A&A 417, 479-486 (2004)
DOI: 10.1051/0004-6361:20034272
P. Battinelli 1 - S. Demers 2
1 - INAF, Osservatorio Astronomico di Roma,
Viale del Parco Mellini 84, 00136 Roma, Italia
2 -
Département de Physique, Université de Montréal,
CP 6128, Succursale Centre-Ville, Montréal,
Québec, H3C 3J7, Canada
Received 2 September 2003 / Accepted 15 December 2003
Abstract
We used the CFH12K wide field camera to survey the
carbon star population of NGC 185 using the CN-TiO technique. 145
C stars are identified with a mean
,
corresponding to
,
a mean luminosity similar
to what was recently found for the C star population of NGC 147.
The stellar surface density profile of NGC 185 follows a power law with an
exponential scale length of 2.53
0.07'. The surface density of
C stars follows also a power law but with a smaller scale length,
,
demonstrating
that the intermediate-age population of NGC 185 is more concentrated
than its old population. A tidal radius,
is
determined from red giant star counts.
The C/M ratio of NGC 185 is
with no apparent sign of a radial
gradient. We compare the carbon star populations
of NGC 185 and NGC 147 to conclude that
it is unlikely that they form a close pair.
Key words: galaxies: individual: NGC 185 - galaxies: stellar content - galaxies: structure
C stars, selected in a specific colour interval,
do have a nearly constant
= -4.59 in galaxies
with at least one hundred such stars (Demers et al. 2003). However,
Battinelli & Demers (2004), see also Nowotny et al. (2003), noted that
the
of the C star population of NGC 147 is
3
fainter than this mean value.
The fact that NGC 147 is the only spheroidal galaxy with an abundant
intermediate-age population, in the sample of galaxies so far surveyed
(Battinelli & Demers 2004)
urges us to investigate another galaxy of similar morphology, namely NGC 185.
It soon, however, became evident that once identified,
C stars can be quite useful to investigate the structure and population
distribution in the outer parts of galaxies, as clearly shown by
Letarte et al. (2002). In this respect, C stars offer two advantages
over red giants: 1- the Galactic contribution to the C star counts is
essentially nil, indeed only one per 200 deg2 late-giant C stars is
observed by Totten & Irwin (1998). Such stars would be brighter than
their extragalactic counterparts, and none of them are dwarf carbon
stars (Totten et al. 2000). This fact allows us to identify C stars
in low density environment; 2- C stars
are brighter, in the I band, that the numerous old red giants seen
in galaxies. They can then be detected with relatively short exposures,
for example,
less than 30 min of open-shutter time was required to observe NGC 185.
We note however that C star surveys in dwarf spheroidals is generally not
very rewarding since the number
of C stars expected in these galaxy
on the basis of the
-
relation (Battinelli & Demers 2000)
may be very low.
C star surveys employing a wide field mosaic have been successful in identifying proven galactic members located in the low density periphery. For example, Battinelli et al. (2003) have seen C stars up to the edge of the observed HI disk of M 31. These outer C stars can then be valuable kinematic probes to investigate the rotation or the mass distribution of galaxies.
The spheroidal galaxy NGC 185 is undoubtedly a satellite of M 31.
Located at (2000) =
and
it is
from M 31. NGC 185 is at a relatively low Galactic latitude
(
). Therefore its colour excess is far from negligible.
Schlegel et al. (1998) obtain
E(B-V) = 0.181, from the COBE and IRAS maps at 100
m. This
corresponds to
E(R-I) = 0.15 and
AI = 0.28 using the
reddening ratios of Rieke & Lebofsky (1985). Direct measurement of
the colour excess of NGC 185 by Lee et al. (1993) yields a similar
value:
.
We adopt this reddening.
One finds in the literature three independent distance determinations
for NGC 185. From 151 RR Lyrae variables Saha & Hoessel (1990) obtained
while Lee et al. (1993) and Martínez-Delgado
& Aparicio (1998) obtained respectively
and
from the tip of the red giant branch (TRGB). A re-evaluation
of the TRGB method by Salaris & Cassisi (1998) shifts the modulus
of NGC 185 to 24.12, the distance adopted by van den Bergh (2000).
This later determination would be our adopted distance for NGC 185.
The mean metallicity of NGC 185 has been estimated from the colour of the
red giant branch by Lee et al. (1993) (
)
and by Martínez-Delgado & Aparicio (1998) (
).
A spread of in the metallicity has been detected by these two
studies. We shall assume that the C star population has a metallicity
of [Fe/H] = -1.33, the average of both determinations.
An intermediate-age population has been known, for sometime, to exist
in NGC 185 ever since Lee et al. (1993) noted the presence of
numerous bright red stars.
Their existence has recently been confirmed by the AGB survey of
Nowotny et al. (2003).
Using essentially the same narrow band technique that we
employ, they identified 154 carbon stars in a
area
centered on NGC 185.
In the next section we describe our observations and the reduction procedure; in Sect. 3 the colour-magnitude and colour-colour diagrams are presented; in Sect. 4 we discuss the photometric properties of the C stars (Sect. 4.1), the C/M ratio (Sect. 4.2), the surface density profile of NGC 185 and its tidal radius (Sect. 4.3), the NGC 185-NGC 147 system (Sect. 4.4) and finally the mass determination of these two galaxies (Sect. 4.5).
Table 1: Summary of the NGC 185 observations.
The data distributed by the CFHT have been detrended. This means that the images have already been corrected with the master darks, biases, and flats. Fringes have been removed on I exposures under 60 s and large scale structures such as the "Skyring'' effect have been removed when relevant. This pre-analysis produces 12 CCD images, of a given mosaic, with the same zero point and magnitude scale.
The photometric reductions were done by fitting model point-spread functions
(PSFs) using DAOPHOT-II/ALLSTAR series of programs (Stetson 1987, 1994).
Instrumental magnitudes are calibrated using equations provided by the
CFHT QSO team
(see http://www.cfht.hawaii.edu/Instruments/Elixir/stds.2003.05.html
for details).
The equations for R and I magnitudes are:
![]() |
Figure 1: Colour-magnitude diagram of the whole CFH12K field. Stars with colour errors <0.10 are plotted. We acquire the first 2.5 mag of the giant branch, quite sufficient to identify the C star population. |
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Figure 2 presents the colour-colour diagram which allows the
discrimination of C stars from M stars.
Only stars with a combined error
smaller than
0.125 mag are retained. Some 29 000 stars are plotted.
The two boxes define the regions of O-rich and C-rich stars as
explained by Letarte et al. (2002). 145 stars fall into the C box,
there are obviously more C stars, with bluer colours, but they
will not be included in our sample. This number of C stars is low
compared to galaxies of similar luminosity.
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Figure 2: Colour-colour diagram showing the boxes of C stars and M stars. |
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Table 2: C stars in NGC 185a.
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Figure 3: The CFH12K field of NGC 185. Carbon stars identified in this study are represented by big dots. North is on top, East is on the left. The average gap, between the CCDs, in the X direction is 7.8''while it is 6.8'' in the Y direction. |
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The lack of C stars younger than 1 Gyr does not appear to us as the explanation for the lower luminosity of the C stars in NGC 185 and NGC 147. In dwarf irregular galaxies, with ongoing star formation, one would certainly expect to see C stars of all ages. The C stars LF's in these galaxies are not wider than the LF's observed in the spheroidal galaxies. In the colour range considered, the spread of magnitude is the essentially the same. Furthermore, the apparent magnitudes of the 900 C stars in NGC 6822 do not show a spatial trend which could be expected if numerous younger stars are located at the center where current star formation takes place.
One needs observations of other spheroidal galaxies
with numerous C stars to investigate their LF.
The 13 C stars in the dwarf spheroidal galaxy Leo I have
(Demers & Battinelli 2002)
a luminosity that goes contrary to what is seen in NGC 147 and NGC 185.
It would be of interest to determine the
of
the C star population of Fornax a galaxy with more C stars than Leo I.
The C star population of the brightest dwarf spheroidals around M 31
may also reveal some clues as to the effect of the morphology on
the
of C stars.
Recently, Nowotny et al. (2003) surveyed the AGB population of NGC 185
where they identified 154 carbon stars. Their definition of a C star
is less restrictive than ours and consequently includes bluer C stars omitted
by us. A cross identification (better than 1'') of our C stars with theirs
resulted in 32 matches. As noted before (Battinelli & Demers
2004), their apparent i magnitudes are 0.2 mag fainter than our
I magnitudes.
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Figure 4: Luminosity function of the C stars in NGC 185. An eye fitted Gaussian is traced over the histogram. |
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The C/M ratio of a stellar population is known to be function of the metallicity of that population (Battinelli & Demers 2004; Groenewegen 1999). The number of C stars is relatively easy to obtain in a galaxy, but on the other hand the number of AGB M stars is much more difficult to properly assess because one has to know the contribution of the foreground M stars to the total counts.
To be consistent with our previous publications of this series and to
follow Brewer et al. (1995), we select as AGB M stars a subset of the M stars (those in the M box of the colour-colour diagram) by applying
a lower and upper magnitude cutoffs. M
must have their
and the I > 18.2. The determination of the
bolometric magnitude follows the procedure detailed by Battinelli et al.
(2003).
The surface density of M
stars, belonging to NGC 185, can be
followed up to 12' from the center. The density then reaches a
plateau of
stars min-2, number that we
adopt for the foreground. Within the 12' ellipse there are 144 C stars
and 1536 M
stars. The estimated number of foreground stars
in this area is 668, a number to be substracted to the M
stars
count. Thus, yielding a
.
In the radial range from 0' to 10' there is little evidence for
a radial gradient of the C/M ratio.
Hodge (1963) was the first to publish a survey of the structure of
NGC 185. Eventhough he used Palomar Schmidt plates Hodge could not
follow the profile further than a major axis distance of 300''.
He
determined that the ellipticity
of the galaxy
increases slightly to reach 0.26 at a major axis distance of 6'.
The position angle of the major axis was found to be 41
.
Hodge (1963) determined also that the tidal limit of NGC 185 is
.
The luminosity profile in the inner regions of NGC 185
has also been determined by Kent (1987), Lee et al. (1993) and
Kim & Lee (1998) they essentially confirm the results of Hodge.
To study the stellar surface distribution of NGC 185 we employ a
file containing 49 944 stars with good photometry and reaching
.
They are the stars plotted in Figs. 1 and 3.
We adopt the PA and
as given by Hodge. We count stars
in elliptical annulii centered on the galaxy. For major axis distances
larger than 15' the annulii spill outside of the CFH12K field. We then
calculate the areas of sectors of elliptical annulii to properly
determine the densities. With this approach, we can reach up to a major axis
distance of 24' when the ellipse touches the east and west borders of
the field.
The outer density profile is displayed in Fig. 5.
The last four counts are averaged and are taken to represent the
foreground surface density: 13.07 stars min-2.
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Figure 5:
Surface density profile of the outer parts of NGC 185.
The foreground is reached at
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Using this estimated foreground density we plot, on Fig. 6, the
logarithm of the density. The profile of NGC 185 follows a
power law with a scale length of
corresponding to 500 pc at 667 kpc, the adopted distance.
This value is larger than the 1.5' quoted by
Caldwell et al. (1992) it is however smaller than the scale length
of NGC 147, recently obtained by Battinelli & Demers (2004).
The density profile of NGC 185 does not fit a de Vaucouleurs' law
as r1/4.
The C stars, plotted as open circles on Fig. 6, have a smaller scale
length than the old halo population of NGC 185. A least square fit through
the points yields a scale length of 1.56
0.06' demonstrating that
the intermediate-age population of NGC 185 is more concentrated than
the old population.
Martínez-Delgado et al. (1999) state that the relative amount
of stars of different ages is similar in each regions of NGC 185.
Figure 6 shows that, within the area surveyed by them (r < 6'), the
density profile of C stars appears essentially similar to the one
of old giants. When the C star profile is plotted over longer distances,
the different slope becomes obvious.
Harbeck et al. (2001) have recently studied the population gradients in
some Local Group dwarf spheroidals finding metallicity driven
gradients in Sextans, Sculptor, Tucana and Andromeda IV, while for Carina
the age is the major factor determining the observed population
gradients. For NGC 185 the different scale lengths we found for the old
and intermediate-age stars, along with
the lack of evidence supporting a clear C/M gradient suggest that
the population gradients in this galaxy are, similarly to Carina,
mainly age driven.
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Figure 6: The density profile of the halo red giant stars follows a power law. The density profile of C stars (where counts have been multiplied by 20 to facilitate comparison), represented by open circles. The two lines correspond to the least square fits of the two profiles. |
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Figure 7:
Surface density profile of the outer parts of NGC 185
yields a tidal radius of
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Carbon stars, being some Gyr old, may have been witnesses of
past interactions. They are then excellent candidates to provide
clues of the gravitational "stirring up'' of galaxies.
One may thing, for example, of the
inter Magellanic Cloud region where a number of C stars have been
seen (Demers et al. 1993). Figure 8 presents, to scale, the two
galaxies with their C star population. The ellipses represent
their tidal radius. The two rectangles correspond to the CFH12K
surveyed fields. There is currently a gap between the two halos.
If the two galaxies had been closer or in interaction, it is then
reasonable to expect a tidal bridge between them. A tenuous
stellar bridge will be hard to detect, their Galactic latitudes
not being particularly high. On the other hand, C stars can be
detected, irrespectively of their number since they are seen
behind a Galactic foreground of zero. Observation of a 9' 9' field did not reveal any C stars (Battinelli & Demers 2004).
We therefore conclude that all evidences point to a case of
optical binary galaxies.
![]() |
Figure 8:
A 100' ![]() |
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Assuming that the two galaxies are gravitationally bound, van den Bergh
(1998) calculated a lower limit for their combined mass from the
difference in their radial velocity, he obtains
.
Bender et al. (1991) determined the central velocity dispersion of
both galaxies to be 23 km s-1. With the luminosity profiles
they obtained the M/L ratios.
Using the newly determined surface density profiles of NGC 185 and
NGC 147 (Battinelli & Demers 2004) we can, following Rood et al.
(1972), (their Eq. (2)), obtain an estimates of their central density,
from their core
radius
and their line of sight velocity dispersion.
,
the radius
where the surface density is half the central value is easily
measured to be 2.82' (619 pc)
for NGC 147 and 1.75' (346 pc) for NGC 185.
Assuming that both galaxies are prolate spheroids, the central mass
is obtained by multiplying the central density by the volume limited to
.
We find
for NGC 185 and
for NGC 147, yielding rather low M/L ratios
close to one.
Our C star surveys have revealed that both galaxies possess a large tenuous stellar halo whose stellar populations are well mixed in NGC 147 but are not well mixed in NGC 185. Indeed, we show that the radial scale length of the intermediate-age population of NGC 185 is significantly smaller, indicating that the younger stars are more concentrated than the older ones. According to Bender et al. (1991) both galaxies have the same velocity dispersion, within a radius of 100''. NGC 147 seems to rotate while NGC 185 does not. Obviously, now that bright members of both galaxies have been positively identified as far as 15'from their center, the kinematics of individual stars is the next logical step to endeavour to understand the population distribution.
Acknowledgements
This research is funded in parts (S.D.) by the Natural Science and Engineering Council of Canada.