The Dwarf Starburst Galaxy NGC 1705 : New H II Region Element Abundances & Reddening Variations Near the Center

NGC 1705 is a nearby dwarf starburst galaxy (D = 5.1 Mpc) whose global and outflow properties have been well-studied (see Lee & Skillman 2004 for references within). Values of the nebular oxygen abundance from the literature were found to be in the range between 12+log(O/H) = 7.8 and 8.5. However, none of the previous measurements include any detection of the temperature-sensitive [O III] 4363 Å emission line. Optical spectroscopy of 16 H II regions were obtained with EFOSC2 on the 3.6-m telescope at ESO La Silla, and [O III] 4363 has been measured for the first time.

Fig. 1 shows the slit placements and the H II regions examined; labeled are H II regions where [O III] 4363 was measured. From [O III] measurements in five H II regions, the resulting mean oxygen abundance is 12+log(O/H) = 8.21 ± 0.05, which corresponds to [O/H] = -0.45, or about 35% of the solar value. No significant spatial inhomogeneities in oxygen abundance were found in a radius about 10’’ from the super star cluster. Although faint and narrow He II 4686 emission was found in two H II regions, the implied contribution from O+3 to the total oxygen abundance is only 0.01 dex. The N/O ratio in the ionized gas agrees with the value for the neutral gas (Heckman et al. 2001), even though the metallicity of the neutral gas is up to 0.7 dex lower than that of the ionized gas; this may indicate the presence of low-metallicity gas in the halo of NGC 1705. This requires exploration in other starburst galaxies; see also John Cannon’s NGC 625 poster. From observed Balmer emission flux ratios, derived values of the extinction in V are found to be in the range 0 ≤ AV ≤ 0.9 mag along various lines of sight in the galaxy (Fig. 2). This may indicate a need to take careful account of possible variations in extinction in order to correct properly the photometry of resolved stars, which can affect the derived star formation history. Fig. 3 shows that the gas-phase metallicity for NGC 1705 is similar to Local Group and other nearby dwarf irregular galaxies at comparable luminosities and gas-to-light fractions. The outflow properties of NGC 1705 are not manifested significantly in these diagrams. Additional insights can be gained from these evolution diagnostics by obtaining measurements at near-infrared and infrared wavelengths to minimize the impact of the luminosity of the most recent starburst on the total luminosity.

Figure 1: Long-slit orientations superimposed on an H image of NGC 1705 (Gil de Paz et al. 2003; their Fig. 3). The position of the super star cluster (SSC) is shown. Left panel - 1.56 by 1.70 arcmin field of view. The central 0.78 by 0.87 arcmin rectangle is shown in the right panel. H II regions where [O III]4363 is detected are labeled.

Figure 3: Diagnostic diagrams. Left panel - Metallicity (nebular oxygen abundance) vs. B galaxy luminosity. Right panel - Metallicity vs. H I gas to B light ratio. NGC 1705 is represented by a large open star. Sculptor group dIs (Skillman et al. 2003) are indicated as “Y” symbols, local dIs (Lee et al. 2003) are represented as filled circles, field dIs (van Zee et al. 1997) are shown as open triangles, and DDO 154 (Kennicutt & Skillman 2001) is marked by an open square. Symbols are the same in both panels. All data symbols here represent [O III] 4363 measurements.

ReferencesGil de Paz et al. 2003, ApJS, 147, 29 Lee & Skillman 2004, ApJ, 614, in press (astro-ph/0406571)

Kennicutt & Skillman 2001, AJ, 121, 1461 Skillman, Côté, & Miller 2003, AJ, 125, 610

Lee et al. 2003, AJ, 125, 146 van Zee et al. 1997, AJ, 114, 2479

Henry Lee (hlee@astro.umn.edu) Starbursts - From 30 Doradus to Lyman Break Galaxies, Cambridge

H. L. and E. D. S. acknowledge partial support from a NASA LTSARP grant NAG 5-9221 and the University of Minnesota.

Henry Lee & Evan D. SkillmanUniversity of Minnesota

Figure 2: Extinction map of central H II regions in NGC 1705. Black labels indicate H II regions whose V extinctions, AV, are less than 0.1 mag. H II region B2 marked in green has AV just below 0.3 mag. The remaining H II regions labeled in red have AV values between 0.3 and 0.9 mag.

ApJ, scheduled 2004 Oct 20 issue, v. 614