Spitzer observations of extragalactic H II regions - III. NGC 6822 and the hot star, H II region connection

Abstract

Using the short-high module of the Infrared Spectrograph on the Spitzer Space Telescope, we have measured the [S IV] 10.51, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71-micron emission lines in nine H II regions in the dwarf irregular galaxy NGC 6822. These lines arise from the dominant ionization states of the elements neon (Ne${}^{++}$, Ne${}^{+}$) and sulphur (S${}^{3+}$, S${}^{++}$), thereby allowing an analysis of the neon to sulphur abundance ratio as well as the ionic abundance ratios Ne${}^{+}$/Ne${}^{++}$ and S${}^{3+}$/S${}^{++}$. By extending our studies of H II regions in M83 and M33 to the lower metallicity NGC 6822, we increase the reliability of the estimated Ne/S ratio. We find that the Ne/S ratio appears to be fairly universal, with not much variation about the ratio found for NGC 6822: the median (average) Ne/S ratio equals 11.6 (12.2$\pm$0.8). This value is in contrast to Asplund et al.'s currently best estimated value for the Sun: Ne/S = 6.5. In addition, we continue to test the predicted ionizing spectral energy distributions (SEDs) from various stellar atmosphere models by comparing model nebulae computed with these SEDs as inputs to our observational data, changing just the stellar atmosphere model abundances. Here we employ a new grid of SEDs computed with different metallicities: Solar, 0.4 Solar, and 0.1 Solar. As expected, these changes to the SED show similar trends to those seen upon changing just the nebular gas metallicities in our plasma simulations: lower metallicity results in higher ionization. This trend agrees with the observations.

arXiv comment22 pages, 13 figures. To be published in MNRAS. reference added and typos fixed. arXiv admin note: text overlap with arXiv:0804.0828, which is paper II by Rubin et al. (2008)

References in corpus (42)

• The chemical composition of the Sun
• The Spitzer Space Telescope Mission
• The Infrared Spectrograph on the Spitzer Space Telescope
• SINGS: The SIRTF Nearby Galaxies Survey
• A new calibration of stellar parameters of Galactic O stars
• A Grid of NLTE Line-Blanketed Model Atmospheres of O-type Stars
• Abundance Gradients and the Formation of the Milky Way
• The 1000 Brightest HIPASS Galaxies: HI Properties
• The SMART Data Analysis Package for the Infrared Spectrograph on the Spitzer Space Telescope
• The Distribution Of Heavy Elements In Spiral And Elliptical Galaxies
• Quantitative Spectroscopy of O Stars at Low Metallicity. O Dwarfs in NGC 346
• A reappraisal of the chemical composition of the Orion nebula based on VLT echelle spectrophotometry
• HerMES: The SPIRE confusion limit
• Oxygen, Carbon and Nitrogen evolution in galaxies
• Star formation sustained by gas accretion
• Ionizing Photon Emission Rates from O- and Early B-type Stars and Clusters
• The Chemical Composition of the 30 Doradus Nebula Derived from VLT Echelle Spectrophotometry
• The metallicity gradient of M 33: chemical abundances of HII regions
• Optical spectroscopy of metal-rich HII regions and circumnuclear hot spots in M83 and NGC 3351
• Faint emission lines in the Galactic H II regions M16, M20 and NGC 3603
• The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822
• Chemical Composition of Two H II Regions in NGC 6822 Based on VLT Spectroscopy
• The [Fe IV] Discrepancy: Constraining the Iron Abundances in Nebulae
• Chemical composition and mixing in giant HII regions: NGC3603, 30Doradus, and N66
• The Spitzer Infrared Nearby Galaxies Survey: A High-Resolution Spectroscopy Anthology
• Red Supergiant Stars as Cosmic Abundance Probes: KMOS Observations in NGC 6822
• Evolution of the Milky Way with radial motions of stars and gas II. The evolution of abundance profiles from H to Ni
• Spitzer IRS Observations of the Galactic Center: Shocked Gas in the Radio Arc Bubble
• Spitzer reveals what's behind Orion's Bar
• Radiation-driven winds of hot luminous stars. XVI. Expanding atmospheres of massive and very massive stars and the evolution of dense stellar clusters
• On the Measurement of Elemental Abundance Ratios in Inner Galaxy H II Regions
• Metallicity and the spectral energy distribution and spectral types of dwarf O-stars
• Elemental Abundances of Blue Compact Dwarfs from mid-IR Spectroscopy with Spitzer
• Spitzer Observations of M83 and the Hot Star, H II Region Connection
• Chemical behavior of the Dwarf Irregular Galaxy NGC 6822. Its PN and HII region abundances
• Electron-Impact Excitation Collision Strengths and Theoretical Line Intensities for Fine-Structure Transitions in S III
• The PNe and H II regions in NGC 6822 revisited. Clues to AGB nucleosynthesis
• A Spitzer Space Telescope survey of massive young stellar objects in the G333.2-0.4 giant molecular cloud
• Testing the existence of non-Maxwellian electron distributions in H II regions after assessing atomic data accuracy
• A large-scale R-matrix calculation for electron-impact excitation of the Ne${}^{2+}$ O-like ion
• Chemical abundances in the protoplanetary disk LV2 (Orion) - II: High dispersion VLT observations and microjet properties
• Three-dimensional modeling of ionized gas. II. Spectral energy distributions of massive and very massive stars in stationary and time-dependent modeling of the ionization of metals in HII regions

Cited by in corpus (7)

• Spitzer Infrared Spectrograph Observations of the Galactic Center: Quantifying the Extreme Ultraviolet/so ft X-ray fluxes
• Dusty Stellar Birth and Death in the Metal-Poor Galaxy NGC 6822
• Numerical Models for the Diffuse Ionized Gas in Galaxies. II. Three-dimensional radiative transfer in inhomogeneous interstellar structures as a tool for analyzing the diffuse ionized gas
• The Young Stellar Population of the metal-poor galaxy NGC 6822
• CO-Dark Star Formation and Black Hole Activity in 3C 368 at z = 1.131: Coeval Growth of Stellar and Supermassive Black Hole Masses
• Investigating episodic mass loss in evolved massive stars: III. Spectroscopy of dusty massive stars in three northern galaxies
• SDSS-V LVM: Resolving Physical Conditions in the Trifid Nebula