CHAOS I. Direct chemical abundances for H II regions in NGC 628

Journal Article · Wed Jun 10 04:00:00 EDT 2015 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/806/1/16· OSTI ID:22883115

Berg, Danielle A. ^[1]; Skillman, Evan D. ^[2]; Croxall, Kevin V.; Pogge, Richard W. ^[3]; Moustakas, John ^[4]; Johnson-Groh, Mara ^[5]

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1. Center for Gravitation, Cosmology and Astrophysics, Department of Physics, University of Wisconsin Milwaukee, 1900 East Kenwood Boulevard, Milwaukee, WI 53211 (United States)
2. Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455 (United States)
3. Department of Astronomy, The Ohio State University, 140 W. 18th Avenue, Columbus, OH 43202 (United States)
4. Department of Physics and Astronomy, Siena College, 515 Loudon Road, Loudonville, NY 12211 (United States)
5. Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8P 1A1 (Canada)

The CHemical Abundances of Spirals (CHAOS) project leverages the combined power of the Large Binocular Telescope (LBT) with the broad spectral range and sensitivity of the Multi Object Double Spectrograph (MODS) to measure “direct” abundances (based on observations of the temperature-sensitive auroral lines) in large samples of H ii regions in spiral galaxies. We present LBT MODS observations of 62 H ii regions in the nearby spiral galaxy NGC 628, with an unprecedentedly large number of auroral lines measurements (18 [O iii] λ4363, 29 [N ii] λ5755, 40 [S iii]λ6312, and 40 [O ii] λλ7320, 7330 detections) in 45 H ii regions. Comparing derived temperatures from multiple auroral line measurements, we find: (1) a strong correlation between temperatures based on [S iii] λ6312 and [N ii] λ5755; and (2) large discrepancies for some temperatures based on [O ii] λλ7320, 7330 and [O iii] λ4363. Both of these trends are consistent with other observations in the literature, yet, given the widespread use and acceptance of [O iii] λ4363 as a temperature determinant, the magnitude of the T[O iii] discrepancies still came as a surprise. Based on these results, we conduct a uniform abundance analysis prioritizing the temperatures derived from [S iii] λ6312 and [N ii] λ5755, and report the gas-phase abundance gradients for NGC 628. Relative abundances of S/O, Ne/O, and Ar/O are constant across the galaxy, consistent with no systematic change in the upper IMF over the sampled range in metallicity. These alpha-element ratios, along with N/O, all show small dispersions (σ ∼ 0.1 dex) over 70% of the azimuthally averaged radius. We interpret these results as an indication that, at a given radius, the interstellar medium in NGC 628 is chemically well-mixed. Unlike the gradients in the nearly temperature-independent relative abundances, O/H abundances have a larger intrinsic dispersion of ∼0.165 dex. We posit that this dispersion represents an upper limit to the true dispersion in O/H at a given radius and that some of that dispersion is due to systematic uncertainties arising from temperature measurements.

• OSTI ID: 22883115
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 806; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United Kingdom
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
COMPARATIVE EVALUATIONS
CONCENTRATION RATIO
CORRELATIONS
DETECTION
DISPERSIONS
ELEMENT ABUNDANCE
GALACTIC EVOLUTION
GALAXIES
HYDROGEN
METALLICITY
OXYGEN
TELESCOPES
TEMPERATURE MEASUREMENT