 display the CMDs of the resolved stars in the ′′ mag), and the effective radius of Reff =0.273 ± inner and outer regions. It is found that the re- ′′ 0.001 (5.73 ± 0.02 pc). These results agree solved stars are mostly RGB stars with a large very well with those from the IRAF/ELLIPSE range of color. task. Adopting the distance to M64, we ob- The most notable feature in the CMDs is tain absolute integrated magnitudes of the clus- that there are two distinguishable RGBs in ter, MF 606W (GC1) = −9.75 ± 0.09 mag, and the bright magnitudes with F 814W . 25.0: 8 Kang et al. one is a vertical blue branch at (F 606W − 23.0 (a) Inner region (a < a ) F 814W ) ≈ 1.0, and the other is a curved 25,B 23.5 red branch with a wide range of color reaching (F 606W − F 814W ) . 2.5. The vertical RGB 24.0 represents a metal-poor RGB, and the curved RGB denotes a metal-rich RGB. The presence 24.5 of these two branches is more clearly seen in the
outer region than in the inner region. The in- F814W 25.0 ner region is dominated by the metal-rich RGB stars, while the contribution from the metal-rich 25.5 RGB stars and metal-poor RGB stars are com- parable in the outer region. 26.0 The metal-poor RGB in the CMDs is very 26.5 similar to the RGB of the new globular cluster -1 0 1 2 3 4 M64-GC1, showing that the metal-poor RGB F606W - F814W stars belong to the halo population. In con- trast, the metal-rich RGB is roughly matched 23.0 with the isochrone for [Fe/H] ≈ −0.4, which is (b) Outer region (a > a25,B) much larger than the mean metallicity of the 23.5 metal-poor RGB ([Fe/H] ≈ −1.5). Thus, the metal-rich RGB represents the disk population. 24.0 The red color of the outer disk presented in Watkins et al. (2016) is mainly due to these 24.5 metal-rich RGB stars. However, taking advan-
F814W 25.0 tage of the resolved stellar photometry, we find that the red outer disk is not made of a single 25.5 Table 2. Basic Parameters of M64-GC1 26.0 Parameter Value 26.5 R.A.(2000) 12h 57m 04s.58 ◦ ′ ′′. -1 0 1 2 3 4 Decl.(2000) +21 38 23 7 F606W - F814W F 606W 18.53 ± 0.01 F 814W 17.80 ± 0.01 F W F W . . Figure 7. CMDs of the resolved stars (a) in the 606 − 814 0 73 ± 0 01 ′ inner region at a < a25,B = 5.0 and (b) the outer V0 18.64 ± 0.03 region at a ≥ a25 . Blue and red boxes denote V I . . ,B ( − )0 0 92 ± 0 13 the boundaries for selecting bright metal-poor RGB MV −9.54 ± 0.09 stars and bright metal-rich RGB stars, respectively. ′′ ′′ Reff 0. 273 ± 0. 001 (5.73 ± 0.02 pc) stellar population with high metallicity, but it is Mean RGB metallicity [Fe/H]= −1.5 ± 0.2 composed of two stellar populations: one with high metallicity and another with much lower metallicity. M64-GC1 and RGB stars 9
23.0 function of the stars, we derive a TRGB mag- (a) Outer region (b) nitude, F 814W = 24.21 ± 0.03 mag. This 23.5 T RGB TRGB magnitude does not change much with F814W TRGB± the choice of Sobel filters. For example, the edge 24.0 = 24.21 0.03 detection algorithm used in Freedman et al. 24.5 (2019), employing the GLOESS smoothing, gives the TRGB magnitude of F 814WT RGB = 24.22 ± 0.02(ran) ± 0.01(sys) mag with the F814W 25.0 optimal smoothing scale of σs = 0.08 mag. 25.5 The weighted edge detection filter suggested by Mager et al. (2008) measures F 814WT RGB = 26.0 24.21 ± 0.04 mag, which is also consistent within uncertainties. Adopting the TRGB cal- 26.5 ibration in Jang & Lee (2017) (for metal-poor 0 1 2 0 100 RGB, MF 814W (TRGB) = −4.03 ± 0.07), the F606W - F814W N foreground reddening in Schlafly & Finkbeiner (2011) (AF 814W = 0.063), and the aperture Figure 8. (a) CMD of the resolved stars in the ′ correction error of 0.03 mag, we obtain a dis- outer region at a ≥ a25,B = 5.0. The shaded grey tance modulus, (m−M)0 = 28.18±0.03(ran)± region represents the stars used for estimating the ± RGB tip. (b) F 814W band luminosity function 0.08(sys) (corresponding to a distance of 4.33 of the metal-poor RGB stars in the outer region. 0.18 Mpc). The edge detection response curve is also plotted 3.4. Radial Distribution of the Metal-poor and (a solid line for the Sobel filter and dashdotted line Metal-rich RGB Stars for the weighted edge detection filter suggested by Mager et al. (2008)). Red lines mark the TRGB In the CMDs of Figure 7, the metal-poor magnitude, F 814WT RGB = 24.21 ± 0.03. RGB and the metal-rich RGB are clearly sep- arated in the bright magnitudes. They are 3.3. TRGB Distance Estimation merged into one as the stars get fainter so it We estimate a distance to M64 using the is difficult to separate them in the faint magni- TRGB method (Lee et al. 1993) to a sample of tudes. For the following analysis we selected metal-poor RGB stars in the outer region of the bright metal-poor RGB stars and metal-rich ACS field. Crowding is much less and the con- RGB stars with F 814W < 24.8 mag using the tribution of halo stars is larger in the outer re- boundaries marked by blue and red boxes in gion than in the inner region. Figure 8(a) is Figure 7. the CMD of the resolved stars in the outer re- We derive the radial number density profiles of gion of M64 which is already shown in Figure the selected metal-poor RGB stars and metal- 7(b). The shaded grey region represents the rich RGB stars as well as all bright RGB stars metal-poor RGB stars in the outer region which with 24.1 < F 814W < 24.8 mag, and show were used to estimate the TRGB magnitude. them in Figure 9. This result was corrected Figure 8(b) shows the luminosity function by the artificial star tests (see Figure 2). For of the stars within the shaded grey region in comparison, we also plot the schematic sur- the CMD. Applying the Sobel filter method de- face brightness profiles of galaxy light given by scribed in Jang & Lee (2017) to the luminosity Watkins et al. (2016), showing the inner/outer disk and the Type III anti-truncation com- 10 Kang et al.
All bright RGB of the surface brightness profile for the outer Inner/Outer disk ± aeff = 2.22 0.05 disk. Third, the slope of the radial number den- ± 10000 n = 0.61 0.03 sity profiles of all bright RGB stars is similar Metal-rich RGB ± to that of the surface brightness profile for the aeff = 2.54 0.04 n = 0.39 ± 0.02 outer disk. Fourth, the radial number density ] -2 Metal-poor RGB profile of the metal-poor RGB stars is slightly 1000 ± aeff = 0.87 0.10 steeper than, but roughly consistent with that ± n = 3.79 0.39 of the surface brightness profile for the Type III [arcmin
Σ Type III anti-truncation component given by Watkins et al. (2016). 100 We fit the radial number density profiles for 3′ . a . 6′ with a S´ersic function (S´ersic 1963), ′ HST/ACS obtaining n = 0.61 ± 0.03 (aeff = 2.22 ± 0.05) coverage for all bright stars, n = 0.39 ± 0.02 (aeff = 10 2.′54 ± 0.04) for the metal-rich RGB stars, and 0 2 4 6 8 10 12 14 ′ a [arcmin] n = 3.79 ± 0.39 (aeff = 0.87 ± 0.10) for the metal-poor RGB stars. Thus, the radial num- ber density profile of the metal-rich RGB stars Figure 9. Radial number density profiles of follows an exponential disk law, while that of the selected metal-poor RGB stars (blue triangles), the metal-poor RGB stars is described well by metal-rich RGB stars (red boxes), and all bright 1/4 RGB stars (black circles) with 24.1 < F 814W < a de Vaucouleurs r law. 24.8 mag. The surface brightness profile of galaxy light given by Watkins et al. (2016) is schematically 4. DISCUSSION shown: black solid line for the inner/outer disk and 4.1. M64-GC1 in Comparison with Other the Type III anti-truncation component. Vertical dashed lines mark the HST/ACS coverage and dot- Globular Clusters ted lines denote S`ersic fitting lines. The fitting re- It is easy to spot M64-GC1 in the ACS im- sults are also shown. ages because it is relatively bright and its outer region is partially resolved into individual stars. ponent. We converted the schematic surface However, it is the only globular cluster we could brightness profiles to be consistent with the find in the ACS field. This implies that the to- unit of the number density profiles, µB divided tal number of globular clusters in M64 is small by 2.5, and shifted them vertically to match and/or M64-GC1 may be unusually brighter ′′ roughly the number density profile at a ≈ 400 . than the other globular clusters in this galaxy. Several features are noted in Figure 9. First, In Figure 10 we compare the effective ra- the radial number density profile of the metal- dius and total magnitude of M64-GC1 with rich RGB stars is much steeper than that of the those of globular clusters in other galaxies in metal-poor RGB stars in the same range. The the literature (the Milky Way Galaxy (Harris flattening or decrease of the radial number den- 1996, 2010), M31 (Peacock et al. 2010), and ′ sity profiles in the innermost bins at a< 3 are M81 (Nantais et al. 2011)) and two intragroup mainly due to incompleteness of our photom- globular clusters (IGGCs) in the M81 group etry although we corrected the incompleteness (Jang et al. 2012). We also plotted a bound- with the result of artificial star tests. Second, ary separating normal and extended globular the slope of the radial number density profile of clusters suggested by van den Bergh & Mackey the metal-rich RGB stars is consistent with that (2004): log(Reff ) = 0.2MV + 2.6. M64- M64-GC1 and RGB stars 11
100 Tully et al. 2013, 2016, and the Extragalactic M64-GC1 (This study) 1 MW GCs (Harris 2010) Distance Database (EDD) ). The previous sur- + 2.6 M31 GCs (Peacock+2010) V M81 GCs (Nantais+2011) face brightness fluctuation (SBF) distance val- M81 IGGCs (Jang+2012) ues show an even larger difference: (m − M)0 = ) = 0.2 M eff 29.37 ± 0.20 in Tonry et al. (2001) and (m − log(R M)0 = 28.60 ± 0.08 in Tully et al. (2013). Note
[pc] 10 that Watkins et al. (2016) adopted a distance ω eff Cen
R M54 4.7 Mpc given in Jacobs et al. (2009), while Brunker et al. (2019) adopted a distance 5.3 Mpc given in Tully et al. (2016). We obtain a distance to M64 applying the TRGB method to the photometry of metal-poor RGB stars in the outer region, (m − M)0 = 1 ± ± -12 -10 -8 -6 -4 28.18 0.09 (4.33 0.18 Mpc). This value is 0.4 mag smaller than those in Mould & Sakai MV (2008), Tully et al. (2013), and Tully et al. (2016), but it is very similar to the one in EDD, Figure 10. Effective radius (Reff [pc]) versus MV (m − M)0 = 28.22 ± 0.02. for M64-GC1 (magenta starlet) in comparison with Note that the F 814W TRGB magnitude in globular clusters in the Milky Way Galaxy (blue ± diamonds) (Harris 1996, 2010), M31 (orange tri- this study, F 814WTRGB,0 = 24.15 0.03, is 0.5 angles) (Peacock et al. 2010), M81 (green boxes) mag brighter than the value in Mould & Sakai (Nantais et al. 2011), and two intragroup globular (2008), 24.64 mag. We used only the metal- clusters (IGGCs) in the M81 group (large green poor RGB stars in the outer region, while boxes) (Jang et al. 2012). A boundary separating Mould & Sakai (2008)’s (as well as Tully et al. normal and extended globular clusters suggested (2013)’s and Tully et al. (2016)’s) estimation by van den Bergh & Mackey (2004) is marked by might have been based mainly on all RGB stars. a dotted line. If we determine the TRGB magnitude using all RGB stars, we obtain F 814W 0 ≈ 24.5 GC1, as well as M54 (NGC 6715), ω Cen- TRGB, mag, which is similar to those obtained by tauri (NGC 5139), and IGGCs, is above the Mould & Sakai (2008) and Jacobs et al. (2009) boundary line. This shows that M64-GC1 be- (probably as well as Tully et al. (2013) and longs to the brightest and largest globular clus- Tully et al. (2016)). ters. M54 and ω Centauri are often consid- EDD provided a value of F 814W 0 = ered to be core remnants of disrupted dwarf TRGB, 24.26 ± 0.02, which is 0.1 mag fainter than ours galaxies (Zinnecker et al. 1988; Freeman 1993; and presented a similar distance value. If we Sarajedini & Layden 1995). Therefore, M64- determine the TRGB magnitude using metal- GC1 is also a possible core remnant of a tidally poor RGB stars in the entire region, we obtain disrupted dwarf satellite around M64. F 814WTRGB,0 ≈ 24.25, which is similar to the value provided by EDD. However, they used all 4.2. Comparison of Distance Estimation RGB stars in the entire region without any color The previous TRGB distance estimates for selection (private communication with Dmitry M64 show a significant variation from (m − M)0 = 28.2 to 28.6, as summarized in Ta- 1 http://edd.ifa.hawaii.edu/ ble 3 (Mould & Sakai 2008; Jacobs et al. 2009; 12 Kang et al.
Table 3. Distance Estimates for M64
Method D[Mpc] (m − M)0 Reference Remarks SBF 7.48 29.37 ± 0.20 Tonry et al. (2001) I-band SBF 5.24 28.60 ± 0.08 Tully et al. (2013)
TRGB 5.27 28.61 Mould & Sakai (2008) F 814WTRGB,0 = 24.64 TRGB 4.66 28.34 ± 0.01 Jacobs et al. (2009) F 814WTRGB,0 = 24.49 Adopted in Watkins et al. (2016) TRGB 5.24 28.60 ± 0.16 Tully et al. (2013) TRGB 5.30 28.62 ± 0.15 Tully et al. (2016) Adopted in Brunker et al. (2019)
TRGB 4.40 28.22 ± 0.02 EDD (2019) F 814WTRGB,0 = 24.26 ± 0.02
TRGB 4.33 28.18 ± 0.09 This study F 814WTRGB,0 = 24.15 ± 0.03
Makarov). Then it is difficult to explain the 0.1 ponential disk law, while that of the metal-poor mag difference. RGB stars is fit well by a S´ersic law with n ≈ 4. In the CMDs, the metal-poor RGB is very sim- 4.3. The Origin of the Type III Component in ilar to the RGB of the new halo globular cluster M64 (M64-GC1). Erwin et al. (2005, 2008) and Pohlen & Trujillo These results show that the metal-poor RGB (2006) introduced the classification of Type III represents a low metallicity ([Fe/H] ≈ −1.5) breaks into two types according to the shape of halo population, while the metal-rich RGB de- outer isophotes: IIId for disk components and notes a disk population with much higher metal- IIIs for spheroidal components. In the case of licity ([Fe/H] ≈ −0.4). The disk is dominated IIIs breaks, the isophotes in the outer region by the metal-rich RGB stars, and no clear sign become progressively rounder than those in the of young stars is found in the ACS field. Thus, inner region. They described the spheroidal the color of the outer disk is dominated by the component in IIIs breaks is either the outer contribution of the metal-rich RGB stars. This extent of the bulge or a separate stellar halo. is consistent with the result that the outer disk From the surface photometry of M64, is red and featureless found in Watkins et al. Guti´errez et al. (2011) classified it as IIIs, not- (2016). The outer region is occupied mainly by ′′ ing that there is a disk break at r ≈ 370 and the metal-poor RGB stars. Thus, the origin of that the ellipticity of the isophotes decreases the Type III anti-truncation component in the ′′ abruptly beyond r ≈ 340 . Watkins et al. outer region of M64 is a halo, neither a disk ′′ (2016) also found a disk break at r ≈ 400 and population nor a bulge population. inferred that the Type III anti-truncation com- ponent in M64 may be due to the spheroidal 4.4. The Origin of the Halo in M64 halo, noting that the isophotes of the outer re- M64 is an isolated galaxy located in a low den- gion of M64 are round and that it is difficult to sity environment (see Brunker et al. 2019, Fig- build outer disks with current models. ure 10). In such an environment, there might We find that the resolved RGB stars in the have been few satellites which merged and en- ACS field are composed of two main stellar pop- larged the halo of M64. This implies that the ulations: metal-poor RGB stars and metal-rich size and mass of the halo in M64 might have RGB stars. The radial number density profile of not been changed much since its formation, or the metal-rich RGB stars is described by an ex- the accretion of dwarf galaxies into M64 was fin- M64-GC1 and RGB stars 13 ished long ago. Thus, the halo in M64 may be progenitor of Coma P had a high speed (v ≈ pristine, as dwarf galaxies or globular clusters 1000 km s−1 ≈ 1 Mpc Gyr−1) fly-by interac- which have much lower mass than M64. This is tion with M64 approximately 1 Gyr ago, and consistent with our finding that the metallicity some of the gas stripped from Coma P resulted of the metal-poor RGB stars in the ACS field is in the counter-rotating gas ring in the outer disk as low as that of the new halo globular cluster of M64. (M64-GC1). The metallicity of the metal-poor If the TRGB distance to M64 derived in this RGB stars is much lower than that of the metal- study is adopted, then the relative line-of-sight rich RGB stars so the origin of the metal-poor distance between Coma P and M64 increases RGB stars cannot be a disk or a bulge. from 200 kpc to 1.2 Mpc (corresponding to the spatial distance from 590 kpc to 1.3 Mpc). It 4.5. Relation between M64 and Coma P would have taken about 1 Gyr if Coma P had Coma P (AGC 229385) is an HI-dominated an encounter with M64 and is receding with 7 blue dwarf galaxy (M(HI)=3.48 × 10 M⊙, v ≈ 1000 km s−1 until today. Therefore, the 5 ◦ M∗ =4.3×10 M⊙) located close (5.86 ) to M64 encounter epoch is around 1 Gyr ago, which in the sky (Brunker et al. 2019). Brunker et al. is in agreement with the scenario suggested by (2019) measured a TRGB magnitude of Coma Brunker et al. (2019). P from HST/ACS F 606W/F 814W images to be F 814W0 = 24.64 ± 0.09 (from maximum 4.6. Evolution History of M64 likelihood method) or 24.60 (from Sobel fil- To explain the counter-rotating gas in the ters), and presented a distance to Coma P, outer disk of M64, it has been suggested that +0.28 +0.11 d = 5.5−0.53 Mpc ((m − M)0 = 28.80−0.21). the progenitor spiral galaxy of M64 under- This value is much smaller than the previous went a merger with a counter-rotating gas-rich distance estimates for Coma P, 11 Mpc based on galaxy (Braun et al. 1992, 1994; Rubin 1994; the TRGB method (Anand et al. 2018) and 25 Walterbos et al. 1994; Rix et al. 1995). Based Mpc based on the flow model (Janowiecki et al. on the result that only the outer gas disk is 2015). Brunker et al. (2019) discussed the com- counter-rotating while the outer stellar disk is parison of their distance estimate with those co-rotating with the inner gas disk and stel- in Janowiecki et al. (2015) and Anand et al. lar disk, they prefer mergers with a low-mass (2018), and concluded that their estimate is gaseous system. more reliable than the others. Alternatively, they also suggested a con- Brunker et al. (2019) discussed the relation tinuous accretion of gas with an opposite between Coma P and M64, adopting a dis- sense of angular momentum. Several stud- tance to M64 from Cosmicflows-3 in Tully et al. ies also showed that a majority of isolated (2016), (m − M)0 = 28.62 ± 0.15 (d =5.3 ± 0.4 S0 galaxies host extended counter rotating gas Mpc). The spatial distance between the two disks originated from external gas (Davis et al. galaxies is as small as 590 kpc for their adopted 2011; Katkov et al. 2014, 2015). Especially, distances. However, the relative line-of-sight ve- Katkov et al. (2014) mentioned that possible −1 locity between Coma P (vhelio = 1348 km s ) sources of external gas accretion are systems of −1 and M64 (vhelio = 410 km s ) is as large as dwarf gas-rich satellites or cosmological cold-gas 938 km s−1. Noting this large relative veloc- filaments (Kereˇset al. 2005; Dekel & Birnboim ity, Brunker et al. (2019) suggested another sce- 2006). nario for the presence of counter-rotating gas Recently, Watkins et al. (2016) supported a in M64 in relation with Coma P: a gas-rich merger scenario again to explain the presence of 14 Kang et al. young stars in the inner disk, a red and feature- to M64 through fly-by interaction about 1 Gyr less outer disk, and counter-rotating gas in the ago. outer disk of M64. The recent star formation is limited only in the inner disk, while the outer 5. SUMMARY AND CONCLUSION disk is quietly evolving. Thus, Watkins et al. We presented deep F 606W/F 814W photom- (2016) pointed out that the merger played two etry of resolved stars in a field located in the opposite roles: it induced star formation in outer disk of M64. We used it to explore the the inner region, while it removed gas and nature of stellar populations in the outer disk quenched star formation in the outer disk. They and find a clue to reveal the origin of the Type also suggested that M64 may be at the end- III component. Primary results are summarized ing phase of a spiral galaxy being transformed as follows. to an S0 galaxy by merger-induced quench- ing. Indeed, the color of M64 is as red as S0a • We discovered a new globular cluster, galaxies (Roberts & Haynes 1994) and Type III M64-GC1. The CMD of this cluster is anti-truncations are more frequently seen in S0 well matched with 12 Gyr isochrones for galaxies (Borlaff et al. 2014). low metallicity [Fe/H]= −1.5 ± 0.2, show- More recently, Brunker et al. (2019) sug- ing that it is a halo population. gested another scenario that M64 had experi- enced a fly-by interaction with a gas-rich galaxy • CMDs of the resolved stars in the ACS rather than a merger. They suggested that a field show two distinguishable stellar pop- gas-rich progenitor of Coma P had a high speed ulations: metal-poor RGB stars and fly-by interaction with M64 approximately 1 metal-rich RGB stars. In the CMDs, Gyr ago, and some of the gas stripped from the metal-poor RGB is very similar to Coma P resulted in the counter-rotating gas the RGB of the new halo globular cluster ring in the outer disk of M64. They also pointed (M64-GC1), showing that the metal-poor out that young stars in Coma P seen in the RGB stars belong to the halo. CMD might have formed during the interaction of Coma P with M64. • The radial number density profile of the From all the previous results, we support metal-rich RGB is described by an expo- the scenario suggested by Brunker et al. (2019). nential disk law, while that of the metal- The presence of old RGB stars in both M64 poor RGB is fit well be a de Vaucouleurs’s (in this study) and Coma P (Brunker et al. law. This shows that the metal-rich RGB 2019) implies that both galaxies were formed represents the disk population with high more than 10 Gyrs ago. The presence of young metallicity, while the metal-poor RGB de- stars in Coma P and the central region of M64 notes the halo population with low metal- strongly indicates a recent interaction between licity. the two galaxies. The counter-rotating gas ring with clumpy structure in M64 must have an ex- • We derive a TRGB distance to M64, d = ternal origin: either via accretion or fly-by in- 4.33 ± 0.18 Mpc ((m − M)0 = 28.18 ± teraction of dwarf gas-rich satellites, or cosmo- 0.03(ran) ± 0.08(sys). logical cold-gas filaments. Coma P is a strong candidate for providing the counter-rotating gas From these results, we conclude that the ori- gin of the Type III break in M64 is a halo rather than a disk or a bulge. M64-GC1 and RGB stars 15
ACKNOWLEDGMENTS Software: DrizzlePac (Gonzaga et al. 2012), DOLPHOT (Dolphin 2000), IRAF/ELLIPSE J.K. was supported by the Global Ph.D. Fel- (Jedrzejewski 1987), GALFIT (Peng et al. 2010) lowship Program (NRF-2016H1A2A1907015) of . the National Research Foundation (NRF). This project was supported by the National Research Foundation grant funded by the Korean Gov- ernment (NRF-2019R1A2C2084019). We thank Brian S. Cho for his help in improving the En- glish in the draft. We also thank the anony- mous referee for useful suggestions to improve the original manuscript.
Facility: HST(ACS)
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