A&A 383, 125-136 (2002)
DOI: 10.1051/0004-6361:20011741
I. D. Karachentsev 1 - A. E. Dolphin 2 - D. Geisler 3 - E. K. Grebel 4 - P. Guhathakurta 5,
- P. W. Hodge 6 -
V. E. Karachentseva 7 - A. Sarajedini 8 - P. Seitzer 9 - M. E. Sharina 1,10
1 - Special Astrophysical Observatory, Russian Academy
of Sciences, N. Arkhyz, KChR, 369167, Russia
2 - Kitt Peak National Observatory, National Optical Astronomy Observatories,
PO Box 26732, Tucson,
AZ 85726, USA
3 - Departamento de Física, Grupo de Astronomía, Universidad de Concepción,
Casilla 160-C, Concepción, Chile
4 - Max-Planck-Institut für Astronomie, Königstuhl 17, 69117
Heidelberg, Germany
5 - UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz,
CA 95064, USA
6 - Department of Astronomy, University of Washington, Box 351580, Seattle, WA, USA
7 - Astronomical Observatory of Kiev University, 04053, Observatorna 3, Kiev,
Ukraine
8 - Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
9 - Department of Astronomy, University of Michigan, 830 Dennison Building,
Ann Arbor, MI 48109, USA
10 - Isaac Newton Institute, Chile, SAO Branch
Received 25 September 2001 / Accepted 5 December 2001
Abstract
We present Hubble Space Telescope/WFPC2 images of the galaxies NGC 2366,
NGC 2976, NGC 4236, IC 2574, DDO 53, DDO 82, DDO 165, Holmberg I, Holmberg II,
Holmberg IX, K52, K73, BK3N, Garland, and A0952+69 in the M 81 complex.
Their true distance moduli, derived from the brightness of the tip of the
red giant branch, lie in the range of 27
52 (NGC 2366) to 28
30 (DDO 165),
with a median of 27
91, which is typical for other known M 81 group members.
Using distances and radial velocities of about 50 galaxies in and around
the M 81/NGC 2403 complex, we find the radius of the zero-velocity surface of
the M 81 group to be
) Mpc, which yields a total mass
and a total
mass-to-luminosity ratio
.
The total mass
within R0 agrees well
with the sum of masses estimated via the virial theorem
(
and
from orbital motions
of
companions around M 81 and NGC 2403.
We suggest that most of the dark matter in the group is concentrated around
the luminous matter, allowing us to explain the observed asymmetry of
the peculiar motions of the M 81 companions. M 81 itself has a peculiar
velocity of about 130 km s-1 with respect to the local Hubble flow, but the
centroid of the M 81/NGC 2403 complex is almost at rest with respect to
Hubble flow
km s-1).
Key words: galaxies: dwarf - galaxies: distances and redshifts - galaxies: structure - galaxies: kinematics and dynamics
The association of galaxies around M 81 is one of the nearest prominent
groups in the vicinity of the Local Group. Both of the groups
have a similar morphological population and binary structure. According to
Tammann & Sandage (1968) and de Vaucouleurs (1978), the M 81 group is an
extended filament with an angular size of
around the two bright galaxies NGC 2403 and NGC 4236.
The space between the M 81 group and the Local Group
is the nearest example of a cosmic
mini-void. It has a diameter of
3 Mpc and is apparently free of any
galaxy brighter than
in absolute magnitude. Until 1998 accurate
distance moduli derived
via Cepheid were known only for the two brightest galaxies M 81
(Freedman et al. 1994) and NGC 2403 (Freedman & Madore 1988). The lack
of distance estimates to other galaxies in the group makes the analysis of its
structure and kinematics difficult. According to Karachentsev (1996)
the physical radius of the M 81 group does not exceed
,
and
all companions to M 81, both dwarf spheroidal (dSph) galaxies and
dwarf irregular (dIrr) galaxies, lie within this radius.
Since 1998 many more distance measurements in the M 81 group have become
available thanks to observations with the Hubble Space Telescope (HST) and
the use of the tip of the red giant branch (TRGB) method. Based on the
brightness of the tip of the RGB, Caldwell et al. (1998), Lynds et al. (1998),
and Sakai & Madore (1999) determined distances to four dwarf galaxies in
the M 81 group: BK5N, F8D1, UGC 6456, and M 82. Many more
probable members of the M 81
group were included in the snapshot survey of nearby galaxies
(programs SNAP 8192 and 8601, PI: Seitzer) with the Wide Field and Planetary
Camera 2 (WFPC2) aboard HST.
As a result, we measured the TRGB distances to nine dSph galaxies
(Karachentsev et al. 1999, 2000, 2001) and to one dIrr (Dolphin et al. 2001).
In this paper we present new distance estimates for 13 late type galaxies
observed with the HST. The new availability
of accurate distances to most galaxies
of the NGC 2403/M 81/NGC 4236 complex allows us to consider the 3D- structure,
kinematics, and dynamics of the complex, a map of which is shown in
Fig. 1. The stellar populations and star formation history of the
observed galaxies will be discussed by us in the next paper.
![]() |
Figure 1: The distribution of galaxies in the NGC 2403/M 81/NGC 4236 complex in supergalactic coordinates. Top: overview of the entire region, bottom: the central part of the M 81 group. |
Open with DEXTER |
WFPC2 images of 15 objects in the M 81 group were obtained during 2000 July 11
to 2001 June 27 as part of our snapshot survey of probable nearby
galaxies (SNAP 8601; Seitzer et al. 1999). Usually our target galaxies
were centered on the WF3 chip, but for some bright targets the WFPC2 position
was shifted towards the galaxy periphery to decrease a stellar crowding
effect. The 600 s exposures were taken in F606W and F814W for each
object. Digital sky survey (DSS) images (POSS-I,E) of the fifteen galaxies are
shown in Fig. 2, on which the HST WFPC2 footprints are superimposed.
The field size of the DSS images is 10
by 10
each.
The photometric reduction was made using the HSTphot stellar photometry
package described in detail by Dolphin (2000a). After removing
cosmic rays with the HSTphot cleansep routine,
simultaneous photometry was performed on the F606W and F814W frames
using multiphot, with aperture corrections for an
aperture with
radius. Charge-transfer inefficiency corrections
and calibrations were then applied based on the Dolphin (2000b) formulae,
producing VI photometry for all stars detected in both images.
Because of the relatively small field of the Planetary Camera (PC) chip,
very few bright stars were available for the computation of an aperture
correction. Thus the PC photometry was omitted from further analysis.
Additionally, stars with a signal-to-noise ratio <5,
,
or
sharpness
in each
exposure were eliminated from the final photometry list.
We estimate the uncertainty of the photometric zeropoint to be within
(Dolphin 2000b).
Figure 3 shows mosaic images of the galaxies, where both filters were combined. The compass in each field indicates the North and East directions.
In Fig. 4, I versus (V-I) color magnitude diagrams (CMDs) for fifteen
observed galaxies are presented. The total number of stars per image ranges
from 20000 (NGC 2976, NGC 4236) to
1000
(for the faintest objects).
The left panels show the CMDs for the central WFC3 field.
The middle panels represent the CMD for the neighbouring
halves of WFC2 and WFC4 (x < 425 pixels in WFC2 and y < 425 pixels
in WFC4), and the right panels are comprised of stars found in the
remaining outer halves of WFC2 and WFC4. Such a representation allows us
to show the relative contribution
of foreground stars when the galaxy diameter is
comparable with the WF3 size.
As demonstrated by Lee et al. (1993), the TRGB
is a reliable distance indicator that is relatively independent
of age and metallicity. For metal-poor systems the TRGB may be assumed
to be at
MI = -4.05 mag (Da Costa & Armandroff 1990). To determine the
TRGB location we obtained the Gaussian-smoothed I-band luminosity
function for red stars with colors V-I within
with respect to
the mean <V-I> for expected RGB stars. Following Sakai et al. (1996),
we used a Sobel edge-detection filter. The position of the TRGB
was identified with the peak in the filter response function. The resulting
luminosity functions and the Sobel filtered luminosity functions are
shown in the upper and lower panels of Fig. 5, respectively. A summary of
the resulting distance moduli for the observed galaxies is given
in Table 1. Its columns contain: (1) galaxy name, (2) equatorial
coordinates corresponding to the WF3 center, (3) Galactic extinction in the
I-band from IRAS/DIRBE data (Schlegel et al. 1998), (4) position of the
TRGB derived with the Sobel filter, (5) true distance modulus.
Galaxy | RA (2000.0) Dec | AI |
![]() |
(m-M)0 |
NGC 4236 | 12 16 43.2 +69 27 56 | 0.03 |
![]() |
28.24 |
IC 2574 | 10 28 22.3 +68 24 58 | 0.07 |
![]() |
28.02 |
NGC 2976 | 09 47 15.6 +67 54 49 | 0.14 |
![]() |
27.76 |
Holm II | 08 19 05.8 +70 42 50 | 0.06 |
![]() |
27.65 |
NGC 2366 | 07 28 51.9 +69 12 18 | 0.07 |
![]() |
27.52 |
DDO 165 | 13 06 26.7 +67 42 14 | 0.05 |
![]() |
28.30 |
DDO 82 | 10 30 34.9 +70 37 09 | 0.08 |
![]() |
28.01 |
Holm I | 09 40 28.1 +71 11 10 | 0.09 |
![]() |
27.92 |
Holm IX | 09 57 32.0 +69 02 45 | 0.15 | ? | |
DDO 53 | 08 34 06.5 +66 10 44 | 0.07 |
![]() |
27.76 |
K 52 | 08 23 56.0 +71 01 45 | 0.04 |
![]() |
27.75 |
Garland | 10 03 42.7 +68 41 27 | 0.13 |
![]() |
27.91 |
K 73 | 10 52 57.1 +69 32 58 | 0.04 |
![]() |
27.91 |
BK3N | 09 53 48.5 +68 57 51 | 0.16 |
![]() |
28.02 |
A0952+69 | 09 57 32.6 +69 17 00 | 0.16 |
![]() |
27.94 |
Some additional comments about the galaxy properties are given below. The galaxies are listed in a sequence of decreasing brightness.
NGC 4236. This galaxy of SBdm type has an angular
dimension of
,
much larger than the WFPC2 field of view. It is well resolved into stars
(Sandage & Bedke 1988). Based on the brightest stars, de Vaucouleurs (1978)
and Tikhonov et al. (1991) estimated its true distance modulus to be 27.65 mag
and 27.56 mag, respectively. The CMD of NGC 4236
in Fig. 4 shows the presence of RGB and asymptotic giant branch (AGB)
stars, as well as bright blue stars on the upper main sequence. We
determined the TRGB position to be
mag, which yields the true
distance modulus
(m-M)0= 28.24 mag. The error can be estimated as 1/2 of
the peak width at 62% of its maximum and turns out to be
.
However, the mean-square scatter of TRGB positions derived separately for
WF2, WF3, and WF4 is only
.
IC 2574 = DDO 81. This galaxy of SABm type with angular dimensions of
has distance modulus estimates of 27.72 mag
(de Vaucouleurs 1978)
and 27.89 mag (Tikhonov et al. 1991) via the brightest stars. In general its
CDM in Fig. 4 looks similar to the NGC 4236 diagram. The derived TRGB position
is
,
but variations of TRGB positions between different chips
do not exceed
.
From our data the true distance modulus
is 28.02 mag.
Holmberg II = DDO 50 = UGC 4305. This irregular galaxy of the
Magellanic (Im) type with numerous bright HII regions and blue stellar
complexes has a size of
.
From the brightest stars
de Vaucouleurs (1978) and Tikhonov et al. (1992) estimated its distance
modulus to be 27.79 mag and 27.78 mag, respectively. Our photometry gives
mag, and
(m-M)0 = 27.65 mag.
NGC 2976. Unlike the two previous galaxies, NGC 2976 has a high surface
brightness and is classified as Sc peculiar. Its central body of
is spotted due to a lot of dust clouds and stellar complexes. Based on the
luminosity of the brightest stars Karachentsev et al. (1991) estimated its
distance modulus to be 28.30 mag. From our data the TRGB position for NGC 2976
corresponds to
mag, which (with an
AI = 0.14 mag)
yields a true distance
modulus 27.76 mag.
NGC 2366 = DDO 42. This is an irregular galaxy with an
angular size of
and with a prominent
star formation region on the southern edge (see Sandage & Bedke 1988),
which is partially covered by the WF4. Photometry of the brightest stars in
the galaxy yielded distance moduli 27.14 mag (de Vaucouleurs 1978) and 27.69
mag (Tikhonov et al. 1991). In our data we find
the TRGB position at
mag, giving the true distance modulus 27.52 mag. Note that the scatter of
the I(TRGB) magnitude between different chips is only
.
DDO 165 = UGC 8201. This irregular galaxy with an angular
size of
has a
very sharp southern boundary. DDO 165 was resolved into stars by
Karachentsev et al. (1991), who derived a distance modulus of 28.44 mag.
In the HST photometry the TRGB is located at
,
which
corresponds to a true distance modulus of 28.30 mag.
DDO 82 = UGC 5692. This
is a compact irregular galaxy with a size of
.
A bright red star is projected onto its southeastern side. In the central part
of DDO 82 some dusty patches were noted (Karachentseva et al. 1985),
which are seen also in the WF3 field (Fig. 3). From the brightest stars
Karachentsev et al. (1994) determined
(m-M)0 = 28.26 mag. The HST
photometry gives
mag
and a corresponding distance modulus of
28.01 mag. The area where DDO 82 is located has been observed repeatedly
in the 21-cm line, but DDO 82 was never detected
due to confusion with Galactic H I. In its optical spectra strong
H
emission with
km s-1 is seen
(Karachentsev & Karachentseva 1984).
Holmberg I = DDO 63 = UGC 5139. This is an irregular dwarf galaxy of
low surface brightness with an angular size of
.
Its distance
modulus estimates via the brightest stars are discrepant: 27.68 mag
(de Vaucouleurs 1978) and 29.11 mag (Tikhonov et al. 1992). The HST photometry
yields
mag and
(m-M)0 = 27.92 mag.
Holmberg IX = DDO 66 = K62 = UGC 5336. This dIrr galaxy has a size of
and an overall shape reminiscent of Holmberg I. It is
situated several arcminutes away from the eastern spiral arm of M 81.
On the H I map of Boyce et al. (2001) its location corresponds
to a local density maximum with a heliocentric velocity of +50 km s-1.
Its distance modulus estimates from photometry of the brightest stars
are rather discrepant: 28.8 mag (Sandage 1984), 30.1 mag (Hopp & Schulte-Ladbeck
1987), 27.5 mag (Davidge & Jones 1989), and 27.67 mag (Georgiev et al. 1991).
Surface photometry of Holmberg IX carried out by Makarova (1999) yields
an integrated magnitude of
mag and an unusually blue integrated color,
mag. Our attempt to determine the distance to Holmberg IX from
the WFPC2 photometry meets with an unexpected problem. The CMD of the
galaxy shows a well-populated red supergiant branch, but without clear
signs of the RGB. Boyce et al. (2001) suggest Holmberg IX might be
a young galaxy, condensing out of the H I tidal material. The apparent
absence of an old stellar population looks like direct evidence for the recent
formation of Holmberg IX. Obviously, this peculiar system requires further
investigation with HST.
DDO 53 = UGC 4459 = VII Zw 238. The blue dwarf galaxy
of
size was resolved into stars by Fisher & Tully (1979). Its distance
modulus from the brightest stars is 27.44 mag (Karachentsev et al. 1994).
Our HST photometry gives for DDO 53
mag,
yielding a true distance modulus of 27.76 mag.
K 52 = M 81dwA. This low surface brightness dwarf galaxy with an angular
size of
was found by Karachentseva (1968), and
re-discovered by Lo & Sargent (1979), who detected it in the H I line.
Tikhonov & Karachentsev (1993) estimated for K52 a distance modulus of
27.35 mag from the brightest blue stars. Our photometry gives
mag, and
(m-M)0 = 27.75 mag.
Garland. The peculiar scattered structure with a size
of
,
consisting of blue stars and H II regions, was found on the southern side
of NGC 3077 by Karachentseva et al. (1985) and nicknamed "Garland''.
Its small part was noted before by Barbieri et al. (1974) as a
probable stellar complex belonging to NGC 3077. Spectral observations by
Karachentsev et al. (1985) showed that the Garland has appreciable internal
motions (
35 km s-1). Its mean radial velocity,
km s-1, is close
to the radial velocity of NGC 3077,
km s-1. Sharina (1991)
carried out photometry of the brightest stars in the Garland and derived
a distance modulus of 27.76 mag. A concentration of neutral hydrogen in the
Garland position was seen on the H I map of the M 81 group obtained by
Appleton et al. (1981). Recently Heithausen & Walter (2000) revealed a
giant molecular cloud in the same region with a mass of about
.
The origin of the Garland remains unclear: either it is a young
stellar complex in the NGC 3077 periphery, a tidal tail,
or a separate dwarf galaxy
disturbed by tidal interaction with NGC 3077.
Our HST photometry gives for the Garland field
mag and
(m-M)0 = 27.91 mag. Unlike the blue stars, the distribution of
RGB stars across the WFPC2 field shows them to be
concentrated strongly towards the NGC 3077. Consequently,
our estimate of the distance modulus applies in fact to NGC 3077, but not to
the Garland. Recently the Garland region has been observed with
WFPC2 by Sakai & Madore (2001). The location of their "Field II'' overlaps
partially with ours. They find the TRGB position to be at
mag in excellent agreement with our estimate.
K 73. This is a small (
)
irregular dwarf galaxy well
resolved into stars (Karachentseva et al. 1985). Its H I spectrum
contains two emission lines with heliocentric velocities -132 km s-1
(Huchtmeier & Skillman 1998) and +115 km s-1 (van Driel et al. 1998).
The recent H I
observations by Huchtmeier (private communication) give a preference to
the value of +115 km s-1 as belonging to the K73 itself rather than to
the Local H I. Based on the luminosity of three the brightest blue stars
Tikhonov & Karachentsev (1993) estimated the distance modulus of K73 to
be
mag. Our HST photometry yields a TRGB position corresponding
to the distance modulus
mag in good agreement with the previous
estimate.
BK3N. This very faint dwarf system with an angular
extent of
is situated
11
to the southwest from the M 81 center. The galaxy was discovered by
Börngen &
Karachentseva (1982) on the Tautenburg Schmidt telescope plates. Tikhonov
& Karachentsev (1993) resolved BK3N into stars and derived a distance
modulus of 27.23 mag via the three brightest blue stars. With
mag (Makarova 1999), BK3N is the galaxy with the faintest absolute magnitude
known
outside the Local Group. Our HST photometry (Fig. 4) shows that
all the detected blue stars are concentrated within BK3N. In addition,
a population of faint red stars is seen in the WF2, WF3, and WF4 fields.
Assuming that they are RGB stars, we obtain
mag,
which yields a distance modulus of 28.02 mag. Because the red stars do not
show any increase in density toward the main body of BK3N, we suggest that
they may not actually be associated with BK3N, but could instead be
part of the extended halo RGB population of M 81.
The TRGB distance modulus agrees within errors with
the distance modulus
derived for M 81 from Cepheids
(Freedman et al. 1994; Ferrarese et al. 2000). Hence BKN3 is very likely
more distant than M 81 and observed through M 81's halo.
A0952+69 = Arp's loop. This ring-like structure of extremely low surface
brightness was found by Arp (1965) around the northern side of M 81. The
brightest part of this structure, A0952+69, situated 17
NE from
the M 81 center, is also seen as overdensity on the H I maps derived by
Gottesman & Veliachev (1975) and Yun et al. (1994). On large-scale
photographs of this part of Arp's loop Efremov et al. (1986) found
a lot of very faint blue stars and diffuse objects, prompting them to
suggest that there is ongoing star formation in the loop,
probably stimulated by the interaction of M 81 with M 82 and NGC 3077.
The CMD derived with WFPC2 shows the presence of blue stars
over the whole field of view. The brightest of them are
.
Apart of them we recognize also red stars, which may belong to
Arp's loop itself or to the periphery of the M 81 disk. The TRGB
position for stars in the WF3 is
mag, yielding a
distance modulus 27.94 mag, which differs very little from the M 81 distance.
Name | SGL | SGB | ![]() |
![]() |
Type | ![]() |
AB | (m-M)0 | D | Rp | R |
![]() |
km s-1 | Mpc | kpc | kpc | km s-1 | ||||||||
M 81 | 41
![]() |
0
![]() |
0
![]() |
106 | Sb | 7
![]() |
0
![]() |
27
![]() |
3.63 | 0 | 0 | 0 |
HoIX | 41.27 | 0.69 | 0.18 | 189 | Ir | 14.53 | 0.35 | 3.7: | 11 | 11 | 82 | |
BK3N | 41.07 | 0.41 | 0.18 | 101 | Ir | 18.78 | 0.35 | 28.02 | 4.02 | 11 | 391 | -5 |
A0952 | 41.10 | 0.87 | 0.28 | 143 | Ir | 16.8 | 0.37 | 27.94 | 3.87: | 18 | 241 | 136 |
K61 | 41.54 | 0.33 | 0.52 | 23 | sph? | 15.25 | 0.30 | 27.78 | 3.60 | 33 | 44 | 57 |
M 82 | 40.72 | 1.05 | 0.62 | 360 | Ir | 9.06 | 0.58 | 27.74 | 3.53 | 39 | 107 | -236 |
N3077 | 41.85 | 0.83 | 0.77 | 153 | Ir | 10.46 | 0.29 | 27.91 | 3.82 | 49 | 196 | 46 |
FM1 | 40.62 | -0.26 | 0.98 | sph | 17.50 | 0.31 | 27.67 | 3.42 | 62 | 218 | ||
BK5N | 42.28 | 0.58 | 1.16 | sph | 17.4 | 0.25 | 27.89 | 3.78 | 73 | 168 | ||
IKN | 42.40 | 0.91 | 1.32 | sph | 17.0: | 0.60 | 3.7: | 84 | 110 | |||
N2976 | 41.33 | -0.78 | 1.38 | 139 | Sm | 10.94 | 0.30 | 27.76 | 3.56 | 87 | 111 | -18 |
U5423 | 40.83 | 2.12 | 1.55 | 493 | BCG | 14.42 | 0.34 | 5.3 | 98 | 1674 | 386 | |
K64 | 42.74 | 0.44 | 1.63 | sph | 15.46 | 0.25 | 27.84 | 3.70 | 103 | 126 | ||
KK77 | 41.80 | -0.90 | 1.64 | sph | 16.3 | 0.65 | 27.71 | 3.48 | 104 | 181 | ||
F8D1 | 41.46 | -1.29 | 1.91 | sph | 15.7 | 0.38 | 27.88 | 3.77 | 121 | 187 | ||
HIJASS | 42.98 | 1.98 | 2.32 | 187 | HI | 0.09 | 3.7: | 147 | 164 | 40 | ||
HoI | 38.76 | 1.34 | 2.47 | 285 | Ir | 13.64 | 0.21 | 27.92 | 3.84 | 156 | 265 | 147 |
DDO 71 | 43.52 | -0.58 | 2.67 | 12 | sph | 15.95 | 0.37 | 27.72 | 3.50 | 169 | 211 | 59 |
HS117 | 41.17 | 3.59 | 3.00 | 116 | Ir | 16.5: | 0.49 | 3.7: | 190 | 204 | 9 | |
IC2574 | 43.63 | 2.31 | 3.05 | 186 | Sm | 10.84 | 0.16 | 28.02 | 4.02 | 193 | 440 | 74 |
DDO 78 | 44.10 | 1.69 | 3.18 | 191 | sph | 15.8 | 0.12 | 27.85 | 3.72 | 201 | 223 | 42 |
DDO 82 | 42.04 | 3.85 | 3.38 | 191 | Im | 13.57 | 0.19 | 28.01 | 4.00 | 214 | 433 | 77 |
BK6N | 45.88 | 1.27 | 4.81 | sph | 16.9 | 0.05 | 27.93 | 3.85 | 304 | 383 | ||
K73 | 44.03 | 4.75 | 5.07 | 263 | Ir | 17.09 | 0.09 | 27.91 | 3.70 | 321 | 380 | 92 |
KKH57 | 45.52 | -3.34 | 5.90 | sph | 17.86 | 0.09 | 27.97 | 3.93 | 373 | 491 | ||
K72 | 47.18 | 2.23 | 6.28 | 466 | Ir | 15.00 | 0.05 | 7.4 | 397 | 3812 | 359 | |
VKN | 35.84 | -3.75 | 6.83 | sph | 17.7: | 0.34 | 3.4: | 432 | 478 | |||
U4483 | 35.02 | -2.65 | 6.90 | 301 | BCG | 14.95 | 0.15 | 27.53 | 3.21 | 436 | 588 | -122 |
K52 | 33.52 | -1.93 | 8.00 | 267 | Ir | 16.35 | 0.09 | 27.75 | 3.55 | 505 | 507 | 0 |
DDO 53 | 36.25 | -6.04 | 8.22 | 150 | Ir | 14.55 | 0.16 | 27.76 | 3.56 | 519 | 520 | 13 |
HoII | 33.26 | -2.36 | 8.40 | 312 | Im | 11.09 | 0.14 | 27.65 | 3.39 | 530 | 567 | -59 |
U6456 | 36.87 | 11.40 | 11.60 | 97 | BCD | 14.28 | 0.16 | 28.23 | 4.43 | 730 | 1139 | 8 |
N4236 | 47.11 | 11.38 | 12.32 | 160 | Sd | 10.06 | 0.06 | 28.24 | 4.45 | 775 | 1190 | 57 |
N2366 | 29.46 | -4.86 | 12.87 | 254 | Im | 11.68 | 0.16 | 27.52 | 3.19 | 809 | 881 | -38 |
DDO44 | 30.31 | -7.09 | 13.26 | sph | 15.64 | 0.19 | 27.52 | 3.19 | 833 | 901 | ||
U7242 | 50.37 | 10.27 | 13.35 | 213 | Ir | 14.60 | 0.09 | 4.3 | 838 | 1137 | 92 | |
N2403 | 30.80 | -8.31 | 13.60 | 267 | Sc | 8.82 | 0.18 | 27.59 | 3.30 | 854 | 884 | -18 |
KKH79 | 55.01 | 9.66 | 16.59 | 446 | Ir | 17.7 | 0.08 | 6.4 | 1036 | 3099 | 334 | |
DDO 165 | 49.62 | 15.59 | 17.18 | 199 | Im | 12.85 | 0.10 | 28.30 | 4.57 | 1072 | 1538 | 92 |
N3738 | 59.56 | 1.79 | 18.47 | 310 | Ir | 12.13 | 0.05 | 28.40 | 4.75 | 1150 | 1741 | 180 |
KKH34 | 22.54 | -0.41 | 18.61 | 298 | Ir | 17.1 | 1.08 | 28.57 | 5.18 | 1158 | 2090 | 183 |
N4605 | 55.47 | 12.02 | 19.67 | 278 | Sm | 10.89 | 0.06 | 5.2 | 1222 | 2160 | 167 | |
N4068 | 62.95 | 4.93 | 22.26 | 290 | Im | 13.19 | 0.09 | 5.3 | 1375 | 2378 | 180 | |
U6541 | 64.19 | -0.79 | 23.12 | 303 | Ir | 14.23 | 0.09 | 27.99 | 3.96 | 1425 | 1555 | 122 |
U7298 | 63.89 | 6.63 | 23.50 | 253 | Ir | 15.95 | 0.10 | 28.10 | 4.17 | 1447 | 1674 | 125 |
N5204 | 59.40 | 17.85 | 24.91 | 341 | Sm | 11.73 | 0.05 | 28.24 | 4.45 | 1529 | 1918 | 184 |
CamA | 16.09 | 1.87 | 25.06 | 157 | Ir | 14.85 | 0.93 | 27.89 | 3.78 | 1538 | 1614 | 62 |
N1560 | 16.03 | 0.79 | 25.09 | 169 | Sd | 12.16 | 0.81 | 27.63 | 3.36 | 1539 | 1541 | 48 |
N2537 | 41.42 | -25.82 | 26.41 | 477 | Sm | 12.32 | 0.32 | 6.9 | 1615 | 3990 | 368 | |
CamB | 15.04 | -4.21 | 26.50 | 264 | Ir | 16.71 | 0.94 | 27.60 | 3.31 | 1620 | 1621 | 54 |
N3741 | 67.96 | -2.08 | 26.97 | 263 | Ir | 14.3 | 0.10 | 27.48 | 3.13 | 1646 | 1650 | 37 |
UA281 | 67.94 | 7.06 | 27.59 | 350 | BCG | 15.15 | 0.06 | 5.7 | 1681 | 2998 | 240 | |
UA105 | 14.99 | -9.26 | 27.82 | 280 | Im | 13.9 | 1.35 | 27.57 | 3.26 | 1694 | 1695 | 44 |
U8508 | 63.09 | 17.91 | 27.98 | 187 | Ir | 14.12 | 0.06 | 27.04 | 2.56 | 1703 | 1821 | 14 |
N4144 | 69.07 | 3.83 | 28.14 | 319 | Scd | 12.16 | 0.06 | 9.8: | 1712 | 6817 | 231 | |
KK109 | 70.24 | -0.92 | 29.16 | 243 | Ir | 17.5 | 0.08 | 28.36 | 4.70 | 1769 | 2339 | 138 |
U4426 | 46.10 | -28.52 | 29.50 | 399 | Im | 15.0 | 0.16 | 5.7 | 1787 | 3106 | 281 | |
N1569 | 11.91 | -4.92 | 29.69 | 105 | BCG | 11.86 | 3.02 | 2.5: | 1798 | 1913 | 45 | |
KKH87 | 60.42 | 23.98 | 29.95 | 475 | Ir | 16.1 | 0.05 | 6.8: | 1812 | 4079 | 367 |
The availability of accurate distance estimates and radial velocities for
most the galaxies around M 81, NGC 2403, and NGC 4236 allows us to study
the 3-D structure and kinematics of the nearest galaxy complex. Basic
data on galaxies in the region are presented in Table 2. Its columns
contain: (1) galaxy name, (2, 3) supergalactic coordinates, (4) angular
separation
from M 81 in degrees, (5) radial velocity in km s-1in the LG rest frame, (6) morphological type, (7) apparent integrated
magnitude from NED or from Makarova (1999) and other recent sources,
(8) Galactic extinction from Schlegel et al. (1998), (9) distance
modulus, (10) linear distance to the galaxy in Mpc, where values with
one decimal number correspond to rough distance estimates derived via
the brightest stars (Karachentsev & Tikhonov 1994); when an individual
distance estimate to a galaxy was absent (for instance, for Holmberg IX or
IKN) the average group distance is adopted, (11, 12) projected (
)
and
de-projected spatial (R) separation of a galaxy from M 81, calculated as
In Fig. 6 we present the resulting 3-D map of the M 81 group and its
surroundings in the volume of
Mpc in the supergalactic
Cartesian coordinates. The three brightest spiral galaxies: NGC 2403,
M 81, and NGC 4236 are shown as ellipsoids, and dIrr and dSph systems
are shown as small filled and light circles, respectively. The observed
3-D distribution proves the suggestion of Tammann & Sandage (1968) regarding
filamentary structure of the galaxy complex around M 81. However, the
observed variation of the mean radial velocity from the nearby end of the
filament (NGC 2403) to the far end (NGC 4236) does not fit the case of
free expansion of that structure: NGC 4236 and its neighbours move away
from the compact subgroup of M 81 companions, and NGC 2403 with its
neighbouring dwarf galaxies approach the M 81. The distribution of dwarf
spheroidal galaxies demonstrates much stronger concentration towards M 81
than the distribution of dIrrs. Such a morphological segregation is well
known in the Local Group, as well as in many clusters of galaxies.
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Figure 6: The 3-dimensional view of the M 81 group and its surroundings in supergalactic Cartesian coordinates. The three brightest spirals NGC 2403, M 81, and NGC 4236 are show as ellipsoids, and dIrr and dSph galaxies are shown as small dark and light dots, respectively. |
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Figure 7: A schematic disposition of a galaxy with respect to the M 81 and the Local Group. |
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According to Sandage (1986) and Lynden-Bell et al. (1988) any sufficiently
dense group or cluster may be characterized by a spherical "zero-velocity
surface'', which separates the group as overdensity against the
homogeneous cosmic expansion. In case of spherical symmetry, the radius
of the sphere, R0, is related to the total mass of the group, M0,
and to the Hubble constant, H0, as
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(1) |
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(2) |
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Figure 8:
Distribution of the radial velocity difference and the spatial
distance of nearby galaxies with respect to M 81. Galaxies with accurate and
with rough distance estimates are indicated by filled and open squares,
respectively. These data yield a radius of the zero-velocity surface of
(
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Note that the number of companions of M 81 with positive and with negative
peculiar velocities is rather asymmetric, N+:N- = 13:3. This asymmetry,
already found by Arp (1982), may be caused by a selection effect when
the H I line emission of galaxies with
km s-1is blended with strong
Galactic emission. This explanation, however, requires a lot of missing
galaxies in the M 81 group. Another possible explanation of the Arp paradox
may lie in the considerable peculiar velocity of M 81 with
respect to the group centroid (
50 km s-1).
The galaxy M 81 with its morphological type Sb, absolute magnitude
MB =-20.47 mag, and amplitude of rotational velocity
km s-1is very similar to the Milky Way and M 31. In the zone of its predominant
gravitational influence,
kpc, 25 probable companions are known.
We distinguished their names in Table 2 with bold print. At the moment,
16 of these objects have measured radial velocities, which allows us to
estimate the group mass.
The total mass of a group can be estimated from the virial balance of
kinetic and potential energy of the system (Limber & Mathews 1960)
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(3) |
Bahcall & Tremaine (1981) and other authors noted that Eq. (3) is an
inefficient mass estimator because the variance of the harmonic projected
separations is logarithmically infinite. If a group contains a dominant
massive galaxy, we may estimate its mass from the orbital motions of its
companions, averaging the value
). In the case of arbitrarily
oriented Keplerian orbits with the eccentricity e we have the more robust
mass estimator
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(4) |
From Eq. (1) with
Mpc and H0 = 70 km s-1 Mpc-1the total mass M0 of the M 81 group within the sphere of the zero-velocity
surface is
The existence of the correlation between the luminosity of spiral galaxies
and the amplitude of their rotation (Tully & Fisher 1977) means that
the distribution of dark matter (halos) follows closely the distribution of
luminous matter. The kinematics of the M 81 subgroup
provides us with unique opportunity to test this assumption.
The main part (90%) of the subgroup luminosity belongs to
M 81 and M 82. Their corrected radial velocities are +106 and +360 km s-1,
respectively. With regard to the mean velocity of the centroid,
km s-1, the galaxies have peculiar velocities of
-58 km s-1 (M 81) and
+196 km s-1 (M 82). To satisfy the law of conservation of motion,
the galaxies should have the mass ratio
.
In
fact, this value is very close to the luminosity ratio of these galaxies,
.
This would not be the case if the dark virial
mass were distributed over the whole volume of the subgroup.
Note, that the total mass-to-total luminosity ratio (
,
derived by us for the M 81 group, is considerably lower than the value of
presented by Tully (1987) in his catalogue. As one can
see, the high excess of virial mass in Tully's estimate is
caused mainly by two reasons: by including the background galaxy NGC 5423
(D = 5.3 Mpc, Sharina et al. 1999) into the group, and by neglecting
the observed dumb-bell structure of the M 81/NGC 2403 group. This considerably
increases both the dispersion of radial velocities and the projected
harmonic radius. In the Tully catalog the median virial mass-to-luminosity
ratio is
.
In this respect the Local Group and M 81 group
with their low values: 23 and
appear to be untypical groups.
However, the application of a new, improved algorithm for group selection
shows that the groups in the Local Supercluster (of number
)
have
the median ratio
(Makarov & Karachentsev 2000).
Consequently, the dynamical situation in the two best studied nearby
groups is not unusual.
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Figure 9:
Distribution of galaxies in the M 81 complex with accurate
distance estimates on the Hubble diagram. The
three lines correspond to the Hubble
relation with H0 = 65, 70, and 75 km s-1 Mpc-1, and the
Local Group mass
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Impressive recent progress in the measurement of accurate distances to
many nearby galaxies (measured independently of their velocities) provides
the opportunity
to study the local peculiar velocity field. Mapping the velocity field
of the known nearby groups, like M 81, Centaurus A, Sculptor etc.,
allows one to determine the total mass of a group via the radius of the
zero-velocity
surface (Sandage 1986), and to compare such estimates with mass estimates
from internal virial motions. As seen from Table 2, our data on accurate
distances to galaxies in the M 81 region are still incomplete.
Among the 59 objects listed in the table, 5 galaxies have indirect distance
estimates based on assumed
membership, and 12 galaxies have distances estimated via
their brightest stars. Comparison of the distance moduli measured through
different methods for the dozen galaxies
discussed in Sect. 3 shows that the method of the brightest stars gives
moduli with a mean-square error of 0.52 mag, but essentially without a
zero-point error, which amounts to
mag respect to the TRGB method.
Important cosmological information can be extracted also from the peculiar
velocity dispersion of the centers of groups and field galaxies with respect
to the Hubble flow. For instance, in the vicinity of the Local Group on the
scale of (1-3) Mpc the mean-square non-Hubble velocity of
galaxies is only 25 km s-1 (Karachentsev & Makarov 2001), which
imposes a strong
limitation on the average density of matter,
,
in the Local volume
(Governato et al. 1997; Klypin et al. 2002).
Figure 9 shows the distribution of the M 81 group galaxies
in the Hubble diagram. Here we present only galaxies with well
determined distances. The three lines correspond to Hubble constants of
H0 = 65, 70, and 75 km s-1 Mpc-1. The effect of deceleration due to the
Local Group with a total mass of
is taken into account.
The companions of M 81 and NGC 2403 are joined with the corresponding
principal galaxies by straight lines. As one can see, the M 81 galaxy
deviates rather significantly from the Hubble regression lines. However,
the M 81 subgroup centroid with
Mpc and
km s-1has a lower peculiar velocity
than M 81 itself. Centroids of the M 81 and NGC 2403 subgroups
are situated on opposite sides with respect to the Hubble regression,
because they approach each other. As a result, the centroid of the whole
M 81/NGC 2403 group has a rather small peculiar velocity,
km s-1,
when the Hubble parameter H0 is 60-70 km s-1 Mpc-1.
The galaxy NGC 4236 and its neighbours DDO 165, UGC 6456 are situated much
below the Hubble regression lines. Apparently, being behind the M 81 group,
they experience acceleration toward both M 81 and the Local Group.
As was emphasized by Peebles (1989), the knowledge of accurate positions and velocities of all the nearest galaxies enables us to make a choice between competing theories of the origin of small-scale structure of the Universe.
Acknowledgements
Support for this work was provided by NASA through grant GO-08601.01-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. D.G. acknowledges financial support for this project received from CONICYT through Fondecyt grant 8000002. This work has also been partially supported by RFBR grant 01-02-16001.The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions.
The National Geographic Society - Palomar Observatory Sky Atlas (POSS-I) was made by the California Institute of Technology with grants from the National Geographic Society.
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Figure 2:
Digital Sky Survey images of 15 dwarf galaxies in the M 81
group. The field size is 10
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Figure 3: WFPC2 images of 15 galaxies: NGC 4236, IC 2574, NGC 2976, Holmberg II, NGC 2366, DDO 165, DDO 82, Holmberg I, Holmberg IX, DDO 53, K52, Garland, K73, BK3N, and A0952+69 produced by combining the two 600 s exposures obtained through the F606W and F814W filters. The arrows point to the North and the East. |
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Figure 3: continued. |
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Figure 3: continued. |
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Figure 4:
Color-magnitude diagrams from the WFPC2 data for
the 15 galaxies shown in Fig. 3
in the M 81 group. The three panels for each galaxy show diagrams
based on stars within the central (WF3) field,
the "medium'' field (the neighbouring halves of the WF2 and WF4 chip), and
the outer field (remaining halves of the WF2 and WF4 chip). Each of
these three field covers an equal area of 800![]() |
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Figure 4: continued. |
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Figure 4: continued. |
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Figure 4: continued. |
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Figure 4: continued. |
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Figure 5: The Gaussian-smoothed I-band luminosity function restricted to red stars (top), and the output of a Sobel edge-detection filter applied to the luminosity function for the 15 galaxies shown in Fig. 3 in the M 81 group. |
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Figure 5: continued. |
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Figure 5: continued. |
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