arXiv:0810.1210v1 [astro-ph] 7 Oct 2008
On the distance and reddening of the starburst galaxy IC101
N. Sanna2,3, G. Bono2,3, P. B. Stetson4, M. Monelli5, A. Pietrinferni6, I. Drozdovsky5, F.
Caputo3, S. Cassisi6, M. Gennaro7, P. G. Prada Moroni7,8, R. Buonanno2, C. E. Corsi3, S.
Degl’Innocenti7,8I. Ferraro3, G. Iannicola3, M. Nonino9, L. Pulone3, M. Romaniello10, and
A. R. Walker11
We present deep and accurate optical photometry of the Local Group star-
burst galaxy IC10. The photometry is based on two sets of images collected with
the Advanced Camera for Surveys and with the Wide Field Planetary Camera
2 on board the Hubble Space Telescope. We provide new estimates of the Red
Giant Branch tip (TRGB) magnitude, mTRGB
ing, E(B-V)=0.78±0.06, using field stars in the Small Magellanic Cloud (SMC)
as a reference. Adopting the SMC and two globulars, ω Cen and 47 Tuc, as ref-
erences we estimate the distance modulus to IC10: independent calibrations give
weighted average distances of µ=24.51±0.08 (TRGB) and µ=24.56±0.08 (RR
Lyrae). We also provide a new theoretical calibration for the TRGB luminosity,
and using these predictions we find a very similar distance to IC10 (µ≈24.60
±0.15). These results suggest that IC10 is a likely member of the M31 subgroup.
F814W=21.90±0.03, and of the redden-
Subject headings: galaxies: IC10—galaxies: Local Group—Stars: distances
1Based on observations collected with the ACS and with the WFPC2 on board of the HST.
2UniToV, via della Ricerca Scientifica 1, 00133 Rome, Italy; Nicoletta.Sanna@roma2.infn.it
3INAF–OAR, via Frascati 33, Monte Porzio Catone, Rome, Italy
4DAO–HIA, NRC, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
5IAC, Calle Via Lactea, E38200 La Laguna, Tenerife, Spain
6INAF–OACTe, via M. Maggini, 64100 Teramo, Italy
7Univ. Pisa, Largo B. Pontecorvo 2, 56127 Pisa, Italy
8INFN, Sez. Pisa, via E. Fermi 2, 56127 Pisa, Italy
9INAF–OAT, via G.B. Tiepolo 11, 40131 Trieste, Italy
10ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany
11CTIAO–NOAO, Casilla 603, La Serena, Chile
– 2 –
The dwarf galaxy IC10 is a very interesting stellar system in the Local Group. It has
been classified as an Ir IV by van den Bergh (1999), but it has also been suggested that it is
the only analog of a post-starburst dwarf galaxy in the Local Group (Gil de Paz et al. 2003).
This means that IC10 is a very good laboratory to investigate episodic star formation over
a broad time interval (Hunter 2001; Demers et al. 2004). Moreover, spectroscopic estimates
based on HII regions indicate that the present-day interstellar medium metal abundance is
[Fe/H] ∼ −0.71 ± 0.10, (Garnett et al. 1990). We cannot exclude the possibility of a range
of metallicity in IC10, but comparison with stellar isochrones (Hunter 2001) also indicates
a mean metallicity of [Fe/H] ∼ −0.7. This metal content is very similar to that of the
Small Magellanic Cloud (SMC; [Fe/H] ≈ −0.7, Zaritsky et al. 1994, based on HII regions;
[Fe/H] ∼ −0.75 ± 0.08, Romaniello et al. 2008, based on classical Cepheids).
However, IC10 is located at very low Galactic latitude (l=119◦0, b=-3◦3), and is there-
fore affected by significant foreground extinction. This circumstance has made estimates
of both reddening and distance quite difficult and controversial. Distance determinations
based on different standard candles cover a wide range, from closer than M31—d = 0.5 Mpc
based on the Red Giant Branch tip (TRGB, Sakai et al. 1999)—to well outside the Local
Group, d = 1.8 Mpc based on the luminosity function (LF) of planetary nebulae (Jacoby &
Lesser 1981). A similar disparity is found among the reddening determinations, with esti-
mates ranging from E(B − V ) ∼ 0.8 based on spectra of HII regions (Richer et al. 2001),
to E(B − V ) ∼ 1.2 based on classical Cepheids (Sakai et al. 1999). Moreover, a spread in
distance and reddening values can be found even when using the same distance indicator and
stellar tracer (see Table 1, in Demers et al. 2004). Therefore, we still lack firm constraints
on the random and systematic errors affecting these important parameters.
2. Observations and data reduction
Our photometric catalog is based on archival Hubble Space Telescope (HST) data sets
collected with both the Advanced Camera for Surveys (ACS, pointings α and β) and with
the Wide Field Planetary Camera 2 (WFPC2, pointing γ)12. Pointing α is located at the
galaxy center and consists of six F606W and six F814W-band images of 360 sec each.
Pointing β is located ∼ 2′NW from the galaxy center and includes 32 F555W-band images
12The pointing coordinates (J2000) are: RA=00 20 19, DEC=59 18 24 (α, GO–9683, PI: F.E. Bauer);
RA=00 20 14, DEC=59 20 23 (β, GO–10242, PI: A.A. Cole); RA=00 20 28, DEC=59 18 00 (γ, GO–6406,
PI: D.A. Hunter).
– 3 –
of 620 sec each and 16 F814W-band images of 595 sec each. Pointing γ is again located
at the galaxy center and includes ten F555W and ten F814W-band images of 1400 sec
each. Pointings α and β (ACS) overlap by about one chip, while pointing γ (WFPC2)
almost entirely overlaps with pointing α. We combined the ACS images using an updated
version of the MultiDrizzle package (Koekemoer et al. 2002), which provides an automated
method for correcting distortion and combining dithered images. The WFPC2 images were
prereduced using the HST pipeline. Initial photometry on individual images was performed
with DAOPHOT IV, followed by simultaneous photometry over the 80 images with ALLFRAME
(Stetson, 1994). We ended up with a catalog including ∼ 720,000 stars with at least one
measurement in each of two different bands. The ACS data in the F555W and F814W
bands were transformed into the VEGAMAG system following Sirianni et al. (2005). To
provide a homogeneous photometric catalog the F606W-band images collected with the
ACS were transformed into the F555W-band using local standards. The same approach
was adopted to transform the F555W and the F814W images collected with WFPC2 to
the corresponding ACS systems. On average the precision of the above transformations is
F555W-F606W(ACS)= -0.006 ± 0.049, F555W(ACS)-F555W(WFPC2)= 0.009 ± 0.054,
F814W(ACS)-F814W(WFPC2)= 0.003 ± 0.050 mag (Sanna et al. 2008, in preparation).
To overcome possible changes in the internal reddening when moving from the center toward
the external regions of the galaxy we split the final catalog into two different regions. Region
C is located at the galaxy center, while the field lying at a radial distance larger than two
arcminutes is our region E. Data plotted in Fig. 1 show that the current photometry ranges
from very bright main sequence stars (F814W ∼ 21, F555W-F814W∼ 1) to faint RG stars
(F814W ∼ 25.5, F555W-F814W∼ 2) with very good precision.
3. Results and discussion
To estimate the TRGB in IC10 we adopted the approach used in Bono et al. (2008,
hereinafter B08). In particular, we selected stars with 21.5 ≤ F814W ≤ 22.4 and F555W −
F814W≥ 2.8. Note that the stellar samples in the central and external regions include,
respectively, ∼ 3,000 and ∼ 2,400 stars within one F814W-band magnitude of the tip.
This is more than an order of magnitude larger than the number required for a robust
detection of the TRGB (Madore & Freedman 1995). The top panels of Fig. 2 show a well
defined jump in the star counts for mF814W∼ 21.90, which we take to mark the position of
the TRGB. This identification is supported by the smoothed LF obtained with a Gaussian
kernel having a standard deviation equal to the photometric error (Sakai et al. 1996; middle
panels). Finally, the bottom panels show the response of the edge detector, a four-point
Sobel filter convolved with the smoothed LF. The dashed vertical lines mark our detection
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of the TRGB at mTRGB
for the external field (right). The error in these TRGB estimates is given by the bin size
adopted in the LF (Fig. 2, top panels).
F814W= 21.90±0.025 for the central field (left) and mTRGB
We adopted empirical calibrators to determine the reddening and the distance to IC10.
This approach is minimally affected by uncertainties in the transformation of theoretical
predictions into the observational plane. Moreover, it provides robust estimates of relative
distances and reddenings. The main drawback is that the empirical calibrators require similar
mean metallicities (or at least trustworthy metallicity corrections). To estimate the reddening
we used the accurate photometry of the region located around the SMC cluster NGC 346
recently provided by Sabbi et al. (2007). This data set offers several advantages: i) the data
were collected with ACS in the same F555W, F814W-bands; ii) the stellar populations in
this region span a very broad age range, namely from the few Myrs of NGC 346 to the several
Gyrs (t = 4.5 Gyr) of the intermediate-age cluster BS90; iii) the region is characterized by
modest reddening (E(B-V)=0.08) and is minimally contaminated by Galactic field stars; iv)
the mean metallicity is very similar to the mean metallicity of IC10.
The relative reddening was estimated by moving the young SMC MS until it matched the
bluest MS stars in IC10 to account for possible differential reddening. Fortunately enough,
the distribution of these stars in the F814W, F555W-F814W CMD is almost vertical and
with the RGs they form a “V” shape. Therefore, this approach minimally depends on
the adopted distance. We found a relative reddening of δE(F555W−F814W)=1.04 ± 0.05
for both the central and the external region of IC10. This indicates that the reddening
in the two fields is, within the errors, the same. Using the analytical relation for stellar
extinction provided by Cardelli et al. (1989) we found that the selective absorptions in the
two ACS filters are: AF555W= 1.03 × AV and AF814W= 0.55 × AV. If we assume an SMC
reddening of E(B-V)=0.08 (Sabbi et al. 2007), this means a mean reddening of E(F555W-
F814W)=1.16±0.06 for IC10, and in turn E(B−V )=0.78±0.06. The error budget includes
generous estimates on the relative reddening (±0.05), on the SMC reddening (∼ 0.02) and
on the reddening law (Fitzpatrick 1999). The left panel of Fig. 3 shows the comparison
between the SMC (green dots) and IC10 (gray dots) using the above differential reddening
and relative distance based on the TRGB (see infra). The new reddening estimate is in very
good agreement with the reddening estimates based on spectra of HII regions (E(B-V)=0.77
± 0.07, Richer et al. 2001), TRGB stars (E(B-V) ∼ 0.85, Sakai et al. 1999), carbon stars
(E(B-V) ∼ 0.79, and a change across the field of ∼ 0.38 mag, Demers et al. 2004), and
Wolf-Rayet stars (E(B-V)=0.75-0.80, Massey & Armandroff 1995). We do not support a
reddening as high as was estimated from classical Cepheid variables.
We also estimated the relative distance between IC10 and the SMC. For the SMC, we
– 5 –
adopted the TRGB estimate provided by Cioni et al. (2000), i.e., mI(SMC) = 14.95±0.03.
With the reddening quoted above for SMC (E(B-V)=0.08), the color-metallicity relation for
TRGB stars provided by Bellazzini et al. (2001), and the transformations from the Cousins I-
band to the ACS VEGAMAG system by Sirianni et al. (2005), we found mF814W,0(SMC) =
14.81 ± 0.05. Using the above reddening, we found that mF814W,0(IC10) = 20.57 ± 0.07
and the error budget includes the same sources of uncertainty as before, plus an additional
uncertainty of ±0.02 mag in the photometric calibration. Therefore, the relative distance is
∆µ = 5.76±0.09. The arrow plotted in the middle panel of Fig. 3 shows that the magnitudes
of TRGB stars in SMC and in IC10 agree within the uncertainties.
To further constrain the relative distance to IC10, we also compared it to the Galactic
Globular Cluster (GGC) 47 Tuc. The reason is threefold: i) accurate spectroscopic mea-
surements indicate that its iron abundance is very similar to IC10, i.e., [Fe/H]=−0.66±0.04
(Gratton et al. 2003); ii) it is minimally affected by reddening: (E(B-V)=0.04±0.02, Salaris
et al. 2007); iii) an accurate estimate of the TRGB has been recently provided by B08. By
transforming the Cousins I-band into the ACS VEGAMAG system (Sirianni et al. 2005),
we found that the TRGB in 47Tuc is located at mF814W,0(47Tuc) = 9.40 ± 0.08. The error
budget accounts for uncertainties in the reddening, in the TRGB detection and in the pho-
tometric transformation. Therefore, the relative distance is ∆µ = 11.17 ± 0.11. A glance
at the data plotted in the middle panel of Fig. 3 shows very good agreement between the
magnitudes of TRGB stars in 47Tuc and IC10. Moreover, the RGs in 47Tuc also attain very
similar colors to RGs in IC10, thus supporting the reddening (and metallicity) estimate.
We also estimated the relative distance between IC10 and the GGC ω Cen. Again, the
reason is threefold: i) it is the most massive GGC and it hosts a sizable sample of bright
RG stars (∼ 220) close to TRGB, so the TRGB detection is particularly robust; ii) accurate
distance estimates are available using several different standard candles (Del Principe et
al. 2006); iii) an accurate estimate of the TRGB has been recently provided by B08. We
found that the TRGB in ω Cen is located at mF814W,0(ωCen) = 9.73±0.09. The error budget
accounts for uncertainties in the reddening (E(B−V ) = 0.11±0.02), in the TRGB detection,
and in the photometric transformation. Moreover, we also accounted for the difference in
metallicity between ω Cen ([Fe/H]∼ −1.7, metal-poor peak, Johnson et al. 2008) and IC10,
and their uncertainties (B08). The relative distance is then ∆µ = 10.84 ± 0.11. The right
panel of Fig. 3 shows that the magnitudes of TRGB stars in ω Cen and in IC10 agree quite
well. The bright RGs in ω Cen are, as expected, bluer when compared with RGs in IC10.
To evaluate the true distance to IC10, we used the TRGB calibrations provided by
Bellazzini et al. (2004) and Lee et al. (1993). For the SMC, we found a true modulus of µ=
18.77 ± 0.13 or µ= 18.73 ± 0.12; µ=18.75 ± 0.09 is the weighted average. Thus, the true
– 6 –
modulus of IC10, based on the TRGB scale of SMC, is µ= 24.51 ± 0.13. Using the same
TRGB calibrations we found for 47 Tuc a true modulus of µ= 13.36±0.14 and µ= 13.32±0.12
(µ=13.34 ± 0.10, weighted average). Thus, the modulus of IC10, based on the TRGB scale
of 47 Tuc, is µ= 24.51 ± 0.15. Finally, using the same TRGB calibrations the true moduli
for ω Cen are: µ= 13.67 ± 0.14 and µ= 13.65 ± 0.13 (µ=13.66 ± 0.10, weighted average).
Therefore, the modulus of IC10, based on the TRGB scale of ω Cen , is µ= 24.50 ± 0.15.
On the other hand, if we use the K-band Period-Luminosity (PL) relations provided by
Del Principe et al. (2006) and Sollima et al. (2008), the true modulus of 47 Tuc based on the
variable V9 is µ = 13.38±0.06 and µ = 13.47±0.11 (µ = 13.40±0.05, weighted average)13.
Therefore, the modulus of IC10, based on the RR Lyrae scale, is µ = 24.57±0.12. Using the
same K-band PL relations, the true modulus of ω Cen is µ = 13.70±0.06 or µ = 13.75±0.11
(µ = 13.71 ± 0.05, weighted average). Therefore, the modulus of IC10, based on the RR
Lyrae scale to ω Cen , is µ = 24.55±0.12. The weighted average true moduli of IC10 agree
within 1σ and are: µ = 24.51 ± 0.08 (TRGB scale) and µ = 24.56 ± 0.08 (RR Lyrae scale).
These estimates agree within 1σ with previous distance determinations based on carbon
stars (µ = 24.35 ± 0.11, E(B-V)=0.79, Demers et al. 2004); on optical (µ = 24.59 ± 0.30,
E(B-V)=0.97, Saha et al. 1996) and NIR (µ = 24.57 ± 0.21, E(B-V)=0.8, Wilson et al.
1996) Cepheid PL relations; and on the TRGB (µ = 24.48±0.08, E(B-V)=0.95, Vacca et al.
2007). However, we found differences of ≈ 0.4–0.5 mag with respect to the moduli estimated
from Wolf-Rayet stars (µ = 24.9, E(B-V)=0.75–0.80, Massey & Armandroff 1995) and red
supergiants (µ = 23.86 ± 0.12, E(B-V)=1.05 ± 0.10, Borissova et al. 2000). This indicates
that the quoted distance estimates might be affected by systematic errors.
As a final test of the systematic errors that might affect the distance estimates to IC10,
we calculated new theoretical TRGB calibrations. We adopted the homogeneous set of cluster
isochrones for scaled-solar abundances provided by Pietrinferni et al. (2004); in particular,
we adopted isochrones with t=12 Gyrs, mass-loss η=0.4, and iron abundances from −2.3 ?
[M/H] ? −0.5 (see the URL http://www.oa-teramo.inaf.it/basti). However, theory
was transformed into the observational plane using only scaled-solar atmospheres based
on ATLAS9 models (Castelli et al. 2003). We linearly extrapolated the surface gravity by
∼0.1 dex and the surface temperature by ∼100 K beyond the available range for the most
metal-rich structures. To account for the new electron-conduction opacities the predicted
TRGB magnitudes were dimmed by 0.075 mag (Cassisi et al. 2007). Moreover, to account
for uncertainties in the color-temperature transformations (Bellazzini 2008) we also adopted
scaled-solar atmospheres based on PHOENIX (Brott et al. 2005).
13We did not estimate the distance to IC10 using the SMC RR Lyrae, since NIR magnitudes for these
objects are not available.
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The new ATLAS9 and PHOENIX TRGB calibrations agree well (Fig. 4, bottom panel):
over most of the metallicity range differences are much less than 0.1 mag, but they do become
perceptibly larger for the most metal-rich structures.
agree (Fig. 4, top panel), within 1σ (see the error bars), with empirical TRGB calibrations
(Lee et al. 1993; Bellazzini et al. 2004; Rizzi et al. 2007). Predicted MTRGB
are still systematically brighter than observed ones, but the difference is a factor of two
smaller than previous TRGB calibrations. The explanation of this mild discrepancy requires
new theoretical and empirical investigations. We performed spline fits of the theoretical
TRGB luminosities14in both I and F814W-band, and from these—by assuming a metal
content for IC10 of [M/H]=-0.70—we derived true distance moduli of µ = 24.59 ± 0.13 and
µ = 24.60 ± 0.15, respectively. The error budget includes uncertainties in the theoretical
predictions (±0.10 mag), the TRGB measurement, and in the metal content (±0.20 dex).
These estimates are in very good agreement with the above distances based on empirical
calibrators, thus further supporting the accuracy of the current theoretical calibrations.
The new TRGB calibrations also
The above distances indicate that IC10 is located, within the errors, at the same distance
as M31, which has true moduli based on the TRGB ranging from 24.47±0.11 to 24.37±0.08
(Rizzi et al. 2007), while those based on Cepheids range from 24.38 ± 0.05 (Sakai et al.
2004) to 24.32 ± 0.12 (Vilardell et al. 2007; Tammann et al. 2008). The new distances are
also in remarkable agreement with the “rotational parallax” for M33 based on water masers
(805±37 vs 730±168 kpc) by Brunthaler et al. (2005). This indicates that IC10 is at the
same distance as M31 and M33, and in turn that it is a likely member of the M31 subgroup.
It is a real pleasure to thank E. Sabbi for sending us her SMC data in electronic form.
This paper was partially supported by PRIN-INAF (PI: M. Bellazzini).
Bellazzini, M., Ferraro, F. R., Pancino, E. 2001, ApJ, 556, 635
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14The predicted TRGB luminosities were estimated for the following metallicities: [M/H]= -2.267, -1.790,
-1.488, -1.266, -0.963, -0.659, -0.481. The set of isochrones for [M/H]= -0.481 was specifically computed for
this project. The anchor points we adopted for the spline fit are: MTRGB
-4.129, -4.088, -4.034, -3.977; MTRGB
(PHOENIX)= -4.066, -4.110, -4.108, -4.106, -4.069, -4.012, -3.870;
-4.149, -4.153, -4.138, -4.106, -4.034, -3.894.
(ATLAS9)= -4.085, -4.137, -4.143,
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This preprint was prepared with the AAS LATEX macros v5.0.
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Fig. 1.— F814W, F555W-F814W CMD of IC10 based on data collected with HST (fields
C and E). Using different selection criteria (sharpness, separation, photometric error) we
ended up with ≈ 150,000 candidate galaxy stars (CGS). The error bars on the right display
the mean intrinsic error.
Fig. 2.— Left – Top – F814W-band LF for the bright end of the RGB in the central region.
Middle – Smoothed LF of the same RGB region obtained using a Gaussian kernel with
standard deviation equal to the photometric error. Bottom – Response of the four-point
Sobel filter to the smoothed LF. The dashed vertical lines indicate the position of the TRGB
and its 1σ error. Right – Same as the left, but for bright RGB stars in the external region.
– 11 –
Fig. 3.— Left – Comparison in the F814W, F555W-F814W CMD between IC10 stars
located in the central region (C, gray dots) and MS, RG stars in the SMC cluster NGC 346
(green dots). The large gray circles mark TRGB stars in IC10. Middle – Same as the left,
but the comparison is between IC10 stars located in the external region (E, gray dots) and
RG stars in 47Tuc (red dots). The large blue circles mark the TRGB stars in 47Tuc. The
horizontal arrow marks the position of TRGB stars in SMC according to the measurement
by Cioni et al. (2000). Right – Same as the middle, but the comparison is between IC10
stars in the external region and RG stars in ω Cen (orange dots). The large blue dots show
TRGB stars in ω Cen according to B08.
– 12 – Download full-text
Fig. 4.— Top – Comparison between theoretical and empirical MTRGB
metallicity. Circles and diamonds display predictions according to ATLAS9 and PHOENIX
atmosphere models. The error bar shows the typical uncertainty on epirical calibrations
(Bellazzini 2008). Bottom – Same as the top, but the theoretical predictions for the TRGB
were transformed into the F814W-band. The error bar shows current uncertainty on theory.