Cam A: a new dwarf low-surface-brightness galaxy on the Local Group outskirts?
I. D. Karachentsev
L. N. Makarova
M. I. Andersen
Nordic Optical Telescope, Observatorio del Roque de los Muchachos
Apartado 474, E-38700 Santa Cruz de La Palma
Special Astrophysical Observatory, Russian Academy of Sciences
N. Arkhyz, KChR 357147
A B S T R A C T We present a deep I-band image of the nearby dwarf irregular galaxy Cam A obtained with the Nordic Optical Telescope with 0.6-arcsec seeing (FWHM). The distance to Cam A, DMW 1:9 Mpc, is estimated from the tip of the red giant branch seen at I 22:8 ^ 0:2 mag: The youngest stellar population of the galaxy seems to be of age about 100 Myr. Relying on its barycentric distance, DLG 1:6 Mpc, Cam A may be considered as a new probable peripheral member of the Local Group, or a member of the small group situated between the IC 342/Maffei group and the Local Group.
I N T R O D U C T I O N
A new low-surface-brightness galaxy with an angular diameter
,3 arcmin was found in the constellation Camelopardalis on the
POSS-I prints by Karachentsev (1994). This exemplified the
incompleteness of existing catalogues, particularly the Uppsala
General Catalogue of Galaxies (UGC) (Nilson 1973, 1974). This
object, called Cam A, is included in the list of nearby dwarf
galaxy candidates by Karachentseva & Karachentsev (1998).
Karachentsev et al. (1997) have noted Cam A as a probable
member of the IC 342/Maffei group. A detailed surface
photometry of the galaxy in the B and V bands was carried out by
Karachentseva et al. (1996) at the Observatoire de Haute-Provence
(OHP) 1.2-m telescope. In 1994 Cam A was observed with the
6m telescope CCD camera, which allows us to resolve its central
part into stars. A reproduction of Cam A from the Digital POSS-I
is presented in Fig. 1. The galaxy has a rather asymmetric shape,
which is also seen on its isophotes presented by Karachentseva
et al. (1996).
O B S E R VAT I O N S
R E S U LT S
Fig. 4 shows the colourmagnitude (CM) diagram derived from
the first pair of images (V and I). The stars located inside and
outside the apparent boundary of the galaxy are indicated by
squares and crosses, respectively. Because Cam A lies at a low
Galactic latitude, b 168: 2, foreground stars dominate among the
6 arcmin2; north is to the top and east to the left.
A new nearby dwarf galaxy
bright I , 21 mag stars. On the CM diagram one can distinguish
a group of bluish stars with kIl . 22 mag and kV 2 Il . 10:35,
which obviously belong to the brightest population of the galaxy.
According to Schlegel, Finkbeiner & Davis (1998), the Galactic
reddening in the Cam A direction is E B 2 V 0:21 or
Figure 5. Differential I-band number counts in the central part of Cam A
(dots) and in the surrounding field regions (dashed line) normalized to the
E V 2 I 0:26, which leads to a corrected colour of these
stars, kB 2 Vl0 . 10:08, typical for the blue supergiant branch.
However, the scanty statistics do not allow a distance estimate to
Cam A from these data.
The recently obtained composite image of the galaxy in the I
band with FWHM 0:61 arcsec reaches a deeper limit. Over the
whole field, daophot detected more than 1400 stars, and about
3/4 of them are concentrated within the apparent boundary of
Cam A. Their differential number counts are shown in Fig. 5. The
dots correspond to the star numbers obtained by a running average
with a window of 0.15 mag, while the dashed line represents the
number of foreground stars normalized to the same area. The data
show that the galaxy luminosity function rises steeply at
I 22:8 ^ 0:2. Assuming that this rise is caused by the red
giant branch, which appears at MI 24:0 (Lee, Freedman &
Madore 1993), we derive the distance modulus m 2 M 0
26:38 ^ 0:2; adopting an extinction AI 0:42.
The V, V 2 I diagram for Cam A was also presented by Gallart
et al. (1999) at the recent IAU Symposium 192 in Capetown. Their
CMD reaches about I 24 and finds the overdensity of red stars
with V 2 I . 2 fainter than I , 23. At the same time, Gallart
et al. note the danger of trying to estimate the distance to Cam A
when only limited data are available, because recent bursts of star
formation in irregulars like Cam A can produce a significant
population of stars brighter than usually found at the tip of the
D I S C U S S I O N
telescope in the velocity range [2600; 11600] km s21 by
Karachentseva et al. (1996). However, H i emission at this radial
velocity is widespread in this area of the sky (e.g. Hartmann &
Burton 1997). A decision about possible H i emission from Cam A
can only be made with synthesis observations.
We know there is no straightforward boundary between the
Local Group and other prominent nearby groups such as the
Sculptor, M81, Centaurus and Maffei/IC 342 groups. Within about
88 from Cam A there are two other nearby dwarf galaxies: NGC
1569 and UGCA 92, with corrected radial velocities 1102 and
189 km s21 and distances 1.7 and 1.8 Mpc, respectively
(Karachentsev et al. 1997). It is probable that NGC 1569,
UGCA 92 and Cam A constitute a small external grouping, just
outside the Local Group boundary, that merges further out with
members of the Maffei/IC 342 group. This small group scenario
lends strong support to the putative radial velocity for Cam A as
well the nearby distance estimate for it.
A C K N O W L E D G M E N T S
We thank J. Lequeux, M. Irwin, W. Huchtmeier and N. Tikhonov
for valuable discussions. This work is supported by INTAS-RFBR
grant 95-IN-RU-1390 and DFG grant KS-9112.
R E F E R E N C E S
This paper has been typeset from a TEX/LATEX file prepared by the author.