Planetary Nebula abundances in NGC 5128 with FORS
description
Transcript of Planetary Nebula abundances in NGC 5128 with FORS
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