A&A 431, 555-563 (2005)
DOI: 10.1051/0004-6361:20041686
R. L. M. Corradi1 - L. Magrini2 - R. Greimel1 - M. Irwin3 - P. Leisy1,4 - D. J. Lennon 1 - A. Mampaso4 - M. Perinotto2 - D. L. Pollacco5 - J. R. Walsh6 - N. A. Walton3 - A. A. Zijlstra7
1 - Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa
Cruz de La Palma, Spain
2 -
Dipartimento di Astronomia e Scienza dello Spazio, Universitá di
Firenze, L.go E. Fermi 2, 50125 Firenze, Italy
3 -
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge
CB3 0HA, UK
4 -
Instituto de Astrofísica de Canarias, c. vía Láctea s/n,
38200, La Laguna, Tenerife, Spain
5 -
APS Division, Dept. of Pure and Applied Physics, Queen's University Belfast,
Belfast BT7 9NN, Northern Ireland, UK
6 -
ST-ECF, ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei
München, Germany
7 -
Physics Department, UMIST, PO Box 88, Manchester M60 1QD, UK
Received 19 July 2004 / Accepted 16 October 2004
Abstract
In the framework of our narrow-band survey of Local
Group galaxies, we present the results of the search for planetary
nebulae (PNe) in three spheroidal companions to the Andromeda
galaxy. We find 9 candidate PNe in NGC 147, 5 in NGC 185, and 75 in
the 0.4 square degree area searched around NGC 205, increasing
the number of PNe known in these galaxies. It is shown that in the
crowded regions of these galaxies continuum-subtracted images are more
effective in detecting PNe than colour-colour diagrams obtained via
automatic photometry.
For NGC 205, the degree of contamination of PNe belonging to the halo
of M 31 is estimated; taking it into account, 35 PNe within 1.5 tidal
radii from the centre of NGC 205 have been used to build its PN luminosity function.
Candidate PNe in NGC 185 are systematically
brighter than those in NGC 147. Considering that star formation is
thought to have been much stronger in NGC 185 than in NGC 147 in the
last 3 Gyr, this might suggest that the bright end of the PN luminosity function is populated by relatively massive stars, as
predicted by some recent theoretical models. This result, however, has
to be taken with some caution, given the small PN population size of
these galaxies and a rather incomplete knowledge of their star
formation history.
Key words: ISM: planetary nebulae: general - galaxies: stellar content
Planetary nebulae (PNe) are valuable tracers of any stellar population of low and intermediate mass. Owing to their selective emission in a small number of strong and narrow emission lines, they can be discovered at significant distances within the nearby Universe. Their study provides accurate information on the luminosity, age, metallicity, and dynamics of the stellar population from which they derive; this makes them very useful to test a number of theories about the evolution of stars and galaxies.
Table 1: Log of the INT+WFC observations.
In order to investigate these issues, the first step is to detect a representative sample of extragalactic PNe. With this aim, we have carried out a narrow- and broad-band imaging survey aimed at cataloguing all classes of emission-line objects in most of the Local Group (LG) galaxies visible from La Palma. The project is called the Local Group Census (LGC), and details of its status can be found at http://www.ing.iac.es/~rcorradi/LGC. So far, the analysis of the LGC data has been focussed on the search for PNe in dwarf irregular galaxies, and resulted in a significant increase of the total population of PNe known in these systems (Magrini et al. 2002, 2003, 2004; Leisy et al. 2004).
The most common morphological type in the LG is however that of the
so-called spheroidal (Sph) galaxies. Most objects in this class are
dwarf galaxies, and their distribution in the LG, total number and
origin is still not well understood (see e.g. Van den Berg
2000, hereafter vdB00). NGC 147, NGC 185 and NGC 205 are
the most luminous Sph galaxies of the LG, and owing to their
relatively high luminosity they should not be be considered as dwarfs.
They are all associated with the Andromeda Galaxy. NGC 205 is fully
embedded in the halo of M 31, and its outer isophotes are clearly
distorted by tidal interaction with the large spiral galaxy. NGC 147
and NGC 185 form a pair at an apparent distance of 12 degrees from
M 31, very close to the projected centre of mass of the LG. Prior to
our work, 5 candidate PNe were detected in NGC 185 (Ford et al. 1973, 1977), 5 in NGC 147 (Ford et al. 1977), and 28 in NGC 205 (Ford et al. 1973; Ciardullo et al. 1989). In this paper we present new [O III], H,
and "continuum'' images of these three galaxies. These allowed us to detect 9 candidate PNe in NGC 147, 5 in NGC 185, and 75 in the
0.4 square degree area searched around NGC 205.
The LGC makes use of the prime focus wide field camera (WFC) of the
2.54 m Isaac Newton Telescope (INT) at La Palma. The detector of the WFC is
composed of four thinned EEV42 CCDs with 2048
4096 pixels each,
with a sampling of 0
33 per pixel, resulting in a total field
of view of
.
The filters used for the
images considered in this paper are:
[O III] (with central wavelength and full width at half maximum of
500.8 nm and 10.0 nm, respectively), H
(656.8/9.5), Strömgren y (550.5/24.0), as well as the broad band
and
filters of the
Sloan photometric system. The Strömgren y,
and
images were used to
evaluate the continuum emission in close proximity to the [O III] and
H
wavelengths. Note that the H
filter includes the contribution
of the [N II] doublet at 654.8 nm and 658.3 nm; for the sake of
conciseness, in the following we label data in this filter only with
the symbol "H
''.
While one WFC field was observed for NGC 147 and NGC 185, two
overlapping adjacent fields were observed for NGC 205 owing to its
larger size and in order to estimate the degree of contamination of
PNe belonging to M 31. These overlapping fields cover a field of
40
40
.
The log of the observations is
presented in Table 1. Seeing ranged from 1.1 to 1.5 arcsec.
Flux calibration of the broad and narrowband images were done by observing the standard photometric stars SA 95-42, SA 95-190, and SA 92-263, the spectrophotometric star G 191-B2B, and the Galactic PN M 2-1, during photometric nights.
Data was reduced using routines available within IRAF. The frames were de-biased, flat-fielded, and
linearity-corrected using the ING WFC data-reduction pipeline (Irwin
& Lewis 2001). The instrumental magnitudes for the
narrowband filters were calibrated by convolving the spectrum of the
spectrophotometric standard star (Oke 1990) or the PN M 2-1 (Wright et al. 2004) with the response curve of each filter, computed for the f/3.3 converging beam of
the INT prime focus. Astrometric solutions were computed using tasks
CCMAP and CCTRAN and the USNO A2.0 catalogue (Monet et al. 1998). The accuracy of astrometry is
0
3 rms. In order to detect PNe, images were then analysed using the
following two methods.
Following the procedure described in Magrini et al. (2002,
2003), [O III] and H
continuum-subtracted images were
obtained. First, all images were corrected for geometrical distortions
and aligned to the [O III] frame. The
images were then used as the
reference continuum for the H
frames, and after proper scaling they
were subtracted from them. For the continuum subtraction of the
[O III] images, we have used the
frames for NGC 185 and NGC 205, and the Strömgren y image for NGC 147.
In the continuum-subtracted frames, candidate PNe were identified as
point-like sources (PNe cannot be resolved at the distance of our
target galaxies) emitting in [O III], although in most cases they were
also detected in H
.
Emission-line fluxes were measured in the continuum-subtracted frames,
using the IRAF task APPHOT. They were corrected for the contribution
of the [O III] and H lines in the
and
continuum images,
respectively. For the [O III] line, fluxes were also corrected for the
contribution in the WFC filter of the companion oxygen line at
495.9 nm (considering the systemic radial velocity of each
galaxy), and transformed to equivalent V magnitudes via the
formula
(Jacoby
1989). The errors of the [O III] and H
fluxes of the
candidate PNe, including photon statistics, background and flux
calibration uncertainties, amount to some 10% for fluxes larger than
3.2
10-15 erg cm-2 s-1 (
= 22.5), to 20-30% for fluxes up to
8.0
10-16s erg cm-2 s-1 (
= 24.0), and are larger for the faintest lines.
Automatic point-spread-function (PSF) photometry in all filters for
all the observed fields were performed using the packages
DAOPHOT/ALLSTAR (Stetson 1994). First, point-like sources
were searched using a detection threshold 6
over the
background. Then the PSF was built using some forty well exposed and
isolated stars in each CCD frame. In very crowded fields, a simple
Gaussian PSF was first computed in order to subtract objects close to
all the stars that were used to build the final PSF. This procedure
was iterated increasing the degree of the polynomials that describe
the spatial variations of the PSF throughout the field. Once built the
PSF, instrumental magnitudes were measured for all unresolved sources
in the various filters.
Different kind of colour-colour diagrams were then produced. A priori,
the most obvious choice to highlight emission-line objects would be to
consider an H-
vs. [O III]-
diagram. An example is
given in Fig. 1 (upper panel) for the two WFC fields of
NGC 205. In this diagram, photometry was obtained for 71 500 stars, and
candidate PNe selected in the continuum-subtracted images clearly show
up as objects with generally strong [O III] and H
excesses (large
dots) with respect to normal stars (small dots). However, using this
diagram some 50% of the candidate PNe were missed because in the
crowded inner regions of the galaxy it is not possible to measure
their
magnitude (even with a lower detection threshold). We have
also considered the less "natural''
H
-
vs. [O III]-
diagram (Fig. 1, lower panel),
because in the
band the galaxy stellar background is fainter than
in
,
as red stars dominate the overall emission of these relatively
old galaxies, and thus PNe are more easily detected. In such a
diagram, the detection rate of PNe increases to 75%, but the degree
of confusion between PNe and stars also increases. The search is
improved if an additional list of objects which are detected by
PSF photometry in the [O III] and H
frames, but not in the
broadband ones, is produced; these sources should be checked
individually in the original images to remove spurious detections in
the narrowband filters. Even so, however, a few PNe are missed.
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Figure 1: Colour-colour diagrams for the two WFC fields around NGC 205. Colours are computed from instrumental magnitudes, so that the zero points of the x and y axes are arbitrary. |
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We conclude that, as expected, in the crowded regions of galaxies the visual inspection of continuum-subtracted images, although somewhat subjective and time consuming, is more effective in detecting PNe than the analysis of colour-colour diagrams obtained via PSF photometry. Even so, the use of these diagrams remains a valid complementary tool to double check identifications of candidate PNe, verifying for example that that no emission line sources have been missed in the visual inspection of the large fields observed. In uncrowded regions, colour-colour diagrams are instead a powerful automatic tool to detect PNe, as shown by Arnaboldi et al. (2003) for the external regions of galaxies and the intergalactic space. Note that other authors (e.g. Feldmeier et al. 2003) apply automated detection algorithms directly to the continuum-subtracted frames in order to detect extragalactic PNe.
![]() |
Figure 2:
Digitized Sky Survey image of an area of
34![]() ![]() ![]() |
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Table 2:
PN candidates in NGC 147. Observed [O III]500.7 and H fluxes are given in 10-16 erg cm-2 s-1.
are corrected for the
contribution of the companion oxygen line at 495.9 nm.
Nine candidate PNe were identified in the continuum-subtracted images
of NGC 147 and colour-colour diagrams. They are listed in Table 2 with
their coordinates, [O III] and H fluxes, and m
.
Their positions in the sky are indicated by crosses in
Fig. 2. They are all located inside the
mpg=26.5 mag arcsec-2 isophote of NGC 147
(12
18
,
Holmberg 1958).
Two of our candidate PNe (id.n. 4 and 7) correspond to sources n. 1
and 4, respectively, detected by Ford et al. (1977). The other
three objects listed by Ford et al. (1977) were found to be
stars as they show a significant continuum emission.
Table 3: As in Table 2, but for NGC 185.
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Figure 3:
Digitized Sky Survey image of an area of
34![]() ![]() ![]() |
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Table 4: As in Table 2, but for NGC 205. The last column indicates whether the candidate PNe are inside ("i'') or outside ("o'') 1.5 tidal radii of the galaxy.
![]() |
Figure 4:
Digitized Sky Survey image of an area of
50![]() ![]() ![]() |
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In this case, we confirm the five candidate PNe originally discovered
by Ford et al. (1977), which include one H source detected by
Martinez-Delgado et al. (1999), while the additional source
found by the latter authors does not emit in [O III], and thus was
conservatively left out from our list of candidate PNe. This is
presented in Table 3, and PNe are indicated by crosses in
Fig. 3. All objects are located in the bright,
innermost regions of the galaxy, well inside its optical size
(12
13
,
Holmberg 1958). The [O III] magnitudes that we have determined are in
good agreement with those measured by Ciardullo et al. (1989), with the exception of PN n. 2 which we measured to be
0.5 mag brighter. This is probably due to its
location in the very central and crowded regions of the galaxy, which
makes it difficult to derive its luminosity in the observations of
Ciardullo et al. (1989), as the detector pixel
projected in the sky was relatively big (0''.86) considering
the rapidly varying galaxy background. Our images also confirm the
presence of a diffuse ionized medium emitting both in H
and [O III] in a region of
60
60
in the centre of
the galaxy (e.g. Martinez-Delgado et al. 1999).
Our narrowband images allowed us to identify 75 PNe in the 0.4 square degree area searched around NGC 205. Forty of them are located
within the
mpg=26.5 mag arcsec-2 isophote (16
26
,
Holmberg 1958), and the remaining ones outside it. The candidates are
listed in Table 4, and their positions within NGC 205
indicated in Fig. 4.
Previous survey for PNe in this galaxy led to the discovery of 24 PNe (Ford et al. 1977; Ciardullo et al. 1989). We have recovered 20 of them, while another 3 were found to be stars (PNe 12, 21, and 28 in Table 6 of Ciardullo et al. 1989), and we did not detect their PN n.14. Outside NGC 205, ten PNe in our list were previously found by Ford & Jacoby (1978) and Meyssonnier et al. (1993). Merrett et al. (2003) included the region around NGC 205 in their survey for PNe in M 31, but in their paper they do not specifically discuss the results for this area.
A simple visual inspection of the distribution of PNe in
Fig. 4 suggests that a number of PNe must be associated
with the disk and halo of M 31. A precise calculation of the amount of
contamination of M 31 PNe at the location of NGC 205 is not easy, as it
depends on the (spatially variable) completeness of our search in
addition to the (also varying) contribution from the disk and halo of
M 31. A first-order estimate was obtained in the following way. In our
images, we have masked an elliptical region centred on NGC 205 and
extending out to 1.5 (9
3, Mateo 1998) and 2 tidal radii. Then
the density of PNe was estimated in stripes parallel to M 31's major
axis, each with a width of 4 arcmin, and excluding the masked
region. Except for the first strip, likely dominated by the disk of
M 31, there is a shallow decrease of the density of PNe with distance
from M 31 (Fig. 5), which allows us to adopt an
average figure of 2.3
10-2 PNe arcmin-2 belonging to
M 31 in this area and at the detection limit of our survey. This would
mean that, among the 27 PNe inside one tidal radius of NGC 205, only
one or two might belong to M 31, being seen in projection over its
spheroidal companion. Contamination by M 31's PNe would become
significant only outside 1.5 tidal radii, which is the "borderline''
that we adopt (see the ellipse in Fig. 4) as the limit at
which we are confident that nearly all the 35 candidate PNe detected
inside it belong in fact to NGC 205.
Consistent results are obtained if M 31's contamination is
estimated from the surface brightness photometry of the halo of M 31
(Irwin et al., in preparation). From this, a bolometric luminosity of
6
is computed for the halo light integrated
within 1.5 tidal radii of NGC 205. This can be transformed into a
total number of PNe using the theoretical expectations of stellar
evolution theories (Renzini & Buzzoni 1986). Adopting
(Buzzoni
& Arnaboldi 2004), this gives a total of 19 PNe belonging to
M 31 and located within 1.5 tidal radii of NGC 205. Correcting this
number for the completeness limit of our survey (see next section)
using the empirical law of the PN luminosity function (PNLF, Ciardullo
et al. 1989), it implies that we should have detected
three or four of M 31's PNe in the area of NGC 205, in very good agreement
with our results from the PN counts (also estimating 3 contaminating
PNe within 1.5 tidal radii). Note, however, that the value of
can vary by more than one order of magnitude depending on the galaxy type, and the value to be used for the halo of M 31 is
uncertain as the population of the PN progenitors is not well known.
The completeness limit of our search for PNe was computed as in
Magrini et al. (2002, 2003). We estimated the number of
"missing'' PNe by adding "artificial stars'' with various m
in both narrowband and continuum images, and then computing
the recovery rate of such artificial objects. As the galaxy
background and crowding become progressively stronger toward the
central regions of these spheroidal galaxies, we have computed the
recovery rate in concentric annuli centred on the galaxy nuclei and
with radial steps of 60 arcsec. We find that, in the inner regions
of the three galaxies, namely around a radius of 120 arcsec, the
recovery rate is
50% (this would define the incompleteness
according to Minniti & Zijlstra
1997) for objects with 23.5
24.5
for NGC 185 and NGC 205, and 24.5
25.5 for
NGC 147. For these magnitudes, the recovery rate decreases in the
innermost regions of the galaxies due to the higher background. For
brighter objects, the recovery rate is 100% at any galactocentric
distance, except for the innermost 60 arcsec where however it decreases
only by some 25%. The difference between NGC 147 and the other two galaxies is due to the fact that the former was observed in better seeing conditions, and (less important) that it has a lower surface brightness than the latter ones, both in terms of its stellar and gas
background emission.
We have also measured the [O III] and H fluxes of the artificial
stars in the same way as we have done for our candidate PNe, and
compared the measured fluxes with the input ones, confirming the
magnitude of the photometric errors listed at the end of Sect. 3.1.
The PN specific luminosity rate for the three galaxies was estimated
in the following way. We have adopted the total absolute V luminosity of the galaxies from vdB00, where distances of 660 kpc for
NGC 147 and NGC 185, and of 760 kpc for NGC 205, are assumed. The
bolometric correction is taken from Buzzoni & Arnaboldi (2004). The
observed number of PNe was then extrapolated to 8 mag down the bright
end of the PNLF using its analytical description (Ciardullo et al. 1989). This allows us to compute the PN specific luminosity rate, that is found to be
1.4
0.3
10-7 PNe
for both
NGC 147 and NGC 185, and
2.3
0.2
10-7 PNe
for NGC 205.
Scaled to our Galaxy, this would imply a total number of 5300 to 8700 Galactic PNe, in agreement with other recent estimates (e.g. Jacoby &
De Marco 2002). Note, however, that the extrapolation to faint
magnitudes using the double-exponential formula of Ciardullo et al. (1989) may overestimate the total number of PNe by up to a factor of two (for the completeness limit of the
observations presented in this paper), if the deep PNe counts in the
SMC presented by Jacoby (2004) are representative of the faint
end of the PNLF for most galaxies. On the other hand, the PNLF of the
bulge of M 31 seems to follow the empirical law, so the issue remains
open and should be further investigated by much deeper surveys in the LG.
The number of PNe found in NGC 147, NGC 185, and NGC 205 also nicely fits into the scaling relation between the absolute V luminosity of LG galaxies and their PN population size (Magrini et al. 2003; Corradi & Magrini 2004).
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Figure 5: The PN surface density at increasing distances from M 31 in strips parallel to its major axis, and outside 1.5 tidal radii from the centre of NGC 205 (see text). The zero point of the x-axis corresponds to the inner limit of our observed field. The dashed lines indicate the range of distances within which NGC 205 is located. |
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In spite of the relatively small number of PNe, we have constructed
the PNLF for the 35 PNe located within 1.5 tidal radii from the centre
of NGC 205 (see discussion in Sect. 4.3). This is presented
in Fig. 6. The Eddington formula (Eddington
1913) was applied to correct for the effects of
observational errors on the number of PNe in each bin. The resulting
luminosity function within the completeness limit was then fit to the
"universal'' PNLF
,
taking into account both errors in
x (associated with the finite width of the histogram bins) and in y(statistical errors).
The derived distance modulus is 24.65
0.21, adopting the
absolute value of
= -4.48 (Ciardullo et al. 2002), and after correcting for foreground reddening using E(B-V) = 0.035 (Burstein & Heiles
1984). The error on the distance modulus was computed
combining the error derived from the fit with the errors on our
photometric zero point and on the adopted extinction. The same
distance modulus, within the errors, is obtained from the PNLF for
objects located outside 1.5 tidal radii from the centre of NGC 205,
presumably belonging to the halo and disc of M 31. This is also in
agreement with the determination of Ciardullo et al. (1989)
and, within the relatively large errors due to the small number of
objects comprising the PNLF, with other determinations of the distance
of NGC 205 (vdB00).
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Figure 6: The luminosity function of the 35 candidate PNe within 1.5 tidal radii from the centre of NGC 205. The fit down to the completeness limit of our images is indicated by the solid line. |
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Figure 7: Histogram of the absolute magnitudes of the candidate PNe in NGC 147 (dashed histogram) and NGC 185 (thick solid line). The completeness limits of our survey are indicated for each galaxy by the arrows in the upper part of the diagram. |
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Figure 7 shows the histogram of the absolute [O III] magnitudes of the candidate PNe in NGC 147 and NGC 185, computed adopting a distance of 660 kpc for both galaxies and a reddening E(B-V) = 0.17 and 0.19, respectively (vdB00). In spite of the small number of objects involved, there is some evidence that in [O III] the PNe in NGC 147 are systematically fainter than those of NGC 185, which populate the bright end of the "universal'' PNLF. Application of a 2-sample Kolmogorov-Smirnov test shows that indeed the two distributions are likely to be different at a significance level (probability of being wrong by saying that the two distributions are not drawn from the same population) smaller than 5%. The same result holds even if we increase the distance modulus of NGC 147 by 0.4 mag (relative to NGC 185), as suggested by certain authors (cf. vdB00).
This difference might be related to the puzzling overall differences
between the two galaxies. In fact, in spite of forming a bound (but
not interacting) physical pair and having very similar morphological
types, luminosities, and metallicities (vdB00), NGC 147 and NGC 185
are strikingly different in terms of their gas and dust content,
likely as a consequence of a different evolution. NGC 147 is gas free
(Sage et al. 1998), and Fig. 8 in Mateo (1998) would suggest
that no star formation has occurred in this galaxy in the last
2-3 Gyr. On the other hand, the detailed photometric study of NGC 185
by Martinez-Delgado et al. (1999) derived a significant star
formation in the last few Gyr in the inner region of this galaxy (see
also Battinelli & Demers 2004), and specifically within an
elliptical isophote of semi-major axis of 240 arcsec, where all
our 5 candidate PNe are located.
The bright PNe in the inner regions of NGC 185 might therefore be the
product of stars formed in the last few Gyr, while the absence of
such bright PNe in NGC 147 would be the consequence of the absence of
such a relatively young stellar population. This is an important
issue, as recent theoretical modelling of the PNLF (Marigo et al. 2004) has indeed predicted its bright cutoff to be extremely
sensitive to the time when star formation has stopped in a
galaxy. According to Marigo et al. (2004), the effect would be
particularly important if the last burst of star formation occurred
more than 1-2 Gyr ago, which is the lifetime of stars with initial
masses around 2-2.5
which are predicted to be those populating
the bright end of the PNLF. Thus our data on NGC 147 and NGC 185 seem
to provide some evidence for the age effects predicted by Marigo et al. (2004). Note also that the average luminosity difference between the PNe of the two galaxies is slightly smaller if the H
fluxes, rather than the [O III] ones, are considered, a fact that is also
predicted by the models of Marigo and collaborators (Girardi, private
communication).
Our result, however, should be taken with extreme caution because of the low number statistics of the PN samples of NGC 147 and NGC 185. A better knowledge of the recent star formation history of NGC 147 would also be desirable. In addition, note that other (but less detailed) simulations of the PNLF show a much milder age effect (e.g. Méndez & Soffner 1997). Moreover, the strong variation of the bright cutoff magnitude of the PNLF predicted by Marigo et al. (2004) has not been observed in the wide variety of galaxian types (spiral discs, bulges, ellipticals, irregulars) observed so far (cf. Ciardullo et al. 2002).
We have presented the result of our search for PNe in the three spheroidal galaxies NGC 147, NGC 185 and NGC 205. This increases the number of candidate PNe known in these galaxies, and provides a valuable resource for follow-up spectroscopy which will allow us to tackle some of the present problems in our understanding of these spheroidal galaxies. New abundances measurements (to improve the works by e.g. Richer & McCall 1995) are in fact needed to shed light into the discrepancy between the metallicity determination from different authors in NGC 205, or to investigate further the difference and similarities between NGC 147 and NGC 185.
For these latter two galaxies, the present data show some evidence for age effects on the magnitude of the brightest PNe along the lines of the theoretical predictions by Marigo et al. (2004), the older stellar population of NGC 147 producing fainter PNe than the younger population of NGC 185. This potentially important result is however weakened by low number of PNe found in these small galaxies, as well as in the general difficulty of determining precisely their star formation history in the last few Gyr. Testing the possible existence of such an age effect in other galaxies (within or outside the LG) would be desirable, but it is likely not be an easy task, as the star formation history is found to be significantly different from galaxy to galaxy, even in systems of the same morphological type, mass and metallicity.
Acknowledgements
We thank Dr. Rino Bandiera for his suggestions on the fitting method of the PNLF of NGC 205. The LGC data are publically available through the Isaac Newton Group Wide Field Camera Survey Programme.