The Oxygen Abundance in the Inner HiiRegions of M101: Implications for the Calibration of Strong‐Line Metallicity Indicators

Abstract

I present deep spectroscopy of four H ii regions in the inner, metal-rich zone of the spiral galaxy M101 obtained with the LRIS spectrograph at the Keck telescope. From the analysis of the collisionally excited lines in two of the target H ii regions, H1013 and H493, I have obtained oxygen abundances 12 þ log (O/ H ) ¼ 8:52 and 12 þ log (O/H ) ¼ 8:74, respectively. These measurements extend the determination of the oxygen abundance gradient of M101 via the direct method to only 3 kpc from the center. The intensity of the C ii k4267 line in H1013 leads to a carbon abundance 12 þ log (C/ H ) ¼ 8:66, corresponding to nearly twice the solar value. From a comparison of the continuum temperature derived from the Balmer discontinuity, T (Bac) ¼ 5000 K, and the line temperature derived from [O iii] k4363/k5007, T ½O iii ¼ 7700 K, an average temperature T 0 ¼ 5500 K and a mean square temperature fluctuation t 2 ¼ 0:06 have been derived. Accounting for the spatial inhomogeneity in temperature raises the oxygen abundance obtained from the oxygen auroral lines to 12 þ log (O/H ) ¼ 8:93. These findings are discussed in the context of the calibration of strong-line metallicity indicators, in particular of the upper branch of R 23 . There is no evidence for the strong abundance biases arising from temperature gradients predicted theoretically for metal-rich H ii regions.

Biases in Metallicity Measurements from Global Galaxy Spectra: The Effects of Flux Weighting and Diffuse Ionized Gas Contamination

“…Thus, the additional temperature variations due to metallicity gradients are likely only significant for the least-massive galaxies in SDSS. After measurement uncertainty is accounted for, the intrinsic scatter of H ii regions about the metallicity gradients in local spirals is ∼ 0.05 − 0.1 dex (Kennicutt et al 2003;Rosolowsky & Simon 2008;Bresolin 2011;Berg et al 2013;Croxall et al 2015Croxall et al , 2016, corresponding to ∼ 0.02 − 0.07 dex in T 3 indicitave of the minimum σ T in the absence of metallicity gradients for local star-forming galaxies. The shallow gradients of the CHAOS galaxies (0.02-0.04 dex kpc −1 ) suggest that stochastic variations in metallicity account for the majority of the width of the electron temeperature distributions in Figure 5.…”

NGC 6845: metallicity gradients and star formation in a complex compact group

“…The break in the solid black line is located in R25 (R∼14kpc). For comparison, we added the schematic representation of the galactocentric oxygen abundance gradients obtained in the literature for NGC 1512, NGC 3621 (Bresolin et al 2012), M101 (Kennicutt et al 2003), (Bresolin 2007) andNGC 92 (Torres-Flores et al 2014). The oxygen abundances for these galaxies were estimated using the N2 method calibrated by Pettini & Pagel (2004).…”

Relative frequencies of supernovae types: dependence on host galaxy magnitude, galactocentric radius, and local metallicity

“…In the past few years, several studies have mentioned the possible errors in abundances obtained from strong lines as in Zaritsky et al (1994) or van Zee et al (1998). Bresolin et al (2004) and Bresolin (2007) performed a more detailed analysis of the gradients in M 51 and M 101. They found flatter gradients than in previous studies for the same galaxies, although their gradients are within the dispersion of those in Fig.…”