1. 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

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) A, R~2.3Å for [O II] 3727 to [O III]5007Å
Red spectra - (300V) Å 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 PN detected < m5007 < 28.1
PN not in Hui catalog
m5007 = 28.1
m5007 = 27.5
λ calib 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/Hβ and 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