CCD photometry and distances of six resolved irregular galaxies in Canes Venatici
Astron. Astrophys. Suppl. Ser.
CCD photometry and distances of six resolved irregular galaxies in Canes Venatici
N.A. Tikhonov 0
I.D. Karachentsev 0
0 Special Astrophysical Observatory, Russian Academy of Sciences, N. Arkhyz , Stavropolsky kraj, 357147 , Russia
The results of detailed B; V imaging of six nearby irregular galaxies with radial velocities V0 < 300 km/s are presented. Except for one all the galaxies have been resolved into stars for the rst time. Based on photometry of the brightest blue stars we derived the following distances to the galaxies: 3.6 Mpc for NGC 4163, 3.5 Mpc for NGC 4190, 8.6 Mpc for UGC 7298, 4.8 Mpc for UGC 7577, 3.7 Mpc for UGC 8308, and 4.0 Mpc for UGC 8320.
galaxies; irregular | galaxies; distances | galaxies; stellar content
1. Introduction
This article continues a series of publications
(Georgiev
et al. 1997; Makarova et al. 1997; Makarova et al. 1998)
devoted to the study of the 3D- structure of the nearest
scattered complex of galaxies in Canes Venatici (=CVn).
Being situated near the Local Supercluster equator, the
CVn cloud is projected onto other galaxy groups, which
complicates the analysis of its structure and kinematics.
About 90% of the population of the CVn cloud are
irregular dwarf galaxies. Application of the
Tully & Fisher
(1977)
method to them gives distance modulus estimates
with a rather low accuracy: (m − M ) = 1:0 − 1:5 mag.
In large-scale images of irregular galaxies in several
bands one can distinguish blue supergiants and use their
luminosity to determine distance moduli with a typical
error of about 0.4 mag
(Karachentsev & Tikhonov 1994)
.
All the six irregular galaxies studied here have corrected
radial velocities V0 < 300 km/s, suggesting close distances.
The results of our photometry con rm this suggestion.
2. Observations and photometry
Images of the six observed irregular dwarf galaxies are
reproduced from the Digital POSS in Fig. 1. We performed
observations of the galaxies in March 10, 1994 at the prime
focus of the 6 meter telescope (scale 8.6 arcsec/mm). A
CCD chip of 580 520 pixels was used with a readout noise
of 12 e and a pixel size of 0:18 0:24 microns, which yielded
a total eld of 120 80 arcsec. For each of the galaxies we
obtained B and V frames with a typical exposure 600 s
under a seeing of F W HM = 1:3 − 1:5 arcsec. The frames
were calibrated using the equatorial standard stars from
Landolt (1992)
.
After the initial processing of the frames we made
aperture photometry of stars using the PC VISTA
package (Tre ers & Richmond 1990) and the UMAG package
of
Georgiev (1995)
, which allows us to reduce the
inhomogeneity of the local background around the measured
stars.
The V images of the observed galaxies are presented
in Fig. 2. To show the structure of the central part as
well as of the periphery of each galaxy, we subtracted a
frame smoothed with a window of 10 F W HM . Based
on the results of our photometry the Colour-Magnitude
diagrams were derived for the brightest stars of the
galaxies. We reproduce them in Fig. 2, where star-like objects
are indicated with lled squares and slightly di use ones
with open squares. The brightest blue (B − V < 0:4) and
red (B − V > 1:6) stars are marked in Fig. 2. Their
apparent B magnitudes and B − V colours are presented in
Table 1. The complete tables of the photometric data are
accessible via FTP address: .
3. Discussion
Following the usual approach developed by
Sandage &
Tammann (1974)
and
de Vaucouleurs (1978
), we
determined the galaxy distance moduli from the mean apparent
magnitude of the three brightest blue stars, < B(3B) >,
from the relation:
0(B) = 1:51 < B(3B) > −0:51Bt − Ab + 4:14;
where Bt is the integral magnitude of the parent galaxy,
Ab is the galactic extinction, and the numerical coe
cients were calibrated
(Karachentsev & Tikhonov 1994)
from galaxies whose distances had been measured from
5 arcmin. North at top, East at
cepheids. In the cases when we found red supergiant
candidates, additional distance estimates were derived from
a similar relation:
0(R) = 1:10V (R1) − 0:10Bt − 0:76Ab + 7:00:
Some basic parameters of the galaxies are presented in
Table 2. Its lines contain:
(1) - the standard angular diameter of the galaxy in
arcmin from the Catalogue PGC-ROM
(Paturel et al. 1992)
;
(2) - the integrated magnitude of the galaxy either
from PGC or from our photometry (marked by \c");
(3) - the integrated colour index of the galaxy within
its standard diameter according our measurements;
(4) - the value of the galactic extinction from the
Catalogue PGC-ROM;
(5) - the radial velocity (in km/s) corrected for motion
with respect to the Local group centroid
(Karachentsev &
Makarov 1996)
;
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(6,7) - the mean magnitude and mean colour of the
three brightest blue stars;
(8,9) - the distance modulus of the galaxy via blue
stars and red one;
(10) - the galaxy distance (in Mpc), corresponding to
the blue modulus. Below we note some properties of the
galaxies observed.
NGC 4163 = UGC 7199. This blue compact galaxy
has a quite regular shape on its faint isophotes. The
integrated colour index increases smoothly from 0.41 in the
center up to 0.53 within the largest visible radii, which
may indicate the presence of two stellar populations in
the galaxy. The brightest blue object (=B0) has a
starlike pro le. However, we excluded it as a probable multiple
system when the distance was derived.
NGC 4190 = UGC 7232 = VV 104. The galaxy
seems to contain two overlapping regions of star formation.
For this reason
Vorontsov-Velyaminov (1959)
included this
peculiar object into the Atlas of interacting galaxies. The
brightest blue stars are concentrated in the SW part of
the galaxy. The distance estimate via the brightest red
star (R1) agrees well with that from the blue ones.
UGC 7298. This small blue galaxy of a low surface
brightness is distinctly resolved into stars, despite the fact
that all of them are fainter than V = 21:5 mag. The galaxy
distance, which is determined both via three blue stars
and via the brightest red one, exceeds distances of both
previous galaxies by more than two times. The distance
estimate, 8.6 Mpc, is unusual for a galaxy with a radial
velocity of only +253 km/s.
UGC 7577 = DDO 125. This and the next two
galaxies are in the list of dwarf irregular galaxies of
van
den Bergh (1966
). In spite of the large dimension of
the galaxy (4:2 2:4 arcmin), almost all its brightest
stars are concentrated within our CCD frame. From the
three brightest blue stars the galaxy distance modulus is
28.42 mag. The brightest red star (R1) yields 28.71 mag,
which is in agreement with the blue modulus. However, its
colour index, 1.58, does not formally satisfy the condition
B − V > 1:6 adopted above.
UGC 8308 = DDO 167. The galaxy has an
asymmetric shape and a low surface brightness. Its integral
colour index increases smoothly from 0.29 in the central
region up to 0.48 within the faintest isophotes. The
distance estimates from the blue stars and the red one agree
well with each other. But we have to note a large scatter
in the apparent magnitudes of red stars, which makes the
red modulus rather unreliable.
UGC 8320 = DDO 168. From the six considered
galaxies this is the only one that have already been
resolved into stars. According to
Bresolin et al. (1993)
its
distance is 3.3 Mpc. Photometry of 86 brightest stars
in B, R bands throughout the galaxy body was carried
out by
Hopp & Schulte-Ladbeck (1995)
. For the
common stars in the B band we nd good agreement between
our and their photometry. Because of its large dimension,
3:7 1:4 arcmin, a large fraction of the galaxy stars fall
outside our CCD frame. Therefore, estimating its distance
via blue stars, we took the star No. 44 from Hopp &
Schulte-Ladbeck's list as the second brightest blue star,
B2. The derived distance value, 4.0 Mpc, exceeds slightly
the estimate of
Bresolin et al. (1993)
.
4. Summary
For six irregular dwarf galaxies having radial velocities
V0 < 300 km/s and situated in the nearby CVn cloud we
derived large-scale CCD images in the B, V bands. All the
galaxies are clearly resolved into stars. Five of them have
been resolved for the rst time. Using the luminosity of
the brightest blue (and, sometimes, red) stars as a distance
indicator, we determined the distances to these galaxies.
Except for UGC 7298 with D = 8:6 Mpc, the distances
lie in the range of [3:5 − 4:8] Mpc. If the majority of the
CVn cloud members take part in the general cosmologic
expansion, the objects with the lowest radial velocities are
expected to be located on the near side of the cloud. The
data derived here do not contradict such a suggestion.
With the distances we have measured all the six
galaxies are actually dwarf systems, whose absolute magnitudes
lie between −12.3 and −15.6 mag. Judging by their total
colours, (B − V )t = [0:31 − 0:48], all the galaxies are at a
stage of active star formation. Two of the six, NGC 4163
and UGC 8308, show a colour increase from the centre
toward the periphery, which may indicate the presence of an
old stellar population. Accumulation of data on the
structure and population of dwarf galaxies in the nearest CVn
cloud, as well as on their location with respect to other
members of the complex,will allow to understand the
conditions which a ect the evolutional status of dwarf stellar
systems.
Acknowledgements. This work was supported by
INTASRFBR grant No. 95-IN-RU-1390 and RFBR grant No.
97-0217163.
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