THE ING NEWSLETTER No. 7, December 2003

The Census of Planetary Nebulae in the

Romano L. M. Corradi (ING), Laura Magrini (University of Firenze, Italy), Pierre Leisy (ING/IAC), Colin Davenport (ING)

lanetary nebulae (PNe), the criterium to select candidate integrated stellar light is low and fate of the vast majority of stars extragalactic PNe is that they are not hardly detectable, like the intergalactic P with a mass similar to the Sun spatially resolved by ground based and intracluster space and in the haloes or a few times higher, represent a short imaging, their sizes being usually a of elliptical (Arnaboldi et but well characterised stage of stellar fraction of a which translates al., 2002). evolution. It is the time at which stars into a couple of hundredth of an arcsec experiment their last thermonuclear at a distance of 1Mpc, approximately Extragalactic PNe also provide burning on the surface of a core that the outer edge of the Local Group. important information on the chemical has been left naked by strong mass evolution of the host galaxies, as the loss during the previous red giant PNe in external galaxies provide a tool nebular abundances of elements like phase. The combination of a hot to investigate some important oxygen, neon, sulphur, or argon, do luminous star (up to 500,000 K and astrophysical problems. First of all, not vary significantly during the more than 10,000 solar luminosities) their number reflects the total mass evolution of low-mass stars (i.e. they and a low density expanding wind, of the underlying stellar population are not significantly produced or allows the formation of an extremely from which they derive. In fact, one destroyed). Therefore the abundances luminous nebula that reprocesses the of the most robust predictions of stellar of these elements probe the initial energetic continuum radiation from the evolution theories allows us to relate metallicity of their environment at the stellar nucleus into specific emission- the number of objects nj in any post time when their progenitors were born. line spectra from atomic ionised gas. main-sequence evolutionary phase to This covers a range in ages that can This makes PNe easily observable in the lifetime of that specific phase, in be hardly covered using other classes our own (being among the the hypothesis of a population of coeval, of stars. preferred targets for amateur chemically homogeneous stars (Renzini telescopes), but equally well detectable & Buzzoni, 1986). The relation is as Moreover, nowadays PNe are used as in external galaxies even with simple as this: reliable extragalactic distance relatively small telescopes. indicators, through the invariance of • their luminosity function with galaxian nj=ξ⋅L T ⋅ t j , The technique used for searching PNe type and metallicity (Jacoby, 1989). in external galaxies is almost • Finally, as they are also detected in where ξ is the so-called specific invariably that of obtaining a narrow- stellar systems of low surface evolutionary flux (number of stars per band, continuum-subtracted image in brightness, they are extremely valuable unit luminosity leaving the main a filter isolating the forbidden emission test particles to map the dynamics of sequence each year), L is the total at 5007Å from double-ionised atomic T stars in galaxies up to very large oxygen [OIII]. A large fraction of the luminosity of the galaxy, and tj the galactocentric distances (the Planetary total luminosity of the star is in fact duration of the evolutionary phase j Nebula Spectrograph at the WHT is ≤ concentrated in this line, and this is ( 10,000• yrs for the PN phase). Note an instrument especially built for the unique property that makes that ξ is only slightly dependent on the this purpose, see e.g. Merrifield et individual stars in the planetary nebula age of the stellar population, its initial al. (2001). phase visible to very large distances: mass function and metallicity. Thus up to several hundred solar counting PNe implies measuring the For the reasons above, we have been luminosities can be emitted in a single total mass of the parent stellar intensively searching for PNe in nearby and very narrow spectral line! population. Once the masses of the galaxies as one of the main objectives Observation of the hydrogen Hα line, progenitors of the PNe are estimated, of the Local Group Census (LGC). The also very bright, is sometimes added it also allows us to discuss the star LGC is a narrow-band survey of the to discriminate against the detection formation history of the host galaxy galaxies of the Local Group observable of highly redshifted galaxies (e.g. [OII] for the range of ages covered by the from La Palma, that was awarded emitting galaxies at z= 0.34, PN progenitors, roughly 1 to 10 Gyr observing time during period two of which shifts the O+ emission to the (it is still not clear which is the lower the ING Wide Field Imaging Survey rest wavelength of [OIII]λ5007, or mass limit for forming a planetary programme (http://www.ing.iac.es/ Lyman-α emitters at redshift 3.1), or nebula). In particular, PNe have proven /WFS/). Observations are being obtained to estimate the ionisation class and to be excellent tracers of stellar with the Wide Field Camera at the discuss possible contamination by populations in large volumes with a 2.5m Isaac Newton telescope, covering compact HII regions. Another basic relatively low density of stars, whose a field of view of 34′×34′. The aim of

11 No. 7, December 2003 THE ING NEWSLETTER

the survey is to find, catalogue and luminous object on the upper-left side chemical properties and of their host study old and young emission-line of the image of A). galaxies. This will be our next objective, populations (e.g. HII regions, PNe, SN together with the analysis of the other remnants, Luminous Blue Variables, The LGC detections provide a more galaxies observed by the LGC. WR stars, symbiotic binaries, etc.) to complete view of the population of PNe unprecedented levels. The value of in the Local Group. These new data Updated information on the status of narrow band [OIII], Hα, [SII], and HeII appear to be consistent with the the project, including the list of all the images is enhanced with complementary predictions of the stellar evolution astronomers and institutions involved, broad band data (g, r, i). This enables, theories mentioned above, as the can be found at: in principle, the linkages between number of observed PNe in each galaxy http://www.ing.iac.es/~rcorradi/LGC/. stellar populations to be probed. scales reasonably well with the ¤ luminosity of the galaxy (Magrini et The first part of the analysis of our al., 2003). In spite of this agreement, References: survey data has been focused on the there are also some interesting search for PNe in dwarf irregular peculiarities. For instance, Sextans A Arnaboldi, M., Aguerri, J. A. L., Napolitano, galaxies of the Local Group. We are and Sextans B have very similar N. R., Gerhard, O., Freeman, K. C., especially interested in these objects as V -band luminosities and mass, but Feldmeier, J., Capaccioli, M., Kudritzki, dwarf galaxies are the most numerous while five PNe were discovered in R. P., Mendez, R. H., 2002, AJ, 123, 760. galaxies in the nearby Universe. Sextans B, only one candidate is Danziger, I. J., Webster, B. L., Dopita, M. A., According to the hierarchical scenarios detected in Sextans A. Statistically, this Hawarden, T. G., 1978, ApJ, 220, 458. of galaxies formation, dwarf galaxies difference is only marginally Dudziak, G., Péquignot, D., Zijlstra, A. A., are the first structures to form and significant, but may suggest some Walsh, J. R., 2000, A&A, 363, 717. from their merging, larger galaxies are differences in their star formation Huchtmeier, W. K., 1979, A&A, 75, 170. built. The Local Group, which appears history, as evidenced by the stronger Jacoby, G. H., 1989, ApJ, 339, 39. to the rest of the Universe as an main-sequence population of Sextans A Jacoby, G. H., Lesser, M. P., 1981, AJ, 86, ordinary collection of dwarf galaxies compared to Sextans B. 185. (90% of its 40 known members) Killen, R. M., Dufour, R. J., 1982, PASP, dominated by two main spiral galaxies, We have also investigated the 94, 444. behaviour of the numbers of planetary is an ideal laboratory as the low- Leisy et al., 2003, in preparation. luminosity dwarf galaxies can be nebulae with galaxy metallicity, and Magrini, L., Corradi, R. L. M., Walton, N. found a possible lack of PN when studied in detail. A., Zijlstra, A. A., Pollacco, D. L., Walsh, [Fe/H]<<–1.0, which might indicate J. R., Perinotto, M., Lennon, D. J., Before our census, only a small number that below this point the formation rate Greimel, R., 2002, A&A, 386, 869. of PNe were known in the dwarf of PNe is much lower than for stellar Magrini, L., Corradi, R. L. M., Greimel, R., irregular galaxies of the Local Group populations of near solar abundances. Leisy, P., Lennon, D. J., Mampaso, A., (3 in Sagittarius, Walsh et al., 1997; This might in turn be related to the Perinotto, M., Pollacco, D. L., Walsh, J. Dudziak et al., 2000; one in Fornax, mass loss mechanism in evolved red- R., Walton, N. A., Zijlstra, A. A., 2003, Danziger et al., 1978; one in Leo A giants, that is governed by radiation A&A, 407, 51. and another one in Sextans A, Jacoby pressure on dust grains, and is Merrifield et al., 2001, ING Newsl., 5, 17. & Lesser, 1981; one in NGC 6822, therefore sensitive to a significant Renzini, A., Buzzoni, A., 1986, in Spectral Killen & Dufour, 1982). With our deficiency of heavy elements in the Evolution of Galaxies, eds. Chiosi, C. & survey, so far 16 PNe in IC 10, 5 in stellar atmosphere. Renzini, A., Reidel, Ap. Space Sci. Lib., Sextans B and 3 in IC 1613 were newly 122, 195. discovered, while the existence of one Another result of our survey is the Walsh, J. R., Dudziak, G., Minniti, D., candidate planetary nebula in Leo A, discovery of candidate planetary Zijlstra, A. A., 1997, ApJ, 487, 651. one in Sextans A, and about 25 in nebulae at large galactocentric NGC 6822 were confirmed (Magrini distances, like in the case of IC 10 Romano Corradi ([email protected]) et al., 2002, 2003; Leisy et al., 2003). where they cover an area of 3.6×2.7kpc, No PNe are instead found in GR8, as much more extended than the expected because of the small 25 mag⋅arcsec–2 diameter (1.1×1.3 kpc). Next page: Three-colour images of luminosity of this galaxy. The data Are these PNe related to the enormous NGC6822, IC1613, IC10, Leo A, GR8, are illustrated in the colour figures in neutral hydrogen envelope surrounding Sextans B, Sextans A and WLM galaxies the next page; in each image, green is IC10 (Huchtmeier, 1979)? of the Local Group. In each image, green is the [OIII] emission, red the Hα one, the [OIII] emission, red the Hα one, while blue corresponds to the broad band while blue corresponds to the broad The new detections of the LGC are Sloan-g images, mainly dominated by band Sloan-g images, mainly clearly a starting point for future continuum stellar emission. In these dominated by continuum stellar spectroscopical studies of individual images, planetary nebulae stand out as emission. In these images, planetary objects, aimed at confirming their green or yellow dots (a striking example nebulae stand out as green or yellow nature as PNe and, more importantly, is the green luminous object on the dots (a striking example is the green at determining their physical and upper-left side of the image of Leo A).

12 NGC 6822

IC 1613 IC 10

Leo A GR 8 Sextans B

Sextans A WLM