THE DENSEST GALAXY

We discovered M60-UCD1 in the Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) imaging of Strader et al. (2012). We have a single orbit of imaging split between F475W and F850LP (hereafter g and z). M60-UCD1 is located at (R.A., decl.) = (190.8999, 11.5347) in decimal J2000 coordinates. This is at a projected distance of only ∼6.6 kpc from the center of M60 (Figure 1; assuming a distance of 16.5 Mpc; Blakeslee et al. 2009). No mention is made of M60-UCD1 in previous Virgo surveys, including the ACS Virgo Cluster Survey (Côté et al. 2004). It is present in the Sloan Digital Sky Survey Data Release 7 photometric catalog (Abazajian et al. 2009) as J124335.96+113204.6, and was classified by Simard et al. (2011) as a background galaxy.

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A spectrum of M60-UCD1 was obtained on the night of 2012 January 17 with Keck/DEIMOS (Faber et al. 2003), utilizing the 1200 l/mm grating centered at 7800 Å and a 1'' slit (resolution ∼1.5 Å). We obtained three 30 minute exposures in 08 seeing. Using the spec2d pipeline (Cooper et al. 2012), the spectra were extracted, calibrated, and combined in the standard manner to produce a final one-dimensional spectrum.

To improve stellar population constraints, further spectroscopy was undertaken with MMT/Hectospec (Fabricant et al. 2005) on 2012 May 16, using the 270 l/mm grating with wavelength coverage from 3700 to 9100 Å and 5 Å resolution. Three 20 minute exposures were taken in 09 seeing. These Hectospec data were pipeline-reduced in a standard manner as described in Mink et al. (2007).

Aperture photometry of M60-UCD1 gives a total integrated magnitude of z = 15.86 ± 0.02 and g = 17.40 ± 0.02, yielding g − z = 1.54 ± 0.03 (the g and z magnitudes in this Letter are AB). The measured half-light radius (see below) is r_e = 24.2 ± 0.5 pc. The inferred total luminosities are L_g = (3.26 ± 0.06) × 10⁷ L_☉; L_z = (7.88 ± 0.14) × 10⁷ L_☉; and L_V = (4.12 ± 0.08) × 10⁷ L_☉. With M_V = −14.2, M60-UCD1 is the most luminous UCD known (see Section 4.1 for further discussion).

We fit the optical imaging with one- and two-component elliptical Sérsic models. These fits, shown in Figure 2, were performed by fitting two-dimensional models convolved with an empirical point-spread function (PSF) using a custom software package as described in Seth et al. (2006). The PSF (10 × subsampled) was derived from point sources in the images. The fits were performed on a 5''×5'' image centered on the UCD, with the background galaxy gradient from M60 itself removed. The fits are not very sensitive to the fitting box size.

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From the residual map (Figure 2), it is clear that a single Sérsic component provides a poor fit. In particular, the ellipticity of M60-UCD1 becomes more circular at larger radii, leaving a residual along the minor axis. The radial shape of the surface brightness profile is also poorly fit.

However, using a two-component Sérsic model, a very good fit ($\chi ^{2}_{\nu }$ = 1.07 in g) is obtained. Table 1 gives the parameters for the single and double Sérsic g-band fits and the double Sérsic z fits. For the two-component fits the Sérsic parameters n and r_e are very similar between the filters. Because the g band provides a much better fit (probably due to PSF modeling issues in z), all structural values cited are from the g fits.

For the best-fit two-component model, the inner component is compact (r_e = 14 pc), with modest ellipticity ( = 0.25), and has about 58% of the total luminosity of M60-UCD1. The outer component is more extended (r_e = 49 pc), round, and with a nearly exponential profile (n ∼ 1.2). The overall half-light radius is r_e = 24.2 ± 0.5 pc, empirically measured using the deconvolved g surface brightness profile. This value is similar to the radius derived from the single-component Sérsic fit; within this radius we estimate L_V ∼ 2.1 × 10⁷ L_☉.

3.2.1. Dynamical Mass, Mass-to-light Ratio, and Resolved Kinematics

3.2.2. Abundances

An X-ray source at the position of M60-UCD1 is present in the Chandra/ACIS catalog of Luo et al. (2013). The central astrometric matching between the Chandra and HST data is excellent due to the large X-ray binary and globular cluster populations of M60, with an rms scatter of 017.

This X-ray source, called XID 144 by Luo et al. (2013), has a position consistent with the optical center of M60-UCD1. There is evidence that it is variable, with its X-ray luminosity (from 0.3 to 8 keV) ranging from ∼6 × 10³⁷ to ∼1.3 × 10³⁸ erg s⁻¹ over timescales as short as a few months. The X-ray spectrum is well fit by a absorbed power law with a photon index of 1.8.

There are two reasonable possibilities for this central X-ray source: it could either be an active galactic nucleus associated with a massive black hole or a low-mass X-ray binary.

The case for a central black hole is straightforward. If the black hole occupation fraction of dwarf galaxies is high, and if UCDs are the products of tidal stripping of dwarf galaxies, then a significant fraction of UCDs should have "overmassive" black holes that could be detected through dynamical or accretion signatures. If UCDs have been stripped of 99% or more of their original mass (we estimate in Section 4.2 that the progenitor of M60-UCD1 was ∼50–200 times more massive), then they could host supermassive black holes that are ≳ 10% of their present-day masses (Mieske et al. 2013). Frank et al. (2011) constrain a putative black hole to be <5% of the total mass of one Fornax cluster UCD through integral-field spectroscopy.

The observed X-ray luminosity would be consistent with a ∼10⁷ M_☉ black hole accreting at 10⁻⁴ of the Eddington rate with a radiative efficiency of 10⁻³. This Eddington ratio of 10⁻⁷ would be typical of nuclei with old stellar populations (Ho 2009).

We can also estimate the odds that M60-UCD1 contains a bright X-ray binary. Sivakoff et al. (2007) derive formulae to estimate the probability that a globular cluster contains a low-mass X-ray binary with L_X > 3.2 × 10³⁸ erg s⁻¹. The odds are higher for metal-rich clusters and those with high encounter rates. Applying their results, but extrapolating to the fainter luminosity observed, suggests a ∼25% chance of having observed a low-mass X-ray binary in M60-UCD1. However, these results are of uncertain relevance for an object with a different structure and formation history than a globular cluster (since what is pertinent is the integrated—not instantaneous—collision rate). Dabringhausen et al. (2012) suggest UCDs have a lower occurrence of low-mass X-ray binaries than expected on the basis of the Sivakoff et al. (2007) results.

Future observations can help clarify the nature of the X-ray source. For example, if M60-UCD1 hosts a ≳ 10⁶ M_☉ black hole that lies on the radio–X-ray fundamental plane for black holes (Plotkin et al. 2012), it should be detectable with the Very Large Array.

Figure 3 shows a plot of log Σ versus log L_V for dispersion-supported stellar systems. Σ is the mean surface luminosity density within r_e. Globular clusters are plotted with different symbols than galaxies. It is clear that M60-UCD1 is an unusual object: it is much denser than any other object classified as a galaxy. It is more massive than any UCD or star cluster of comparable size, but is much more compact than other galaxies of similar luminosity.

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M60-UCD1 is not the densest stellar system known. That honor goes to any of a number of nuclear star clusters, which can reach mean surface densities of >10⁵ M_☉ pc⁻² within r_e (Walcher et al. 2005; these are not plotted in Figure 3). Many massive globular clusters are also extremely dense. However, M60-UCD1 is arguably the densest galaxy known in the local universe. Using the M/L_V from Section 3.2.1, its mean effective surface density is Σ = 5.4 × 10⁴ M_☉ pc⁻², a factor of 2.5–3 higher than for M32. The inner component of M60-UCD1, with r_e ∼ 14 pc, has a mean Σ ∼ 9 × 10⁴ M_☉ pc⁻², comparable to that of many nuclear star clusters. The central volume density of M60-UCD1 is not well constrained by the present data.

The object most similar to M60-UCD1 is HUCD1, a Hydra Cluster UCD, which has r_e = 25 pc and M_V = −13.4 (Misgeld et al. 2011), though M59cO (Chilingarian & Mamon 2008) and several Coma Cluster UCDs (Chiboucas et al. 2011) are also similar, if less extreme. It seems likely that ongoing surveys for UCDs will turn up additional objects with properties comparable to M60-UCD1.

The extreme mass, multiple structural components, high metallicity, and possible presence of a central black hole make it unlikely that M60-UCD1 is a star cluster or merged cluster complex. It is most plausible that the object is the tidally stripped remnant of a more massive progenitor galaxy.

Pfeffer & Baumgardt (2013) present new simulations of the formation of UCDs through tidal stripping of nucleated galaxies in a massive, Virgo-like cluster. They show that it is possible to reproduce the sizes and luminosities of typical UCDs. These simulations were not intended to match the most massive UCDs, and hence are not directly comparable to the properties of M60-UCD1. However, a general conclusion from this study is relevant: the nucleus is largely unaffected by the stripping process, so the inner core of the resultant UCD should have similar properties to the original nucleus.

The inner component of M60-UCD1 has M_g = −13.1, g − z = 1.54, and r_e = 14 pc. The size, luminosity, and red color are similar to nuclei in the Virgo galaxies NGC 4379 and NGC 4387 (Côté et al. 2006) and the Fornax Cluster galaxies NGC 1389 and IC 2006 (Turner et al. 2012). These galaxies have M_B ∼ −18 to −19 and stellar masses ∼1–3 × 10¹⁰ M_☉. We conclude that M60-UCD1 could have originated in the tidal stripping of a Virgo galaxy in this luminosity range. Since the present luminosity of M60-UCD1 is M_B ∼ −13.2, we infer that it is a factor of ∼50–200 less massive because of the stripping. The projected distance of M60-UCD1 from the center of M60 (∼6.6 kpc) is consistent with the small pericenter needed for efficient stripping (Pfeffer & Baumgardt 2013). Galaxies in this mass range host significant globular cluster populations (∼40–100; Brodie & Strader 2006) that would also be stripped during UCD formation and might still be detectable in phase space (e.g., Romanowsky et al. 2012).

We caution that UCDs may originate in a biased subset of host galaxies that have been largely destroyed, so it is possible that there is no correspondence between UCD progenitors and a subpopulation of surviving galaxies.

The Pfeffer & Baumgardt (2013) simulations show that extended debris suggestive of tidal stripping becomes challenging to observe after relatively short (∼1 Gyr) timescales, so the lack of evidence for such debris in Figure 2 does not disfavor this scenario. The dynamical friction timescale (Binney & Tremaine 1987) for M60-UCD1 is ∼5 Gyr, so it is plausible that its progenitor was stripped long ago and the remnant has "stalled" at its current radius. In this case no observable tidal tails would be expected.

The stellar density of M60-UCD1 is much higher than expected for dark matter with a standard Navarro–Frenk–White profile (a factor of ∼15 for even for extreme, cluster-scale halo masses; Tollerud et al. 2011). If the galaxy has undergone as much tidal stripping as inferred, this is likely to have strongly modified the dark matter profile; nonetheless, M60-UCD1 is probably the worst UCD in which to search for dark matter. As discussed by Willman & Strader (2012; see also Hilker et al. 2007), the UCDs most likely to show evidence for dark matter are the least massive and most extended UCDs.

Future observations will help constrain the detailed properties of M60-UCD1, including its two-dimensional kinematics and whether it hosts a central supermassive black hole.