Mon. Not. R. Astron. Soc. 356, L17–L22 (2005) doi:10.1111/j.1745-3933.2005.08584.x

Upper limits on central black hole masses of globular clustersfrom radio emission and a possible black hole detectionin the Ursa Minor dwarf galaxy

Thomas J. Maccarone,1� Robert P. Fender1 and Anastasios K. Tzioumis2

1Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, the Netherlands2Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia

Accepted 2004 November 3. Received 2004 November 2; in original form 2004 August 20

ABSTRACTIntermediate-mass black holes (IMBHs) have been alternatively predicted to be quite commonin the centres of globular clusters or nearly impossible to form and retain in the centres ofglobular clusters. As it has been recently shown that radio observations are currently the mostsensitive observational technique for detecting such objects, we have obtained new deep radioobservations of Omega Cen, and have reanalyzed older observations of M 15 in the hope ofconstraining the masses of possible black holes in their centres. In both cases, upper limits ofabout 100 µJy are found at GHz frequencies. We find that if the Bondi–Hoyle accretion ratetruly represents the spherical accretion rate onto a black hole, then the masses of the black holesin the centres of these two galaxies are severely constrained – with mass limits of less thanabout 100 solar masses in both cases. If more realistic assumptions are made based on recentwork showing the Bondi rate to be a severe overestimate, then the data for Omega Cen aremarginally consistent with a black hole of about 1/1000 of the mass of the cluster (i.e. about1000 M�). The data for M 15 are then only marginally consistent with previous reports of a∼2000 solar mass black hole, and we note that there is considerable hope for either detectingthe black hole or improving this upper limit with current instrumentation. Finally, we discussthe possibility that the radio source near the core of the Ursa Minor dwarf spheroidal galaxyis a ∼104-M� black hole.

Key words: accretion, accretion discs – black hole physics – globular clusters: general –globular clusters: individual: Omega Cen – radio continuum: general.

1 I N T RO D U C T I O N

Intermediate-mass black holes (IMBHs) may represent the link be-tween the stellar mass black holes seen in the Milky Way (which areprobably all less massive than 20 M�) and the supermassive blackholes thought to exist in the centres of galaxies [which are mostlymore massive than 106 M�, but see e.g. Greene & Ho (2004) fora sample of active galactic nuclei (AGN) whose black hole massesare likely to be a few hundred thousand solar masses and Fillipenko& Ho (2003) for the discussion of an especially low-mass AGN]. Itis possible that the AGN have grown from ∼100–1000 M� formedfrom core collapses of Population III stars (see e.g. Fryer, Woosley& Heger 2001). On the other hand, another natural place to pro-duce these IMBHs is in the centres of globular clusters (Miller &Hamilton 2002) or young dense star clusters (Portegies Zwart &

�E-mail: tjm@science.uva.nl

McMillian 2002; Portegies Zwart et al. 2004; Gurkan, Freitag &Rasio 2004).

Several methods have been considered for proving the existenceof these IMBHs, but to date there is no conclusive evidence forthe existence of a black hole in the 102–104 M� range. Attemptshave been made to associate some or all of the ultraluminous X-ray (ULX) sources with IMBHs on various bases – X-ray spectrashowing cool discs yet high luminosities compared to what wouldbe expected from a stellar mass black hole (e.g. Miller, Fabian &Miller 2004, quasi-periodic oscillations (QPOs) similar to thoseobserved from stellar mass black holes, except at a frequency anorder of magnitude lower (Strohmayer & Mushotzky 2003), andthe association of one of the brightest ULXs (in fact the same oneshowing the low-frequency QPO) with a young dense star clusterwhich is a plausible birth location for this source (Portegies Zwartet al. 2004), and a plausible scenario for the evolution of an X-raybinary with such a black hole has been laid out as well.

Some work has suggested that gravitational radiation recoilshould lead to the ejection, rather than merger of unequal mass black

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L18 T. J. Maccarone, R. P. Fender and A. K. Tzioumis

holes (e.g. Redmount & Rees 1989; Favata, Hughes & Holz 2004;Merritt et al. 2004; Madau & Quataert 2004), which would suggestthat build-up of black holes through mergers in the centres of globu-lar clusters should be ineffective. The most recent work (Favata et al.2004) suggests that the recoil kicks should be of the order of 10–100 km s−1; because several of the Milky Way’s globular clustershave escape velocities from their cores as large as 50–100 km s−1

(Gnedin et al. 2002), it is plausible that some clusters can grow blackholes through mergers, although the fraction of such clusters mightbe rather small (see e.g. Merritt et al. 2004). Newtonian dynamicalinteractions may also eject black holes by releasing gravitationalpotential energy when binaries are hardened (Portegies Zwart &McMillan 2000). However, these calculations are usually simpli-fied, and neglect factors such as the possible effects of gas, whichcan be quite important, for example, in the mergers of supermassiveblack holes in galaxies (e.g. Escala et al. 2004). Given that whetherglobular clusters can grow IMBHs is an important theoretical ques-tion, attempts to search for observational signatures of such objectsare well motivated.

Attempts to identify signatures of the influence of a black holeon stellar orbits have a rather spotty record. Claims have been madefor IMBHs in the globular cluster G1 in M 31 and in M 15 inour own Galaxy (Gebhardt, Rich & Ho 2002; van der Marel et al.2002; Gerssen et al. 2002), but the statistical significance of thesedetections is less than 3σ , and more sophisticated treatments ofstellar dynamics found that the mass concentration observed at thecentre of M 15 could be explained by a concentration of heavystellar remnants (i.e. neutron stars and white dwarfs – Baumgardtet al. 2003; Gerssen et al. 2003). In fact, it has been argued thatmany globular clusters may simply not have enough stars to probethe gravitational potential well enough ever to conclusively provethe existence of a black hole (Drukier & Bailyn 2003), motivatingother search techniques.

Long ago it was suggested that black holes in the centre of globularclusters should accrete gas, and perhaps produce observable X-rayemission (Bahcall & Ostriker 1975). Their original suggestion thatIMBHs should dominate the X-ray emission has been proven falseby the detection of orbital periods and type I X-ray bursts (see Liu,van Paradijs & van den Heuvel and references within); however, thebasic idea that an accreting black hole should be detectable fromits X-ray emission has been re-stimulated by the capability of theChandra X-ray Observatory to resolve out sensitively the X-rayemission from the very centre of globular clusters. Searches havebeen made for X-ray emission from the accretion onto possible cen-tral black holes, but with only upper limits found (Grindlay et al.2001; Ho, Terashima & Okajima 2003). Unfortunately, these upperlimits are not currently particularly constraining of theoretical mod-els, and they already closely approach the limits of current X-rayinstrumentation.

Recent advances in our understanding of accretion and its relationwith relativistic jet formation indicate that the most sensitive wave-band for detecting low-luminosity IMBHs is, in fact, the radio. Asthe luminosity of accretion on to a black hole decreases, the ratio ofradio to X-ray power increases (Gallo, Fender & Pooley 2003); theratio of radio to X-ray power also increases with increasing blackhole mass (Falcke & Biermann 1996, 1999; Merloni, Heinz & DiMatteo 2003, hereafter MHD03; Falcke, Kording & Markoff 2004,hereafter FKM04). Furthermore, as accretion theory is now sug-gesting that the Bondi & Hoyle (1944) rate overestimates the actualaccretion rate by 2–3 orders of magnitude (e.g. Perna et al. 2003), theX-ray luminosities from accretion of the interstellar medium (ISM)by IMBHs in globular clusters are likely to be well below detection

limits of currently existing X-ray observatories, but given the pre-diction of Miller & Hamilton (2002), that the black holes should beabout 0.1 per cent of the total cluster mass, the radio luminositiesof the brightest cluster central black holes may be detectable withexisting instrumentation (Maccarone 2004). The idea of constrain-ing the masses of black holes based on combined measurements oftheir X-ray and radio emission has also been put forth by Merloni(2004) in the context of high-redshift AGN, where dynamical mea-surements are also not likely to be possible for quite some time. Inthis paper, we present the upper limits on the radio luminosity ofthe nucleus of Omega Centauri from an Australian Telescope Com-pact Array (ATCA) observation, and reconsider the implications ofpast upper limits on radio emission from Very Large Array (VLA)searches for pulsars, and consider the implications of these resultsfor the possible masses of central black holes. We also apply thesame methodology in order to discuss the possibility that a sourcefound by the NRAO VLA Sky Survey (NVSS) may be an IMBHnear the centre of the of the Ursa Minor dwarf spheroidal galaxy.

2 P R E D I C T E D L U M I N O S I T I E S

The method for predicting the radio and X-ray luminosities of anaccreting black hole in a globular cluster was set forth in Maccarone(2004). There it was shown that radio emission provides a muchmore sensitive means of detecting low-luminosity, intermediate (orhigher) mass black holes, as have been speculated to exist in globularclusters. Here we briefly review the assumptions and summarize theresults as follows.

(i) Black hole mass of 1/1000 of the stellar mass of the globularcluster (Miller & Hamilton 2002).

(ii) Gas density of 0.15 H cm−3, approximately the value esti-mated from pulsar dispersion measures in M 15 and 47 Tuc, andexpected from stellar mass loss (Freire et al. 2001).

(iii) Accretion at 0.1–1 per cent of the Bondi rate, due to discwinds (e.g. Blandford & Begelman 1999) and/or convection (e.g.Quataert & Gruzinov 2000), calculated from numerical simula-tions of low-luminosity flows (e.g. Hawley, Balbus & Stone 2001;Igumenshchev, Nrayan & Abramowicz 2003), and constrained byobservations of low-luminosity AGN in elliptical galaxies and thelack of observations of isolated neutron stars accreting from the ISM(Perna et al. 2003 and references within). The low end of this rangeis in good agreement with the constraints on the accretion rate inSagittarius A∗, derived from millimetre polarization measurements(see e.g. Bower et al. 2003).

(iv) A radiative efficiency in the X-rays of

η = 0.1

(1 + A2

2L tot− A

√A2

4L2tot

+ 1

L tot

), (1)

where A is a constant to be fitted from observations (and being largerwhen the kinetic power of the jet is a larger fraction of the total ac-cretion power), and Ltot is the radiative plus kinetic luminosity ofthe system in Eddington units, as used by Fender, Gallo & Jonker(2003) to explain the L X–L R correlation observed by Gallo, Fender& Pooley (2003), with the idea being that enough kinetic poweris pumped into a jet to make the accretion flow radiatively ineffi-cient (see also, e.g. Malzac, Merloni & Fabian 2004). Whether theradiative inefficiency is due to mass and energy loss into a jet oralso partially due to advection into the black hole (e.g. Ichimaru1977; Narayan & Yi 1994) is not clearly established by these rela-tions, and does not affect the results presented here. We have setA = 6 × 10−3 for these calculations, which is based on a

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Upper limits on central black hole masses L19

conservative estimate of the jet power. This relation is used to con-vert a calculated accretion rate (from the assumed fraction of theBondi–Hoyle rate) into a calculated X-ray luminosity.

(v) The Fundamental Plane relationship among X-ray luminos-ity, radio luminosity and black hole mass of Merloni et al. (2003),parameterized for convenient applications to Galactic globularclusters:

F5 GHz = 10

(Lx

3 × 1031 erg s−1

)0.6 (MB H

100 M�

)0.78

×(

d

10 kpc

)−2

µJy. (2)

We note that the relation found by Falcke et al. (2004), which con-sidered only flat spectrum, low-luminosity radio sources such asthose we expect to see in the centres of dwarf galaxies or globularclusters, is consistent with this relation within the uncertainties.

We then take the numerical data for the masses of the globu-lar clusters and distances from the Harris catalogue (Harris 1996),compute the X-ray luminosity under the assumptions given abovefor the gas density, fraction of the Bondi–Hoyle accretion rate, andradiative efficiency, and convert the X-ray luminosity into the radioluminosity using the Fundamental Plane relation. The typical fluxvalues predicted are less than 40 µJy, even with the accretion rateset to 10−2 of the Bondi rate, with the exception of that for OmegaCen, where the cluster is quite large and reasonably close (see thetabulation of results in Maccarone 2004).

3 O B S E RVAT I O NA L DATA

Omega Cen was observed with ATCA simultaneously at 4.8 and8.6 GHz over 12 h on 2004 May 8. The array was in the ‘6C’ con-figuration, with baselines up to 6 km. Data reduction was performedwith the MIRIAD software package (Sault, Teuben & Wright 1995).Flux and bandpass calibration was performed on PKS 1934 − 638,and the phase calibrator was 1315 − 46 (SUMSS J131829-462034).No source was detected (limits given below).

We also consider the upper limits on the flux from a central blackhole in M 15 based on the deep VLA observations of Kulkarniet al. (1990) and Johnston, Kulkarni & Goss (1991). These obser-vations were made at 1.4 GHz and reached a noise level of 43 µJy.Finally, we also consider the NVSS detection of a 7.1-mJy radiosource within the 1σ error box of the centre of the Ursa Minordwarf spheroidal galaxy.

4 C O M PA R I S O N O F O B S E RVAT I O N SW I T H P R E D I C T I O N S

4.1 Omega Cen

The ATCA data showed no sources within 10 arcsec of the centre ofOmega Cen. In naturally weighted maps, the rms noise was 57 µJyat 8.6 GHz, and 38 µJy at 4.8 GHz. We assumed a flat (i.e. f ν ∼constant) spectrum (as is likely for a low-luminosity accreting blackhole) in order to combine the data sets and reduce the noise. Afterdoing so, the noise level becomes 32 µJy, giving a 3σ upper limitof just under 100 µJy. For what seems to be the most likely set ofparameters – a gas density which is the same as that in M 15 and47 Tuc, an accretion rate of 0.1 per cent of the Bondi rate, and ablack hole mass of 0.1 per cent of the stellar mass of the cluster – theprediction of Maccarone (2004), using the methodology outlined in

Section 2, was that the radio flux should be about 0.15 mJy. That nodetection is made while the predicted level value is 4.5σ should notas yet be taken as evidence against the hypothesis that IMBHs canform in globular clusters. There are several factors of ∼ a few thatcould be contributing scatter in the theoretical predictions – mostnotably the gas density and temperature, but also the scatter in theFundamental Plane relations of MHD03 and FKM04, and possiblescatter in the black hole mass–globular cluster mass relation. We cansay with reasonable confidence that there cannot be both a black holewith a mass of 1/1000 of the cluster mass and accretion at ∼ 10−2 ofthe Bondi rate (considered to be the likely upper limit of the accretionrate in the work of Perna et al. 2003), as the predicted level of radioemission in that case would be more than 20 times the observed level.Still, in order to make a strong statement about whether Omega Cencontains an IMBH, we would need either a detection, or an upperlimit about 10 times lower than the current upper limit. BecauseOmega Cen is far south of the declinations which can be reachedwith the VLA, this does not seem likely unless and until the SquareKilometer Array is built in the Southern hemisphere.

4.2 M 15

Given that Omega Cen is by far the best candidate in the South-ern hemisphere for detecting an IMBH in a globular cluster, andthat it seems unlikely that it will provide useful limits without newinstrumentation, we turn our attention to the Northern hemisphere.Because M 15 had been claimed to show evidence for a ∼2000-M�black hole, and has been the subject of deep radio observations tosearch for pulsars, we consider it as well. The 3σ upper limit on theflux from the centre of M 15 is 130 µJy (Johnston et al. 1991). Asthe nearest ‘bright’ source, AC 211, is about 2.2 arcsec away, andthe data was taken in VLA A configuration, with 1.4-arcsec resolu-tion, confusion is not likely to be a major problem here, as can beverified from looking at the image of M 15 presented in Johnstonet al. (1991). Furthermore, M 15 is a core-collapse globular clus-ter, so its central position is quite well defined. We again assumethat the putative IMBH should have a flat spectrum, and that henceits radio flux should be below about 130 µJy at 5 GHz as well asat 1.4 GHz. This flux level is already well above that predicted inMaccarone (2004), even for accretion at 10−2 of the Bondi rate.On the other hand, the predicted radio luminosity in the model ofMaccarone (2004) scales roughly as M3.2, and because the claimsexist for a black hole much larger than 1/1000 of the cluster mass(i.e. a 2500 M� black hole has been claimed, while 1/1000 thecluster mass is only about 440 M�), we can come close to mak-ing a useful prediction about the black hole mass with these data.These data rule out a black hole more massive than about 600 M�,provided that the systematic factors discussed above are small.

Because the gas density is well measured in M 15 from pulsars,one source of uncertainty can be eliminated. Given that a large frac-tion of the scatter in the MHD03 relation probably comes from theinclusion of some steep spectrum radio sources (which any blackhole accreting from the ISM of a globular cluster is very unlikelyto be), and that the more restrictive sample of FKM04 shows some-what less scatter despite imposing a theoretical mass–X-ray–radiorelation rather than fitting to the data, it seems likely that even thisscatter is not too large. We are cautious about interpreting the cur-rent upper limits as evidence against a ∼ 2000 M� black hole in M15, but we note that this upper limit, unlike the upper limit on theradio luminosity of Omega Cen, can easily be reduced by lookingat higher frequency (where the VLA is more sensitive), for longerintegrations. A factor of 5 improvement of the upper limit could

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L20 T. J. Maccarone, R. P. Fender and A. K. Tzioumis

be obtained in a quite reasonable 4-h integration, or in a few yearswith a much shorter integration from the Expanded VLA (EVLA).The project to expand the MERLIN array should also result in thecapability to reach similar flux limits.

4.3 Ursa Minor dwarf

The Ursa Minor dwarf spheroidal galaxy is one of the closest dwarfgalaxies to the Earth, at a distance of about 66 kpc. It is one ofthe largest of the Milky Way’s dwarf spheroidal galaxies in termsof mass, with an estimated mass of 2.3 × 107 M� (see Mateo1998 and references within for the properties of the Milky Way’sdwarf galaxies, including all the data on dwarf spheroidal galaxies’masses and distances used in this paper). The NVSS survey (Condonet al. 1998) indicates the presence of a 7.1 mJy source 20 arcsecfrom the centre of the Ursa Minor dwarf. While this may seemrather far from the centre of the galaxy, we note that this is only0.02 core radii for this galaxy, and only 1σ from the measuredcentral position of the galaxy, using the method of Webbink (1985) toestimate the uncertainties. We note that this is an inherent uncertaintyin the centre of the galaxy when measured from stellar light due tothe finite number of bright stars.

Recent work has suggested that the dwarf spheroidal galaxiesmight actually have black holes substantially larger than 1/1000 oftheir total mass, if they are the remnants of Population III stars andthey began accreting rapidly soon after forming, and contributed toreionizing the universe (Ricotti & Ostriker 2004). Given this sug-gestion, we have repeated the calculations of Maccarone (2004), forblack holes of 0.1, 1 and 10 per cent of the mass of the galaxy. Weretain the assumption made for globular clusters that the gas densityshould be about 0.15 H cm−3. This is likely to be an overestimate, asthe dwarf spheroidal galaxies are less dense than globular clustersand their gas should be more spread out, but we note that at thepresent, the constraints on gas densities of dwarf spheroidal galax-ies are rather poor (see e.g. Gallagher et al. 2003). As we find thatfor a fraction of the Bondi rate of 10−2 the radio flux levels are allsubstantially higher than could possibly be allowed by the obser-vations, we make calculations for only 10−3 of the Bondi rate. Wefind that the three predicted radio fluxes are 1.7 mJy, 510 mJy and97 Jy, respectively. The predicted X-ray luminosities are 7 × 1034

5 × 1037 and 1.5 × 1040 erg s−1. We note that Einstein observedthe Ursa Minor dwarf for about 5.5 ks, and detected no sources withpositions coincident with the NVSS source. The upper limit of thoseobservations was ∼ 1035 erg s−1, if at the distance of the Ursa Minorgalaxy (Markert & Donahue 1985; see also ROSAT upper limits at asimilar flux level from Zang & Meurs 2001). These data are thus allin accord with a black hole of about 40000 M�; if the gas densityis ∼30 times lower than in the globular clusters, then a black holemass about 4–5 times as large can be accommodated.

4.3.1 The other dwarf spheroidal galaxies

To determine the false source probability, we have also looked at theNVSS catalogue around the centre of the Ursa Minor galaxy witha search radius of 20 arcmin. In this region, 20 sources are found,leaving a spurious source probability of about 5 per cent under theassumption that any source within 1 arcmin of the centre of thegalaxy (the 3σ error circle) would be a candidate central IMBH. Weargue that there are few hidden trials here by noting that we wouldnot expect to detect most of the other Milky Way dwarf spheroidals;scaling the flux from the Ursa Minor dwarf galaxy, the only other

dwarf spheroidal galaxies massive enough and near enough andfar enough north to be detected in the NVSS are those in Draco,Sagittarius, Sextans and Fornax, and none of these are detected.The other four Milky Way dwarf spheroidal galaxies are those inSculptor and Carina (which are too far south to have been observedby the VLA); and Leo I and Leo II, which are at distances of 205and 250 kpc, respectively, and have masses of about half the massand the same mass of the Ursa Minor, respectively, leading to fluxesexpected to be about 100 times and about 15 times lower than UrsaMinor’s, assuming that L ∝ M3.2, as explained above.

Draco’s flux would be expected to be a factor of about 2 lowerthan that of the Ursa Minor dwarf because of its greater distance (82vs 66 kpc), while the Sextans galaxy is further than Draco (86 kpc)and less massive (by about 20 per cent), their expected fluxes arethus pushed into the region where the NVSS suffers from seriousincompleteness effects. The expected flux of Sagittarius would bequite hard to determine; if the mass is assumed just to be the mass inM 54, the globular cluster-like object thought to be the core of theSagittarius dwarf galaxy, then the calculation of Maccarone (2004) isvalid and the expected flux should be only a few µJy. If, on the otherhand, one scales from the mass of the dwarf, then the fact that thisgalaxy is in the process of being tidally destroyed makes estimatingits mass quite difficult. Furthermore, the gas in the Sagittarius galaxyis likely to have been stripped during its ongoing encounter with theMilky Way. The Fornax dwarf is more problematic; assuming thatgalaxy mass and distance are the only important parameters, thisgalaxy should actually have a brighter radio source than does UrsaMinor by a factor of about 3 (it is about three times as massive andabout twice as far away). No detection was made in the NVSS withinabout 2 arcmin of the centre of the galaxy, but a factor of 3 in flux iswell within the uncertainties of our calculations. Furthermore, wenote that the hidden trial problem is less severe than 3 (to accountfor the reasonable chance of detecting a radio source in the centresof Ursa Minor, Draco & Fornax) times 5 per cent (the probability ofdetecting a source by chance in the 3σ error circle of the Ursa Minorgalaxy); the Ursa Minor galaxy has an error radius for its centreabout ten times larger than those of Fornax and Draco, the two otherbest candidates (largely because it has a much lower core stellardensity, so a small number of bright stars can affect its centre-of-light location much more); the false detection probabilities in thosegalaxies are close to zero, so searching the other two galaxies addsalmost nothing to the probability of finding a source by chance. Wethus believe that the central IMBH hypothesis is more likely thanthe background AGN hypothesis, but only marginally so, and thatthe best way to make this identification more secure is by localizingit well enough to identify a unique optical counterpart to the radiosource, and then to take a spectrum of that object.

5 C O N C L U S I O N S

At present, we have shown merely that stricter observational up-per limits can be placed on the maximum mass of a possible ac-creting black hole in globular clusters through measurements madein the radio than through measurements made in the X-rays. Un-der the assumptions which most strictly constrain the presence ofIMBHs without violating other observational constraints, we canrule out the presence of a black hole in Omega Cen larger than about20 M� – that is we can rule out the presence of an IMBH. Undermore realistic assumptions, though, we cannot rule out a black holeof a few hundred solar masses.

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Upper limits on central black hole masses L21

One other additional caveat is that that accreted gas might havesome angular momentum and hence might form an accretion disc.In this case, gas might pile up at some outer radius, and then undergooutbursts, much like the soft X-ray transients in the Milky Way. Inthis case, the assumption we have made, that the instantaneous accre-tion rate equals the time-averaged accretion rate, would be violated,and substantially lower X-ray and radio luminosities than those wehave predicted might be expected most of the time. However, theinterstellar gas would have to cool down and condense in order toform an accretion disc. This seems to be plausible in galaxies suchas the Milky Way, where there may exist a thin disc as a remnantfrom a past strong accretion episode (Nayakshin 2003), but seemsless likely in a globular cluster where it is thought that any IMBHswould have grown through mergers rather than accretion.

Upper limits which would truly constrain the theoretical sugges-tions of Miller & Hamilton (2002) are likely to come only with theSquare Kilometer Array (SKA), or possibly from extremely deepobservations made by the High Sensitivity Array or the VLA. InMaccarone (2004), the 15 best candidates for detecting an IMBHthrough its radio emission are listed; 13 of these are in the SouthernHemisphere, including the six best candidates. If the detection ofIMBHs in globular clusters is considered a key science goal of SKA,then this provides a strong justification for locating the array southof the equator.

One of the key causes for systematic uncertainty in these mea-surements is the uncertainty in the gas density in any globular clusteror gas-poor galaxy which does not contain a large number of pul-sars. While it is comforting to note that the two globular clusters thatcontain large number of pulsars have gas densities in close agree-ment with the predictions of stellar mass-loss (Freire et al. 2001),passages through the plane of the Galaxy can strip the clusters ofmost or all of their gas, so this will not necessarily be the case in allstellar systems. In the ROSAT era, X-ray images were obtained oftwo of the Milky Way’s dwarf spheroidal galaxies in order to findbackground quasars, primarily in order to establish an extragalacticreference frame for astrometric studies of these galaxies, but it wasalso noted that these objects might be useful as probes of the ISM inthe dwarf galaxies, in much the same way that high-redshift quasarsare used to study the Lyman α forest (Tinney, Da Costa & Zinnecker1997), but it seems that this technique as not yet produced any de-tections of gas. If this technique can be taken better advantage of,then it should be possible to convert upper limits on radio emissionfrom the cores of the dwarf galaxies directly into mass upper limitsin the same way as we have done for the globular clusters; at thepresent time, it would be premature to do so, given the greater uncer-tainties in the dwarf galaxies’ gas densities. It should be noted thatone dwarf galaxy, that in Sculptor (i.e. the closest one) does have asubstantial gas content (Bouchard, Carignan & Maschenko 2003),with a central density at least 10−3 cm−2 in neutral gas, and higherconcentrations off-centre, as the gas distribution is asymmetric. Thesuggestion above that the gas density might be about 1/30 of that inthe globular clusters is thus in reasonable agreement with the datafor the one galaxy with the best observational constraints.

We have found suggestive evidence for an IMBH in the UrsaMinor dwarf spheroidal galaxy. This should be followed up first inX-rays, to determine whether its ‘radio mass’, to use the terminologyof Merloni et al. (2003), is of the order of 104 M� before a strongclaim of a detection is made, and an optical counterpart should besearched for as well. As the gas density is probably a lower in thisgalaxy than in a typical globular cluster, the true mass may be higherthan suggested by the observed radio luminosity in the context of ourcalculation, and hence closer to the 106 M� suggested by Ricotti &

Ostriker (2004). If an X-ray source in the 1033–1035 erg s−1 range isfound, the case for an IMBH would be strengthened dramatically,and X-ray luminosities in the lower end of that range would be mostlikely to indicate the highest-mass black holes.

AC K N OW L E D G M E N T S

We thank Dave Meier for pointing out that SKA could be espe-cially useful for future work in this area. We also are grateful tothe referee Heino Falcke for useful suggestions which have helpedimprove the clarity and content of this paper, and to Eva Grebel forbrief but useful discussions of the gas content of dwarf spheroidalgalaxies.

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