Radio Emission from the Intermediate-mass Black Hole in the Globular
  Cluster G1
Radio Emission from the Intermediate-mass Black Hole in the
Globular Cluster G1
James S. Ulvestad
National Radio Astronomy Observatory
P.O. Box O, Socorro, NM 87801
julvesta@nrao.edu
Jenny E. Greene1
Department of Astronomy, Princeton University
Princeton, NJ
jgreene@astro.princeton.edu
Luis C. Ho
The Observatories of the Carnegie Institute of Washington
813 Santa Barbara St., Pasadena, CA 91101
lho@ociw.edu
ABSTRACT
We have used the Very Large Array (VLA) to search for radio emission from
the globular cluster G1 (Mayall-II) in M31. G1 has been reported by Geb-
hardt et al. to contain an intermediate-mass black hole (IMBH) with a mass of
∼ 2 × 104 M⊙. Radio emission was detected within an arcsecond of the cluster
center with an 8.4 GHz power of 2× 1015 W Hz−1. The radio/X-ray ratio of G1
is a few hundred times higher than that expected for a high-mass X-ray binary in
the cluster center, but is consistent with the expected value for accretion onto an
IMBH with the reported mass. A pulsar wind nebula is also a possible candidate
1Hubble Fellow
http://arxiv.org/abs/0704.1458v1
– 2 –
for the radio and X-ray emission from G1; future high-sensitivity VLBI observa-
tions might distinguish between this possibility and an IMBH. If the radio source
is an IMBH, and similar accretion and outflow processes occur for hypothesized
∼ 1000 M⊙ black holes in Milky Way globular clusters, they are within reach of
the current VLA and should be detectable easily by the Expanded VLA when it
comes on line in 2010.
Subject headings: accretion, accretion disks — galaxies: individual (M31) —
globular clusters: individual (G1) — radio continuum: galaxies
1. Introduction
Most galaxies with massive spheroidal components appear to harbor central black holes
(BHs), with masses ranging from a few ×106 M⊙ to over 10
9 M⊙. These BH masses are
well correlated with both the luminosity and the velocity dispersion of the galaxy spheroid
(Gebhardt et al. 2000; Ferrarese & Merritt 2000), implying that the formation of the central
BHs is connected intimately with the development of the galaxy bulges. However, a bulge
is not necessarily a prerequisite for a massive BH. On the one hand, neither the late-type
spiral galaxy M33 (Gebhardt et al. 2001) nor the dwarf spheroidal galaxy NGC 205 (Valluri
et al. 2005) shows dynamical evidence for a massive BH. On the other hand, both the Sd
spiral galaxy NGC 4395 and the dwarf spheroidal galaxy POX 52 contain active BHs with
masses ≈ 105 M⊙, although neither contains a classic bulge (Fillipenko & Ho 2003; Barth
et al. 2004). Greene & Ho (2004, 2007) find that optically active intermediate-mass BHs
(IMBHs, MBH < 10
6 M⊙), while rare, do exist in dwarf galaxies, but optical searches are
heavily biased toward sources accreting at high Eddington rates. Alternate search techniques
are needed to probe the full demographics of IMBHs.
Direct dynamical detection of IMBHs is currently impossible outside of the Local Group.
However, Gebhardt, Rich, & Ho (2002, 2005) have found dynamical evidence for an excess
dark mass of (1.8±0.5)×104 M⊙ at the center of the globular cluster G1 in M31; the evidence
for this IMBH was questioned by Baumgardt et al. (2003), but supported by the improved
data and analysis of Gebhardt et al. (2005). The physical nature of the central dark object
is difficult to prove: it could be either an IMBH or a cluster of stellar remnants. While
the putative presence of a BH in a globular cluster center may appear unrelated to galaxy
bulges, the properties of G1, including its large mass, high degree of rotational support, and
multi-aged stellar populations, all suggest that G1 is actually the nucleus of a stripped dwarf
galaxy (Meylan et al. 2001). Most intriguingly, the inferred BH mass for G1 is about 0.1%
of the total mass, consistent with the relation seen for higher mass BHs, and consistent with
– 3 –
predictions based on mergers of BHs (Miller & Hamilton 2002) or stellar mergers in dense
clusters (Portegies Zwart & McMillan 2002).
Recently, Pooley & Rappaport (2006) reported an X-ray detection of G1, with a 0.2–
10 keV luminosity of LX ≈ 2 × 10
36 ergs s−1. Although this may represent accretion onto
a central BH, it is within the range expected for either accretion onto an IMBH or for a
massive X-ray binary. Unfortunately, the most accurate X-ray position determined recently
by Kong (2007) does not have sufficient accuracy to determine whether the X-ray source
is located within the central core of G1, which would help distinguish between these two
possibilities. The radio/X-ray ratio of G1 provides an additional test of the nature of the
G1 X-ray source. As pointed out by Maccarone (2004) and Maccarone, Fender, & Tzioumis
(2005), deep radio searches may be a very effective way to detect IMBHs in globular clusters
and related objects, since, for a given X-ray luminosity, stellar mass BHs produce far less
radio luminosity than supermassive BHs. The relation between BH mass, and X-ray and
radio luminosity empirically appears to follow a “fundamental plane,” in which the ratio of
radio to X-ray luminosity increases as the ∼ 0.8 power of the BH mass (Merloni, Heinz, & di
Matteo 2003; Falcke, Körding, & Markoff 2004). For an IMBH mass of 1.8× 104 M⊙ in G1,
one thus would expect a radio/X-ray ratio about 400 times higher than for a 10 M⊙ stellar
BH. In this paper, we report a deep Very Large Array (VLA) integration on G1 and a radio
detection that apparently confirms the presence of an IMBH whose mass is consistent with
that found by Gebhardt et al. (2002, 2005).
2. Observations and Imaging
We obtained a 20-hr observation of G1 using the VLA in its C configuration (maximum
baseline length of 3.5 km) at 8.46 GHz. The observation was split into two 10-hr sessions, one
each on 2006 November 24/25 and 2006 November 25/26. Each day’s observation consisted
of repeated cycles of 1.4 minutes observation on the local phase calibrator J0038+4137 and
6 minutes observation on the target source G1. In addition, each day contained two short
observations of 3C 48 (J0137+3309) that were used to calibrate the flux density scale to that
of Baars et al. (1977). Thus, the total integration time on G1 was 14.1 hr. We also obtained
a total of 9.5 hr of observing in C configuration at 4.86 GHz on 2007 January 13/14 and
2007 January 14/15, using a similar observing strategy, and achieving a total of 7.3 hr of
integration on source.
All data calibration was carried out in NRAO’s Astronomical Image Processing System
(Greisen 2003). Absolute antenna gains were determined by the 3C 48 observations, then
transferred to J0038+4137, which was found to have respective flux densities of 0.52 mJy and
– 4 –
0.53 mJy at 8.4 and 4.9 GHz. In turn, J0038+4137 was used to calibrate the interferometer
amplitudes and phases for the target source, G1. Erroneous data were flagged by using
consistency of the gain solutions as a guide and by discarding outlying amplitude points.
The VLA presently is being replaced gradually by the Expanded VLA (EVLA), which
includes complete replacement of virtually all the electronic systems on the telescopes. Since
antennas are refurbished one at a time, the VLA at the time of our observations consisted
of 18–20 “old” VLA antennas and 6 “new” (actually, refurbished) EVLA antennas, having
completely different electronics systems. Although all antennas were cross-correlated for our
observations, we found subtle errors in some of the EVLA data. Thus, to be conservative,
we discarded the data from all EVLA antennas except for 3 antennas that were confirmed
to work very well on 2006 November 24/25.
The radio data were Fourier transformed and total-intensity images were produced in
each band, covering areas of 17′×17′ at each frequency.2 These images were CLEANed in
order to produce the final images. At 8.4 GHz, the rms noise was 6.2 µJy beam−1 for a beam
size of 2.′′94×2.′′72; at 4.9 GHz, the noise was 15.0 µJy beam−1 for a beam size of 5.′′09×4.′′43.
A few radio sources with strengths of hundreds of microjansky to a few millijansky were
found in the images, but we discuss only G1 in this Letter.3 At 8.4 GHz, an apparent source
with a flux density of 28 ± 6 µJy [corresponding to 2 × 1015 W Hz−1 for distance modulus
of (m − M) = 24.42 mag (Meylan et al. 2001)] was found approximately one arcsecond
from the G1 optical position reported by Meylan et al. (2001); this radio source has J2000
coordinates of α = 00h32m46.54s, δ = 39◦34′39.2′′. Figure 1 shows our 8.4 GHz image of the
20′′ by 20′′ region centered on G1; this image includes a 1 sigma error circle of 1.′′5 radius
for the X-ray position found by Kong (2007). The radio position has an estimated error of
0.′′6 in each dimension (not shown in the figure), derived by dividing the beam size by the
signal-to-noise ratio.4
The a priori probability of finding a 4.5 σ noise spike or background source so close
to G1 is quite small, as indicated by the lack of any other contours of similar strength in
2The image areas covered were much larger than the primary beams of the individual VLA antennas, in
order to provide the best possible subtraction of confusing sources.
3The strongest nearby radio source is a 1 mJy (at 8.4 GHz) object located approximately 100′′ from G1;
this object and the other weak sources might be X-ray binaries or supernova remnants if located at the
distance of M31, but their numbers also are consistent with the possibility that some could be background
extragalactic sources.
4Transfer of the phase from the local phase calibrator makes an insignificant contribution to the radio
source position error, relative to the error imposed by the limited signal strength.
– 5 –
Figure 1. If we hypothesize that there are 9 independent beams (roughly 8′′ by 8′′) within
which a source would be considered to be associated with G1, then the probability of a 4.5 σ
noise point close to G1 is less than 10−4. Similarly, the expected density of extragalactic
radio sources at 28 µJy or above is 0.25 arcmin−2 (Windhorst et al. 1993), or 4 × 10−3 in a
box 8′′ on a side, making it unlikely that we have found an unrelated background source. In
order to search for possible data errors that might cause a spurious source, we have subjected
our data set to additional tests, imaging data from the two days separately, and also imaging
the two different intermediate frequency channels separately. The G1 radio source remains
in the images made from each data subset, with approximately the same flux density and
position. The overall significance is reduced by 21/2 to approximately 3 σ in each image made
with about half the data, as expected for a real source with uncontaminated data. Other
2.5 σ–3 σ sources appear in the central 20′′ box in some subsets of half the data, consistent
with noise statistics, but none is above the 3.5 σ level in the full data set. Thus, all tests
indicate that the detection of G1 is real, and we will proceed on that basis for the remainder
of this paper. At 4.9 GHz, we find no detection at the G1 position, but the much higher
noise level provides us only with very loose constraints on the source spectrum (see below).
3. Origin of the G1 Radio Emission
Merloni et al. (2003) and Falcke et al. (2004) have quantified an empirical relation (or
“fundamental plane”) among X-ray and 5 GHz radio luminosity and BH mass; we use the
Merloni et al. (2003) relation LR ∝ L
M0.78
. Merloni et al. (2003) analyzed this relation
in the context of accretion flows and jets associated with massive BHs. One might expect
some general relationship among these three quantities, if an X-ray-emitting accretion flow
onto a massive BH leads to creation of a synchrotron-emitting radio jet, with the detailed
correlation providing some insight into the nature of that flow. By comparing the empirically
determined relation with expectations from theoretical models, Merloni et al. (2003) deduced
that the data for BHs emitting at only a few percent of the Eddington rate are consistent with
radiatively inefficient accretion flows and a synchrotron jet, but inconsistent with standard
disk accretion models.
Maccarone (2004) scaled the fundamental-plane relation to values appropriate for an
IMBH in a Galactic globular cluster; we rescale their equation here to find a predicted radio
flux density of
S5 GHz = 52
1036 ergs s−1
)0.6 ( MBH
104 M⊙
)0.78 ( d
600 kpc
µJy . (1)
– 6 –
Using the previously cited X-ray luminosity and BH mass for G1 and our adopted
distance modulus, this predicts a 5 GHz flux density of 77 µJy for G1. However, taking into
account the 30% uncertainty in the IMBH mass, the unknown spectral index of the radio
emission, and the dispersion of 0.88 in logLR (Merloni et al. 2003), the predicted 8.4 GHz
flux density for G1 is in the range of tens to a few hundred microjansky. Thus, our radio
detection of 28 µJy at 8.4 GHz is consistent with the predictions for a 1.8× 104 M⊙ IMBH,
but strongly inconsistent with a 10 M⊙ BH. Since neutron star X-ray binaries in a variety
of states have radio/X-ray ratios much lower than BH X-ray binaries (Migliari & Fender
2006), and thus another 2 orders of magnitude below the observed value, stellar-mass X-ray
binaries of any type are ruled out as the possible origin of the radio emission in G1.
We can use the radio/X-ray ratio to assess other possible origins for the radio emission.
Here, we use the ratio RX = νLν(8.4 GHz)/LX(2− 10 keV) as a fiducial marker. For G1,
RX ≈ 5× 10
−5, which is considerably lower than RX ≈ 10
−2 that is common to the Galactic
supernova remnant Cas A, low-luminosity active galactic nuclei (supposing G1 might be a
stripped dwarf elliptical galaxy), and most ultraluminous X-ray sources (cf. Table 2 of Neff,
Ulvestad, & Campion [2003], and references therein).
It is of interest to compare the G1 source to various relatives of pulsars as well. For
instance, G1 is within the wide range of both luminosity and radio/X-ray ratio observed
for pulsar wind nebulae (PWNs) (Frail & Scharringhausen 1997), less luminous than the
putative PWN in M81 (Bietenholz, Bartel, & Rupen 2004), but considerably more luminous
than standard pulsars or anomalous X-ray pulsars (Halpern et al. 2005). The 8.4 GHz
luminosity of G1 is similar to that of the magnetar SGR 1806 − 20 about 10 days after its
outburst in late 2004, and the lack of a 4.9 GHz detection would be consistent with the fading
of SGR 1806−20 two months after the outburst (Gaensler et al. 2005). However, there is no
published evidence for a gamma-ray outburst from G1, and the relatively steady apparent
X-ray flux (Pooley & Rappaport 2006) also argues against a transient source. Thus, the
only stellar-mass object that might account for the radio and X-ray emission would be a
PWN; using the scaling law given by Frail & Scharringhausen (1997), we find a likely size
of ∼ 10 milliarcseconds for a PWN radio source in G1, implying that high-sensitivity VLBI
observations could distinguish between a PWN and IMBH origin for the radio emission from
Knowledge of the radio spectrum of G1 could provide more clues to the character of
the radio emission, although either a PWN or an IMBH accretion flow might have a flat
spectrum. In any case, our 5 GHz observation simply is not deep enough. If we choose
a 2 σ upper limit of 30.0 µJy at 4.9 GHz (2 σ chosen since we know the position of the
8.4 GHz source with high accuracy), we derive a spectral index limit of α > −0.12 ± 0.99
– 7 –
(for Sν ∝ ν
+α, 1 σ error in spectral index), which has little power to discriminate among
models.
The X-ray emission from G1 may be due to Bondi accretion on the IMBH, either from
ambient cluster gas or from stellar winds (Pooley & Rappaport 2006). Ho, Terashima, & Okajima
(2003) and Pooley & Rappaport (2006) give approximate relations for the Bondi accretion
on an IMBH in a globular cluster; for an ambient density of 0.1 cm−3, an ambient speed of
15 km s−1 for the gas particles relative to the IMBH, and a radiative efficiency of 10%, the
Bondi accretion luminosity for the G1 IMBH would be ∼ 3× 1038 ergs s−1. The X-ray lumi-
nosity of 2 × 1036 ergs s−1 measured by Pooley & Rappaport (2006) thus implies accretion
at just under 1% of the Bondi rate. Given that LX/LEdd ≈ 10
−6, a more likely scenario is
that G1 accretes at closer to 10% of the Bondi rate but with a radiative efficiency under 1%.
In this context, we note that the radio/X-ray ratio for G1 is logRX > −4.3, which is above
the value of −4.5 used to divide radio-quiet from radio-loud objects (Terashima & Wilson
2003).5 G1 therefore should be considered radio-loud, as inferred for BHs in galactic nuclei
that radiate well below their Eddington luminosities (Ho 2002).
If the globular clusters in our own Galaxy also have central BHs that are 0.1% of their
total masses, and they accrete and radiate in the same way as G1, many would have expected
5 GHz radio flux densities in the 20–100 µJy range; flux densities often would be in the 1–
10 µJy range even for less efficient accretion and radiation (Maccarone et al. 2005). As
Maccarone et al. (2005) summarize, there are few radio images of globular clusters that go
deep enough to test this possibility. Fender (2004) points out that the Square Kilometer
Array (SKA) will be able to test for the existence of IMBHs in many globular clusters.
However, based on our results for G1, we suggest that it is not necessary to wait for the
SKA; the current VLA can reach the hypothesized flux densities with some effort. The
EVLA (Ulvestad et al. 2006), scheduled to be on line in about 2010, will have 40 times the
bandwidth and 6.3 times the sensitivity of the current VLA in the frequency range near
8 GHz. This will enable the EVLA to reach the 1 µJy noise level in approximately 12 hours
of integration, thus probing the range of radio emission predicted by Maccarone et al. (2005)
for many globular clusters.
5A lower limit is given for RX because this quantity traditionally is given in terms of the 2–10 keV
luminosity, whereas logRX = −4.3 would correspond to the value computed for the 0.2–10 keV luminosity
given by Pooley & Rappaport (2006).
– 8 –
4. Summary
We have detected faint radio emission from the object G1, a globular cluster or stripped
dwarf elliptical galaxy in M31. The emission has an 8.4 GHz power of 2× 1015 W Hz−1. As-
suming that the radio source is associated with the X-ray source in G1 (Pooley & Rappaport
2006), the radio/X-ray ratio is consistent with the value expected for an accreting ∼ 2 ×
104 M⊙ BH. Thus, the radio detection lends support to the presence of such an IMBH within
G1. The other possible explanation, a pulsar wind nebula, could be tested by making very
high-sensitivity VLBI observations of G1.
The National Radio Astronomy Observatory is a facility of the National Science Foun-
dation operated under cooperative agreement by Associated Universities, Inc. We thank the
staff of the VLA that made these observations possible. Support for JEG was provided by
NASA through Hubble Fellowship grant HF-01196, and LCH acknowledges support from
NASA grant HST-GO-09767.02. Both were awarded by the Space Telescope Science Insti-
tute, which is operated by the Association of Universities for Research in Astronomy, Inc.,
for NASA, under contract NAS 5-26555. We also thank Dale Frail for useful discussions
about pulsar wind nebulae, and an anonymous referee for useful suggestions.
Facilities: VLA.
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arcsec
10 5 0 -5 -10
Fig. 1.— VLA C configuration 8.4 GHz image of the vicinity of G1 in M31, with contours
at intervals of 0.5 times the r.m.s noise. The lowest contour is at 3 times the noise of
6.2 µJy beam−1, and negative contours are shown dashed; the synthesized beam is shown in
the box in the lower-left corner. The (0,0) point is at a J2000 position of α = 00h32m46.60s,
δ = 39◦34′40.0′′. The 1.′′5-radius circle about this central point represents the 1 σ error in
the X-ray position of G1 (Kong 2007).
	Introduction
	Observations and Imaging
	Origin of the G1 Radio Emission
	Summary
