On the radial extent of the dwarf irregular galaxy IC10

Abstract

We present new deep and accurate space (Advanced Camera for Surveys -- Wide Field Planetary Camera 2 at the Hubble Space Telescope) and ground-based (Suprime-Cam at Subaru Telescope, Mega-Cam at Canada-France-Hawaii Telescope) photometric and astrometric data for the Local Group dwarf irregular IC10. We confirm the significant decrease of the young stellar population when moving from the center toward the outermost regions. We find that the tidal radius of IC10 is significantly larger than previous estimates of $r_t \lesssim$ 10\min. By using the $I$,\vmi\ Color Magnitude Diagram based on the Suprime-Cam data we detect sizable samples of red giant (RG) stars up to radial distances of 18-23$'$ from the galactic center. The ratio between observed star counts (Mega-Cam data) across the tip of the RG branch and star counts predicted by Galactic models indicate a star count excess at least at a 3$\sigma$ level up to 34-42\min\ from the center. This finding supports the hypothesis that the huge H{\size{I}} cloud covering more than one degree across the galaxy is associated with IC10 \citep{huchtmeier79,cohen79}. We also provide new estimates of the total luminosity ($L_V\sim9\times$$10^7$ $L_\odot$, $M_V$$\sim$-15.1 mag) that agrees with similar estimates available in the literature. If we restrict to the regions where rotational velocity measurements are available (r$\approx13'$), we find a mass-to-light ratio ($\sim$10 $M_\odot$ $L_\odot$) that is at least one order of magnitude larger than previous estimates. The new estimate should be cautiously treated, since it is based on a minimal fraction of the body of the galaxy. Comment: 7 pages, 4 figures, ApJ Letters accepted

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arXiv:1009.3917v1 [astro-ph.SR] 20 Sep 2010
Draft version 2010 September 21
Preprint typeset using LATEX style emulateapj v. 11/10/09
ON THE RADIAL EXTENT OF THE DWARF IRREGULAR GALAXY IC101
N. Sanna2,3, G. Bono3,4, P. B. Stetson5, I. Ferraro4, M. Monelli6, M. Nonino7, P. G. Prada Moroni8,9, R.
Bresolin10, R. Buonanno3,11, F. Caputo4, M. Cignoni12,13, S. Degl’Innocenti8,9, G. Iannicola4, N. Matsunaga14, A.
Pietrinferni15, M. Romaniello16, J. Storm17, and A. R. Walker18
(Dated: drafted 2010 September 21 / Received / Accepted)
Draft version 2010 September 21
ABSTRACT
We present new deep and accurate space (Advanced Camera for Surveys – Wide Field Planetary
Camera 2 at the Hubble Space Telescope) and ground-based (Suprime-Cam at Subaru Telescope,
Mega-Cam at Canada-France-HawaiiTelescope) photometric and astrometric data for the Local Group
dwarf irregular IC10. We confirm the significant decrease of the young stellar population when moving
from the center toward the outermost regions. We find that the tidal radius of IC10 is significantly
larger than previous estimates of rt? 10′. By using the I,V–I Color Magnitude Diagram based on
the Suprime-Cam data we detect sizable samples of red giant (RG) stars up to radial distances of
18-23′from the galactic center. The ratio between observed star counts (Mega-Cam data) across the
tip of the RG branch and star counts predicted by Galactic models indicate a star count excess at
least at a 3σ level up to 34-42′from the center. This finding supports the hypothesis that the huge
HI cloud covering more than one degree across the galaxy is associated with IC10 (Huchtmeier 1979;
Cohen 1979). We also provide new estimates of the total luminosity (LV ∼9×107L⊙, MV∼-15.1
mag) that agrees with similar estimates available in the literature. If we restrict to the regions where
rotational velocity measurements are available (r≈13′), we find a mass-to-light ratio (∼10 M⊙/L⊙)
that is at least one order of magnitude larger than previous estimates. The new estimate should be
cautiously treated, since it is based on a minimal fraction of the body of the galaxy.
Subject headings: galaxies: dwarf — galaxies: individual (IC10) — galaxies: stellar content — Local
Group — stars: evolution
1. INTRODUCTION
Photometric investigations of the stellar populations
in Local Group (LG) dwarf galaxies provide firm con-
straints on cosmological parameters and the unique
opportunity to investigate galaxy formation models
1This research used the facilities of the Canadian Astron-
omy Data Centre operated by the National Research Council of
Canada with the support of the Canadian Space Agency. This
research is based in part on data collected at Subaru Telescope,
which is operated by the National Astronomical Observatory of
Japan.
2University of Virginia, 530 McCormick Road 22903 Char-
lottesville, VA, USA; ns2as@virginia.edu
3Univ.Roma Tor Vergata, via della Ricerca Scientifica 1,
00133 Rome, Italy
4INAF–OAR, via Frascati 33, Monte Porzio Catone, Rome,
Italy
5DAO–HIA, NRC, 5071 West Saanich Road, Victoria, BC
V9E 2E7, Canada
6IAC, Calle Via Lactea, E38200 La Laguna, Tenerife, Spain
7INAF–OAT, via G.B. Tiepolo 11, 40131 Trieste, Italy
8Univ. Pisa, Largo B. Pontecorvo 2, 56127 Pisa, Italy
9INFN, Sez. Pisa, via E. Fermi 2, 56127 Pisa, Italy
10Institute for Astronomy, 2680 Woodlawn Drive, Honolulu,
HI 96822, USA
11ASI–Science Data Center, ASDC c/o ESRIN, via G. Galilei,
00044 Frascati, Italy
12Dipartimento di Astronomia, Universit` a di Bologna, via
Ranzani 1, 40127 Bologna, Italy
13INAF–OAB, via Ranzani 1, 40127 Bologna, Italy
14Institute of Astronomy, University of Tokyo, 2-21-1 Osawa,
Mitaka, Tokyo 181-0015, Japan
15INAF–OACTe, via M. Maggini, 64100 Teramo, Italy
16ESO, Karl-Schwarzschild-Str.
Munchen, Germany
17AIP, An der Sternwarte 16, D-14482 Potsdam, Germany
18NOAO–CTIO, Casilla 603, La Serena, Chile
2, 85748 Garching bei
(Mateo 1998; Tolstoy et al. 2009; Wyse 2010). In this
context dwarf irregulars (dIs) play a key role, since we
still lack firm empirical and theoretical constraints con-
cerning their evolution and the possible transition into
dwarf spheroidal galaxies (Bekki 2008; Woo et al. 2008;
Kormendy et al. 2009). Although the number of dwarf
galaxies known in the LG is rapidly growing in the last
few years, current statistics indicate that the dIs are at
least one quarter of LG galaxies (McConnachie et al.
2008; Sanna et al. 2009).
Among the dIs of the LG, IC10 is an interesting system,
since it underwent strong star formation activity during
the last half billion years and it is considered the only
LG analog of starburst galaxies. Even though IC10 has
been the subject of several investigations ranging from
the radio (Wilcots & Miller 1998) to the near-infrared
(NIR, Vacca et al.2007), to the UV (Hunter 2001;
Richer et al.2001), and to the X-ray (Wang et al.
2005), its structural parameters and in particular its ra-
dial extent are poorly defined. Massey & Armandroff
(1995) found that the major axis of IC10 is ∼ 7′. A sim-
ilar diameter (∼ 6′) was found by Jarrett et al. (2003)
using the isophotal radii from 2MASS NIR images. More
recently Tikhonov & Galazutdinova (2009), using both
ground-based and space images, suggested that the ex-
tent of the thick disk along the minor axis is ≈10.5′. It
has also been suggested by Demers et al. (2004), using
asymptotic giant branch and red giant branch (RGB)
stars, that IC10 should have a halo of ∼ 30′diameter.
On the other hand, radio measurements by Huchtmeier
(1979, hereinafter H79) indicated that IC10 has a huge
envelope of neutral hydrogen extending over more than

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2Sanna et al.
1 square degree (62′× 80′) across the sky.
We are also facing a significant uncertainty in the
total mass of IC10.By using HI regions H79 found
Mtot∼ 1.8x109M⊙, assuming a distance of 1 Mpc and a
Holmberg diameter of ∼ 10′. Also, Shostak & Skillman
(1989, hereinafter SS89), using high resolution maps of
HI regions, measured an inclination of 45◦and a max-
imum in the rotation curve of 30 km s−1(42 km s−1
deprojected) and the same Holmberg diameter (depro-
jected angular diameter ∼ 13′), from which they found
Mtot∼ 1x109M⊙. More recently, van den Bergh (2000),
following H79, but assuming a smaller distance (660 kpc,
Sakai et al. 1999), found Mtot∼ 6x108M⊙.
2. PHOTOMETRIC DATA
The Hubble Space Telescope (HST) data were col-
lected with the Advanced Camera for Surveys (ACS,
pointings α,β) and with the Wide Field Planetary Cam-
era 2 (WFPC2, pointings γ,δ).
α, β and γ were already presented (Sanna et al. 2008,
2009). Pointing δ includes 24 F555W-band and 24
F814W-band images of 500 s each.
located ∼ 3′NE of the galaxy center; it is outside
the disk identified by Jarrett et al. (2003, see the red
ellipse in Fig. 1) and inside the thick disk identified
by Tikhonov & Galazutdinova
based data were collected with the prime focus camera
(Suprime-Cam, pointing ǫ) on the Subaru telescope and
with Mega-Cam on the CFHT (pointing ζ). Pointing ǫ
(see Fig. 1) is located across the galaxy center and in-
cludes both shallow (3V , 3R, 3I; 3×60 s per band) and
deep (12V , 12×480 s; 13R, 13×360 s; 23I, 23×240 s)
images. The pointing ζ (see Fig. 1) is also located across
the galaxy center and includes 3g (3×700 s) and 3i-band
(3×400 s) images.
Photometry on individual images was performed
with DAOPHOT IV/ALLSTAR (Stetson 1987).
ground-based images were simultaneously reduced with
ALLFRAME (Stetson 1994); the same applies to the 392
space images.We ended up with a catalog including
∼ 1,200,000 stars with at least one measurement in
two different bands. The ground-based data were trans-
formed into the Johnson-Kron-Cousins system using the
standard stars provided by Landolt (1983, 1992) to cal-
ibrate local standards. The typical accuracy is 0.04 for
I and 0.05 mag for the V band. Some external chips of
the Mega-Cam include a limited number of local stan-
dards and they were not included in the final calibrated
catalog. To provide a homogeneous photometric cata-
log the ACS and the WFPC2 were transformed into the
V,I Johnson-Kron-Cousinssystem using prescriptions by
Sirianni et al. (2005). The typical accuracy is of a few
hundredths of magnitude in both V and I. The conclu-
sions of this investigation are not affected by the precision
of the absolute zero-points.
Data from pointings
This pointing is
(2009).The ground-
The 786
3. RESULTS AND DISCUSSION
The ground-based data cover a sky area of ∼ 1◦× 1◦
while the high angular resolution of HST images al-
lowed us to perform accurate photometry in the inner-
most crowded regions. We selected eight different re-
gions, the region “C” is located across the galaxy cen-
ter and includes data of pointings α(ACS@HST) and γ
(WFPC2@HST), while the region “W” is located ∼ 3′
from the center and includes the data of the pointing
δ (WFPC2@HST). Regions “S1”, “S2”, “S3”, and “S4”
cover the corners of the Suprime-Cam data (pointing ǫ)
and are located at ∼ 18′from the galaxy center, while re-
gions “M1” and “M2” are two regions of the Mega-Cam
data (pointing ζ) located at ∼ 30′from the galaxy center
in the SW and NE directions, respectively. Fig. 2 shows
the I,V–I Color-Magnitude Diagrams (CMDs) of the se-
lected regions. Data plotted in this figure show several
interesting features.
i) The photometry based on HST data is deep and very
accurate, and indeed the CMDs (regions “C” and “W”)
reach limiting magnitudes of I ∼ 25.5 − 26 and V ∼
26.5−27 mag. The same outcome applies to the Subaru
data, and indeed the CMDs reach limiting magnitudes
of I ∼ 25 and V ∼ 26 mag. The CMDs based on CFHT
data are shallower with limiting magnitudes I ∼ 22.5
and V ∼ 24.5 mag.
ii) Young MS stars (18 ≤ I ≤ 25.5, 1 ≤ V–I ≤ 1.5
mag) show a strong radial gradient. Their number de-
creases rapidly when moving from the center to the out-
ermost galaxy regions. A handful of them are visible in
region “W”, while the blue objects of region “S” might
be field galaxies.
iii) The different apparent colors of RGB stars when
moving from the center to the outermost regions further
support the occurrence of differential reddening. We es-
timated the reddening of region “W” using the same ap-
proach adopted in Sanna et al. (2008). The ridgeline of
the RGB in this field was adopted to estimate the redden-
ing in the regions covered by Subaru data. The ridgeline
of the contaminating blue field stars located in region
“S4” was used to estimate the reddening in the regions
covered by CFHT data. We found that the reddening is
higher along the semi-major axis (E(B–V ) = 0.78±0.10
mag) and attains an almost constant value (E(B–V) =
0.63 ± 0.10 mag) in the regions covered by the Subaru
data external to the HST data. In the regions covered by
the CFHT data external to the Subaru data the redden-
ing attains either similar (E(B–V) = 0.63±0.10 mag) or
smaller (E(B–V ) = 0.40 ± 0.10 mag) values in the SW
direction (Sanna et al. 2010, in preparation). The pres-
ence of a significant reddening variation up to 20′from
the center supports previous investigations based on the
radial distribution of HI regions (SS89; Wilcots & Miller
1998).
iv) Old RGB stars are ubiquitous and they can be
easily identified in HST and Subaru CMDs. To prop-
erly identify RGB stars we also plotted an α-enhanced
isochrone (green line) of 13 Gyr at fixed metal con-
tent (total metallicity, [M/H]=-0.66 dex; helium content,
Y=0.251) from the BaSTI data base (Pietrinferni et al.
2006)19. We also adopted the same true distance modu-
lus (µ=24.60±0.15 mag, Sanna et al. 2008) and individ-
ual reddening estimates. The comparison between theory
and observations further supports the evidence that stars
located in the range 21 ? I ? 26 and 1 ? V–I ? 2.5 mag
are bona fide old and intermediate-age RGB stars. The
CMDs based on these data show also strong contamina-
tion by field stars (see the region “S1”).
v) The CMDs based on CFHT data show the same
field contamination as the Subaru data, and probably a
19See also http://www.oa-teramo.inaf.it/BASTI

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On the radial extent of the dwarf irregular galaxy IC103
Fig. 1.— Location of ground-based and space pointings. The background is a reference image of IC10 based on randomly selected
stars from the Digitized Sky Survey (DSS) catalog. The polygon labeled ”C” shows the sky area covered by pointings α (ACS@HST)
and γ (WFPC2@HST), the polygon “W” the pointing δ (WFPC2@HST). The large rectangular polygon shows the pointing ǫ (Suprime-
Cam@Subaru), and the largest irregular polygon the pointing ζ (Mega-Cam@CFHT). The boxes labeled “S1”, “S2”, “S3”, “S4” and “M1”,
“M2” display the regions adopted for the CMDs. The red ellipse displays the estimate of major and minor axis according to (Jarrett et al.
2003). The blue circle has a diameter of 13′.
small overdensity of stars in the region across the tip of
the RGB (TRGB, 21 ? I ? 22, 2 ? V–I ? 3.5 mag).
The above results indicate that the radial extent of
IC10 has been significantly underestimated, and indeed
according to the Subaru data the diameter is at least of
the order of 36-46′and probably larger than one degree
according to the CFHT data.
To further constrain the radial extent of IC10, we
decided to compare the observed star counts with star
counts of foreground field stars predicted by Milky Way
(MW) models. This approach presents several advan-
tages when compared with the method based on the sta-
tistical subtraction of an external control field. i) It is
not affected by reddening differences between the galac-
tic and the control field. ii) It is not affected by com-
pleteness problems of the control field, thus saving tele-
scope time. iii) The real radial extent of these stellar
systems it is not known in advance. Therefore, the con-
trol fields might still be located inside their halo. The
main drawback is that MW models need to be validated
using deep and accurate star counts covering broad sky
regions (Reyle et al. 2009, and references therein).
However, the ground-based and space data sets are
characterized by different limiting magnitudes. To pro-
vide a robust estimate of the completeness of the former
data sets, we compared their luminosity function (LF)
with the LF of the WFPC2 data for pointing “W”. We
adopted this approach since we are interested in esti-
mating the completeness down to 1.5 magnitudes fainter
than the TRGB, i.e., I ? 23 mag. In this magnitude
range the WFPC2 data are minimally affected by com-
pleteness problems. We have found that the complete-
ness is ∼ 65% for the Subaru at the limiting magnitude
of I = 23.0 mag, while it is ∼ 65% for the CFHT at
the limiting magnitude of I = 22.6 mag. We have cho-
sen the above limiting magnitudes to apply conservative
completeness corrections to both the Suprime-Cam and
the Mega-Cam data.
In order to compare theory and observations, we
used the Pisa (Castellani et al.
2006) and the Padova (Girardi et al. 2005) MW model.
We focused our attention on two outer regions cover-
ing the same sky area, with the same mean reddening
(E(B–V) = 0.63 ± 0.10 mag) and located in the NE
direction, namely the “S4” (N(stars)∼4000) and “M2”
(N(stars)∼ 3000) regions. The top panels of Fig. 3 show
the I,V–I CMD for the Pisa (left) and the Padova (right)
MW models for a sky area of 1◦× 1◦at the position of
IC10 (l = 119◦; b = −3.3◦) and assuming a reddening of
E(B–V) = 0.63 ± 0.10 mag.
The bottom panels of Fig. 3 show the ratio between
the number of observed stars and the number of can-
didate field stars predicted by the quoted MW models.
Star counts were smoothed using a Gaussian kernel at
fixed σ. We also estimated the number of unresolved
background galaxies, with reddened I-band magnitudes
2002; Cignoni et al.

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4Sanna et al.
Fig. 2.— I,V-I CMDs for eight regions located at different radial distances (see labeled values and Fig. 1). Blue young, main sequence stars
decrease quite rapidly when moving from the center to the outermost regions. The old RGB stars are ubiquitous in HST and in Subaru data.
The reddening also changes when moving across the different regions (see labeled values). The green line shows a α-enhanced isochrones
of 13 Gyr, at fixed chemical composition ([M/H]=-0.66 dex, Y=0.25) from the BaSTI database. The blue arrows display the location of
contaminating field stars, while the red lines plotted in panel “W” and “S4” display the ridgeline used to determine the reddening.
and V–I colors typical of the tip of RGB stars, using
empirical galaxy counts (Fukugita et al. 1995; Benitez
2000; Capak et al. 2004; Ferguson et al. 2004) and it
was subtracted to the number of observed stars. To avoid
spurious fluctuations caused by the limited sky area cov-
ered by observations, theory and observations were nor-
malized in the bright end (17.9≤I≤18.4 mag). The bot-
tom left panels display the comparison with the Pisa MW
model. There is evidence of IC10 stars across the TRGB
region (I = 21.66 ± 0.25 mag) in both the “S4” (left,
1.59±0.13) and the “M2” (right, 1.38± 0.12) field. The
star count excess is at 3σ level. The above evidence fur-
ther supports the occurrence of IC10 stars in the region
covered by the Mega-Cam data, since in the “S4” region
we clearly identified IC10 RG stars. The bottom right
panels of Fig. 3 display the comparison between obser-
vations and the Padova MW model. There is once again
a clear evidence of a star excess across the TRGB region
in both the “S4” (left, 1.74±0.14) and the “M2” (right,
1.53±0.14) field (∼ 4σ level). The two Galactic models
were constructed assuming similar input parameters20.
The difference in the star count ratios between the two
models is due to the different evolutionary inputs and to
the normalization of the star counts in the Solar neigh-
20Galactic model input parameters: Kroupa initial mass func-
tion not corrected for binaries; double exponential thin disk
(hz(height)=250 pc, hR(scale length)=3000 pc, constant star for-
mation rate [SFR] for t≤ 7 Gyr, Z(mean metallicity)=0.02); ex-
ponential thick disk (hz=1000 pc, hR=3500 pc, constant SFR
for 5≤t≤12 Gyr, Z=0.006); oblate halo with r1/4(hR=2800
pc, hq(semiaxis ratio)=0.8, constant SFR for 11≤t≤13 Gyr,
Z=0.0002).

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On the radial extent of the dwarf irregular galaxy IC105
Fig. 3.— Top–Simulated CMDs for the field stars adopting a
reddening of E(B − V ) = 0.63 ± 0.10 mag in a field of view of
1◦x 1◦, according to the Pisa (left) and to the Padova (right) MW
model. Only a small fraction of the total number of stars is plotted.
Bottom–Left–Star count ratios between the stars located in the
“S4” and in the “M2” region with the Pisa MW model. There
is evidence of IC10 stars at the position of the TRGB, i.e. I =
21.66 ± 0.25 mag (see the vertical arrows). Bottom–Right–Same
as the left, but the ratio is between observations and the Padova
MW model.
bourhood.
The above findings indicate that the radial distribu-
tion of IC10 old and intermediate-age stellar populations
agrees quite well with the size of the huge hydrogen
cloud detected by Huchtmeier (1979) and by Cohen
(1979), and cover more than one square degree across
the galaxy (r ≈ 34 − 42′). This means that the stel-
lar halo and the hydrogen cloud have, within the er-
rors, similar radial extents and resolve this peculiar fea-
ture of IC10 (Tikhonov & Galazutdinova 2009). More-
over, this evidence further supports the hypothesis that
the hydrogen cloud is associated with the galaxy (stellar
mass loss, pristine gas; Huchtmeier 1979; Cohen 1979;
Wilcots & Miller 1998).
To estimate the total luminosity we need to select can-
didate IC10 stars.To describe the procedure, Fig. 4
shows the I,V–I CMDs of the stars located inside a circle
of 13′diameter across the galaxy center. The top panels
display the photometry of space data (pointings α, β, γ,
δ), while the bottom ones show ground data (pointing ǫ,
Suprime-Cam). The difference between ǫ1 and ǫ2 is in
the mean reddening (see Fig. 4). For the stars located
in the overlapping regions we use the HST photometry.
The candidate IC10 stars were selected using different
boxes in the aforementioned CMDs. The green box in-
cludes young MS stars (V–I ∼ 1.5 mag), the cyan box
the intermediate-age stars (V–I ∼ 3, 17.5 ? I ? 21.5
mag), while the pink box includes old and intermediate-
age RG stars (3 ? V–I ? 6, I ∼ 22 mag). The position
of the boxes in the four CMDs was shifted according to
the local reddening (see Fig. 1). The limiting magnitude
of the boxes is I = 23.0 according to the completeness
experiment.
The dashed and the dashed-dotted lines plotted in the
bottom right panel of Fig. 4, show two young scaled So-
lar abundance isochrones (Pietrinferni et al.
fixed metallicity ([M/H]=-0.66 dex) and ages of t = 6
and 200 Myr, while the solid red line shows the old α-
enhanced isochrone with the same total metallicity and
an age of t = 13 Gyr. These isochrones validate the
position of the boxes we adopted to pinpoint the dif-
ferent subpopulations of IC10. The same approach was
adopted to select candidate IC10 stars located between
the blue circle of Fig. 1 and the outermost regions of the
ǫ pointing (r ? 23′). On the basis of these data and
of the recent IC10 distance based on the TRGB (830
kpc, Sanna et al. 2008) we estimated a total V -band lu-
minosity of LV ∼ 9.13 × 107L⊙and a total magnitude
of MV=-15.11 mag. This estimate agrees within a fac-
tor of two with similar estimates available in the litera-
ture (LV ∼ 1.6 × 108L⊙, Mateo 1998; MV=-16.0 mag,
Richer et al.2001).The current estimate is a lower
limit, since we are not including the stars located in the
outermost regions covered by our photometry (pointing
ζ).However, the estimates available in the literature
only cover the innermost galactic regions. The difference
is mainly due to the fact that the current photometry al-
lows us a robust identification of field stars (1 ? V–I ? 2,
15 ? I ? 22 mag, see the blue arrow in the bottom left
panel of Fig. 4). If they are even partially included, these
objects introduce a systematic bias in the estimate of the
total luminosity. The different assumptions concerning
the adopted distance and reddening variation also help
to explain the above difference.
To estimate the mass-to-light (M/L) ratio of IC10 we
restricted ourself to the galactic regions where rotational
velocity measurements are available (see the blue circle
with a diameter of 13′in Fig. 1). The luminosity inside
this area is LV ∼ 5.88 × 107L⊙ (MV = −14.63 mag).
By using the quoted true distance and diameter together
with the rotation velocity based on HI regions (SS89)
we found, following Huchtmeier & Richter (2001) and
Casertano & Shostak (1980), a total mass of Mtot ∼
6.2x108M⊙ that agrees quite well with similar esti-
mates available in the literature (Huchtmeier
van den Bergh 2000; Woo et al. 2008). Eventually, we
found M/L ∼10 M⊙/L⊙. Although this estimate is ham-
pered by several empirical limitations it is at least one
order of magnitude larger than the value recently pro-
vided by Woo et al.(2008). The quoted authors use
two independent methods to estimate the M/L ratios of
LG dwarf galaxies: colors and inferred star formation
history. They found that the median M/L ratio of dIs
based on the latter approach is slightly smaller than on
the former one (0.7 vs 0.8, see their Table2). However,
the difference needs to be investigated in more detail,
particularly in view of the severe limitations affecting
the estimates of the rotational velocity and of the total
2004) at
1979;

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6Sanna et al.
Fig. 4.— CMDs in I, V-I bands for the stars located within 6.5′across the galaxy center. Top – space data of the pointings α, β and
γ (left, E(B − V ) = 0.78 ± 0.06 mag) and δ (right, E(B − V ) = 0.63 ± 0.09 mag). The colored boxes mark the CMD regions adopted to
select candidate IC10 stars. The green box includes young MS stars, the cyan box the intermediate-age stars and the pink one the old
and intermediate-age RGs. The position of the boxes was changed according to the local reddening. Bottom – ground based data of the
pointing ǫ (ǫ1, E(B − V ) = 0.78 ± 0.10; ǫ2, E(B − V ) = 0.63 ± 0.10 mag). The blue arrow marks the position of candidate field stars
(left). The dashed and the dashed-dotted lines (right) display two young, scaled Solar isochrones (t=6, 200 Myr), while the solid line an
old α-enhanced isochrone (t=13 Gyr). Young and old isochrones were constructed at fixed total metallicity ([M/H]=-0.66 dex).
luminosity over the entire body of the galaxy.
We are facing empirical evidence that dIs seem to
show smaller M/L ratios when compared with dwarf el-
lipticals (see Fig. 1 and Table 2 in Woo et al.
The new data will allow us to constrain whether this
evidence might be affected by observational biases.
Moreover, they can shed new lights on the prediction
2008).
that dwarf galaxies might have tidal radii significantly
larger than empirical estimates (Hayashi et al.
Kazantzidis et al. 2004).
2003;
It is a real pleasure to thank the referee, Dr. I. Chilin-
garian, for his constructive suggestions.
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