L. Magrini 1 - R. L. M. Corradi 2 - R. Greimel 2 - P. Leisy 2,3 - D. J. Lennon 2 - A. Mampaso 3 - M. Perinotto 1 - D. L. Pollacco 4 - J. R. Walsh 5 - N. A. Walton 6 - A. A. Zijlstra 7
1 - Dipartimento di Astronomia e Scienza dello Spazio, Universitá di
Firenze, L.go E. Fermi 2, 50125 Firenze, Italy
2 -
Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa
Cruz de La Palma, Canarias, Spain
3 -
Instituto de Astrofísica de Canarias, c. vía Láctea s/n,
38200, La Laguna, Tenerife, Canarias, Spain
4 -
School of Pure and Applied Physics, Queen's University Belfast, Belfast BT7
9NN,
Northern Ireland, UK
5 -
ST-ECF,
ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
6 -
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge
CB3 0HA, UK
7 -
Physics Department, UMIST, PO Box 88, Manchester M60 1QD, UK
Received 28 March 2003 / Accepted 6 May 2003
Abstract
In the framework of our narrow-band survey of the Local
Group galaxies, we present the results of the search for planetary
nebulae (PNe) in the dwarf irregular galaxies
IC 10, Leo A and Sextans A.
Using the standard on-band/off-band technique, sixteen new candidate
PNe have been discovered in the closest starburst galaxy, IC 10. The
optical size of this galaxy is estimated to be much larger than
previously thought, considering the location of the new PNe in an area
of 3.6 kpc 2.7 kpc. We also confirm the results of previous
studies for the other two dwarf irregular galaxies, with the detection
of one candidate PN in Leo A and another one in Sextans A. We review the
number of planetary nebulae discovered in the Local Group to date and
their behaviour with metallicity. We suggest a possible fall in the
observed number of PNe when [Fe/H]
-1.0, which might indicate that
below this point the formation rate of PNe is much lower than for
stellar populations of near Solar abundances. We also find
non-negligible metallicity effects on the [O III] luminosity of the
brightest PN of a galaxy.
Key words: planetary nebulae: general - galaxies: individual: Leo A, Sextans A, IC 10
Most of the galaxies in the Local Group (LG) are dwarf irregulars and spheroidals. These morphological types also represent the most numerous objects in the nearby universe, but can be studied in great detail only at the close distance of the LG. We are performing a narrow- and broad-band filter survey, the Local Group Census, which is mainly aimed at studying all classes of emission-line populations in the LG. The first results were presented by Magrini et al. (2002, hereafter M02), Magrini et al. (2003), Wright et al. (2003), while the status of the project is described at http://www.ing.iac.es/~rcorradi/LGC. M02 presented the detection of planetary nebulae (PNe) in Sextans B. Our next targets were the dwarf irregular galaxies IC 10 (morphological type IrIV according to van den Bergh 2000, hereafter vdB00), Leo A (IrV) and Sextans A (IrV), that we discuss in the present work.
IC 10 is a highly obscured galaxy
,
Sakai et al. 1999) located at a low Galactic latitude
.
Its distance (660 kpc, Sakai et al. 1999)
and its position (only
18
apart from M 31 on the
sky) suggest a possible membership to the M 31 subgroup (vdB00). It
is the only starburst galaxy in the LG, and the presence of a large
number of H II regions (Hodge & Lee 1990) proves that it
is undergoing massive star formation. IC 10 is a rather small galaxy
with an effective radius
kpc (de Vaucouleurs & Ables
1965), only one half the effective radius of the Small
Magellanic Cloud (SMC), whereas their luminosities are comparable.
Its oxygen abundance is higher than that of
compared with 7.98 in SMC; Skillman et al.
1989), showing a higher past rate of star formation. This
galaxy is clearly resolved in stars on ground-based images, and a
large number of Wolf-Rayet stars is known (Massey et al. 1992). The presence of a large foreground extinction due
to its location in a direction close to the Galactic plane has
prevented so far deep studies of the stellar populations. A first
search for PNe was undertaken by Jacoby & Lesser (1981,
hereafter JL81), and resulted in the discovery of one PN.
Leo A is a small irregular galaxy (
,
Young
& Lo 1996) at a distance of 800 kpc (Dolphin et al. 2002), now firmly considered as a member of the LG
(vdB00). It contains both old and young population components (Tolstoy
et al. 1998), the older one amounting to approximately 10%
of the stellar population located near to the centre of the galaxy.
Strobel et al. (1991) have detected several H II regions
excited by hot stars and an unresolved emission-line object, probably
a planetary nebula. Heavy element abundance of this object shows a
very low metallicity (
2.4% solar; Skillman et al.
1989). In this galaxy, JL81 discovered two candidate PNe.
The membership of the LG of Sextans A, located at the distance of 1.45 Mpc
(Sakai et al. 1996), is instead doubtful. It
could form a possible group with NGC 3109, Antlia
and Sextans B (vdB00). Sextans A seems to contain a conspicuous
intermediate-age population, as suggested by its prominent red giant
branch (Dohm-Palmer et al. 1997). Star formation, at present,
is concentrated in a H II region complex observed by Hodge et al.
(1994). Skillman et al. (1989) found an oxygen abundance 3%
of Solar. A PN was identified by JL81.
In this paper we present [O III] and H+[N II] continuum-subtracted
images of these three galaxies. These lead to the discovery of 16 new
candidate PNe in IC 10. In Leo A, the two candidate PNe found by JL81
were shown instead to be normal stars, while the unresolved emission-line
object seen by Strobel et al. (1991) is confirmed as a
possible PN. We also detect a candidate PN in Sextans A, confirming the
detection of JL81. Observations are described in Sect. 2. Data
reduction and analysis are presented in Sect. 3. In Sect. 4, we
discuss the results and in Sect. 5 we review the present knowledge
about PNe in the LG, discussing their behaviour with metallicity.
Summary and conclusions are given is Sect. 6.
Sextans A (DDO 75, 10h 11m 00.5s -04d 41m 29s, J2000.0),
Leo A (DDO 69, 09h 59m 26.4s +30d 44m 47s, J2000.0) and
IC 10 (00h 20m 23.2s +59d 17m 30s, J2000.0) were observed
using the prime focus wide field camera (WFC) of the 2.54 m Isaac
Newton Telescope (La Palma, Spain), on February 2001, February 2002
and June 2002. The detector of the WFC is composed of four thinned
EEV CCDs with
pixels each, with a pixel scale of
.
The large size of the field of view of the camera,
,
allows to cover each galaxy in a single
WFC pointing. The filters used are: [O III] (500.8/10.0 nm), H
+ [N II] (656.8/9.5), Strömgren y (550.5/24.0),
(Sloan r, 624.0/134.7). The
Strömgren y and
filters were used as off-band images for
continuum subtraction of [O III] and H
+[N II] images, respectively.
Each exposure was split into three sub-exposures. The total exposure
times were 3600 s for [O III] and H+[N II] for all galaxies. We
also exposed for 3600 s in Strömgren y, except for Sextans A for which we
exposed for 1800 s. The exposure times in the
filter were
1200 s for Leo A and Sextans A, and 1800 s for IC 10. The nights of
February 2002 were not photometric, thus short exposures of IC 10 and of the Galactic PN IC 5117 (Wright et al., in preparation)
were taken on June 2002 to allow absolute flux calibration.
The nights of February 2001 were however photometric and several
observations of the spectrophotometric standard stars: BD+33 2642 and
G191-B2B (Oke 1990) were made each night.
Data reduction was done using IRAF. The
frames were de-biased, flat-fielded, and linearity-corrected using the
ING WFC data-reduction pipeline (Irwin & Lewis
2001). Then we corrected for geometrical distortions and
aligned all frames to the [O III] one. The sky background, measured in
regions far from the galaxies, was subtracted from each frame. During
the observations of Leo A the seeing was 1
3 through all
filters. For Sextans A, it was 0
9 in the H
+[N II] and Strömgren y,
and 1
1 in the [O III] and
filters. For IC 10, it was 1
1 in the H
+[N II] and Strömgren y and 1
3 in the [O III] and
filters.
We used the standard on-band/off-band technique (cf. Magrini et al. 2000) to identify emission-line objects. The
and
Strömgren y frames, properly scaled, were subtracted from the H
+[N II] and
[O III] frames, respectively. The flux of the emission-line objects
were computed in the continuum-subtracted frames, using the IRAF task
APPHOT. For the [O III] line, fluxes were transformed to equivalent
V-mag magnitudes using
(Jacoby 1989).
The errors of the [O III] and H+[N II] fluxes of the candidate PNe in
Sextans A and Leo A, including photon statistics, background and flux
calibration uncertainties, amount to a few percent. The fluxes of
PNe in IC 10 have typical errors of
10% for PNe with
,
15-20% for
and of 30% or
more for the faintest PNe. These errors take into account photon
statistics and the error on the zero point (2-3%) for the flux
calibration.
The instrumental magnitudes were calibrated by convolving the spectrum of the spectrophotometric standard star (Oke 1990) or the PN IC5117 (Wright et al., in preparation) with the response curve of each filter.
The astrometric solutions were computed using the IRAF tasks CCMAP and
CCTRAN, using the USNO A2.0 catalogue (Monet et al. 1998) for IC 10 and the APM POSS1 for Sextans A and Leo A. The accuracy of all
solutions is 0
3 rms
Candidate PNe were identified using the same criterion as in Magrini
et al. (2000), i.e. as spatially unresolved
emission-line objects detected in the [O III] and H+[N II]
continuum-subtracted frames. The candidate PNe are listed in Table 1,
with their positions, their [O III] and
H
+[N II] fluxes and the equivalent V-mag
.
Fluxes in the [O III] line at
nm were corrected for the contribution of the
companion oxygen line at
nm. This contribution varies
depending on the different heliocentric radial velocities of the
galaxies (-348 km s-1 for IC 10, 20 km s-1 for Leo A, 324 km s-1 for Sextans A
from the Nasa Extragalactic Database) and amounts to 6%, 11% and 14% respectively.
Table 1:
PN candidates in IC 10, Leo A and Sextans A.
[O III]500.7 and H+[N II] observed fluxes are given in units of
10-16 erg cm-2 s-1. [O III] fluxes are corrected for the
contribution of the companion oxygen line at 495.9 nm (see text).
= [O III]/(H
+[N II]) is computed after correcting the fluxes for the
reddening with
E(B-V)=0.85 for IC 10, 0.015 for Sextans A, 0.02 for
Leo A(vdB00), following Mathis' prescription (1990).
![]() |
Figure 1:
Digitized Sky Survey images of the galaxies IC 10,
Leo A and Sextans A. The size of the IC 10 frame is
![]() ![]() |
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Sixteen new candidate PNe were identified in the galaxy IC 10. This is
the first identification of PNe in IC 10, except for one PN discovered
by JL81, whose position was not reported. Our candidate PNe are
detected both in the [O III] and H+[N II] continuum-subtracted images,
so they cannot be confused with highly redshifted background
galaxies. For each PN, we compute the R = [O III]/(H
+[N II]) flux ratio
after correcting for a mean foreground extinction
E(B-V)=0.85 (vdB00)
following Mathis (1990). All candidate PNe have values of
the excitation index R between 1.1 and 7.0, which is comparable with
values for Galactic PNe (cf. Magrini et al. 2000,
Fig. 3). The PNe distribution looks spatially uniform, and no bias
against high/low R or faint/bright PNe can be detected.
The PNe lie in a large area of
(see
Fig. 1), that corresponds to a linear size of 3.6 kpc
2.7 kpc
for a distance of 660 kpc (Sakai et al. 1999). No PNe are
found very close to the centre of this starburst galaxy, presumably
because of the presence of numerous extended H II regions that cover a
large fraction of that area. Eight PNe are situated around the
starburst region, within 0.9 kpc to the centre, the closest ones (PN4
and PN8) being at
0.35 kpc. The remaining PNe are located in
the outskirts of IC 10, outside the 25 mag arcsec-2 diameter
(
,
or
kpc; Massey &
Armandroff 1995). The farthest of our PNe (PN1) is at the
distance from the centre of 12
7 (2.4 kpc). On the other hand,
the large extent of the gaseous component of IC 10 is well known.
From 21-cm line observations, Huchtmeier (1979)
found the presence of an enormous neutral hydrogen envelope
(
)
surrounding the galaxy. PNe have proven to
be excellent tracers of stellar populations in large volumes with a
relatively low density of stars, whose integrated stellar light is low
or even hardly detectable, like the intergalactic and intracluster
space and in the haloes of elliptical galaxies (Arnaboldi et al. 2002). Thus, if spectroscopic studies confirm the nature
as PNe of our candidates as well as their belonging to IC 10 (via
their radial velocities), they would reveal the presence of a
conspicuous stellar population at galactocentric distances much larger
than considered so far. Moreover, Shostak & Skillman
(1989) found that both the outer hydrogen envelope and the
core, which coincides with the small optical size of the galaxy, show
a rotation along the same axis, but with opposite sign. It would be
extremely interesting to verify whether this kinematical dichotomy
also applies to PNe and to the stellar populations that they
represent.
![]() |
Figure 2: The number of PNe in galaxies in or near the Local Group, versus the V-band luminosity in solar units. The dotted line shows the expected numbers based on a total number of PNe in the MW of 23 000. The dashed line is fitted to the known population in the LMC. Filled squares indicate LG gaseous galaxies, triangles indicate spheroidal galaxies and open circles show the NGC 3109 group. |
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The number of candidate PNe detected in IC 10 is roughly consistent
with the expected population size of this galaxy and its large
foreground extinction (
E(B-V)=0.85; vdB00). The population size can
be estimated using a model describing a simple (i.e. coeval and
chemically homogeneous) stellar population (Renzini & Buzzoni
1986). As described by M02, the number of stars nj in
any post-main-sequence phase j is given by
![]() |
(1) |
As shown in Fig. 2, the number of our candidate PNe in IC 10 is
also only slightly smaller than expected by scaling its luminosity
with that of the LMC, and normalizing with the known number of LMC PNe
where the survey for PNe is the most complete. The expected detectable
PN population for a galaxy with the V-band luminosity of IC 10 would
in fact be 30 PNe. Our survey might therefore be slightly
less complete owing to the greater distance of IC 10 compared to the
LMC, the large reddening, and the presence of the large central
complex of H II regions. Particularly, the star formation regions contribute
considerably to the V-band luminosity, but they are too young to
produce planetary nebulae. Our method for estimating the number of
PNe in a galaxy makes use of the V-band luminosity, without
considering that the galaxy luminosity must depend on the star
formation history. The "effective'' V-band luminosity is thus lower
than the total V-band luminosity and consequently the dashed line in
Fig. 2 slightly overestimates the expected number of PNe in IC 10.
We have also analyzed the "incompleteness'' of our survey as a function
of
and of the distance to the centre of the galaxy,
as described in M02. We estimated the number of "missing'' PNe by
adding "artificial stars'' with various
,
as expected
for the PN population of IC 10, in both [O III] and Strömgren y frames. The
incompleteness (i.e. a recovery rate of artificial stars less than 50%, Minniti & Zijlstra 1997) is due to the probability of
missing an object in the [O III] or H
+[N II] images and/or to the
probability of a wrong identification of a star in the continuum
images. We found that our survey of IC 10 is incomplete for
emission-line objects with
and there is a
probability of
5% to find a star on the same line of sight and
consequently to miss the emission-line object. The loss of faint
emission-line objects is quite uniform across the whole field, whereas
the probability of overlapping with a star or with a large H II
region increases towards the centre, particularly in
H
+[N II] images where it reaches 40-50% within 4
from the
centre. Considering the fraction of the total mass inside the central
4
4
region, we conclude that at least 5 PNe
brighter than the completeness limit may have been missed there.
Previous estimates of the distance to IC 10 locate it to a distance
ranging from 0.5 to 3 Mpc (Roberts 1962; de Vaucouleurs &
Ables 1965; Wilson et al. 1996; Tikhonov
1999; Sakai et al. 1999).
The planetary nebulae luminosity function (PNLF) is widely used as an
extragalactic distance indicator (Jacoby 1989). When the PN
population size is small, the absolute magnitude of the bright cut-off
(-4.53 for a large sample of PNe, Ciardullo et al. 2002)
of the luminosity function increases because the brightest PNe could
not be observed (Méndez et al. 1993). In this case the PNLF
cannot be used as a distance indicator. The
of the
brightest PN can give, however, an upper limit to the distance of
IC 10,
1.8 Mpc.
In Sect. 5 we will discuss how the metallicity correction applies to
the
of the brightest PN.
![]() |
Figure 3:
The ratio between the observed and expected number of PNe (
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Our observations identified one candidate PN in Leo A (see Table 1
and Fig. 1). The survey by JL81 allowed the discovery of two
candidate PNe, which are shown in Fig. 2 of their paper. These two
candidate appear clearly both in our continuum and emission-line
frames, therefore we conclude that they are normal stars. During an
H
survey of Leo A, Strobel et al. (1991)
have detected three H II regions and an unresolved emission region that they
identified as a PN. This object corresponds to our candidate PN. A
spectrum of this object was obtained by Skillman et al. (1989), confirming its nature as a PN. From our [O III] and H
+[N II] fluxes, corrected for the foreground extinction
E(B-V)=0.02 (Strobel et al. 1991), we computed an
excitation index R=1, typical of a relatively low excitation PN, and/or
of low oxygen abundance.
This is lower than the value R=1.4 computed using the line fluxes
in the spectrum by Skillman et al. (1989). Note however that
their data would also measure an unrealistic H
/H
ratio of 2.3,
suggesting that line fluxes in the blue region are overestimated. In
fact, adjusting their line fluxes in the blue to achieve the
theoretical value H
/H
= 2.85, [O III] would be scaled so as to
provide
,
in good agreement with our value.
Using Eq. (1), the expected PN population of Leo A is 1 to 3 PNe, thus
consistent with the single PN observed. The upper limit to the
distance modulus to Leo A given by the
of the PN,
without correcting for its low metallicity, is
26.3 (1.8 Mpc), very
close to the distance obtained by Sandage (1986) using the
three brightest stars in Leo A, but both methods are not reliable for
galaxies with such a small population size. Our survey is fairly
complete up to
and no PN brighter than 24.5 mag
remains to be discovered. Further deep spectroscopic study, as the one
by Skillman et al. (1989) who measured the O/H abundance,
will give fundamental information about the metal content in one of
the lower metal abundance galaxies of the Local Group.
Several emission-line objects are shown in the [O III] and H+[N II] continuum subtracted images of Sextans A. The extended emission-line
sources are H II regions previously studied by Hodge et al.
(1994), Dohm-Palmer et al. (1997) and supergiant
ionized filaments studied by Hunter & Gallagher (1997). We
identified one unresolved emission-line source present both in the
[O III] and H
+[N II] continuum-subtracted images (see Table 1 and Fig. 1).
This candidate PN was previously discovered by JL81, and the
[O III] flux presented in their paper is in agreement,
within the errors, with our measure. The R ratio for this candidate PN, computed after correcting for reddening,
E(B-V)=0.015 (vdB00), is R=0.5, indicating that
either that it is a low-excitation PN, a compact H II region or that it has low
oxygen abundance, as we can expect from the low metallicity of Sextans A
from Skillman et al. 1989). It lies near
to the centre of the galaxy and has
which sets
an upper limit to its distance modulus of 26.7 (2.5 Mpc), without
correction for low-metallicity. The completeness of our survey is up
to
.
No PN brighter than the 24.5 mag remain to
be discovered.
Sextans A and Sextans B have similar distances and are separated
on the sky by only 10$.^$4. Considering also their small velocity
difference ( km s-1, vdB00), it is quite possible that
they had formed together and then drifted apart over a Hubble time (vdB00). Their V-band luminosities and their mass are also similar,
and thus their expected PN populations would be alike. Five PNe were
discovered in Sextans B (M02), while only one candidate PN is detected in
Sextans A. Statistically, this difference is only marginally significant,
but may suggest some differences in their star formation history. In
fact, M02 argued that Sextans B has a large number of intermediate-age
stars formed in the last 5 Gyrs, while Sextans A has a stronger main
sequence population (low- and intermediate-mass stars, Dohm-Palmer
et al. 1997) likely due to a stronger recent star formation.
A total of 2500 extragalactic PNe have been discovered
over the last thirty years in almost all the LG galaxies whose total
luminosity implies a population size large enough to allow the
presence of PNe (cf. Ford et al. 2002; Magrini et al. 2002).
Figure 2 shows the number of PNe discovered to date in galaxies of the LG or in nearby groups, versus the V-band luminosity in solar units. This is an updated version of Fig. 2 in M02 and includes the PNe discovered in this paper (16 in IC 10 and 1 in Sextans A), as well as 19 PNe in NGC 6822 confirmed spectroscopically by P. Leisy (in preparation), 1284 candidate PNe in M 31 (D. Carter, private communication, see also Ford et al. 2002), one PN in WLM (Zijlstra, private communication), 2389 PNe in the Milky Way (Acker et al. 2002), 2 in IC 1613 (Magrini et al., in preparation). Compared to the previous version of this plot, the overall dispersion of data is reduced and a better relationship between the number of PNe and the V-luminosity of the galaxies can be seen. M 31 and M 33 are rather short of known objects, probably because of extinction effects and incompleteness in the existing surveys.
We have also investigated possible metallicity effects on the number
of PNe in a galaxy. The upper panel of Fig. 3 shows the [Fe/H]
abundance of the galaxies vs. (
)), which is the logarithm of the ratio between the
observed number of PNe (
)
and the expected population size
(
,
scaled from the data on Sextans B where M02 found
that one PN is expected per
LV=106.92 solar luminosities).
The adopted number of PNe per unit luminosity is consistent with the
statement by vdB00 that 6 carbon (C) stars are found at MV=-10,
making the expected ratio between PNe and C stars approximately 0.1.
Given that PNe are bright until they become optically thin at the
radius approximately of 0.05 pc, which occurs after 2500 yr for a
nebular expansion velocity of 20 km s-1, Fig. 3 implies that
the carbon star life time must be ten times longer or
yrs, which is qualitatively correct. Note that
in this paper we use two different relations for the number of PNe:
Eq. (1) and
.
Equation (1) derives the total number
of expected PNe, while
is based on the observations
(so excluding faint objects, confusion, etc.) and thus infers the
expected observable number of PNe. Note that adopting a value
different from 6.92 as the normalization factor would not affect the
discussion, as it would only shifts the relation
up
or down.
Figure 3 shows that there is a slight tendency for the number of
detected PN to decrease with metallicity, if M 31 and M 33, whose
total observable population size in uncertain, are excluded (encircled
squares at bottom). In this graph, we have also excluded galaxies
where the expected number of PNe is <1 PN (Leo A and Pegasus) and
IC 1613 which still lacks a proper survey for PNe.
In particular, in Fig. 3 there are some hints of a shortfall in the
observed number of PNe for [Fe/H] -1.0. As
corresponds
to the point where the AGB wind is expected to be driven no longer by
dust, but only by pulsations (Zijlstra 1999), the lack of
PNe might suggest that below this point the PN formation rate is
largely reduced. Note that Mira variables are only seen in globular
clusters when
,
suggesting there is significant change in
AGB evolution at this point.
The bottom panel of Fig. 3 shows the PN deficit against the ratio between carbon and M stars (C/M3+) for the galaxies where this information is available, taken from Cook et al. (1986). The number of M stars here is those with spectral type M 3 or later. This number increase rapidly with lower metallicity. The trend in the figure confirms the suggestion in the top panel.
As a matter of caution, it should however be noticed that part of the
deficit of detected PNe in metal poor galaxies might also depend on
the technique that is generally used to find PNe, i.e. [O III] 500.7 nm
imaging, as the [O III] emission decreases for low abundances of this
element, as shown in the LMC and SMC by Leisy (2003, in preparation).
On the other hand, our detection technique makes use in addition of
H + [N II] images.
We have considered also low-R candidate,
as the one discovered in Sextans A, thus reducing the probability to loose PNe in
low-metallicity galaxies.
The first studies of the PNLF showed no strong evidence for
metallicity dependence (Jacoby et al. 1988; Jacoby et
al. 1990). Theoretical models by Jacoby (1989)
suggested that the dependence of the bright cutoff of the PNLF with
metallicity is modest, Z0.5. This effect is relatively
unimportant when metallicity differs from galaxy to galaxy by 30% or
less, but it could lead to significant differences when a wider range
of metallicities is considered, as in the case of the LG. Dopita et al. (1992) examined the effects of metallicity on the
luminosity of PNe, modeling the variation of the [O III] magnitude
with the oxygen abundance as
![]() |
(2) |
![]() |
Figure 4: The expected number of PNe vs. the absolute magnitude (M*) of the brightest PN, before (panel on the left) and after (panel on the right) metallicity correction following Dopita et al. (1992) prescription. For M 31, LMC and SMC we report the magnitude of the cutoff of their PNLFs with errors from Ciardullo et al. (2002). For M 33 the magnitude of the cutoff is from Durrell et al. (2003). The horizontal line show the magnitude of the PNLF cutoff M*=-4.53 as expected for a galaxy with a large population size. Dashed lines mark 1-mag around M*=-4.53. |
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The correction given in Eq. (2) is significant when the metallicity is much lower than solar. The LG is a good environment in which investigate this issue, as oxygen abundance spans over a large range: from 7.30 of Leo A to 9.0 of M 31 (from vdB00). With the data presented in this paper and in the previous surveys (Danziger et al. 1978; JL81; Killen & Dufour 1982; Ciardullo et al. 1989; Morgan 1995; Zijlstra & Walsh 1996; Leisy et al. 1997; M02; Durrell et al. 2003), an extensive sample of the brightest planetary nebulae of the LG is available. In Fig. 4, we present the PN population size versus M* or the cutoff of the PNLF (when available, i.e. M 31, M 33, SMC, LMC, cf. Ciardullo et al. 2002), Durrell et al. 2003 before and after correcting for metallicity using Eq. (2). Distances used to compute the absolute magnitudes are from vdB00, observed magnitudes are from the above cited papers. The solid horizontal line show the magnitude of the PNLF cutoff as expected for a galaxy with a large population size and Solar metallicity (M*=-4.53, Ciardullo 2002).
After correcting for metallicity using Eq. (2), a number of galaxies of the LG, especially those with a PNe population size larger than 102, are tightly grouped around the "universal'' value M*=-4.53. This seems to confirm that the metallicity correction by Dopita et al. (1992) is at least qualitatively correct, and that the effects of metallicity are non-negligible for galaxies in the LG.
For galaxies with a population size of 102 or less, M* is spread over a large range; it is likely that in these galaxies we start seeing the effect of having a small population size, as discussed in Sect. 4.1.2, together with some other effects, like for instance incompleteness of the existing surveys due to extinction or to the existence of areas of high stellar density or with large systems of H II regions (cf. e.g. with Sect. 4.1.1. for IC 10). In this respect, it is somewhat surprising to find small galaxies like Leo A, Sextans B, and Sextans A near M*=-4.53, in spite of their very small PN population size.
The corrected magnitudes of the brightest PNe allow us to revise our upper limits to the distances of these galaxies presented previously in this paper and in M02. After correcting for metallicity, the upper limits to the distance moduli to Sextans B would be 25.5 (1.3 Mpc), 25.4 (1.2 Mpc) for Sextans A, and 24.3 (0.7 Mpc) for Leo A, in good agreement with other estimates (1.32 and 1.45 from vdB00, 0.8 from Dolphin et al. 2002), respectively, whereas for IC 10 the correction is not remarkable. We stress once more, however, that caution should be used when determining distance for such low-luminosity systems using PNe.
In this paper, we have presented the discovery of sixteen new
candidate PNe in IC 10, and confirmed two previously known candidate
PNe, one in Leo A and the other one in Sextans A. These observations,
together with other ones that will be presented in the future for
other LG galaxies, provide a further improving in our understanding of
the PN population of the Local Group. The behaviour of the numbers of
PNe with galaxy metallicity has been discussed, finding a possible
lack of PN when
.
The magnitudes of the brightest PNe
of the LG have been reviewed and the possibility to measure distance
using the magnitude of the brightest PN, after correcting for
metallicity, has been discussed.
Future spectroscopic studies of individual objects will confirm their nature as PNe and allow detailed chemical abundance studies. In addition, they will provide unique information about the star formation history and chemical evolution of the parent galaxies, using tracers of stellar populations of intermediate to old age, and thus fully complementary to the information obtained from older (e.g. population II red giants) and younger (e.g. HII regions) classes of objects.
Acknowledgements
The data are made publically available through the Isaac Newton Groups' Wide Field Camera Survey Programme. This research has made use of the NASA/IPAC Extragalactic Database (NED), the APM and USNO-A2.0 Sky Catalogues, and the ESO Online Digitized Sky Survey. We would like to thank the anonymous referee for comments and suggestions that improved our paper.