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New Insights into the Irregular Dwarf Galaxy NGC 6822: Planetary Nebulae and Metallicity

This study explores the characteristics of the irregular dwarf galaxy NGC 6822 (distance: 490 kpc) by analyzing imaging and spectroscopic data. It highlights the low metallicity (Log(O/H) + 12 ~ 8.1) and the ongoing star formation within its huge HI envelope. We report 15 new planetary nebulae (PNe) discoveries, bringing the total to 19. The research discusses the distribution of PNe candidates, extinction corrections, and the chemical composition of the galaxy, emphasizing its significance for understanding local group galaxies.

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New Insights into the Irregular Dwarf Galaxy NGC 6822: Planetary Nebulae and Metallicity

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  1. Local Group CensusNGC 6822 P. Leisy (1,3) L. Magrini (2), R. Corradi (1), A. Mampaso (3) (1) Isaac Newton Group (La Palma, SPAIN) (2) Universita’ di Firenze (ITALY) (3) Instituto Astrofisico Canarias (Tenerife, SPAIN)

  2. Irregular Dwarf galaxy Ir IV-V Distance 490 Kpc (m-M=23.48) Low metallicity Log(O/H)+12~8.1 (HII) 7.91Chandar et al. 2000 8.25Skillman et al. 1989 or Pagel et al 1980 -2.0 < [Fe/H] ~-1.0 Tolstoy et al. 2001 Gas rich galaxy – Huge HI envelope currently ongoing “low” star formation taking place Introduction

  3. Several set of data Imaging ESO/WFIHa, [OIII], [SII], B, continuum ING/WFCHa, [OIII], [SII], r, Str Y Spectra ESO / NTT (8 long slit spectra) ING / WHT-AF2 (fiber multi-spec 40’ )

  4. WFC WFI Filters used

  5. H V

  6. Red giant extension (6 scale length) Letarte et al. 2002 Supergiant HI shell r>40’ or ~6Kpc Blok & Walter 2000 NGC 6822 PNe / outliers

  7. PNe candidates Criteria Ha – [OIII] emitter Point-like (at 500Kpc 1” = 2.5pc) No Continuum Not using R=[OIII]/(Ha+[NII]) > 1 loose some PN Ratio-[OIII]/Ha Ratio Ha/Cont PNe>100-200 - HII ~ 30-50 Most PNe distributed in the bar young population ? 2-3 intergalactic PN candidates ? (~15 Kpc)

  8. HII PN Diagram Ha/NII Ha/SII First introduced Sabbadin & D’ Oddorico 1978 Ha/[OIII]-Ha/[SII].jpg Great potential to select candidates; place HII vary inside the Local Group galaxies G. Delgado 2003, IAC Master Thesis, 2003 Delgado, Viironen, Mampaso, Corradi II Internat. workshop of science with GTC 2004

  9. PNLF with 19 PNe • Completeness ~3 mag 2 ind. sets of data (Jacoby) • Dist = 23.91+/- 0.19 • Extinction correction Av=0.26-0.45 mean~0.32 8 PNe : 0.45<C(Hb)<1.5 or Av ~ 0.3-1.0 Dist = 23.6 +/- 0.4 23.48 +/- 0.06 (2003)

  10. Spectra NTT spectra of PNe / HII 10-20mn exposure time high S/N ratio (brightest objects) need observations with 8-10m class telescopes

  11. Diag. N/O - He LMC – SMC Large N/O NGC 6822 (M33) more He ? or much less N Galactic

  12. Diag. N/O - O NGC 6822 Oxygen ? sample too small LMC – SMC dest O 2nd Type I slight prod O 2x ? (Carbon 10-100x) M33 same trends MC

  13. Conclusions 15 new PNe in NGC6822 (19 in total) Difficulty distinguish PNe/compact HII  spectra distance PNLF ~23.6 +/- 0.4 (~500 Kpc) individual extinction derived from spectra High He, faint N(like in M33) N/O ratio SMALL(compare to LMC-SMC)

  14. Laura MAGRINI Sextans A / Sextans B VLT m-o spectroscopy

  15. Diag. N/O - N NGC 6822 - M33 Faint N (N/O)

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