For refcode 1997MNRAS.284..286d: Retrieve 72 NED objects in this reference. Please click here for ADS abstract
NED Abstract
Copyright by Royal Astronomical Society.
1997MNRAS.284..286d
Are there binary galaxies in clusters?
Roland den Hartog
Accepted 1996 July 26. Received 1996 June 6; in original form 1995 July 7
ABSTRACT
We investigate a sample of 73 rich clusters containing in total 7754
galaxies and find, after a thorough assessment of the projection effects,
that 6 per cent of the galaxies are part of a `binary' system. Apparent
`binaries' are defined as nearestneighbour pairs for which the velocity
difference {DELTA}V of the galaxies does not exceed 300 km s^1^ and the
maximum separation {DELTA}R is less than 0.1 h^1^ Mpc. This results in
1514 pairs. Monte Carlo simulations show that 1047 of these must be
accidental, leaving 467 binaries. These binaries cannot be identified
individually, but their properties can be studied statistically.
The binaries found in this way are dynamically `soft', i.e. {DELTA}V
is less than the global {sigma}_V_ of the cluster. Despite this, they
only seem to avoid the inner 0.4 h^1^ Mpc of clusters, suggesting that
outside a radius of 0.4 h^1^ Mpc the velocity dispersion must locally be
lower than the global value of {sigma}_V_ that we measure in projection,
or that many of the found pairs are unbound.
There is a significant correlation between the presence of binaries and
the detection of substructure with the Dressler & Shectman test. Binaries
in binaryrich clusters seem to be directly associated with the low
{sigma}_V_ subclusters for which the DStest is sensitive. The fraction
of binaries in the 19 binaryrich clusters is nearly as high (~12 per
cent) as in the population of fore and background galaxies
(`interlopers') that is normally removed from the cluster surveys (~14.5
per cent). In the remaining 54 clusters the binary fraction is
considerably lower (~3 percent).
Depending on the assumed form of the orbits of the galaxies in a
binary, the average mass of the binaries lies in the range of 25 x
10^11^ h^1^ M_sun_, while the average M/Lratio lies in the range of 20
40 h^1^ for circular orbits to about 45100 h^1^ for eccentric orbits,
which is consistent with the results for field binaries. Monte Carlo
models of the binaries suggest that the orbits are likely to have low
eccentricities. Both the masses and the M/Lratios of binaries are larger
inside 1.2 h^1^ Mpc than outside, which argues against the tidal
stripping of galaxy haloes in the central regions of clusters.
We have compared our Monte Carlo methods with recent Nbody models of
clusters which incorporate galaxy formation in a crude way. These models
reproduce the main statistical properties of the binaries. When applied
to the models, our statistical method detects about 75 per cent of the
bound pairs of galaxies that satisfy our selection criteria for binaries.
In comparison with the binaries in the data, the masses of bound pairs in
these models are systematically larger by a factor of 6. This prevents
more detailed comparisons between the data and the models.
Key words: galaxies: clusters: general  galaxies: kinematics and
dynamics  cosmology: observations  dark matter.
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