Keck Hires Spectroscopy of Extragalactic H Ii Regions: C and O Abundances From Recombination Lines

Abstract

We present very deep spectrophotometry of 14 bright extragalactic H II regions belonging to spiral, irregular, and blue compact galaxies. The data for 13 objects were taken with the HIRES echelle spectrograph on the Keck I telescope.We have measured C II recombination lines in 10 of the objects and O II recombination lines in 8 of them. We have determined electron temperatures from line ratios of several ions, specially of low ionization potential ones. We have found a rather tight linear empirical relation between T e ([N II]) and T e ([O III]). We have found that O II lines give always larger abundances than [O III] lines. Moreover, the difference of both O ++ abundance determinations -the so-called abundance discrepancy factor-is very similar in all the objects, with a mean value of 0.26 ± 0.09 dex, independently of the properties of the H II region and of the parent galaxy. Using the observed recombination lines, we have determined the O, C, and C/O radial abundance gradients for 3 spiral galaxies: M 33, M 101, and NGC 2403, finding that C abundance gradients are always steeper than those of O, producing negative C/O gradients accross the galactic disks. This result is similar to that found in the Milky Way and has important implications for chemical evolution models and the nucleosynthesis of C.where the auroral lines become very faint, or to measure recombination lines (hereafter RLs) useful for abundance determinations of heavy-element ions (Peimbert 2003;López-Sánchez et al. 2007;Bresolin 2007).The detection of C II and O II lines produced by pure recombination in EHRs was firstly reported by Esteban et al. (2002) from deep spectra taken with the 4.2 m William Herschel Telescope. In principle, these lines have the advantage that their intensity is much less dependent on the value of T e than the collisionally excited lines (hereafter CELs), which are the lines commonly used for abundance determinations in nebulae. The brightest C II RL is C II λ4267, with typical fluxes of the order of 10 −3 × I(Hβ). This line permits to derive the C ++ abundance, which is the dominant ionization stage of C for the typical conditions of EHRs. There are only a few C abundance determinations available for EHRs, most of them derived from UV CELs that can only be observed from space (Garnett et al. 1995(Garnett et al. , 1999Kobulnicky & Skillman 1998), and more recently from RLs (Esteban et al. 2002;Peimbert 2003;Tsamis et al. 2003;Peimbert et al. 2005;López-Sánchez et al. 2007;Bresolin 2007). The C abundance determinations based on UV CELs are severely affected by uncertainties in the reddening correction. To further complicate the situation, the STIS spectrograph aboard the Hubble Space Telescope, the only instrument capable to detect the UV CELs of C in bright EHRs, stopped science operations in 2004, so that nowadays the observation of the optical CII RLs provides the only possibility for determining C abundances in EHRs. The study of the behavior of C/H and C/O ratios and their galactocentric gradients in galaxies of di...

Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution★

“…The peculiar position of NGC 2363 is indicated in Figure 6 because it will be discussed later. The figure shows the wellknown C/O vs. O/H correlation found in previous works by, for example, Garnett et al (1995Garnett et al ( , 1999) from observations of CELs or Esteban et al (2002Esteban et al ( , 2009) from observations of RLs. Figure 6 includes the time evolution of the abundances predicted by the chemical evolution models of the Milky Way disc presented by Esteban et al (2013).…”

“…Figure 6 plots the C/O vs. O/H ratios for the complete sample of H ii regions where C and O abundances have been determined from RLs: HV (NGC 6822) and NGC 5408 from this work, Galactic H ii regions Esteban et al 2013), H ii regions in external spiral and irregular galaxies (Peimbert 2003;Esteban et al 2009) and the blue compact dwarf galaxy NGC 5253 (López-Sánchez et al 2007). The peculiar position of NGC 2363 is indicated in Figure 6 because it will be discussed later.…”

Massive star formation in Wolf-Rayet galaxies

“…Indeed, if that is correct, the majority of the abundance determinations in extragalactic objects following the direct method, including those provided in this work, have been underestimated by at least 0.2 to 0.3 dex. Deeper observations of a large sample of star-forming galaxies -that allow us to detect the faint recombination lines, such as those provided by Esteban et al (2009) and more theoretical work -including a better understanding of the photoionization models, such as the analysis provided by Kewley & Ellison (2008) -are needed to confirm this puzzling result.…”

Toward a Removal of Temperature Dependencies From Abundance Determinations: NGC 628

“…In nearby bright H ii regions, optical recombination lines are also detected. These different sets of lines, optical recombination and collisionally excited, yield significantly different temperatures for the same cloud (e.g., Esteban et al 2009;García-Rojas & Esteban 2007). Different elements also indicate different temperatures for a given ionization zone.…”