The Hubble Space Telescope (HST) will allow astronomers to obtain luminosity profiles, rotation curves, and velocity dispersions at angular scales that are an order of magnitude superior to those obtained previously. This enhanced spatial resolution will greatly improve the sensitivity for detecting centrally condensed matter in nearby galactic nuclei including, possibly, black holes

Harms, Richard J.

English

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SPACE TELESCOPE SEARCHES FOR BLACK HOLES IN GALACTIC NUCLEI
RICHARD J. HARMS
Applied Research Corporation
ABSTRACT
The Hubble Space Telescope (HST) will allow astronomers to
obtain luminosity profiles, rotation curves, and velocity dispersions at
angular scales that are an order of magnitude superior to those obtained
previously. This enhanced spatial resolution will greatly improve our
sensitivity for detecting centrally condensed matter in nearby galactic
nuclei including, possibly, black holes.
I. INTRODUCTION
The determination of the total masses and mass distributions of various types of
galaxies has been an active area of astronomical research throughout the past
approximately 15 years. Reviews by Faber and Gallagher (1979) and by Trimble
(1987) summarize many of the results obtained from ground-based observations and
their implications for the existence of black holes in the nuclei of some galaxies.
Many of these observations concentrate on determining the mass content of galaxies
out to large distances from their nuclei in order to estimate their total masses and its
forms. However, other observations have been directed at determining the mass and
luminosity distributions of galaxies as far toward their nuclei as practicable. Because
they are so nearby, M31 and M32 have been especially good candidates for seeking
black holes in their nuclei. Observations by Tonry (1987), Dressier and Richstone
(1988), and Kormendy (1988) strongly suggest the possible presence of black holes in
both these nearby galaxies.
The Hubble Space Telescope (HST) will routinely obtain images with spatiaJ
resolution of 10s of milliarcseconds throughout the UV to near-IR regior
Spectroscopy of regions through apertures as small as 0.1 arcseconds will also I
possible throughout most of the same spectral region. This increase in angu'
resolution will enable astronomers using the HST to obtain luminosity profil
rotation curves, and velocity dispersion profiles of the central regions of gala_
with spatial resolution at least an order of magnitude improved over what has b_
possible to date from the ground. Limited angular resolution is the primary limiting-
factor for detecting the presence of centrally condensed matter in the nuclei of
galaxies, so that the availability of the HST Observatory will greatly improve our
sensitivity for detecting black holes in the nuclei of galaxies.
II. APPROACH
From images of a galaxy taken through one or more broad-band filters, along
with certain assumptions about the galaxy's orientation and symmetry, one directly
obtains the luminosity profile L(R). From these data and additional models of the
fractions of stellar types, the luminosity distribution yields an estimate of that
portion of the mass distribution due to stars. Non-uniqueness of the conversion from
observed luminosities and spectra to stellar sources, and uncertainty in the true
orientations and symmetry of the galaxy, produce rather large uncertainties in the
determination of Mstellar(R) for any particular galaxy.
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The total mass distribution of the galaxy can be estimated from the effects of
gravity acting on the luminous matter. Spatially-resolved spectra of spiral galaxies
produce rotation curves, V(R), of the (line-of-sight) velocity of stars versus position
in the galaxy. Because elliptical galaxies generally are not supported by rotation,
one sees in their spectra primarily a dispersion, c v(R), in the (line-of-sight)
velocities with position. Within uncertainties set by unknown inclination effects,
symmetry, and dominant orbit types (radial, circular, isotropic), one can obtain
estimates of M(R), the total mass within radius R, for both spirals and ellipticals.
(Note that the velocity dispersion, gv(R), as R-->0 will provide useful information
about the matter in both spiral and elliptical galaxy nuclei.)
The signature of a black hole in the nucleus of a galaxy will be a normal
luminosity and a high value of mass as R-->0. If the inferred density of invisible
material within the nucleus becomes great enough, it can be argued that the matter
very probably has collapsed into a black hole simply because separate masses so
closely crowded will not be stable against gravitational collapse. Such an argument
is unlikely to be absolutely conclusive in any given case, but the likelihood of a black
hole can become extremely high if one can detect the existence of substantial mass
within a small enough volume. Thus, the measurements of L(R) and M(R), the latter
through either V(R) or _rv(R), at small R are crucial for detecting any possible black
holes in galactic nuclei.
III. SENSITIVITY OF HST FOR FINDING NUCLEAR BLACK HOLES
It is the factor-of-10 increase in angular, and therefore spatial, resolution
that will make the HST a powerful tool for finding black holes (if they are there) in
the nuclei of galaxies, or for setting improved upper limits to their presence. Two
major reasons explain why increased spatial resolution enhance so dramatically our
sensitivity for detecting black holes in galaxies:
(1) The measured spectral effects become stronger as R-->0 since both V(R) and
gv(R) scale as (M/R) 1/2, and
(2) Observations at smaller values of R enhance the intrinsic contrast between
total mass M(R) and the mass Mstellar(R) due to stars.
The functional dependence of the spectral signature strength, proportional to
(M/R) 1/2 (Item 1), suggests that the HST will be about 10 times more sensitive than a
ground-based telescope for detecting a black hole in a particular galactic nucleus.
However, it is the contrast enhancement (Item 2) arising from the ability to "home
in" more sharply on the central region of the galaxy that really increases our
sensitivity to a central black hole. Sampling R 10 times closer reduces the
contribution to M(R) from stars by a factor of 1,000 if the central stars are uniformly
distributed, and even more if (as is likely) their density increases toward the center
of the galaxy.
Table 1 provides examples of the spatial resolution which will become possible
with the HST. Images sampled on an angular scale as fine as 7 milliarcseconds will be
possible, while spectra will be obtained through apertures as small as 0.1 arcseconds
in diameter. Spectra taken from the HST will allow determination of V(R) and gv(R)
to at least 100 km/sec accuracy. Table 2 illustrates typical masses of black holes
which will be detectable with the HST at various distances.
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RESOIMIION
(arcsec)
TABLE 1
SPATIAL SAMPLING IN PARSECS FOR VARIOUS BLACK HOLE DOMICEF_
i
RESO_N (pc) ON CERTAIN TARGETS
M31 AND M32 M87 NGC 6251
D - 0.35 Mpc 9 Mpc 70 Mpc
AVAILABLE INSTRUMENTS
0.007
0.022
0.044
0.10
0.2
1.0
0.03 0.6 5
0.08 2 15
0.15 4 31
0.35 9 70
0.88 23 175
3.5 90 700
fOC
FOC
FOC,WF/PC
FOC,WF/PC,FOS
FOC,WF/PC,FOS,GHRS
HST and GROUND-BASED
WF/PC =
FOS =
GHRS =
Faint Object Camera (on HST)
Wide-Field/Planetary Camera (on HST)
Faint Object Spectrograph (on HST)
Goddard High Resolution Spectrograph (on HST)
TABLE 2
HST CAPABILITY FOR DE-'IECrlNG BLACK HOLES 1NGALAXIES AT VARIOUS RANGES
DISTANCE (Mpc) RV_/G., MIN DETECTABLE BH (MO)
0.35 8 * 105
9 2 * 107
70 2* 10 a
I
For M31 and M32, we will be sensitive to black holes of the size (_ 106 M O)
suspected to exist at the nucleus of our own galaxy [see, for example, Serabyn and
Lacy (1985)]. The HST observations will be able to prove conclusively the presence or
absence of black holes in M31 and M32 of the masses (106.5-8.0 M®) suggested by the
observations of Tonry (1987), Dressier and Richstone (1988), and Kormendy (1988).
IV. CONCLUSIONS
The HST will provide a powerful tool for finding black holes in the nucleus of
nearby galaxies. The factor-of-10 increase in angular resolution of the HST,
compared to ground-based telescopes, will increase our sensitivity to detect black
holes in galactic nuclei by factors of 10 to 1,000. For the neighboring galaxies M31
and M32, we will be able to detect black holes of the same size (=106 MO) as may exist
in our own Milky Way Galaxy, and will conclusively confirm or refute the presence
of M = 106.5-8.° MO black holes hinted at by current ground-based data.
112
REFERENCES
Dressier, A., and Richstone, D. O. 1988, "Stellar Dynamics in the Nuclei of M31 and M32: Evidence for Massive
Black Holes," Astrophys. J., 324, 701-713.
Faber, S. M., and Gallagher, J. S. 1979, "Masses and Mass-to-Light Ratios of Galaxies," Ann. Rev. Astron.
Astrophys.,17, 135-187.
Kormendy, J. 1988, "Evidence for a Supermassive Black Hole in the Nucleus of M31," Astrophys. J., 325, 128-
141.
Serabyn, E., and Lacy, J. H. 1985, "[Nell] Observations of the Galactic Center: Evidence for a Massive Black
Hole," Astrophys. J., 293, 445-458.
Tonry, J. L. 1987, "A Central Black Hole in M32," Astrophys. J., 322, 632-642.
Trimble, V. 1987, "Existence and Nature of Dark Matter in the Universe," Ann. Rev. Astron. Astrophys., 25,
425-472.
zf
DISCUSSION
SONNABEND: Will HST give sufficient resolution at, say, the center of M31 to see
individual high-velocity stars (if they exist), and thus provide a better test for a
central black hole than just the velocity curve?
HARMS: Very high velocity stars (projected) near galactic nuclei would be very good
indicators of condensed central matter which might be black holes, so we will
certainly be looking for them. The cameras on the HST will provide images with
spatial resolutions ranging from 7 to 44 milliarcseconds per pixel. At the distance of
M31, this corresponds to spatial sampling on a scale of 0.03 parsecs to 0.15 parsecs,
assuming a distance to M31 of 0.7 Mpc. Spectroscopy throughout a broad wavelength
region from the UV through the near-IR is possible on a spatial scale of 100
milliarcseconds, corresponding to 0.35 parsecs at M31. While HST will not truly
resolve individual stars in M31, individual bright stars near the nucleus may
dominate the light emission in a given spatial sample, which would allow us to
measure velocities of individual stars. If these stars show high velocity dispersions,
this would provide strong evidence for centrally condensed matter in the galactic
nucleus. The images from HST will allow us to determine whether such central
matter is luminous or not with spatial resolution generally even better than will be
attainable for spectra.
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Space telescope searches for black holes in galactic nuclei

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