Up to 10% of the total luminosity of a planetary nebula, ~500 L⊙, can be emitted in the dominant cooling line, [O iii] λ5007. This, coupled with the narrowness of the line (~15-25 km s-1), makes it extremely easy to detect PNe in external galaxies using a narrow-band lter tuned to the galaxy redshift. The availability of multiple independent distance indicators for our closest neighbouring galaxies, the Magellanic Clouds and M 31, means that the luminosities of the PNe in these galaxies (in particular the large numbers of PNe in the LMC) can be used to calibrate PN luminosity functions, which have been used over the past 15 years as probes of the Hubble constant. Given the fact that we still do not have accurate distances for most planetary nebulae in the Milky Way, this has been an astonishing development. Unlike H ii regions, which cannot be used to probe elliptical galaxies, planetary nebulae can be used to probe the dynamics and metallicity of any type of galaxy. Today, via accurate radial velocity measurements, extragalactic PNe are being used as dynamical mass probes of galaxies, and even of the stellar mass content of the intracluster regions of galaxy clusters, with dedicated ‘planetary nebula spectrographs’ being built to further such studies. The angular resolution of the Hubble Space Telescope has turned out to be ideally suited to the study of PNe in the LMC and SMC, and the exploitation of the accurately known distances to these two galaxies has allowed reliable luminosities and masses to be derived for the central stars and nebulae, thereby putting PN research on a much more quantitative footing

Barlow, MJ

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UCL Discovery

Planetary Nebulae Beyondthe Milky Way – Historical OverviewMichael J. BarlowDepartment of Physics & Astronomy,University College London,Gower Street, London WC1E 6BT, U.K.1 IntroductionUp to 10% of the total luminosity of a planetary nebula, ∼500 L, can be emittedin the dominant cooling line, [O iii] λ5007. This, coupled with the narrownessof the line (∼15-25 km s−1), makes it extremely easy to detect PNe in externalgalaxies using a narrow-band filter tuned to the galaxy redshift. The availabilityof multiple independent distance indicators for our closest neighbouring galax-ies, the Magellanic Clouds and M 31, means that the luminosities of the PNein these galaxies (in particular the large numbers of PNe in the LMC) can beused to calibrate PN luminosity functions, which have been used over the past15 years as probes of the Hubble constant. Given the fact that we still do nothave accurate distances for most planetary nebulae in the Milky Way, this hasbeen an astonishing development. Unlike H ii regions, which cannot be used toprobe elliptical galaxies, planetary nebulae can be used to probe the dynam-ics and metallicity of any type of galaxy. Today, via accurate radial velocitymeasurements, extragalactic PNe are being used as dynamical mass probes ofgalaxies, and even of the stellar mass content of the intracluster regions of galaxyclusters, with dedicated ‘planetary nebula spectrographs’ being built to furthersuch studies. The angular resolution of the Hubble Space Telescope has turnedout to be ideally suited to the study of PNe in the LMC and SMC, and theexploitation of the accurately known distances to these two galaxies has allowedreliable luminosities and masses to be derived for the central stars and nebulae,thereby putting PN research on a much more quantitative footing.In this overview of past and recent studies of extragalactic planetary nebulae(PNe), I have divided the research into two main areas: (a) surveys for plane-tary nebulae in other galaxies, PN luminosity functions and intracluster PNe;(b) detailed studies of planetary nebulae in the Magellanic Clouds, particularlystudies carried out with the HST.2 Surveys for Extragalactic Planetary NebulaeThe low dust extinctions to the LMC and SMC mean that virtually all neb-ulae present can be detected by a survey down to a given intrinsic luminositylimit. The low reddening also makes them well suited to study in the ultraviolet.Sanduleak, MacConnell & Philip (1978: SMP) published a catalogue of all PNeM.J. Barlow, Planetary Nebulae Beyond the Milky Way – Historical Overview. In: Planetary NebulaeBeyond the Milky Way, L. Stanghellini et al. (eds.), ESO Astrophysics Symposia, pp. 3–14 (2006)c© Springer-Verlag Berlin Heidelberg 2006DOI 10.1007/11604792 14 M.J. Barlowthat had been discovered in the Magellanic Clouds up till then. Of the 28 SMCPNe in their list, 18 had been discovered by Henize (1956), 7 were from Lindsay(1961) and 3 came from SMP’s own objective prism survey. Of the 102 LMCPNe in their list, 59 were from Henize (1956), 21 came from Lindsay & Mullan(1963) and 22 came from their own objective prism survey.Ford et al. (1973) detected 26 PNe in the dwarf elliptical companion galaxiesto M 31: NGC 185, NGC 205 and NGC 221, via filter imaging in [O iii] andHα. Ford & Jenner (1975) followed up with deeper observations of NGC 221,detecting 21 PNe in its field (see Fig. 1). Three of the 14 PNe with observedradial velocities belonged to M 31, the rest to NGC 221 (they were distinguishedby means of the 200 km s−1 difference between the two galaxies).Fig. 1. Hα (left) and [O iii] (right) images of NGC 221 (= M 32). The central wave-lengths and widths (in Å) of the filters are indicated at the bottom of the figures. FromFord & Jenner (1975).A deep narrow-band [O iii] and Hα filter survey by Jacoby (1980) led to thedetection of 53 faint new SMC and LMC PNe in selected fields and the deriva-tion of a PN luminosity function (PNLF) that reached six magnitudes below thebrightest. This work led the foundations for subsequent studies that exploitedPNLF’s for more distant galaxies. Ciardullo et al. (1989a) detected 429 PNe inthe bulge of M 31, of which 104 were in the first 2.5 mags of its PNLF (Fig. 2).A steep turnover observed at the high luminosity end was interpreted as corre-sponding to a sharp cutoff in the upper mass limit of PN central stars, consistentwith the expectation that no central star should exceed the Chandrasekhar masslimit and that they should evolve increasingly quickly as this mass is approached.Ciardullo, Jacoby & Ford (1989b) detected 249 PNe in three early type galax-ies in the Leo I cluster: NGC 3377 (E6), NGC 3379 (E0) and NGC 3384 (S0).Planetary Nebulae Beyond the Milky Way – Historical Overview 5Fig. 2. Image of M 31 (left), showing the region surveyed for PNe using an [O iii]5007 Å filter. The plot on the right shows the resulting PN luminosity function for the429 PNe discovered, from Ciardullo et al. (1989a)All had similar PN [O iii] luminosity functions, indistinguishable from those ob-served in the spiral galaxies M 31 and M 81. The PNLF invariance with galaxytype and metallicity demonstrated the PNLF to be an excellent standard candle.Model simulations indicated a central star mass distribution highly peaked at∼0.6 M, similar to that of nearby white dwarfs.PNe are now routinely used to study the internal dynamics and mass distri-butions of both early and late type galaxies, e.g. Arnaboldi et al. (1998) havemeasured the rotation curve and velocity dispersions along two axes of the earlytype galaxy NGC 1316 (Fornax A) from radial velocity measurements of 43 PNe(Fig. 4). The total galaxy mass inside 16 kpc radius was derived to be 2.9×1011solar masses.Méndez et al. (2001) detected 535 PNe in the elliptical galaxy NGC 4697using filter images and slitless spectra. The former gave the [O iii] PNLF (Fig. 3,right), which yielded a distance of 10.5 Mpc. The latter gave radial velocities for531 of the PNe, allowing an accurate mass to luminosity ratio to be derived forthe galaxy.Turning to more local regions, Magrini et al. (2003) have surveyed a numberof Local Group dwarf galaxies for PNe. They found that the number of PN perunit stellar luminosity appears to decline steeply for metallicities below 0.1 solar(Fig. 5).6 M.J. BarlowFig. 3. [O iii] PNLF’s for three Leo I galaxies (left), derived by Ciardullo et al. (1989b).The plot on the right shows the [O iii] PNLF derived by Méndez et al. (2001) for535 PNe in the elliptical galaxy NGC 4697, which is located in the Virgo southernextension. The three lines show PNLF simulations from Méndez & Soffner (1997), forthree different sample sizes: 2500, 3500, and 4900 PNe.Fig. 4. Velocity dispersions (upper) and rotation curves (lower) for NGC 1316. FromArnaboldi et al. (1998).Planetary Nebulae Beyond the Milky Way – Historical Overview 7Fig. 5. Number of planetary nebulae per unit galactic luminosity as a function ofgalaxy metallicity (upper) and as a function of the ratio of carbon stars to O-richlate-type AGB stars (lower). From Magrini et al. (2003).3 Planetary Nebulae in the Intracluster Regionsof Galaxy ClustersArnaboldi et al. (1996) measured radial velocities for 19 PNe in the outer regionsof the giant elliptical galaxy NGC 4406, in the southern Virgo extension region.Although this galaxy has a radial velocity of –227 km s−1, three of the PNehad radial velocities close to 1400 km s−1, the mean radial velocity of the Virgocluster. It was concluded that they were intracluster PNe. Theuns & Warren(1997) discovered ten PN candidates in the Fornax cluster, in fields well awayfrom any Fornax galaxy – consistent with tidal stripping of cluster galaxies. Theyestimated that intracluster stars could account for up to 40% of all the stars inthe Fornax cluster.Méndez et al. (1997) surveyed a 50 arcmin2 area near the centre of the Virgocluster, detecting 11 PN candidates. They estimated that a stellar populationcomprising 4×109 M lay in their survey area and that such a population couldaccount for up to 50% of the total stellar mass in the Virgo cluster. Feldmeier,Ciardullo & Jacoby (1998) searched for intracluster PNe in several Virgo Clusterfields. Comparison with the normalised M 87 PNLF showed an excess at thebright end, attributed by them to intracluster PNe (see Fig. 6).8 M.J. BarlowFig. 6. Left: Virgo cluster fields searched for PNe by Feldmeier et al. (1998). Right:the resulting PN luminosity functions.4 Observations of Magellanic Cloud Planetary NebulaeMagellanic Cloud PNe are bright enough that Dopita et al. (1988) were ableto obtain echelle spectra with 11.5 km s −1 resolution of 94 LMC PNe in the[O iii] 5007 Å line, using 1.0 m and 2.3 m telescopes. The nebulae were spatiallyunresolved from the ground but those with asymmetric velocity profiles (Fig. 7)indicate non-spherically symmetric nebulae. The radial velocities were used byMeatheringham et al. (1988) to conduct a dynamical study of the PN populationin the LMC.Monk et al. (1988) analysed 3.9-m AAT spectra of 21 SMC and 50 LMCPNe. They found that the PN and H ii region abundances of oxygen and neonagreed within the same galaxy but that nitrogen in the planetary nebulae wasenhanced by 0.9 dex in both the SMC and LMC. This was interpreted by themas due to CN cycle processing of almost all of the original carbon into nitrogenat the time of the first dredge-up. This process appears to have operated muchPlanetary Nebulae Beyond the Milky Way – Historical Overview 9Fig. 7. High spectra resolution profiles of the [O iii] 5007 Å line for four LMC PNe.Asymmetric or double-peaked profiles indicate non-spherically symmetric nebulae.From Dopita et al. (1988).more efficiently in the metal-poor Clouds than in the Galaxy, in agreement withtheoretical expectations (e.g. Becker & Iben 1980).Meatheringham & Dopita (1991) obtained optical spectra of 73 LMC andSMC PNe and Dopita & Meatheringham (1991) applied a grid of photoioniza-tion models to these observations to derive nebular abundances and central stareffective temperatures, luminosities and masses (Fig. 8).4.1 The HST EraThe first spatially resolved images of Magellanic Cloud PNe were published byBlades et al. (1992), who presented pre-COSTAR Faint Object Camera [O iii]5007 Å images of two SMC PNe and one LMC PN, with 50 milli-arcsec resolution(the FOC was the only instrument that reached the HST’s optical diffraction-limited resolution). One LMC nebula N 66 (=WS 35), was found by them tohave a particularly unusual morphology, showing multiple lobes and a brightcentral star.In a series of papers, Dopita, Vassiliadis and collaborators have publishedWFPC and WFPC-2 [O iii] images of Magellanic Cloud PNe obtained overseveral HST cycles, mostly pre-COSTAR, while Stanghellini et al. (1999) have10 M.J. BarlowFig. 8. Hertzsprung-Russell diagram for LMC PN central stars, together with H-burning evolutionary tracks for three different core masses. From Dopita & Meather-ingham (1991).published pre-COSTAR (and some post-COSTAR) HST FOC images of 27 Mag-ellanic Cloud PNe taken in the [O iii] line, some examples of which are shownin Fig. 9.Vassiliadis et al. (1996) presented HST FOS UV spectra for a number ofMagellanic Cloud PNe, while Dopita et al. (1997) have fitted photoionizationmodels to the combined UV+optical spectra of 10 of these PNe in the LMC.Central star effective temperatures and luminosities were derived, with a starbeing classed as an H-burner or a He-burner according to which evolutionarytrack gave an age in agreement with the nebular expansion age (given by themeasured nebular radius and expansion velocity). Fig. 10 shows one of theirspectral fits, as well as the HR diagram for the objects assigned as He-burners.The addition of STIS to its instrument suite powerfully increased the HST’scapabilities for studies of Magellanic Cloud PNe, with STIS’s ability to obtainslitless spectral imaging in multiple emission lines making a particularly strongimpact. Shaw et al. (2001) have presented STIS images for 29 LMC PNe, ob-tained as part of an HST Snapshot survey. The overall sample of 60 LMC PNewhich had HST images showed a much higher incidence of non-symmetric neb-Planetary Nebulae Beyond the Milky Way – Historical Overview 11Fig. 9. HST Faint Object Camera images of five LMC PNe taken in the [O iii] 5007 Åline. Each image is 2.2 arcsec on a side. From Stanghellini et al. (1999).ulae than do comparable Galactic PN samples. Stanghellini et al. (2002) havepresented STIS slitless spectra of 29 more LMC PNe (some examples are shownin Fig. 11), while Stanghellini et al. (2003) presented STIS images and slitlessspectra for 27 more SMC PNe.Pẽna et al. (1994) discovered that the central star of LMC N66 was muchbrighter at IUE UV wavelengths in the 1990’s than it was in the 1980’s. Sincethen, it has been monitored closely. It reached maximum brightness in 1994 andby 2000 had faded to a fainter level than in 1983. Since 1994 its optical and UV12 M.J. BarlowFig. 10. Left: model stellar + nebular continuum fit to the UV+optical energy dis-tribution of LMC SMP 76. Right: Hertzsprung-Russell diagram showing central starlocations, together with He-burning stellar evolutionary tracks that are labelled bytheir initial main sequence masses. From Dopita et al. (1997).Fig. 11. HST STIS slitless spectra of the LMC PNe SMP 59 (LM 2-25) and SMP 93(N 181). The images on the left are in the [O iii] 5007 Å line, those in the middle showthe Hα plus [N ii] 6548, 6584 Å blend, and those on the right are deblended [N ii]images. From Stanghellini et al. (2002).spectrum has been that of a WN star, the only known case amongst PN nuclei.Fig. 12 shows an HST FOC [O iii] image of LMC N66, from Pẽna et al. (2004),while Fig. 13 shows a sequence of IUE and HST UV spectra, from Hamann et al.(2003), which nicely illustrates the rise and fall of the central star’s brightness.Hamann et al. suggested that rather than this being a final thermal pulse event,its nucleus may be a white dwarf accreting matter from a companion and couldbe the precursor to a Type Ia supernova.Planetary Nebulae Beyond the Milky Way – Historical Overview 13Fig. 12. HST Faint Object Camera (not WFPC) image of LMC N66 (=WS35) in the[O iii] 5007 Å line, acquired in February 1994 when the central star was near peakbrightness. From Pẽna et al. (2004).Fig. 13. A time sequence of 1200-2000 Å IUE and HST spectra of the central starof LMC N66, showing the transient brightening event that peaked in 1993-4. FromHamann et al. (2003).14 M.J. BarlowReferences1. M. Arnaboldi et al.: ApJ, 472, 145 (1996)2. M. Arnaboldi et al.: ApJ, 507, 759 (1998)3. S. A. Becker, I. Iben Jr.: ApJ, 237, 129 (1980)4. J. C. 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Planetary nebulae beyond the milky way - Historical overview

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Planetary nebulae beyond the milky way - Historical overview

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