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Light element abundances in NGC 5128 from planetary nebulae
Jeremy R. Walsh, ESO George H. Jacoby, WIYN Reynier Pelletier, Kapteyn Lab., Groningen Nic A. Walton, IoA, Cambridge


Planetary nebulae
· Low mass (~0.7-7M^¼ ) main sequence stars undergo a planetary nebula (PN) phase at ages 0.1-10Gyr depending on initial mass, following the red-giant branch (RGB) and asymptotic g iant branch (AGB) to the white dwarf cooling track · L eng th of PN phase ~104 yr (depending on core mass) · Emission from the expanding shell ionized by the hot (50-150,000 K) central star · Ionization level falls with distance from star (and density) giving a Nebulium [O III] w ide range ­ neutral species (O0) to > 100ev (Ne 4+)


Planetary nebula spectra
· · O is the most efficient coolant for the nebula < 15% of the stellar luminosity (1-2x104 L^¼) is emitted in a singl e collisionally excited line O++ 1D2 ­ 3P2 [O III]5007å This strong line allows detection of PN; h igh [O III]/H ratio discriminates against HII regions Other lines of H , He, N, Ne, Ar, S prominent in optical spectra of PN Line ratios dependent on Te, Ne and abundance Some line ratios of same species depend e nt on Te and/or Ne. Example: [O III]5007/4363 => Te Determine O abundance:
[O II]3727å / H => O+/H+ [O III]5007å / H => O++/H+ O IV 1401å / H => O3+/H+
+ + ++ + 3+ +

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Planetary nebulae in NGC 5128
· 785 PN catalogued in early 1990's (Hui et al, 1993); further 350 by Peng et al. 2004; another ~500 from NTT imaging survey => 1600 PN . All from on-band/off-band imaging The strong emission lines allow three probes:
Detection and photometry of PN from [O III] emission line objects. Luminosity function (PNLF) fitting => distance. E.g. Hui et al. (1993) measured 3.5Mpc to NGC 5128 Measurement of radial velocity of many PN from emission line(s) => rotation curve, mass estimate, dark matter Measurement of many lines in a PN => abundances of light elements

[O III]

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Three pronged programme:
­ Extensive imaging with NTT EMMI (imager / spectrometer) to obtain a very large sample over whole extent of galaxy (to <60kpc) ­ VLT FLAMES multi-fibre spectra of the strongest lines [O III]4959,5007å for radial velocities (~1070 PN measured) Poster ­ FORS multi-slit for detailed optical spectra (3500-7000å) for abundance measurements (50 PN)

Cont.


FORS1 MOS observations
FORS1 MOS and WF/PC2 fields ESO 2.2m WFI [O III] · · Three fields at a range of radial offsets of 4' (F42), 7.5' (F56) and 14.5' (F34) Observing catalogued PN from Hui et al. (1993) maximizing number observed per field with18 slitlets of FORS1 Blue spectra ­ (600B) 3500-5500å, R~2.3å for [O II] 3727 to [O III]5007å Red spectra - (300V) 4500-8500å R~5å for H/H, He I, [N II], etc. Exposures to 3hrs per field per spectral range

HH8 HH21 HH31

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Extracting PN spectra
· · · · · · PN at distance of NGC 5128 are point sources so extract PN spectra from background galaxy distribution Faint PN not Galaxy continuum strong in inner in image fields (e.g. H absorption strong) catalogues No. of PN extracted: m5007 = 27.5 Field F56: 20 PN F42: 21 PN F34: 9 PN 50 PN detected in total 23.5 < m5007 < 28.1 [m5007 = -13.74 ­ 2.5 log F5007 ] Sky+background subtracted

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calib field F56 FORS1 MOS spectrum


Radial trends from PN emission
· Extinction correction (from H/H and Case B). Limit is Schlegel et al. (1998) EB-V=0.11 [O III]/H shows no trend, except some higher values near centre (Field 42). Assuming most PN have similar stellar temperature and most of O in O++ => O/ H abundance [Ne III]/[O III] rather constant as found in other PN studies (MW, MC's). Ne/O ratio `fixed' by high mass star enrichment (SN II), thus marginal evidence for O enrichment by PN central stars

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Stellar continuum
· · · · Stellar continuum around PN measured in multislits Inner fields show absorption lines with metal rich signatures - Fe, Mg b, H Lick indices measured (but without templates!). Possible gradient with radius for Mg but not Fe (to 11 kpc) Comparison to stellar evolutionary tracks (Thomas et al. 2003) compatible with -enhanced Z=-0.3 track Extra-PN galaxy continuum F42 F56


Abundance measurements
· Te sensitive [O III]4363å line detected in 4 PN; 3 in outer field (low background) Summed all spectra in each field for higher S/N line detections. Apply mean Te, Ne diagnostics to all PN in each field Determined He/H, O/H, N/H and Ne/H for each PN where lines detected PN without detected [O II] => lower limit to O/H. He++/H to correct for unseen ionization stages (e.g. O3+) Abundances in HII regions in dust lane suggested to be high ([O/H] ~ +0.2)

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HII

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Oxygen Distribution Function (ODF)
· Metallicity Distribution Function (MDF) for the RGB, AGB and red clump stars in three fields from HST WFPC2 (Harris2 1999, 2000, 2002) and Rejkuba et al. (2005) for outer field PN give the Oxygen Distribution Function (ODF) for similar range of radii Peak of ODF shifted by +0.35 wrt peak of MDF => [O/Fe] = +0.35 at Z = -0.5 Can compare [O/Fe] with models From Pipino et al 2008 at 1Re

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PN and star formation history
· Photometric studies indicat e youngest stars in NGC 5128 5-8 Gyr old Corresponding to entry to AG B for a ~2M^¼ star from evolutionary tracks Models not in agreement with data ­ difficulties of modelling the PN evolution and associating a n [O III] flux with an initial MS stellar mass No evidence for PN from high mass progenitor s (enhanced He and N abundance ­Type I)

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Observed PN

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From Marigo et al. 2004


PN in NGC 5128 - Conclusions
· Emission lines measured in 50 PN in NGC 5128 over offset range 2-20 kpc · O/H measured in 6 PN, lower limits in further 35 PN · -0.35 < [O/H] < +0.20 · ODF displaced by +0.3 dex with respect to MDF · No evidence of a radial gradient in [O/H] from the PN, but some higher values at low galactocentric offset · PN probably from the last major star formation episode