SBF distances to Hydra and Centaurus:
Consequences on the Great Attractor model
1
S.
1
Mieske , M.
1
Hilker ,
2
L. Infante
Sternwarte der Universit ät Bonn, Germany 2Departamento de Astronomı́a y Astrofı́sica, Pontificia Universidad Católica de Chile
Peculiar velocities towards Hydra/Centaurus
In the past few years, a lot of effort has been put into
a precise determination of cosmological parameters.
The accuracy in determining the Hubble constant H0
is approaching 5%. This enormous improvement in
precision has the consequence that deviations from an
undisturbed Hubble flow can also be determined to a
higher precision. This is of special importance for the
Hydra-Centaurus region, as high peculiar velocities
have been measured towards this direction (LyndenBell et al. 1988, Tonry et al. 2000, T00 in the following), leading to the proposal of a massive “Great At16
tractor” (' 10 M∗)at a distance of about 40-50 Mpc,
slightly behind the Centaurus cluster.
the following) model by T00, who had not included
distances to the Hydra cluster in their flow analysis.
The SBF measurements for both Hydra and Centaurus
come from deep VLT-FORS1 imaging in the I band of
seven fields in the central region of each cluster. See
Fig.1 for the location of the investigated galaxies.
Fig.2 summarizes the results of our SBF distance
measurements. We only include distances of galaxies within the empirically well calibrated range of
1.0 < (V − I) < 1.3 mag into the subsequent analysis. We also indicate the mean distance obtained when
restricting the Hydra sample to the same absolute magnitude range than the Centaurus sample, denoted as luminosity restricted Hydra sample in the following.
The mean distance to Hydra is 33.03 ± 0.07 mag,
to Centaurus it is 33.17 ± 0.12 mag, yielding (m −
M )Cen − (m − M )Hyd = 0.14 ± 0.14 mag. For the luminosity restricted Hydra sample we get 33.16 ± 0.06
mag, yielding (m − M )Cen − (m − M )Hyd = 0.01 ± 0.13
mag.
angular region centered on Hydra of about 5 degrees
radius (4 Mpc) is allowed. For a 10% higher GA distance, the diameter of this region almost doubles. For
20% higher distance (not shown in the plot), the allowed region remains the same. The distance of 42.9
Mpc to the luminosity restricted Hydra sample cannot
be accomodated for a GA at 43 Mpc, only for a 10%
higher GA distance.
Fig.1: DSS images of the central Centaurus cluster (left)
and Hydra cluster (right) with the investigated galaxies indicated. The images have a 20 arcminutes sidelength.
1.3
1.2
Fig.3: Mean CMB radial velocity plotted vs. mean SBF
distance modulus for Hydra (blue) and Centaurus (red). Radial velocities come from Christlein&Zabludoff (2003) and
Stein et al. (1998), respectively. The lines indicate the Hubble flow for different values of H0. The filled green circle gives the Hydra distance when excluding dwarfs with
MV > −15 mag.
1.1
1
0.9
-22
-20
Is the Great Attractor really that attractive?
-18
-16
-14
32
32.5
33
33.5
34
32
32.5
33
33.5
34
Fig.2: SBF distances for the investigated galaxies in Hydra (left panels) and Centaurus (right panels) plotted vs.
V-I (top) and MV (bottom). The bottom plots include
only galaxies within the empirically calibrated colour range
1.0 < V − I < 1.3 mag. This range is indicated in the
top plots by dashed horizontal lines. The vertical lines in
the bottom plots indicate the mean distance (solid line) and
its 1σ uncertainty (dashed lines). The green vertical line
gives the mean Hydra distance when excluding dwarfs with
MV > −15 mag.
SBF distances to Hydra/Centaurus
We present new SBF-distances to 16 early type galaxies in Hydra (Mieske, Hilker& Infante 2004, submitted) and revised distances to 15 galaxies in Centaurus
(from Mieske & Hilker 2003). These new distances
are used to test the spherical Great Attractor (GA in
Fig. 3 shows that our distance value to the Centaurus
cluster implies a quite small peculiar velocity of only
about 200 km/s. Our low peculiar velocity for Centaurus implies that either the GA is much less massive
than estimated by Tonry et al. or that the Centaurus
cluster falls into it almost perpendicular to the line of
sight. The fact that for the Hydra cluster we do find a
quite large peculiar velocity of about 1000 km/s (see
Fig. 3) implies that the second possibility holds: there
is a massive Great Attractor, but closer to the projected
position of Hydra than to Centaurus.
To quantify this shift, we indicate a series of projected
GA positions in Fig. 4 for which we show the expected
CMB velocities of Hydra and Centaurus. These are
calculated from the flow model of T00, only varying
the projected position of the GA.
From Fig. 4 it is clear that at the original position of the
GA determined by Tonry et al., the measured distances
and CMB velocities of Hydra and Centaurus cannot be
accomodated. In case of a GA distance of 43 Mpc, an
Fig.4: Top: Positions in super-galactic coordinates of the
indicated objects. The Centaurus and Hydra distances are
from this presentation. Crosses indicate assumed projected
positions of the GA for which the expected CMB radial velocities of Hydra and Centaurus are shown in the bottom
panels. The dashed circles delimit the allowed region for a
GA at 43 Mpc and 47.5 Mpc distance, respectively. Bottom: CMB radial velocities expected from the flow model
by Tonry et al. (2000) for the different GA positions as indicated in Fig. 4. Left panel: GA distance=43 Mpc. Right
panel: GA distance=47.5 Mpc. Red symbols: expected
CMB velocity for the Centaurus cluster at (m-M)=33.17
mag. Blue symbols: expected CMB radial velocities for Hydra at (m-M)=33.03 mag. Green symbols: expected CMB
radial velocities for the distance of (m-M)=33.16 mag to the
luminosity restricted Hydra sample. Blue horizontal lines:
CMB velocity of the Hydra cluster. Red horizontal lines:
CMB velocity of the Centaurus cluster.
Conclusions
Our SBF-distances to the Hydra and Centaurus cluster
confirm the presence of a massive Great Attractor in
the Hydra-Centaurus region, at a distance of about 45
Mpc. They imply that the GA be closer to the Hydra
than to the Centaurus cluster in projected position, at
least 10-15 degrees (7-10 Mpc) away from the GA position estimated by Tonry et al. (2000), who had not
included the Hydra cluster in their flow analysis.
Acknowledgements
We thank the ESO User Support Group for carrying out the DDT spectroscopy (program
273.B-5008xs) in service mode. SM was supported by DAAD Ph.D. grant D/01/35298 and
DFG Projekt Nr. HI 855/1-1. LI acknowledges support from ”proyecto Fondap # 15010003”