Using a spectrum obtained under moderate (of order 1 arcsecond) seeing, we
show that the double nucleus in M31 produces a strong kinematic signature even
though the individual components are not spatially resolved. The signature
consists of a significant asymmetric wing in the stellar velocity distribution
close to the center of the system. The properties of the second nucleus derived
from this analysis agree closely with those measured from high-spatial
resolution Hubble Space Telescope images. Even Space Telescope only has
sufficient resolution to study the structure of very nearby galactic nuclei
photometrically; this spectroscopic approach offers a tool for detecting
structure such as multiple nuclei in a wider sample of galaxy cores.Comment: 4 pages of uuencoded compressed postscript, figures included.
  Accepted for publication in MNRA

Gerssen, J.

Kuijken, K.

Merrifield, M. R.

English

arXiv

Using a spectrum obtained under moderate (of order 1 arcsecond) seeing, we show that the double nucleus in M31 produces a strong kinematic signature even though the individual components are not spatially resolved. The signature consists of a significant asymmetric wing in the stellar velocity distribution close to the center of the system. The properties of the second nucleus derived from this analysis agree closely with those measured from high-spatial resolution Hubble Space Telescope images. Even Space Telescope only has sufficient resolution to study the structure of very nearby galactic nuclei photometrically; this spectroscopic approach offers a tool for detecting structure such as multiple nuclei in a wider sample of galaxy cores

Gerssen, J

Kuijken, K

Merrifield, M R

CERN Document Server

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Mon. Not. R. Astron. Soc. 000, 000{000 (0000) Printed 17 August 1995 (MN L
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Kinematic detection of the double nucleus in M31
Joris Gerssen
1
, Konrad Kuijken
1?
and Michael R. Merrield
2
1
Kapteyn Institute, Groningen 9700 AV, The Netherlands
2
Department of Physics, University of Southampton, Higheld, UK.
17 August 1995
ABSTRACT
Using a spectrum obtained under moderate ( 1 arcsecond) seeing, we show that
the double nucleus in M31 produces a strong kinematic signature even though the in-
dividual components are not spatially resolved. The signature consists of a signicant
asymmetric wing in the stellar velocity distribution close to the center of the system.
The properties of the second nucleus derived from this analysis agree closely with those
measured from high-spatial resolution Hubble Space Telescope images. Even Space Tele-
scope only has sucient resolution to study the structure of very nearby galactic nuclei
photometrically; this spectroscopic approach oers a tool for detecting structure such
as multiple nuclei in a wider sample of galaxy cores.
Key words: galaxies: nuclei { line: proles { galaxies: individual: M31 { galaxies:
kinematics and dynamics.
1 INTRODUCTION
Because of its proximity, M31 provides us with a unique op-
portunity to study the detailed structure of galactic nuclei at
high spatial resolution. In the 1970s, high resolution images
obtained using the Stratoscope II telescope showed that the
nucleus of M31 contains signicantly asymmetric structure
(Light et al. 1974). Conrmation of this unexpected result
has come from Hubble Space Telescope (HST) images which
show that the nucleus consists of two components separated
by 0:49 arcsec (corresponding to 2 pc at the distance of M31;
Lauer et al. 1993). The brighter peak, dubbed P1, appears
to be unresolved and of a colour similar to the M31 bulge,
whereas the second peak, P2, has a morphology which is
reminiscent of the power-law cusps seen at the centres of
many galactic nuclei. P2 has also been found to have a UV
excess like other galactic nuclei (King et al. 1995), and it
sits right at the centre of the bulge isophotes, so it seems
likely that this fainter source is the \true" nucleus of M31.
Various explanations for the nuclear structure of M31
have been proposed, but so far it has proved dicult to nd
a scenario in which the double nucleus is not a very transient
structure. Dynamical timescales at 2pc are extremely short
( 10
5
years), arguing against an out-of-equilibrium lopsid-
edness which is yet to phase-mix. Further, dynamical friction
should have caused the orbit of any massive object to decay
relatively quickly [although not as fast as naive dynamical
friction arguments might suggest (Miller & Smith 1995)].
?
Visiting Scientist, Dept. of Theoretical Physics, University of
the Basque Country, Bilbao, Spain
Tremaine (1995) has recently suggested that P1 represents
the high-density part of an eccentric disk in near-Keplerian
orbit about the point mass potential of P2. Crane (1995),
on the other hand, has argued that the data are consistent
with the possibility that the bright P1 peak is a dynami-
cally insignicant entity on the point of merging with the
true galaxy nucleus at P2.
Unfortunately, M31 is one of the few galaxies in which a
structure of this kind could have been resolved, even with the
HST. It is therefore dicult to know whether such structures
are rare, and hence plausibly explained as a coincidence, or
whether they are suciently numerous to require a more
generic explanation.
The goal of the present investigation is to test whether
such structures might be detected not by resolving them spa-
tially, but by doing so spectroscopically, from their line-of-
sight stellar kinematics. To this end, we have analysed high-
dispersion absorption-line spectra of M31 taken in moderate
( 1 arcsec) seeing. While these data oer no hope of a spa-
tial detection of the two components, we will show that the
signal of a two-component nucleus is readily visible in the
spectra. This result therefore augurs well for a search for
such components in more distant galaxies, beyond the reach
of even high spatial resolution imaging instruments.
2 THE DATA
The data described here were obtained at the end of the
night of 1993 December 10 at the Multiple Mirror Telescope
using the Red Channel Spectrograph with a 1.25 arcsec wide
2 J. Gerssen, K. Kuijken and M.R. Merrield
Figure 1. Greyscale representation of the velocity distributions
in the core of M31. Darker shading indicates higher densities of
stars at the given velocities and positions along the slit. Note the
excess of stars at high velocities in the central arcsecond.
long-slit and a 1200 lines per mm grating. The dispersion
per pixel with this setup is  0:7

A ( 40 kms
 1
at 5000

A),
which adequately samples the FWHM of 1.8

A. The spatial
resolution was limited by the seeing, which was  1 arc-
second FWHM. We obtained a single 30 minute integration
with the slit aligned at a position angle of 148

. Calibrating
arc lamp exposures were taken before and after the galaxy
spectrum.
The long-slit spectrum was reduced to log-wavelength
bins in the standard way, using iraf packages. Adjacent
spectra along the slit were then averaged to obtain a signal-
to-noise ratio of at least 75 per bin, and the absorption line
proles of these co-added spectra were analysed.
3 ABSORPTION LINE KINEMATICS
It is now possible to derive the intrinsic shape of the broad-
ening function of a high signal-to-noise ratio galaxy absorp-
tion line spectrum (Bender 1990, Rix & White 1992, van
der Marel & Franx 1993, Gerhard 1993, Kuijken & Merri-
eld 1993, Saha & Williams 1994, Statler 1995). This func-
tion provides a direct measure of the line-of-sight velocity
distribution (LOSVD) of the stars along that line of sight
through the galaxy. To investigate the stellar kinematics in
the core of M31, we have employed the unresolved gaus-
sian decomposition (UGD) technique of Kuijken & Merri-
eld (1993), which returns non-negative smooth broadening
functions with well-dened error bars, but does not assume
a particular few-parameter description of the LOSVD.
The cumulative velocity distributions derived by the
UGD method for the central regions of M31 along PA 148

are shown in Fig. 1. This slit position lies close to the minor
axis of the M31 bulge, and there is little variation of the
LOSVD with distance from the center outside the central
few arcsec. We only show the central part of the data, since
no sky-subtraction was attempted (M31 being too large for
this to be possible from a single slit exposure). In the central
few arcseconds of radius, a highly signicant high-velocity
`blip' can be seen in these cumulative velocity curves. This
feature is found independently in ve adjacent spectra, and
indicates an asymmetry in the central LOSVDs. It was also
seen in the analysis of van der Marel et al. (1993). It is this
feature in the high-velocity wing of the LOSVD which pro-
vides the signature of a secondary nucleus that we study in
this paper.
4 IDENTIFICATION OF THE EXTRA
COMPONENT
The asymmetric structure seen in Fig. 1 cannot occur in
axisymmetric (or even bi-symmetric) systems, since any
positive-velocity feature at a given radius should always be
accompanied by a negative-velocity one, placed symmetri-
cally with respect to the nucleus. There can therefore be
no one-sided distortions to the velocity distribution in the
center of a galaxy with such symmetry.
As a simple explanation for the observations (also moti-
vated by the known photometric results) we therefore adopt
a two component model. Since there is little detectable vari-
ation in the shape of the velocity distribution outside the
central two arcsec along our slit position, we constructed a
`background' distribution by averaging the velocity distri-
butions at distances between 12.3" and 20.1" on either side
of the nucleus. We then made a least-squares t simultane-
ously to the central ve LOSVDs. As a kinematic model,
we adopted the sum of the background LOSVD and an ad-
ditional component which we rather arbitrarily assigned a
gaussian velocity distribution whose parameters were to be
determined. Both components were further required to have
amplitudes which varied in a gaussian manner with position
along the slit. We therefore tted a total of eight parameters
to the observed velocity distributions: three parameters for
the background component's radial amplitude dependence,
and ve for the extra component, which is a bivariate gaus-
sian in position and velocity.
The results of the t are illustrated in Fig. 2 and Ta-
ble 1. It is evident that the data are well described by an
extra kinematic component (henceforth called K1), spatially
unresolved, and kinematically oset by +160 kms
 1
from a
background component with a spatially-invariant LOSVD
and a centrally peaked density prole (henceforth K2). The
brightnesses of the two components peak at slightly dierent
positions along the slit, separated by 0.1 arcsec. The best-
t velocity dispersion of the extra component is 100 km s
 1
,
but simulations have shown that this number is an artefact
of the smoothness imposed on the outcome of our unresolved
gaussian t to the LOSVD, and only provides an upper limit
on the true dispersion of this component.
It is tempting to identify K1 and K2 directly with the
photometric decomposition in to P1 and P2: in the following
subsections, we assess the viability of this identication.
THE NUCLEUS OF M31 3
-1000 -500 0 500
Figure 2. The 5 central LOSVDs with the best-t two component
model. The dotted line is K2, the dashed line is K1, and the
solid line is their sum. The small error bars represent the 1-
range of LOSVDs, smooth on a scale of 80kms
 1
, which are
consistent with our high signal-to-noise ratio data. The indicated
radii are measured with respect to the center of the CCD pixel
which contained the highest ux. Vertical scales are normalized
in the various panels.
Table 1. Best-t parameters for the two component t to M31's
core kinematics
K1 K2
Position of maximum ux 0.4" 0.5"
Total relative ux 0.12 1.0
Gaussian spatial dispersion 0.95" 2.65"
mean velocity +150kms
 1
|
velocity dispersion 100kms
 1
|
4.0.1 Brightness
Our central ve line proles cover a rectangular region of
1.25 arcsec by 5.2 arcsec on the sky. From Bacon et al.'s
(1994) t to the central light distribution in M31, we obtain
a ux ratio P1:P2 of 0.09 in this region. This value agrees
very well with the relative uxes in our kinematic K1 and
K2 components: integrating the tted contributions of both
components to the line proles in Fig. 2, we obtain a ux
ratio K1:K2 of 0.12. Analysis of simulated spectra has shown
that if in reality K1 has a very small velocity dispersion,
its ux will be overestimated from our analysis by up to a
factor two, in which case we would deduce a lower limit for
the kinematic ux ratio of 0.06. These limits neatly bracket
the photometric ratio.
4.0.2 Position
According to the analysis of the HST data by Lauer et
al. (1993), the photometric components are separated by
0.49" along a line with position angle 43

. Our slit was ori-
ented at a position angle of 148

, at 75

to the P1-P2 line.
Projected onto the slit, P1 and P2 are therefore separated by
0:49 cos(75

) = 0:13 arcsec, in good agreement with the sep-
aration obtained between our tted positions of maximum
ux in K1 and K2.
4.0.3 Velocity
K1 has a velocity of +150 km s
 1
with respect to the sur-
rounding regions of M31. The data of Bacon et al. (1994)
show a radial velocity of +100 km s
 1
at the peak of P1, and
after correcting for seeing, these authors deduce a rotation
velocity of +160 km s
 1
at that location, very close to our
velocity oset for K1.
5 DISCUSSION
We have shown that it is possible to unambiguously iden-
tify the multiple components in the nucleus of M31 using
kinematic observations which make no attempt to spatially
resolve the components. Further, we have been able to show
that the properties of the two nuclei such as their relative
uxes and velocities can be obtained reliably from such data.
From these kinematic observations, it appears that the
nuclear region of M31 can be understood by postulating that
P1 is simply an extra bright object in motion around the true
center of the galaxy, which lies at P2. However, the dynam-
ical friction timescale for such an arrangement is short, so
(unless the object P1 has a very low mass-to-light ratio) we
must be observing M31 at an unusual moment. Furthermore,
any object without an unusually large mass-to-light ratio
would be tidally shredded at that position, further shorten-
ing its lifetime. Perhaps a black hole, not suciently massive
to perturb the dynamics of the center severly or produce a
strong UV excess, but massive enough to accrete a substan-
tial stellar halo of its own, may oer an explanation. Once
again, though, such an object would spend most of its time
at large radii, and so we are still observing M31 in an un-
usual state. One way around this problem has been explored
by Xu & Ostriker (1994) who considered models in which
a steady supply of massive black holes make their way into
the nucleus.
An alternative possibility has been considered by
Tremaine (1995), who has constructed a model for the cen-
tral asymmetries in the light prole consisting of a lopsided
disk in nearly Keplerian motion. Our data can essentially
rule out such a model: by conservation of angular momen-
tum, we know that the luminosity-weighted velocity of any
lled Keplerian orbit about a stationary object must be zero.
Since the putative disk would fall entirely within the slit of
our observation, we would not expect to nd the observed
non-zero mean velocity. Non-zero velocities could result if
the disk were precessing, but this scenario would require a
non-Keplerian central potential which would be unlikely to
support an eccentric disk over many precession periods.
4 J. Gerssen, K. Kuijken and M.R. Merrield
A further possibility is that P1 is an M31 globular clus-
ter which is not physically associated with the nucleus, but
just lies along the same line of sight. Such a model obvi-
ates the need for a contrived dynamical model, but the very
small probability of such a chance alignment renders this
explanation equally unsatisfactory.
The wealth of possible explanations for the double nu-
cleus in M31 illustrates the need for observations which
will show whether such structures are rare or commonplace.
Tackling this question with an imaging survey of other more
distant galaxies would require challenging ultra-high resolu-
tion observations. However, we have shown that spectro-
scopic data can reveal the strong signature of a double nu-
cleus even when the nuclei are unresolved, and so such ob-
servations can be used to search for more distant systems
like M31 without requiring very high spatial resolution.
At very large distances (say, more than ten times the
distance to M31) the spectroscopic method will also break
down because of dilution of the nuclear light by scattered
emission from surrounding stars. This limitation is not
driven by resolution but by background noise, which makes
it fundamentally dierent from, and less severe than, the res-
olution limit that applies to direct imaging of more distant
galaxies.
ACKNOWLEDGMENTS
The data presented in this paper were obtained using the
Multiple Mirror Telescope, which is a joint facility of the
Smithsonian Institute and the University of Arizona. Much
of the analysis was performed using iraf, which is dis-
tributed by NOAO. MRM is supported by a PPARC Ad-
vanced Fellowship (B/94/AF/1840). We thank the referee
for his helpful comments.
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Kormendy J., 1988, ApJ, 325, 128
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Kinematic detection of the double nucleus in M31

Using a spectrum obtained under moderate (similar to 1 arcsec) seeing, we show that the double nucleus in M31 produces a strong kinematic signature even though the individual components are not spatially resolved. The signature consists of a significant asymmetric wing in the stellar velocity distribution close to the centre of the system. The properties of the second nucleus derived from this analysis agree closely with those measured from high-spatial-resolution Hubble Space Telescope images. Even Space Telescope has sufficient resolution only to study the structure of very nearby galactic nuclei photometrically; this spectroscopic approach offers tool for detecting structure such as multiple nuclei in a wider sample of galaxy cores.</p

GERSSEN, J

KUIJKEN, K

MERRIFIELD, MR

ARTS repository - University of Groningen

KINEMATIC DETECTION OF THE DOUBLE NUCLEUS IN M31

University of Groningen

Using a spectrum obtained under moderate (similar to 1 arcsec) seeing, we show that the double nucleus in M31 produces a strong kinematic signature even though the individual components are not spatially resolved. The signature consists of a significant asymmetric wing in the stellar velocity distribution close to the centre of the system. The properties of the second nucleus derived from this analysis agree closely with those measured from high-spatial-resolution Hubble Space Telescope images. Even Space Telescope has sufficient resolution only to study the structure of very nearby galactic nuclei photometrically; this spectroscopic approach offers tool for detecting structure such as multiple nuclei in a wider sample of galaxy cores

Kinematic detection of the double nucleus in M31

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