Discovery of two M32 twins in Abell 1689
S.
1
1
Mieske , L.
2
Infante , M.
1
Hilker ,
3
N. Benitez ,
3
J. Blakeslee ,
3
H. Ford ,
3
K. Zekser
Sternwarte der Universit ät Bonn, Germany 2Departamento de Astronomı́a y Astrofı́sica, Pontificia Universidad Católica de Chile 3Department of Physics and Astronomy, Johns Hopkins University, Baltimore
this is confirmed. Also a single Sersic profile with
reff = 300 pc and n=1.85 fits quite well the observed
CG profiles. In order to explore the acceptable range
for a single component reff , we use the ISHAPE task
(Larsen 1999, A&AS, 139, 393L). The resulting range
for reff is 370 pc ± 180 pc for CGA1689,1 and 225 pc ±
75 pc for CGA1689,2.
M32 and UCDs
One of the most extreme outliers in the magnitudesurface brightness diagram of stellar systems (Ferguson & Binggeli 1994, A&ARv, 6, 67) is the compact
elliptical (cE) galaxy M32. It has MV ' −17 mag
(Graham 2002, ApJL, 568, 13), but at much brighter
µeff than dwarf ellipticals of comparable MV . Only
very few galaxies with similar properties to M32 have
been found (e.g. Bender & Ziegler 1998, A&A, 330,
819), amongst which M32 is the faintest and most
compact one. Closer to the magnitude regime of globular clusters, Hilker et al. (1999, A&AS, 134, 75) and
Drinkwater et al. (2000, PASA, 17, 227) have discovered in the Fornax cluster the so called ”ultracompact
dwarf galaxies” (UCDs), having MV ' −13 mag. Up
to now, no compact galaxy intermediate in luminosity
between M32 and UCDs has been found.
Fig.2: Spectra of the five UCD candidates, numbers as from
Table 1.
No.
1
2
3
4
5
i
22.18
22.38
22.77
23.24
23.80
zspec
0.186
0.001
0.201
?
0.196?
zphot
0.164
0.001
0.180
0.090
0.001
Comment
CGA1689,1
Star
CGA1689,2
Close neighbor
Close neighbor
Table 1: Properties of the spectroscopically investigated
UCD candidates in Abell 1689. “CG” in the comment column stands for “compact galaxy”.
The resulting spectroscopic redshifts of the five brightest UCD candidates are shown in Table 1. We confirm the cluster membership for UCD candidates
No. 1 and 3 in Abell 1689. The spectra of the two
faintest UCD candidates were partially blended with
neighbouring galaxies, inhibiting an unambigous redshift determination.
A note on nomenclature: the absolute luminosities of
the two confirmed A1689 UCD candidates are almost
identical to M32, such that one might also call them
“compact elliptical” (cE) or “compact dwarf elliptical”
(cdE). We choose to assign the two cluster members
the more general term “compact galaxies” (CGs) or
“M32-like galaxies”.
Fig.1: Top: ACS image of Abell 1689 plus thumbnails of
the five brightest UCD candidates, numbered by decreasing luminosity according to Table 1. Bottom: CMD of UCD
candidates in Abell 1689. Blue circles: Photometric redshift
zphot < 0.5. Red crosses: zphot > 0.5. The green vertical
lines indicate the colour range of Fornax UCDs expected at
A1689’s distance. The black vertical tick denotes the absolute magnitude range of UCDs in Fornax. The location of
M32 at A1689’s distance is indicated by the blue cross.
M32 vs. A1689 compact galaxies
In Figs. 3 and 4 the morphology and surface brightness
profiles of the two A1689 compact galaxies (CGs) are
compared with a seeing convolved, simulated image of
M32, as taken from the fit by Graham (2002). This fit
consists of a bulge component described by a Sersic
profile with n=1.5 and reff = 105 pc plus an exponential disk with reff = 806 pc.
The search for UCDs in A1689
The search for Ultra Compact Dwarf (UCD) galaxy
candidates in Abell 1689 (z=0.183, m-M' 39.75 mag)
has been presented in Mieske et al. (2004, AJ in press,
astro-ph/0406613), based on deep high resolution ACS
images, see Fig. 1. The search resulted in the discovery of several very luminous UCD candidates in
the luminosity range between Fornax UCDs and M32.
Two of the brightest UCD candidates (i ' 22.5 mag,
MV ' −17 mag) are marginally resolved, implying reff ' 300pc at Abell 1689’s distance, similar to
M32. In order to spectroscopically confirm their cluster membership, Director’s Discretionary Time (program 273.B-5008) was assigned to our group at the
VLT. The five brightest UCD candidates were targeted
with MXU/FORS2 at 6Å/pixel resolution and a total
on-source integration time of 6500 seconds, yielding
S/N between 4 and 15 (see Fig.2).
Fig.4: Surface brightness profiles of objects indicated. Top
panel: Direct comparison between the two CG candidates
and the projected M32 model. Bottom panel: All profiles
normalized to have the same surface brightness at r=0. Scale
is 0.0500 or 155pc/pixel.
Fig.3: Top panel: Morphological comparison between
(from left to right): CGA1689,1 (i=22.2 mag); M32 modelled according to the fit by Graham (2002), projected
to A1689’s distance and seeing convolved (i=22.4 mag);
CGA1689,2 (i=22.8 mag). Bottom panel: An unresolved point
source (i=22.4 mag). The thumbnails are from ACS i-band
exposures of Abell 1689 and have the same intensity cuts.
On the ACS image in Fig. 3, the two A1689 CGs and
the simulated M32 appear very similar and are notably
more extended than a pure stellar source of comparable luminosity. In the surface brightness plots of Fig.4,
Discussion
We have discovered two M32-like galaxies in the
galaxy cluster Abell 1689, resulting from our search
for ultra compact dwarf galaxies. M32-like objects
are extremely rare. Finding two of them in the massive cluster Abell 1689 – each one in close projection
to a giant cluster member – indicates that tidal forces
seem to be important to form M32-like galaxies. If
fainter compact cluster members would be confirmed,
this would partially fill the magnitude gap between
Fornax UCDs and the M32-analoga, possibly implying similar formation mechanisms for both classes of
objects. M32-like galaxies might be the extreme end
of the newly discovered UCD class.
The next steps to address this very interesting issue are:
• (Re-)targetting the fainter UCD candidates to check
their cluster membership
• A photometric check whether or not the M32 twins
in Abell 1689 are at the faint end of a continous distribution of brighter compact ellipticals.
Acknowledgements
We thank the ESO User Support Group for carrying out the DDT spectroscopy (program 273.B-5008xs) in service mode. ACS was developed under NASA contract
NAS 5-32864, and this research has been partially supported by NASA grant NAG5-7697 and by an equipment grant from Sun Microsystems, Inc. The Space
Telescope Science Institute is operated by AURA Inc., under NASA contract NAS5-26555. 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”