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Planetary Nebula abundances in NGC 5128 with FORS

Planetary Nebula abundances in NGC 5128 with FORS. J. R. Walsh, ST-ECF, ESO G. H. Jacoby, WIYN R. Pelletier, Kapteyn Lab. N. A. Walton, IoA, Cambridge. Motivation. Measure PN abundances in an elliptical galaxy (Z > Milky Way)

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Planetary Nebula abundances in NGC 5128 with FORS

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  1. Planetary Nebula abundances in NGC 5128with FORS J. R. Walsh, ST-ECF, ESO G. H. Jacoby, WIYN R. Pelletier, Kapteyn Lab. N. A. Walton, IoA, Cambridge

  2. Motivation • Measure PN abundances in an elliptical galaxy (Z > Milky Way) • Determine abundance gradient (α-elements, e.g. O) from PN; compare with stellar values ([Fe/H]) • From spectra determine if the brightest PN are pathological (upper L cut-off of PNLF) • Compare kinematics with abundances – halo PN with lower Z or groups of PN with similar abundances (merger remnant?) • Input to galaxy evolution models and star formation history

  3. 4m-class PN spectra in NGC 5128 • ESO 3.6m EFOSC long-slit spectra • 3 slit positions, max exp. 9hrs • 5 PN selected from Hui et al. (1993)

  4. FORS1 MOS Observations • Three fields at a range of radial offsets (4, 7.5 & 14.5’) • PN selected from Hui et al. (1993) catalogue maximizing PN observed with18 slitlets • Blue spectra – (600B) 3500-5500A, R~2.3Å for [O II] 3727 to [O III]5007Å • Red spectra - (300V) 4500-8500Å R~5Å for Hα/Hβ, He I, [N II], etc. • Exposures to 4hrs per field per spectral range

  5. Results • λ & flux calibrate each slitlet, extract all PN spectra • Field F56: 20 PN F42: 21 PN F34: 9 PN 50 PN detected 23.5 < m5007 < 28.1 PN not in Hui catalog m5007 = 28.1 m5007 = 27.5 λcalib F56 MOS spectrum PN #5601 300V spectrum Sky+background subtracted

  6. Spectra and abundances • Tie blue and red spectra together with [O III]5007Å • Three brightest spectra analysed in each field • [O III]4363Å detected in 4 PN; 3 in outer field (low background) and in #5601 • No [O III]4363Å, use 5007Å/Hβand Excitation class – Teff relation of Dopita et al (1992) to give Te • Determined O/H, N/O and He/H where lines available • PN spectra ‘normal’. One with N/O>1. One PN with c=0.4, most low (<0.2). One lower excitation

  7. Galaxy scale abundances • O/H v. r – gradient? • Harris et al. (1999, 2000, 2002) determined old Red Giant Branch metallicities from HST photometry in 3 fields (two halo, one central) • Find broad Metallicity Distribution Function (MDF) with peak Z declining with radius • Naïve [O/Fe] ~ +0.2 • Point comparison of O v. Fe in NGC 5128 GC’s PN#4211 ΔV=-190km/s

  8. Prospects/Conclusions • Analysis still in progress. About 40/50 PN have most strong lines (H, [O II], [O III], [N II], etc) detected, m5007<25.5, for at least O abundance determination • NGC 5128 PN show a range of metallicity (cf. stellar MDF) but no obvious radial gradient • Previous puzzling result that PN show sub-solar O/H confirmed, but some higher values (to solar) • Velocity data (rotation curve) required to determine ‘true’ radial distance for abundance gradient • 8-10m multi-object low resolution spectra feasible ([O III]4363Å detection) for PN abundance samples to ~10Mpc

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