Planetary Nebula abundances in NGC 5128
with FORS
J. R. Walsh, ST-ECF, ESOG. H. Jacoby, WIYN
R. Pelletier, Kapteyn Lab.N. A. Walton, IoA, Cambridge
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
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)
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
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 #5601 300Vspectrum
λ calib F56 MOSspectrum
Sky+background subtracted
PN not in Hui catalogm5007 = 28.1m5007 = 27.5
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
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
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