MNRAS 000,1–9 (2019) Preprint 19 December 2019 Compiled using MNRAS LATEX style file v3.0

A consistency-test for determining whether ultra-compact dwarf could be the remnant nuclei of threshed galaxies

Alister W. Graham? Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia.

Accepted XXX. Received YYY; in original form ZZZ

ABSTRACT

It has been suggested that ultra-compact dwarf (UCD) galaxies are the “threshed” re- mains of larger galaxies. Simulations have revealed that extensive tidal-stripping may pare a back to its tightly-bound, compact nuclear cluster. It has therefore been pro- posed that the two-component nature of UCD galaxies may reflect the original nuclear surrounded by the paltry remnants of its host galaxy. A simple quantitative test of this theory is devised and applied here. If the mass of the central black hole in UCD galaxies, relative to the mass of the UCD galaxies’ inner stellar component, i.e. the suspected , matches with the (black hole mass)-(nuclear star cluster mass) relation observed in other galaxies, then it would provide quantitative support for the stripped galaxy scenario. Such consistency is found for four of the five UCD galaxies reported to have a massive black hole. This (black hole mass)-(nuclear star cluster mass) relation is then used to predict the central black hole mass in two additional UCD galaxies, and to reveal that NGC 205 and pos- sibly NGC 404 (which only has an upper limit to its black hole mass) also follow this scaling relation. Key words: galaxies: dwarf – galaxies: nuclei – galaxies: star clusters: general – galaxies: structure – galaxies: evolution

1 INTRODUCTION by a massive neighbour to form a “compact elliptical” (e.g., M32 and NGC 4486B), of which at least one is now known Several origins for ultra-compact dwarf (UCD) galaxies (Harris to have an (Paudel et al. 2016). The strip- et al. 1995; Hilker et al. 1999; Drinkwater et al. 2000; Gómez ping explains why these rare1 galaxies are overly metal rich for et al. 2006) have been proposed. The apparent lack of dark mat- their luminosity (e.g. Chilingarian et al. 2009; Price et al. 2009), ter in UCD galaxies (e.g. Hilker et al. 2007; Chilingarian et al. underluminous for their colour (e.g. Graham & Soria 2019, , their 2011; Frank et al. 2011) ruled out the notion of them being compact Figure 11, and references therein), and should be excluded when galaxies formed in small, compact halos. This left al- establishing the M –M and M –M scaling relations. Fur- ternatives such as: large globular clusters possibly built over two or bh galaxy bh bulge ther stripping might reduce a galaxy to its more tightly-bound, nu- multiple epochs (e.g. Norris 2000; Bedin et al. 2004; Pfeffer et al. clear star cluster (Chilingarian & Mamon 2008; Koch et al. 2012; 2014); mergers (e.g. Norris et al. 1997); compact Pfeffer & Baumgardt 2013). Bekki & Freeman(2003) and Ideta young massive clusters formed during past galaxy interactions (e.g. & Makino(2004) investigated how dwarf elliptical and dwarf disc

arXiv:1912.08346v1 [astro-ph.GA] 18 Dec 2019 Kroupa 1998; Maraston et al. 2004; Linden et al. 2017; Maji et al. galaxies might be pared down by strong external gravitational tides, 2017); direct formation of star clusters from supergiant molecular leaving an object similar to ω Centauri and the UCD galaxies. clouds (Goodman & Bekki 2018); failed galaxies surrounding the The commonly-observed, two-component nature of a UCD remnant black hole of a Population III star or a primordial black galaxies’ structure might reflect the original nuclear star cluster hole (Dolgov & Postnov 2017); or perhaps the remnant nuclei of surrounded by the some of the remnants of its former host galaxy tidally-stripped, low-mass galaxies (Zinnecker et al. 1988; Freeman (e.g. Evstigneeva et al. 2007; Jennings et al. 2015; Voggel et al. 1993; Bekki et al. 2001a, 2003; Drinkwater et al. 2003; Georgiev 2016; Wittmann et al. 2016). Such objects would still house the et al. 2009; Norris et al. 2015; Zhang et al. 2015). The latter sce- original galaxy’s (SMBH), or perhaps nario is of interest because it can be tested. intermediate-mass black hole (IMBH: 102 < M /M < 105). Bekki et al.(2001b) show how an early-type can bh have much of its stellar disc, and some of its bulge, stripped away 1 While grossly over-represented in some published scaling diagrams, at a given luminosity, just 0.5 percent of dwarf galaxies are “compact elliptical” ? E-mail: [email protected] galaxies (Chilingarian, private communication).

c 2019 The Authors 2 Graham

Georgiev et al.(2009) have shown that the nuclear star clusters in (mainly) late-type dwarf galaxies resemble massive globular clus- ters, which they note might not be globular clusters but nuclei largely stripped of their former galaxy (Böker 2008). There has been an abundance of claims, usually followed by counter-claims, for the existence of IMBHs in large globular clus- ters. For example, see the studies of: G1, the most massive globular cluster around M31 (Gebhardt et al. 2005; Miller-Jones et al. 2012); ω Centauri, the most massive globular cluster around the (Noyola et al. 2008; Anderson & van der Marel 2010; van der Marel & Anderson 2010; Haggard et al. 2013; Zocchi et al. 2017, 2019); plus other globular clusters around the Milky Way such as M15 (Gebhardt et al. 2000; Gerssen et al. 2002; Baumgardt et al. 2003; Kirsten & Vlemmings 2012; den Brok et al. 2014; Kirsten et al. 2014); M54 (Ibata et al. 2009); 47 Tucanae (Kızıltan et al. 2017; Abbate et al. 2018; Mann et al. 2019); and NGC 6624 (Per- era et al. 2017; Gieles et al. 2018; Baumgardt et al. 2019). To date, there remains no conclusive evidence for IMBHs in globular clus- ters (Hurley 2007; Vesperini & Trenti 2010; Lanzoni & Cosmic- Lab Team 2016; Maccarone 2016; Wrobel et al. 2016; Ferraro et al.

2018; Tremou et al. 2018), although see (Lützgendorf et al. 2016). Figure 1. Mcmo–Mspheroid,∗ diagram, where the central massive object This current status may simply reflect the observational difficulty (cmo) may be a nuclear star cluster (NC: black line, equation1), a larger, in detecting low mass black holes in a relatively gas poor envi- flatter nuclear disc (ND: region denoted by the green pentagon), or a su- ronment. While few IMBHs are currently known (e.g. Farrell et al. permassive black hole (BH: black line, equation2, for galaxies with large- 2009; Baldassare et al. 2015; Graham et al. 2016; Chilingarian et al. or intermediate-scale discs). The three (morphological type)-specific black 2018), many have recently been predicted at the centres of low- hole relations shown by the blue dotted line and the two red dashed lines mass galaxies in the cluster (Graham & Soria 2019; Graham have been taken from Sahu et al.(2019a). They represent spiral (Sp) galax- ies, ellicular (ES, see Graham 2019) and lenticular (S0) galaxies, and dis- et al. 2019), with follow-up Chandra X-ray data in tow (PI: R. So- cless elliptical (E) galaxies. When combined, the E, ES and S0 galaxies ria. Proposal ID: 18620568), and beyond (Martínez-Palomera et al. occupy the region traced by the red parallelogram. 2019). Given that UCD galaxies may represent the nuclei of low- mass galaxies, UCD galaxies may thus be good targets for IMBH research (e.g. Moran et al. 2014; Sartori et al. 2015; Chilingarian 5 9 hole masses of 10 M , and bulge masses of 10 M (Graham & et al. 2018; Graham et al. 2019; Manzano-King et al. 2019). Scott 2015). The super-quadratic Mbh–Mbulge relation observed at Nuclear star clusters (e.g. Böker et al. 2002, 2004; Leigh et al. low masses has since been confirmed for spiral galaxies (which 2012) differ from globular star clusters in that they reside at the bot- are all Sérsic galaxies, as opposed to core-Sérsic galaxies with par- tom of the gravitational potential well of their host galaxy, which tially depleted stellar cores: Graham et al. 2003) using the latest can help to retain stellar winds and funnel down all manner of samples with directly measured black hole masses and careful mul- material that has lost its orbital angular momentum either through ticomponent decompositions of the galaxy light (Savorgnan et al. shocks in the case of gas (e.g. Martini & Pogge 1999; Milosavljevic´ 2016; Davis et al. 2019). This revised view, captured in Sahu et al. 2004), or dynamical friction in the case of denser bodies (e.g. Chan- (2019a), see also (Salucci et al. 2000; Laor 2001; Graham 2012b), drasekhar 1943; Tremaine et al. 1975; Inoue 2011; Arca-Sedda departs dramatically from the early picture of a single near-linear & Capuzzo-Dolcetta 2014). For over a decade, many nuclear star Mbh–Mbulge relation (Dressler 1989; Kormendy & Richstone 1995; clusters with supermassive black holes have been known to coexist Magorrian et al. 1998; Kormendy & Ho 2013), see Figure1. (e.g. González Delgado et al. 2008; Seth et al. 2008; Shields et al. Low-mass (non dwarf spheroidal)2 galaxies tend to have nu- 2008; Graham & Spitler 2009, and references therein). The Milky clear star clusters rather than depleted cores (e.g. Sandage et al. Way and M32 were among the first galaxies recognised to house 1985; Graham & Guzmán 2003; Balcells et al. 2003, 2007; Côté both (Lynden-Bell 1969; Sanders & Lowinger 1972; Tonry 1984), et al. 2006; Ferrarese et al. 2006). Not surprisingly, mass scaling and other examples are reported in Neumayer & Walcher(2012); relations for nuclear star clusters and their host bulge also exist, al- Georgiev et al.(2016). If UCD galaxies are indeed remnant nuclear though they have not received the same attention as the black hole star clusters, after their host galaxy has largely been removed, then mass scaling relations. Combining the low-mass Mbh–Mbulge and some may harbour a massive black hole. Mnc–Mbulge relations to cancel out the bulge mass yields a relation Obviously, given the array of formation scenarios noted above, between the black hole and nuclear cluster mass (Graham 2016a). the presence of a massive black hole in a UCD galaxy does not One can then test if UCD galaxies do, or do not, follow this rela- imply that they are the nuclear star clusters of stripped galaxies. tion. If they do, it will provide quantitative support for the threshing However, if the (black hole)-to-(inner star cluster) mass ratio in scenario. UCD galaxies matches that observed in nuclear star clusters, it Section2 describes this method in more detail, and the use of does then become suggestive of this scenario. The various scal- the Mbh–σ and Mnc–σ relations to construct an additional Mbh–Mnc ing relations connecting supermassive black holes with the differ- ent physical properties of their host bulges were reviewed in Gra- ham(2016b), with references to many key, but often over-looked, 2 Dwarf spheroidal galaxies are fainter than dwarf elliptical galaxies, and papers. The relation between the logarithm of the black hole mass have long been known not to be particularly nucleated (van den Bergh 1959; and the logarithm of the host bulge mass now extends down to black Mailyan 1973).

MNRAS 000,1–9 (2019) Ultra-compact dwarf galaxies 3

relation that can be, and is, used. Section3 applies this to all five — which is consistent with the symmetric Mbh–σ relations in Sahu UCD galaxies with a published black hole mass and available inner- et al.(2019b) 3 — gives: stellar-component mass. The black hole mass is also predicted for ± 8.22 ± an additional two UCD galaxies with available inner-component log(Mnc/M ) = (0.38 0.06) log(Mbh/[10 M ]) + (7.83 0.20) 7.89 stellar masses. In Section4, two additional nuclear star clusters — = (0.38 ± 0.06) log(Mbh/[10 M ]) + (7.70 ± 0.20) (7) in one dwarf galaxy recently reported to have a black hole, and in (8) another with an upper limit on the black hole mass — are checked Inverting this equation, one obtains M ∝ M2.6±0.4, consis- to see if they too conform to the Mbh–Mnc relation. A brief dis- bh nc cussion is provided in Section5, while noting some of the many tent with the exponent 2.7 reported in Graham(2016a). These re- implications arising from UCD galaxies with massive black holes. lations involving nuclear star cluster mass are observed over the 5 7 range 10 . Mnc/M . 5 × 10 and with half light radii less than ≈20 pc and generally around 4 pc. More massive nuclear star clus- ters likely blend into the population of nuclear discs, which depart 2 METHODOLOGY from these scaling relations (see Scott & Graham 2013, their Fig- ures 1 and 2). The test herein is straightforward, bar one twist that we shall get to, In the log M –log M diagram, a slope of 2.7 will span 7.3 and which surprisingly has not been performed for UCD galaxies. bh nc orders of magnitude in black hole mass across the above mentioned It inputs the published black mass of the UCD galaxy into the M – bh 2.7 orders of magnitude in nuclear cluster mass. It also corresponds M scaling relations (see below) to calculate the expected host nu- nc to an M /M mass ratio that increases by 4.6 orders of magnitude clear star cluster mass. This expected mass is then compared with bh nc from M /M = 105 to 5 × 107 M . At this higher-mass juncture in the observed mass of the UCD galaxies’ inner-component, i.e., the nc the M –M diagram, two different evolutionary scenarios can be suspected nuclear star cluster. If UCD galaxies are not the threshed bh nc envisioned, leading to a bifurcation. A dry merger event may bring remains of a galaxy, but formed via a different formation channel in a second massive black hole (and star cluster) forming a massive (for example, failed galaxies around SMBH seeds), then the pre- BH pair which scours away the nuclear star cluster(s) along with dicted nuclear cluster mass and the observed inner-cluster mass the core of the host galaxy (Bekki & Graham 2010; Begelman et al. may not agree. Put another way, if the masses are found to dis- 1980). Alternatively, a wet merger or accretion event may build a agree, it would disfavour the threshing origin of UCD galaxies, larger, flatter, nuclear disc (creating the upturn seen in Figures 1 while agreement would offer support for such a scenario. and 2 in Scott & Graham(2013); see also Spengler et al.(2017) Following Graham(2016a), the M –M relation can be nc spheroid and Sánchez-Janssen et al.(2019, , their Figure 4)). 4 These two united with the Mbh–Mspheroid relation (for galaxies without depleted processes may also account for some of the scatter in the Mbh–Mnc cores) to eliminate the quantity Mspheroid and produce an Mnc–Mbh relation observed from 5 < log M /M < 7.7. scaling relation. The M –σ and M –σ relations can also be com- nc nc bh The above non-linear M –M scaling relations imply a range bined to eliminate the quantity σ and yield an additional M –M bh nc nc bh of (black hole)-to-(nuclear star cluster) mass ratios. This is evident scaling relation. This is done here. by comparing the Milky Way and M32, with their similar mass ratio First, combining the relation of 1-to-10, with galaxies like NGC 1023 that have a mass ratio of 10-to-1, or NGC 3115 with a ratio approaching 100-to-1 (e.g. log(M /M ) = (0.88 ± 0.19) log(M /[109.6 M ]) + (7.02 ± 0.10) nc sph Graham & Spitler 2009). Furthermore, Georgiev et al.(2016) report (1) a range of ratios from 10−4 to 104 when including nuclear discs. If most massive UCD galaxies host high-mass fraction BHs (Ahn from Scott & Graham(2013) together with the relation et al. 2017), then UCD galaxies would appear to be inconsistent 10 with the notion that they are the tidally-stripped, remnant nuclear log(Mbh/M ) = (2.22±0.58) log(Msph/[2×10 M ])+(7.89±0.18) star clusters of galaxies which have a broad range of (black hole)- (2) to-(nuclear star cluster) mass ratios. from Scott et al.(2013) for the Sérsic, i.e. not core-Sérsic, galaxy sample — which is consistent with the Mbh–Msph relation for spiral galaxies from Davis et al.(2019) and Sahu et al.(2019a), and which 3 RESULTS 5 has been shown to extend to black hole masses of 10 M (Graham & Scott 2015) — gives our first Mbh–Mnc relation: 3.1 M60-UCD1

7.89 log(Mnc/M ) = (0.40 ± 0.13) log(Mbh/[10 M ]) + (7.64 ± 0.18)(3)The measurement of the black hole mass in M60-UCD1 comes from Seth et al.(2014), and is (2 .1±0.4)×107 M (1σ uncertainty). (4) This is reported there to be 15% of the total mass, which is thus 8 One advantage with the above combination is that any systematic 1.4 × 10 M . errors in the stellar mass of the spheroid should cancel out. The g-band and z-band luminosity ratio of the inner- Building on Graham(2012a), Scott & Graham(2013), (see component relative to the total luminosity is 0.58 (Strader et al. also Capuzzo-Dolcetta & Tosta e Melo 2017) reported that 2013, , their table 1). Assuming this luminosity comes from stellar light, a 0.58 fraction of 85% of the total mass yields a stellar mass log(Mnc/M ) = (2.11 ± 0.31) log(σ/54) + (6.63 ± 0.09). (5)

Combining this with the relation 3 Sahu et al.(2019b) observe consistent Mbh–σ relations for their larger log(Mbh/M ) = (5.53 ± 0.34) log(σ/200) + (8.22 ± 0.05) (6) sample of barred and non-barred galaxies. 4 This is not meant to imply a discontinuity between star clusters and nu- for non-barred galaxies from Graham & Scott(2013, , their Table 3) clear discs.

MNRAS 000,1–9 (2019) 4 Graham

7 for the inner-component equal to 6.9 × 10 M . This calculation as- black hole masses until lower bulge masses — the uncertainty in sumes equal stellar mass-to-light ratios for the inner- and outer- these bulge mass estimates results in consistency. component of M60-UCD1. This gives a mass ratio of the black hole relative to the inner component of 2.2/6.9 or about one-third. In passing, it is noted that the Sersic(1968) index (see Gra- 3.3 M59-UCD3 ham & Driver 2005, for a review of the Sérsic model) of the inner A fourth UCD galaxy reported to have a massive component is 3.3. This compares favourably with the Sérsic index black hole is M59-UCD3 Ahn et al.(2018). Although their tri- of the nuclear star cluster in the Milky Way (n = 3) and in M32 axial Schwarzschild model has a minimum χ2 value at a black (n = 2.3) (Graham & Spitler(2009); see also Nguyen et al.(2019) hole mass of zero, from their axisymmetric Schwarzschild model, who report n = 2.7 ± 0.3 for M32). 7 and Jeans anisotropic model (JAM), they determine a black hole Plugging in Mbh = (2.2 ± 0.4) × 10 M (Seth et al. 2014) +2.1 6 mass of 4.2−1.7 × 10 M (1σ uncertainties). From equation8, into equation4, and assuming an intrinsic scatter of 0.5 dex in this would suggest a host star cluster of stellar mass given by the log Mnc direction, gives log(Mnc/M ) = 7.42 ± 0.53, or Mnc = 4.2 7 log M = 7.22 ± 0.55, or M = 1.7 × 10 M . (2.6+6.3) × 107 M . This predicted nuclear cluster mass is just 2.65 nc nc −1.2 −1.8 Ahn et al.(2018) decomposed the image of M59-UCD3 into times smaller than the stellar mass of the inner component in M60- 7 three components. The inner (Re,inner = 16.8 pc), middle (Re,middle = UCD1 (= 6.9 × 10 M ), i.e. the suspected nuclear star cluster. 7 40.0 pc), and outer (Re,outer = 99.2 pc) components were reported to Plugging Mbh = (2.2±0.4)×10 M into equation8, and again 8 8 have a stellar mass of (1.73±0.28)×10 M , (1.06±0.15)×10 M , assuming an intrinsic scatter of 0.5 dex in the log Mnc direction, and (0.12 ± 0.1) × 108 M , respectively. The inner-component has ± gives the consistent result log(Mnc/M ) = 7.49 0.54, or Mnc = a mass which is an order or magnitude greater than expected from (3.1+7.6) × 107 M . −2.2 the above calculation. Oddly, the inner-component also has an axis This simple test, which was not previously performed for ratio of 0.74 while the outer components are round rather than M60-UCD1, provides support for the stripped-galaxy scenario, in stretched out. The colour of the inner- and middle components are which a threshing process may leave behind the dense central seed rather similar, and significant rotation (24-30 km s−1) is observed (nuclear cluster plus black hole) of the pared galaxy. across the inner ∼40 pc. Conceivably, M59-UCD3 may represent the remnant nuclear disc of a threshed galaxy. Balcells et al.(2007) 3.2 Virgo cluster: VUCD3 and M59cO have reported on the commonality of nuclear discs (tens of in size) in early-type disc galaxies. Given that the scaling relations Following Seth et al.(2014), Ahn et al.(2017) claimed the detec- in Section2 were established for nuclear star clusters (with masses 7 tion of massive black holes in two UCD galaxies (VUCD3 and less than 5 × 10 M ), it may therefore be inappropriate to apply M59cO) residing in the Virgo . VUCD3 is particu- the consistency test in this instance, as it is not designed to pre- larly interesting given that Liu et al.(2015) had detected a pos- dict the masses of nuclear discs, or it may represent a failure of the sible tidal stream associated with this target. While Ahn et al. consistency test. (2017) noted that high levels of orbital anisotropy in these two UCD galaxies could nullify their suspected signature of a black +2.5 6 hole, they reported Mbh = 4.4−3.0 × 10 M in VUCD3 and Mbh = 3.4 -UCD3 +2.5 6 5.8−2.8 × 10 M in M59cO (3σ uncertainties). As done above for M60-UCD1, we can employ (the more ac- Fornax-UCD3 (Frank et al. 2011) was observed with a Hubble curate) equation8 to determine what the expected mass should be Space Telescope / Advanced Camera for Surveys F606W image for the inner-component of VUCD3 and M59cO if they are the rem- and modelled as the sum of two Sérsic components by Evstigneeva nant nuclear star clusters of galaxies. Doing so, one obtains stellar et al.(2008). However, the magnitude of the inner Sérsic compo- nent was not reported, only the combined/total UCD galaxy mag- mass expectations of log(Mnc/M ) = 7.23 ± 0.56 and 7.27 ± 0.55, 00 +4.4 7 +4.7 7 nitude was given, along with Re,total = 86.5 ± 6.2 pc (0. 94). It was or Mnc = (1.7−1.2) × 10 M and (1.9−1.4) × 10 M , respectively. These masses, based upon the black hole masses, can be com- also noted there that a possible in the background of pared with the stellar masses reported by Ahn et al.(2017, , their Fornax-UCD3 complicated the analysis. Section 3) for the inner-component of their two-component decom- In this UCD galaxy, Afanasiev et al.(2018) have reported a BH mass similar to that in M59-UCD3 and equal to 3.3+1.4 ×106 M positions of each UCD galaxies’ image. For VUCD3, they reported −1.2 7 (1-sigma uncertainty), from which one would expect, using equa- M∗,inner = (1.1 ± 0.3) × 10 M , while for M59cO they reported +0.2 7 tion8, a nuclear star cluster stellar mass given by log M nc = M∗, = (1.4 ) × 10 M . This remarkable agreement, in both inner −0.2 7.18 ± 0.55, or M = 1.5+3.9 × 107 M . UCD galaxies, further supports the notion that these UCD galaxies nc −1.1 could be the nuclei and remnants of threshed galaxies. Once again, Afanasiev et al.(2018) fit a two-component model to Fornax- this test had never been performed for these two UCD galaxies. UCD3, reporting an F606W magnitude of 20.17 (AB mag) for the ∼ inner component, to which they apply a stellar mass-to-light ratio The Mbh-to-M∗,inner−cpt mass ratios are 40 percent in both of 6 of 3.35. This gives a stellar mass of 9.7 × 10 M , for their adopted these UCD galaxies. The smaller Mbh-to-M∗,UCD ratio is of course somewhat irrelevant, depending simply on how much of the host distance of 20.9 Mpc, in good agreement with the predicted mass. galaxy remains around the original nuclear star cluster. Using the mass of the suspected former nuclear star clusters, equation1 sug- 9 3.5 A: UCD 320 and UCD 330 gests a former spheroid/bulge mass of 4 and 5×10 M with an uncertainty of a factor of ∼4 which is dominated by the intrinsic Voggel et al.(2018), see also Rejkuba et al.(2007), have ob- scatter in the Mnc–M∗,sph relation. While these estimates are sev- served two low-mass UCD galaxies (UCD 320 and UCD 330: 7 eral times more massive than the bulge mass estimates of 1.2 and MUCD < 10 M ), first discovered by Harris et al.(1992), around 9 1.7×10 M in Ahn et al.(2017) — arising from their use of a near- the galaxy Centaurus A (NGC 5128). They did not, however, de- linear Mbh–M∗,sph scaling relation which does not reach these low tect the presence of a central black hole in either of these galaxies,

MNRAS 000,1–9 (2019) Ultra-compact dwarf galaxies 5 and thus there are, to date, just five UCD galaxies with a reported black hole mass measurement. Nontheless, for UCD 330, they reported that the absolute mag- nitude is MV = −11.03 mag, and that M∗/LV = 3.30, giving a total 6 stellar mass of 7.15 × 10 M when using M ,V = +4.81 mag. They additionally fit a two-component structure to the distribution of light in UCD 330, reporting that 62.4 per cent of the light resides in the inner component, which therefore has a stellar mass equal to 6 4.46 × 10 M . From equation9, which represents the inversion of equation8. the expected black hole mass based on this speculated 5 nuclear star cluster mass is 1.3×10 M , albeit with over an order of magnitude uncertainty. For reference, Voggel et al.(2018) reported a 3σ upper limit on any potential black hole in this UCD galaxy to 5 be less than 1.0×10 M , suggesting that they may have just missed out on the required spatial resolution to detect the suspected black hole in this UCD galaxy. Voggel et al.(2018, , their Section 6.1) wrote that UCD 330, with its Mbh/MUCD mass ratio of just a few percent, is clearly differ- ent from other UCD galaxies with Mbh–to–MUCD mass ratios with percentages in the teens. However, it makes more sense to compare the Mbh–to–Minner−cpt mass ratio when testing the galaxy threshing scenario. Given that the M /M ratio is not constant across nu- bh nc Figure 2. M –M relations: equation4 (dashed line) and equation8 (solid clear clusters of different masses, different M /M mass ra- bh nc bh inner−cpt line, with the 1-sigma uncertainty denoted by the dotted lines). The data tios in UCD galaxies — if they are stripped nuclei — is expected. points represent: the inner (stellar mass) component of six UCD galaxies, These ratios are reported in Table1. thought to be remnant nuclear star clusters (orange squares); the inner (stel- For UCD 320, Voggel et al.(2018) reported a single- lar mass) component of one UCD galaxy (M59-UCD3), which may be a component structure (perhaps indicative of a globular cluster rather remnant nuclear disc (blue circle); and the nuclear star cluster in NGC 205 than a UCD galaxy), with MV = −10.39 mag and M∗/LV = 2.64, and NGC 404 (2 red ). Three of these entries only have upper limits on 6 and therefore one obtains a total stellar mass of 3.17 × 10 M . If their black hole mass. A ±25 percent uncertainty has been assigned to those this is a remnant nuclear star cluster, then from equation9 one may stellar mass measurements without an error bar in Table1. The 14 small 4 black and grey stars are nuclear star clusters with dynamically measured expect it to contain a black hole with a mass equal to 5.5 × 10 M , i.e. 20 times less than the 3σ upper limit of 106 M reported in black hole masses (Graham & Spitler 2009; Graham 2012a). In galaxies with black hole masses greater than ≈108 M , the nuclear star clusters start Voggel et al.(2018). to erode at the expense of the black holes (Bekki & Graham 2010). For convenience, the above mentioned masses for all of the UCD galaxies are provided in Table1 and shown in Figure2. One can immediately appreciate that the measured (black hole)- because this method of scientific analysis is so readily employable, to-(inner star cluster) mass ratios for some of the UCD galaxies yet has not been widely explored, a first Case Study shall be made would make it challenging to separate the contributions from the using the galaxy NGC 205. In essence, this will test black hole and star cluster to the inner root-mean-square veloc- q if the black hole and nuclear star cluster in NGC 205 follow the 2 2 ity Vrms = sigma + Vrot spike. The black hole’s gravitational Mnc–Mbh relation used here. sphere-of-influence would loom large when the cluster’s stellar The NGC 205 dwarf early-type galaxy has a well-resolved 6 mass is just 2 to 3 times greater than the black hole’s mass. The nuclear star cluster with a stellar mass of (1.4 ± 0.7) × 10 M structure and motion of these inner star clusters will be partly dic- (De Rijcke et al. 2006; Graham & Spitler 2009), or more recently 6 tated by the black hole (Bahcall & Wolf 1976, 1977; Merritt 2015). (1.8 ± 0.8) × 10 M (Nguyen et al. 2018). Bypassing any need to know the of NGC 205’s main spheroidal component, which is distinct from the velocity dispersion of NGC 205’s nuclear star cluster, equa- 4 NUCLEAR STAR CLUSTERS tion8 can be inverted to estimate the expected black hole mass in 4.1 NGC 205 NGC 205’s star cluster. This equation for the black hole mass sim- ply depends on the star cluster mass, and is such that Having provided the above analysis for all five UCD galaxies with 7.83 a directly measured black hole mass to date, things shall now be log(Mbh/M ) = (2.62 ± 0.42) log(Mnc/[10 M ]) + (8.22 ± 0.20). reversed. That is, rather than investigating if a reported black hole (9) mass is consistent or not with the speculation that it resides in a remnant nuclear star cluster, the scaling relations will now be used The above equation gives an expected black hole mass in 3 6 to predict the mass of a black hole within two bona fide nuclear NGC 205 equal to 6.4 × 10 M if Mnc = 1.4 × 10 M , or 4 6 star clusters which have just had their central black hole masses Mbh = 1.2 × 10 M if Mnc = 1.8 × 10 M . This expectation from measured. However, because the shallow slope (0.38 ± 0.06) of equations established a few years ago is in remarkable agreement the Mnc–Mbh relation (equation8) corresponds to a steep slope with Nguyen et al.(2019) who report a dynamically-determined +95.6 3 (≈ 2.62 ± 0.42) for the Mbh–Mnc relation, the uncertainty on Mbh black hole mass in NGC 205 of 6.8−6.7 ×10 M (±3σ uncertainty), 3 is notably greater for a given measurement of Mnc. As such, the consistent with the upper limit of 3.8 × 10 M (±3σ uncertainty) predictive power with this approach is less certain. Despite this, from Valluri et al.(2005).

MNRAS 000,1–9 (2019) 6 Graham

Table 1. Black hole and star cluster masses.

Object Mbh,obs Mnc,obs Mbh,obs/Mnc,obs Mnc,pred Mbh,pred M M M M (1) (2) (3) (4) (5) (6) ± × 7 × 7 +7.6 × 7 M60-UCD1 (2.1 0.4) 10 6.9 10 0.30 (3.1−2.2) 10 ... +2.5 × 6 ± × 7 +4.4 × 7 Virgo-UCD3 (4.4−3.0) 10 (1.1 0.3) 10 0.40 (1.7−1.2) 10 ... +2.5 × 6 +0.2 × 7 +4.7 × 7 M59cO (UCD) (5.8−2.8) 10 (1.4−0.2) 10 0.41 (1.9−1.4) 10 ... a +2.1 × 6 ± × 8 a M59-UCD3 (4.2−1.7) 10 (1.73 0.28) 10 0.02 ...... +1.4 × 6 × 6 +3.9 × 7 Fornax-UCD3 (3.3−1.2) 10 9.7 10 0.34 (1.5−1.1) 10 ... CenA-UCD330 < 1.0 × 105 4.46 × 106 < 0.02 ... 1.3 × 105 CenA-UCD320 < 1.0 × 106 3.17 × 106 < 0.32 ... 5.5 × 104 +95.6 × 3 ± × 6 × 4 NGC 205 (6.8−6.7 ) 10 (1.8 0.8) 10 0.004 ... 1.2 10 NGC 404 < 1.5 × 105 (3.4 ± 0.2) × 106 < 0.05 ... 6.3 × 104 NGC 3319 ... ≈6 × 106 ...... ≈3 × 105 Column 2: Observed, measured black hole mass. Column 3: Observed, measured star cluster mass; either the inner-component of UCD galaxies or the nuclear star clusters of (non-stripped) galaxies. References to these masses are provided in the relevant subsections. Column 5: Predicted nuclear cluster mass a based on Mbh,obs and equation8. Column 6: Predicted black hole mass based on Mnc,obs and equation9. M59-UCD3 may represent a threshed nuclear disc. The uncertainty on the predicted black hole masses are large: 2.62 × 0.5 dex.

The agreement may reveal that the Mnc–Mbh relation can be an X-ray point-source in the NGC 3319. They 4 extended to black hole masses less than 10 M , or it may be fortu- find that the X-ray SED is more consistent with an AGN rather itous. While the expected black hole mass is more than two orders than a super-Eddington, stellar-mass, ultra-luminous X-ray source. of magnitude less than the nuclear star cluster mass, the uncertainty They find the X-ray point-source to be coincident with the nuclear 6 on the expected black hole mass is approaching a factor of 40 — star cluster, whose mass they report to be around 6 × 10 M . From 5 dominated by the assigned intrinsic scatter (1.31 dex) which is 2.62 equation9, the expected black hole mass is 3 × 10 M . times greater along the log Mbh direction than compared with the The Virgo cluster, dwarf early-type galaxies, IC 3442 and log Mnc direction. IC 3292 similarly have X-ray point-sources coincident with nuclear 5 star clusters that are expected to house ∼10 M black holes (Gra- ham & Soria 2019). Pushing to yet lower masses, the early-type 4.2 NGC 404 galaxies IC 3602 and IC 3633 have been predicted to house black hole masses of ∼104 M (Graham & Soria 2019), although this is One can similarly ask the question: What black hole mass is based on their galaxy luminosity and velocity dispersion rather than expected in the nuclear star cluster of the nearby dwarf galaxy a nuclear cluster mass or X-ray emission. NGC 404 (Seth et al. 2010). Nguyen et al.(2017) constrain the 5 The spiral galaxy NGC 4178 may harbour an intermediate mass of this black hole to be less than 1.5 × 10 M . From the stel- 6 mass black hole (Satyapal et al. 2009; Secrest et al. 2012), and an lar mass (3.4±0.2)×10 M of this galaxy’s innermost component, 3 M referred to as the “Central excess Sérsic” by Nguyen et al.(2017,, estimate of just 10 has been made (Graham et al. 2019). How- see their Table 4 and Figure 10), one can predict the expected black ever, this galaxy’s centrally-located, X-ray point-source has a soft, hole mass. This approach differs from Nguyen et al.(2017, , their probably thermal, spectrum suggestive of a stellar-mass black hole Section 6.2) who refer to the inner two components as the nuclear in a high/soft state. Further investigation is required. star cluster. From equation9, one expects log( Mbh/M ) = 4.8 ± 1.5 4 (Mbh = 6.3×10 M ) if the intrinsic scatter is 2.62×0.5 dex. This to- tal uncertainty of 1.5 dex, which was calculated in quadrature from the various smaller uncertainties, would be halved if the intrinsic 5 DISCUSSION scatter was excluded. There is a slight twist to all of this. One may ask, were the UCD This is an intriguing case study as the “Central excess Sér- galaxies once in early- or late-type galaxies, or in galaxies with or sic” is thought to be 1 Gyr old and accreted (Nguyen et al. 2017), without a substantial stellar disc? Some of the analysis performed while the second (larger) component has a half-light radius of 20 pc here has implicitly assumed that the UCD galaxies were once in and would be considered a nuclear disc by Balcells et al.(2007) galaxies with (intermediate- or large-scale) stellar discs. This is be- and Scott & Graham(2013) due to its size. As such, the M –M nc bh cause early- and late-type galaxies appear to follow different M – equations used here are not applicable to the second component. In bh M∗ and M –M∗ relations (Savorgnan et al. 2016; van den passing, it is noted that UCD1 around NGC 4546 was also forming ,bulge bh ,galaxy Bosch 2016; Terrazas et al. 2017; Davis et al. 2019; Sahu et al. stars just 1 to 2 Gyr ago (Norris et al. 2015). The topic of the stellar 2019a). The use of equation2 — which is consistent with the M – populations in nuclear star clusters (e.g. Böker et al. 2001; Paudel bh M∗ relation for both late-type galaxies and early-type galax- & Lisker 2009; Leigh et al. 2016), is, however, beyond the scope of ,bulge ies with either an intermediate- or a large-scale disc (Davis et al. this paper. 2019; Sahu et al. 2019b) — effectively creates a link to galaxies with a substantial stellar disc (see Figure1). One might consider use of the latest M –M relations from Sahu et al.(2019a), 4.3 Other bh ∗,bulge however this does not seem particularly appetising given the ap- As for additional nuclear star clusters that have been suggested to parent order of magnitude difference in black hole mass depending house an intermediate mass black hole, Jiang et al.(2018) report on whether the original early-type galaxies contained a disc or not.

MNRAS 000,1–9 (2019) Ultra-compact dwarf galaxies 7

Given that the Mbh–M∗,bulge relations depend on galaxy type, fu- higher Galactic latitudes (Sofue 1977, 2000; Bland-Hawthorn & ture progress would benefit from establishing if the Mnc–M∗,bulge Cohen 2003; Su et al. 2010; Heywood et al. 2019). The presence relations also depend on either the galaxy type (see Figure 5 in of weakly-active massive black holes in UCD galaxies may further Georgiev et al.(2016) in regard to the Mnc–M∗,gal relations) or the diminish prospects for life in such systems (e.g. Gonzalez et al. presence/absence of a substantial disc. Establishing and partnering 2001; Jiménez-Torres et al. 2013; Thompson 2013; Kane & De- these (morphological type)-specific relations should shed further veny 2018), but see Di Stefano & Ray(2016). This is especially light on this topic and yield improved predictive power. However, true given the close proximity of the stars and potential planets to our preferred use of the scaling relations involving the velocity dis- the massive black hole (Chen et al. 2018; Schnittman 2019). persion may have circumvented this issue. This is because nucle- 5 ated early- and late-type Sérsic galaxies may follow the same Mbh– σ relation (Sahu et al. 2019b). Whether they follow different M –σ nc ACKNOWLEDGEMENTS relations is yet to be established. Another implicit assumption has been that the local (z ≈ 0) re- The result for M60-UCD1 was first presented by the author at the lation between nuclear star cluster mass and black hole mass held Wilhelm and Else Heraeus Seminar 631 titled “Stellar aggregates when the UCD galaxy was formed. This may not have been so. over mass and spatial scales”, and held over 5-9 Dec. 2016. The However, given the good agreement between the UCD galaxies and author is grateful for their support and pleased to finally publish the local (black hole)–(nuclear star cluster) mass scaling relation, that result here. This research was additionally supported under the there may have been little evolution in this relation, which does not Australian Research Council’s funding scheme (DP17012923). discount evolution along the relation for those nuclear star clusters and black holes that remained at the centre of a galaxy. The distri- bution in Figure2, showing the mass of the UCD galaxies’ black REFERENCES hole and inner stellar component, supports the notion that UCD galaxies are the remnant nuclei of threshed galaxies. Abbate F., Possenti A., Ridolfi A., Freire P. C. C., Camilo F., Manchester The increased number density of black holes at the low-mass R. N., D’Amico N., 2018, MNRAS, 481, 627 end of the black hole mass function (e.g. Shankar et al. 2004; Gra- Afanasiev A. V., et al., 2018, MNRAS, 477, 4856 ham et al. 2007; Vika et al. 2009; Kelly & Shen 2013), due to Ahn C. P., et al., 2017, ApJ, 839, 72 the (expected) central massive black holes in UCD galaxies (Pf- Ahn C. P., et al., 2018, ApJ, 858, 102 Amaro-Seoane P., Gair J. R., Freitag M., Miller M. C., Mandel I., Cutler effer et al. 2014, 2016), has a couple of immediate consequences. C. J., Babak S., 2007, Classical and Quantum , 24, R113 It will increase the expected number of extreme mass-ratio inspi- Amaro-Seoane P., et al., 2017, arXiv e-prints, p. arXiv:1702.00786 ral (EMRI) events — involving stellar mass black holes and neu- Anderson J., van der Marel R. P., 2010, ApJ, 710, 1032 tron stars merging with the massive black hole — and therefore Arca-Sedda M., Capuzzo-Dolcetta R., 2014, ApJ, 785, 51 increase the expected number of gravitational wave events (e.g. Baganoff F. K., et al., 2001, Nature, 413, 45 Amaro-Seoane et al. 2007; Gair et al. 2010; Merritt 2010; Fra- Bahcall J. N., Wolf R. A., 1976, ApJ, 209, 214 gione et al. 2018). As detailed in Mapelli et al.(2012), the steeper Bahcall J. N., Wolf R. A., 1977, ApJ, 216, 883 (super-quadratic rather than near-linear) Mbh–Mbulge scaling rela- Baker J., et al., 2019, arXiv e-prints, p. arXiv:1907.06482 tion implied an order of magnitude reduction to the number of Balcells M., Graham A. W., Domínguez-Palmero L., Peletier R. F., 2003, EMRI events that the Laser Interferometer Space Antenna (LISA: ApJ, 582, L79 Amaro-Seoane et al. 2017; Baker et al. 2019) could detect in galac- Balcells M., Graham A. W., Peletier R. F., 2007, ApJ, 665, 1084 Baldassare V. F., Reines A. E., Gallo E., Greene J. E., 2015, ApJ, 809, L14 tic nuclei hosting nuclear star clusters and massive black holes. As Baumgardt H., Hut P., Makino J., McMillan S., Portegies Zwart S., 2003, such, the promise of additional EMRI events from UCD galaxies is ApJ, 582, L21 likely to be exciting and welcome news for the new LISA mission Baumgardt H., et al., 2019, MNRAS, 488, 5340 and Taiji program (Ruan et al. 2018). Bedin L. R., Piotto G., Anderson J., Cassisi S., King I. R., Momany Y., Second, it will enhance the expected number of stellar tidal Carraro G., 2004, ApJ, 605, L125 disruption events (TDE: Hills 1975; Komossa 2013; Stone & Met- Begelman M. C., Blandford R. D., Rees M. J., 1980, Nature, 287, 307 zger 2016) given the abundance of UCD galaxies (Jones et al. 2006; Bekki K., Freeman K. C., 2003, MNRAS, 346, L11 Madrid et al. 2010; Chiboucas et al. 2011; Voggel et al. 2019). Bekki K., Graham A. W., 2010, ApJ, 714, L313 Third, and related, it opens the question as to whether UCD galax- Bekki K., Couch W. J., Drinkwater M. J., 2001a, ApJ, 552, L105 ies may simmer away due to weak, sporadic, AGN activity. The Bekki K., Couch W. J., Drinkwater M. J., Gregg M. D., 2001b, ApJ, 557, L39 massive black hole in the Milky Way’s nuclear star cluster6 sput- Bekki K., Couch W. J., Drinkwater M. J., Shioya Y., 2003, MNRAS, 344, ters and bubbles according to its time-variable fuel supply. This is 399 evidenced by both the daily/yearly variations in flux at different Bland-Hawthorn J., Cohen M., 2003, ApJ, 582, 246 wavelengths (e.g. Riegler et al. 1981; Zhao et al. 2001; Baganoff Böker T., 2008, ApJ, 672, L111 et al. 2001; Genzel et al. 2003; Ghez et al. 2004; Mauerhan et al. Böker T., van der Marel R. P., Mazzuca L., Rix H.-W., Rudnick G., Ho 2005) and the ∼200 pc radio bubble discovered by Sofue & Handa L. C., Shields J. C., 2001,AJ, 121, 1473 (1984), see also (Tsuboi et al. 1985), with known extensions to Böker T., Laine S., van der Marel R. P., Sarzi M., Rix H.-W., Ho L. C., Shields J. C., 2002,AJ, 123, 1389 Böker T., Sarzi M., McLaughlin D. E., van der Marel R. P., Rix H.-W., Ho 5 Sahu et al.(2019b) find that (non-nucleated) core-Sérsic early-type galax- L. C., Shields J. C., 2004,AJ, 127, 105 ies and (typically-nucleated) Sérsic early-type galaxies define different Capuzzo-Dolcetta R., Tosta e Melo I., 2017, MNRAS, 472, 4013 Mbh–σ relations. Chandrasekhar S., 1943, ApJ, 97, 255 6 Excitingly, the Milky Way’s nuclear star cluster seems set to be moni- Chen H., Forbes J. C., Loeb A., 2018, ApJ, 855, L1 tored in the near- by the planned Japanese satellite Small-JASMINE Chiboucas K., et al., 2011, ApJ, 737, 86 (Yano et al. 2013; Yano & JASMINE-WG 2018). Chilingarian I. V., Mamon G. A., 2008, MNRAS, 385, L83

MNRAS 000,1–9 (2019) 8 Graham

Chilingarian I., Cayatte V., Revaz Y., Dodonov S., Durand D., Durret F., Graham A. W., 2019, Publ. Astron. Soc. Australia, 36, e035 Micol A., Slezak E., 2009, , 326, 1379 Graham A. W., Driver S. P., 2005, Publ. Astron. Soc. Australia, 22, 118 Chilingarian I. V., Mieske S., Hilker M., Infante L., 2011, MNRAS, 412, Graham A. W., Guzmán R., 2003,AJ, 125, 2936 1627 Graham A. W., Scott N., 2013, ApJ, 764, 151 Chilingarian I. V., Katkov I. Y., Zolotukhin I. Y., Grishin K. A., Beletsky Y., Graham A. W., Scott N., 2015, ApJ, 798, 54 Boutsia K., Osip D. J., 2018, ApJ, 863, 1 Graham A. W., Soria R., 2019, MNRAS, 484, 794 Côté P., et al., 2006, ApJS, 165, 57 Graham A. W., Spitler L. R., 2009, MNRAS, 397, 2148 Davis B. L., Graham A. W., Cameron E., 2019, ApJ, 873, 85 Graham A. W., Erwin P., Trujillo I., Asensio Ramos A., 2003,AJ, 125, 2951 De Rijcke S., Prugniel P., Simien F., Dejonghe H., 2006, MNRAS, 369, Graham A. W., Driver S. P., Allen P. D., Liske J., 2007, MNRAS, 378, 198 1321 Graham A. W., Ciambur B. C., Soria R., 2016, ApJ, 818, 172 Di Stefano R., Ray A., 2016, ApJ, 827, 54 Graham A. W., Soria R., Davis B. L., 2019, MNRAS, 484, 814 Dolgov A., Postnov K., 2017, J. Cosmology Astropart. Phys., 2017, 036 Haggard D., Cool A. M., Heinke C. O., van der Marel R., Cohn H. N., Dressler A., 1989, in Osterbrock D. E., Miller J. S., eds, IAU Symposium Lugger P. M., Anderson J., 2013, ApJ, 773, L31 Vol. 134, Active Galactic Nuclei. p. 217 Harris G. L. H., Geisler D., Harris H. C., Hesser J. E., 1992,AJ, 104, 613 Drinkwater M. J., Jones J. B., Gregg M. D., Phillipps S., 2000, Publ. Astron. Harris W. E., Pritchet C. J., McClure R. D., 1995, ApJ, 441, 120 Soc. Australia, 17, 227 Heywood I., et al., 2019, Nature, 573, 235 Drinkwater M. J., Gregg M. D., Hilker M., Bekki K., Couch W. J., Ferguson Hilker M., Infante L., Vieira G., Kissler-Patig M., Richtler T., 1999, A&AS, H. C., Jones J. B., Phillipps S., 2003, Nature, 423, 519 134, 75 Evstigneeva E. A., Gregg M. D., Drinkwater M. J., Hilker M., 2007,AJ, Hilker M., Baumgardt H., Infante L., Drinkwater M., Evstigneeva E., Gregg 133, 1722 M., 2007, A&A, 463, 119 Evstigneeva E. A., et al., 2008,AJ, 136, 461 Hills J. G., 1975, Nature, 254, 295 Farrell S. A., Webb N. A., Barret D., Godet O., Rodrigues J. M., 2009, Hurley J. R., 2007, MNRAS, 379, 93 Nature, 460, 73 Ibata R., et al., 2009, ApJ, 699, L169 Ferrarese L., et al., 2006, ApJS, 164, 334 Ideta M., Makino J., 2004, ApJ, 616, L107 Ferraro F. R., et al., 2018, The Messenger, 172, 18 Inoue S., 2011, MNRAS, 416, 1181 Fragione G., Leigh N. W. C., Ginsburg I., Kocsis B., 2018, ApJ, 867, 119 Jennings Z. G., et al., 2015, ApJ, 812, L10 Frank M. J., Hilker M., Mieske S., Baumgardt H., Grebel E. K., Infante L., Jiang N., et al., 2018, ApJ, 869, 49 2011, MNRAS, 414, L70 Jiménez-Torres J. J., Pichardo B., Lake G., Segura A., 2013, Astrobiology, Freeman K. C., 1993, in Smith G. H., Brodie J. P., eds, Astronomical So- 13, 491 ciety of the Pacific Conference Series Vol. 48, The globular clusters- Jones J. B., et al., 2006,AJ, 131, 312 galaxy connection. Astronomical Society of the Pacific Conference Se- Kane S. R., Deveny S. J., 2018, ApJ, 864, 115 ries, Volume 48, Proceedings of the 11th Santa Cruz Summer Work- Kelly B. C., Shen Y., 2013, ApJ, 764, 45 shop in and Astrophysics, held July 19-29, 1992, at the Uni- Kirsten F., Vlemmings W. H. T., 2012, A&A, 542, A44 versity of California, Santa Cruz, San Francisco: Astronomical Society Kirsten F., Vlemmings W., Freire P., Kramer M., Rottmann H., Campbell of the Pacific (ASP), |c1993, edited by Graeme H. Smith, and Jean P. R. M., 2014, A&A, 565, A43 Brodie, ISBN # 0-937707-67-8. LC # QB856 .S26 1992, p.608. p. 608 Kızıltan B., Baumgardt H., Loeb A., 2017, Nature, 542, 203 Gair J. R., Tang C., Volonteri M., 2010, Phys. Rev. D, 81, 104014 Koch A., Burkert A., Rich R. M., Collins M. L. M., Black C. S., Hilker M., Gebhardt K., Pryor C., O’Connell R. D., Williams T. B., Hesser J. E., 2000, Benson A. J., 2012, ApJ, 755, L13 AJ, 119, 1268 Komossa S., 2013, in Zhang C. M., Belloni T., Méndez M., Zhang S. N., Gebhardt K., Rich R. M., Ho L. C., 2005, ApJ, 634, 1093 eds, IAU Symposium Vol. 290, Feeding Compact Objects: Accretion Genzel R., Schödel R., Ott T., Eckart A., Alexander T., Lacombe F., Rouan on All Scales. pp 53–56, doi:10.1017/S1743921312019199 D., Aschenbach B., 2003, Nature, 425, 934 Kormendy J., Ho L. C., 2013, &A, 51, 511 Georgiev I. Y., Hilker M., Puzia T. H., Goudfrooij P., Baumgardt H., 2009, Kormendy J., Richstone D., 1995, ARA&A, 33, 581 MNRAS, 396, 1075 Kroupa P., 1998, MNRAS, 300, 200 Georgiev I. Y., Böker T., Leigh N., Lützgendorf N., Neumayer N., 2016, Lanzoni B., Cosmic-Lab Team 2016, Mem. Soc. Astron. Italiana, 87, 563 MNRAS, 457, 2122 Laor A., 2001, ApJ, 553, 677 Gerssen J., van der Marel R. P., Gebhardt K., Guhathakurta P., Peterson Leigh N., Böker T., Knigge C., 2012, MNRAS, 424, 2130 R. C., Pryor C., 2002,AJ, 124, 3270 Leigh N. W. C., Antonini F., Stone N. C., Shara M. M., Merritt D., 2016, Ghez A. M., et al., 2004, ApJ, 601, L159 MNRAS, 463, 1605 Gieles M., Balbinot E., Yaaqib R. I. S. M., Hénault-Brunet V., Zocchi A., Linden S. T., et al., 2017, ApJ, 843, 91 Peuten M., Jonker P. G., 2018, MNRAS, 473, 4832 Liu C., et al., 2015, ApJ, 812, L2 Gómez M., Geisler D., Harris W. E., Richtler T., Harris G. L. H., Woodley Lützgendorf N., et al., 2016, in Meiron Y., Li S., Liu F. K., Spurzem K. A., 2006, A&A, 447, 877 R., eds, IAU Symposium Vol. 312, Star Clusters and Black Holes González Delgado R. M., Pérez E., Cid Fernand es R., Schmitt H., 2008, in Galaxies across Cosmic Time. pp 181–188 (arXiv:1501.07441), AJ, 135, 747 doi:10.1017/S1743921315007784 Gonzalez G., Brownlee D., Ward P., 2001, Icarus, 152, 185 Lynden-Bell D., 1969, Nature, 223, 690 Goodman M., Bekki K., 2018, MNRAS, 478, 3564 Maccarone T. J., 2016, Mem. Soc. Astron. Italiana, 87, 559 Graham A. W., 2012a, MNRAS, 422, 1586 Madrid J. P., et al., 2010, ApJ, 722, 1707 Graham A. W., 2012b, ApJ, 746, 113 Magorrian J., et al., 1998,AJ, 115, 2285 Graham A. W., 2016a, in Meiron Y., Li S., Liu F. K., Spurzem R., Mailyan N. S., 1973, Astrophysics, 9, 33 eds, IAU Symposium Vol. 312, Star Clusters and Black Holes in Maji M., Zhu Q., Li Y., Charlton J., Hernquist L., Knebe A., 2017, ApJ, Galaxies across Cosmic Time. pp 269–273 (arXiv:1412.5715), 844, 108 doi:10.1017/S1743921315008017 Mann C. R., et al., 2019, ApJ, 875, 1 Graham A. W., 2016b, in Laurikainen E., Peletier R., Gadotti D., eds, As- Manzano-King C. M., Canalizo G., Sales L. V., 2019, ApJ, 884, 54 trophysics and Space Science Library Vol. 418, Galactic Bulges, As- Mapelli M., Ripamonti E., Vecchio A., Graham A. W., Gualandris A., 2012, trophysics and Space Science Library, Volume 418. ISBN 978-3-319- A&A, 542, A102 19377-9. Springer International Publishing Switzerland, 2016, p. 263. Maraston C., Bastian N., Saglia R. P., Kissler-Patig M., Schweizer F., Goud- p. 263, doi:10.1007/978-3-319-19378-6-11 frooij P., 2004, A&A, 416, 467

MNRAS 000,1–9 (2019) Ultra-compact dwarf galaxies 9

Martínez-Palomera J., Lira P., Bhalla-Ladd I., Förster F., Plotkin R. M., Tonry J. L., 1984, ApJ, 283, L27 2019, arXiv e-prints, p. arXiv:1912.02860 Tremaine S. D., Ostriker J. P., Spitzer L. J., 1975, ApJ, 196, 407 Martini P., Pogge R. W., 1999,AJ, 118, 2646 Tremou E., et al., 2018, ApJ, 862, 16 Mauerhan J. C., Morris M., Walter F., Baganoff F. K., 2005, ApJ, 623, L25 Tsuboi M., Inoue M., Handa T., Tabara H., Kato T., 1985, PASJ, 37, 359 Merritt D., 2010, ApJ, 718, 739 Valluri M., Ferrarese L., Merritt D., Joseph C. L., 2005, ApJ, 628, 137 Merritt D., 2015, ApJ, 814, 57 Vesperini E., Trenti M., 2010, ApJ, 720, L179 Miller-Jones J. C. A., et al., 2012, ApJ, 755, L1 Vika M., Driver S. P., Graham A. W., Liske J., 2009, MNRAS, 400, 1451 Milosavljevic´ M., 2004, ApJ, 605, L13 Voggel K., Hilker M., Richtler T., 2016, A&A, 586, A102 Moran E. C., Shahinyan K., Sugarman H. R., Vélez D. O., Eracleous M., Voggel K. T., et al., 2018, ApJ, 858, 20 2014,AJ, 148, 136 Voggel K. T., Seth A. C., Baumgardt H., Mieske S., Pfeffer J., Rasskazov Neumayer N., Walcher C. J., 2012, Advances in Astronomy, 2012, 709038 A., 2019, ApJ, 871, 159 Nguyen D. D., et al., 2017, ApJ, 836, 237 Wittmann C., Lisker T., Pasquali A., Hilker M., Grebel E. K., 2016, MN- Nguyen D. D., et al., 2018, ApJ, 858, 118 RAS, 459, 4450 Nguyen D. D., et al., 2019, ApJ, 872, 104 Wrobel J. M., Miller-Jones J. C. A., Middleton M. J., 2016,AJ, 152, 22 Norris J. E., 2000, in IAU Joint Discussion. p. 4 Yano T., JASMINE-WG 2018, in Recio-Blanco A., de Laverny P., Norris J. E., Freeman K. C., Mayor M., Seitzer P., 1997, ApJ, 487, L187 Brown A. G. A., Prusti T., eds, IAU Symposium Vol. 330, Norris M. A., Escudero C. G., Faifer F. R., Kannappan S. J., Forte J. C., van Astrometry and Astrophysics in the Gaia Sky. pp 360–361, den Bosch R. C. E., 2015, MNRAS, 451, 3615 doi:10.1017/S1743921317005476 Noyola E., Gebhardt K., Bergmann M., 2008, ApJ, 676, 1008 Yano T., Gouda N., Kobayashi Y., Tsujimoto T., Niwa Y., Yamada Y., 2013, Paudel S., Lisker T., 2009, Astronomische Nachrichten, 330, 969 in de Grijs R., ed., IAU Symposium Vol. 289, Advancing the Physics Paudel S., Hilker M., Ree C. H., Kim M., 2016, ApJ, 820, L19 of Cosmic Distances. pp 433–436, doi:10.1017/S1743921312021898 Perera B. B. P., et al., 2017, MNRAS, 468, 2114 Zhang H.-X., et al., 2015, ApJ, 802, 30 Pfeffer J., Baumgardt H., 2013, MNRAS, 433, 1997 Zhao J.-H., Bower G. C., Goss W. M., 2001, ApJ, 547, L29 Pfeffer J., Griffen B. F., Baumgardt H., Hilker M., 2014, MNRAS, 444, Zinnecker H., Keable C. J., Dunlop J. S., Cannon R. D., Griffiths W. K., 3670 1988, in Grindlay J. E., Philip A. G. D., eds, IAU Symposium Vol. Pfeffer J., Hilker M., Baumgardt H., Griffen B. F., 2016, MNRAS, 458, 126, The Harlow-Shapley Symposium on Globular Cluster Systems in 2492 Galaxies. p. 603 Price J., et al., 2009, MNRAS, 397, 1816 Zocchi A., Gieles M., Hénault-Brunet V., 2017, MNRAS, 468, 4429 Rejkuba M., Dubath P., Minniti D., Meylan G., 2007, A&A, 469, 147 Zocchi A., Gieles M., Hénault-Brunet V., 2019, MNRAS, 482, 4713 Riegler G. R., Ling J. C., Mahoney W. A., Wheaton W. A., Willett J. B., den Brok M., van de Ven G., van den Bosch R., Watkins L., 2014, MNRAS, Jacobson A. S., Prince T. A., 1981, ApJ, 248, L13 438, 487 Ruan W.-H., Guo Z.-K., Cai R.-G., Zhang Y.-Z., 2018, arXiv e-prints, p. van den Bergh S., 1959, Publications of the David Dunlap Observatory, 2, arXiv:1807.09495 147 Sahu N., Graham A. W., Davis B. L., 2019a, ApJ, 876, 155 van den Bosch R. C. E., 2016, ApJ, 831, 134 Sahu N., Graham A. W., Davis B. L., 2019b, ApJ, 887, 10 van der Marel R. P., Anderson J., 2010, ApJ, 710, 1063 Salucci P., Ratnam C., Monaco P., Danese L., 2000, MNRAS, 317, 488 Sánchez-Janssen R., et al., 2019, ApJ, 878, 18 This paper has been typeset from a TEX/LATEX file prepared by the author. Sandage A., Binggeli B., Tammann G. A., 1985,AJ, 90, 1759 Sanders R. H., Lowinger T., 1972,AJ, 77, 292 Sartori L. F., Schawinski K., Treister E., Trakhtenbrot B., Koss M., Shirazi M., Oh K., 2015, MNRAS, 454, 3722 Satyapal S., Böker T., Mcalpine W., Gliozzi M., Abel N. P., Heckman T., 2009, ApJ, 704, 439 Savorgnan G. A. D., Graham A. W., Marconi A. r., Sani E., 2016, ApJ, 817, 21 Schnittman J. D., 2019, arXiv e-prints, p. arXiv:1910.00940 Scott N., Graham A. W., 2013, ApJ, 763, 76 Scott N., Graham A. W., Schombert J., 2013, ApJ, 768, 76 Secrest N. J., Satyapal S., Gliozzi M., Cheung C. C., Seth A. C., Böker T., 2012, ApJ, 753, 38 Sersic J. L., 1968, Atlas de Galaxias Australes. Observatorio Astronomico, Cordoba, Argentina Seth A., Agüeros M., Lee D., Basu-Zych A., 2008, ApJ, 678, 116 Seth A. C., et al., 2010, ApJ, 714, 713 Seth A. C., et al., 2014, Nature, 513, 398 Shankar F., Salucci P., Granato G. L., De Zotti G., Danese L., 2004, MN- RAS, 354, 1020 Shields J. C., Walcher C. J., Böker T., Ho L. C., Rix H.-W., van der Marel R. P., 2008, ApJ, 682, 104 Sofue Y., 1977, A&A, 60, 327 Sofue Y., 2000, ApJ, 540, 224 Sofue Y., Handa T., 1984, Nature, 310, 568 Spengler C., et al., 2017, ApJ, 849, 55 Stone N. C., Metzger B. D., 2016, MNRAS, 455, 859 Strader J., et al., 2013, ApJ, 775, L6 Su M., Slatyer T. R., Finkbeiner D. P., 2010, ApJ, 724, 1044 Terrazas B. A., Bell E. F., Woo J., Henriques B. M. B., 2017, ApJ, 844, 170 Thompson T. A., 2013, MNRAS, 431, 63

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