DRAFT VERSION NOVEMBER 6, 2018 Preprint typeset using LATEX style emulateapj v. 5/2/11 NGC 3628-UCD1: A POSSIBLE ! CEN ANALOG EMBEDDED IN A STELLAR STREAM ZACHARY G. JENNINGS1 ,AARON J. ROMANOWSKY1,2 ,JEAN P. BRODIE1 ,JOACHIM JANZ3 ,MARK A. NORRIS4 ,DUNCAN A. FORBES3 , DAVID MARTINEZ-DELGADO5 ,MARTINA FAGIOLI3 ,SAMANTHA J. PENNY6 Draft version November 6, 2018 ABSTRACT Using Subaru/Suprime-Cam wide-field imaging and both Keck/ESI and LBT/MODS spectroscopy, we iden- tify and characterize a compact star cluster, which we term NGC 3628-UCD1, embedded in a stellar stream around the spiral galaxy NGC 3628. The size and luminosity of UCD1 are similar to ! Cen, the most luminous Milky Way globular cluster, which has long been suspected to be the stripped remnant of an accreted dwarf 6 galaxy. The object has a magnitude of i = 19:3 mag (Li = 1:4 × 10 L ). UCD1 is marginally resolved in our ground-based imaging, with a half-light radius of ∼ 10 pc. We measure an integrated brightness for the stellar stream of i = 13:1 mag, with (g - i) = 1:0. This would correspond to an accreted dwarf galaxy with an 8 approximate luminosity of Li ∼ 4:1 × 10 L . Spectral analysis reveals that UCD1 has an age of 6:6 Gyr , [Z=H] = -0:75, and [α=Fe] = -0:10. We propose that UCD1 is an example of an ! Cen-like star cluster possi- bly forming from the nucleus of an infalling dwarf galaxy, demonstrating that at least some of the massive star cluster population may be created through tidal stripping. Subject headings: galaxies: star clusters: general, galaxies: interactions, galaxies: individual: NGC 3628 1. INTRODUCTION derstanding the origins of the UCD population has important Since the discovery of ultra-compact dwarfs (UCDs) a implications for understanding sub-halos in a ΛCDM context. decade and a half ago (Hilker et al. 1999; Drinkwater et al. The most massive Milky Way (MW) GC, ! Cen, is an out- 2000), there has been considerable discussion in the literature lier among the MW population and may be an example of a regarding their origin. The conversation can be distilled down sripped dwarf-galaxy nucleus. The cluster has a large velocity to a simple question: are UCDs the largest star clusters, or the dispersion (e.g. Anderson & van der Marel 2010), rapid rota- smallest galaxies? tion leading to flattening (e.g. Merritt et al. 1997), unusual abundance patterns revealing multiple populations (e.g. King The earliest UCDs discovered have Rh∼20 pc and lumi- 7 et al. 2012), and odd orbital properties (e.g. Dinescu et al. nosities > 10 L . These objects represent a middle-ground between globular clusters (GCs), which have R of ∼3 pc and 1999). However, definite confirmation of this formation sce- h nario has remained elusive. luminosities of ∼ 106 L , and dwarf galaxies, with R > 100 h In this work, we identify and describe a star cluster, which pc. Expanded observational studies have found that UCDs we call NGC 3628-UCD1 (hereafter UCD1), embedded in occupy a sequence with similar luminosity to GCs, but larger a stellar stream around the nearby spiral galaxy NGC 3628. R (Brodie et al. 2011; Misgeld & Hilker 2011; Norris et al. h NGC 3628 is an Sb galaxy with M = 21:37. It is located in 2014). We adopt the definition of UCDs from Brodie et al. V - the Leo Triplet, a loose group with two other large compan- (2011): UCDs are objects with Rh ranging from ∼10 to 100 5 ions, NGC 3623 and NGC 3627. An obvious stellar stream pc, and luminosities Mi < -8:5 mag (or Li & 10 L ). extends ∼140 kpc away from the galaxy, shown in Fig. 1. The simplistic galaxy vs. cluster distinction breaks down UCD1 is located within the plume nearest to NGC 3628. As further. UCDs could include objects resulting from mergers shown in §2, UCD1’s size and luminosity are very similar to of globular clusters (e.g. Fellhauer & Kroupa 2002; Kissler- those of ! Cen. Patig et al. 2006), or objects formed primordially in intense The stream itself has been studied extensively since its first star formation episodes (Murray 2009). Characterizing the characterization (Zwicky 1956; Kormendy & Bahcall 1974). UCD population would have implications for cluster forma- The stream contains significant neutral hydrogen (Rots 1978; tion physics. In the galaxy scenario, these objects could form Haynes et al. 1979). Chromey et al. (1998) identified two blue primordially in association with distinct dark-matter halos, or arXiv:1509.04710v1 [astro-ph.GA] 15 Sep 2015 clumps along the stream and estimated young ages for both. they could be the remnant nuclei of larger galaxies which have There are several known examples in the literature of UCDs undergone tidal stripping during accretion onto larger galaxy connected with tidal stripping events. Norris & Kannappan halos (e.g. Bekki et al. 2001; Pfeffer & Baumgardt 2013). Un- (2011) identified a young UCD around NGC 4546 and argued, 1 University of California Observatories, Santa Cruz, CA 95064, USA; based on the properties of the galaxy, that it is a result of strip- [email protected] ping. Foster et al. (2014) identified an object likely to be the 2 Department of Physics and Astronomy, San José State University, One nucleus of the dwarf galaxy forming the "umbrella stream" Washington Square, San Jose, CA, 95192, USA around NGC 4651. Mihos et al. (2015) identify a nucleus of 3 Centre for Astrophysics & Supercomputing, Swinburne University, an ultra diffuse galaxy in Virgo, which they argue is in the Hawthorn VIC 3122 4 Max Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidel- process of tidal threshing. berg, Germany Using Subaru/Suprime-Cam imaging, we measure photom- 5 Astronomisches Rechen-Institut, Zentrum für Astronomie der Univer- etry and size of both UCD1 and the full stellar stream. We sität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany propose that UCD1 is an example of a UCD in formation 6 Institute of Cosmology and Gravitation, University of Portsmouth, through tidal stripping. By measuring the UCD and infer- Dennis Sciama Building, Burnaby Road, Portsmouth PO1 3FX, UK 2 Jennings et al. Figure 1. Smoothed image of stellar stream next to NGC 3628 from our i-band Subaru/SuprimeCam imaging. The left-most edge of NGC 3628 is visible at the far right of the image. We highlight the location of UCD1 in zoom-in panels. North is up and east is left. The limiting surface brightness in the large image is -2 roughly µi ∼ 28:5 mag arcsecond . The stretch is modified in each image. Angular sizes are approximately 26x10 arcmin, 4.5x1.5 arcmin, and 0.9x0.4 arcmin from largest to smallest scale. UCD1 Properties ring the properties of the potential progenitor galaxy, we are Parameter Value Uncertainty able to draw an evolutionary line in size/luminosity parame- R.A. (J2000, from SDSS) 170.25493 - ter space between the original parent galaxy and the stripped Dec (J2000, from SDSS) 13.60813 - cluster. In §2, we explain our imaging analysis and results, Suprime-Cam Photometry and in §3, we do the same for our spectroscopy. We discuss g 19.98 mag 0.05 mag our results in §4. r 19.57 mag 0.04 mag NGC 3627 has a Cepheid distance measurement of 10.6 i 19.29 mag 0.04 mag Mpc (Kanbur et al. 2003). We adopt this value for NGC 3628 (g - r) 0.41 mag 0.05 mag and include an approximate distance uncertainty of ±1 Mpc (g - i) 0.69 mag 0.05 mag Rh, r-band 10 pc 3 pc on distance-dependent properties, given the potential offset of Ellipticity 0.9 - 6 6 NGC 3628 from NGC 3627. Luminosity Li = 1:4 × 10 L ±0:2 × 10 L 2. IMAGING ESI Spectroscopy Vel. 815 km s-1 4 km s-1 2.1. Data Reduction -1 Vel. Dispersion . 23 km s - +1:9 We imaged NGC 3628 in r-band for 425s on 2009 April Age 6.6 Gyr -1:5 Gyr 20th with Subaru/Suprime-Cam. UCD1, a marginally- [Z=H] -0.75 0.12 resolved source in the center of plume just east of NGC 3628, [α=Fe] -0.10 0.08 was first noticed in this pointing. We subsequently acquired MODS Spectroscopy +1:4 imaging centered on UCD1 in the i-band on 2014 March 3 Age 6.6 Gyr -1:2 Gyr (425s exposure time) and in the g-band on 2014 December [Z=H] -0.77 0.16 19 (1225s exposure time). Seeing was ∼ 0:8000, ∼ 0:7000, [α=Fe] -0.08 0.15 and ∼ 0:7500 in g;r;i respectively. We employed a modified version of the SDFRED-2 pipeline7 to reduce our Suprime- Cam data. AB Zeropoints were calculated by comparing pho- Surface Photometry tometry for bright, unsaturated stars in both the SDSS cata- Filter Apparent Mag Luminosity log and the Suprime-Cam imaging. We used the Schlafly & Full Stream 8 Finkbeiner (2011) values from NED to correct for Galactic g 14.15 mag (2:7 ± 0:5) × 10 L 8 extinction. i 13.14 mag (4:1 ± 0:8) × 10 L (g - i) 1.01 mag - 2.2. Photometry and Size of UCD1 Plume Containing UCD1 +1:8 7 g 15.34 mag (9:-1:7) × 10 L 8 We performed aperture photometry of UCD1 using an aper- i 14.19 mag (1:6 ± 0:3) × 10 L ture roughly twice the size of the FWHM for each image, cho- (g - i) 1.15 mag - sen to maximize the S/N. Aperture corrections were measured using several bright, unsaturated stars in the field.