Carbon and oxygen abundance gradients in NGC 300 and M33 from optical recombination lines

Abstract

We present deep spectrophotometry of several H ii regions in the nearby low-mass spiral galaxies NGC 300 and M 33. The data have been taken with UVES and OSIRIS spectrographs attached to the 8 m VLT and 10.4 m GTC telescopes, respectively. We have derived precise values of the physical conditions for each object making use of several emission line-intensity ratios. In particular, we have obtained direct determinations of the electron temperature in all the observed objects. We detect pure recombination lines (RLs) of C ii and O ii in several of the H ii regions, permitting to derive their C/H and C/O ratios. We have derived the radial abundance gradient of O for each galaxy making use of collisionally excited lines (CELs) and RLs, as well as the C and N gradients using RLs and CELs, respectively. We obtain the first determination of the C/H gradient of NGC 300 and improve its determination in the case of M 33. In both galaxies, the C/H gradients are steeper that those of O/H, leading to negative C/O gradients. Comparing with similar results for other spiral galaxies, we find a strong correlation between the slope of the C/H gradient and M V . We find that some H ii regions located close to the isophotal radius (R 25 ) of NGC 300 and M 33 show C/O ratios more similar to those typical of dwarf galaxies than those of H ii regions in the discs of more massive spirals. This may be related to the absence of flattening of the gradients in the external parts of NGC 300 and M 33. Finally, we find very similar N/H gradients in both galaxies and a fair correlation between the slope of the N/H gradient and M V comparing with similar data for a sample of spiral galaxies.

Carbon and oxygen in H ii regions of the Magellanic Clouds: abundance discrepancy and chemical evolution

“…Fig. 10 reinforces the results obtained by Toribio San Cipriano et al (2016). They presented the first indication that the slopes of the C/H -and to a lesser extent C/O -gradients show a clear correlation with the M V of the galaxies.…”

“…The ADFs obtained in the SMC are at the higher end of these typical values. Esteban et al (2014) and Toribio San Cipriano et al (2016) have found ADFs of about 0.3-0.4 dex in the case of star-forming dwarf galaxies and H ii regions in the spiral galaxy NGC 300, respectively. Tsamis et al (2003) determined the ADF(O 2+ ) for 30 Doradus and N11B in the LMC and for N66 in the SMC.…”

“…4 shows the ADF(O 2+ ) determined in this work and other previous papers as a function of the total O abundances derived from CELs (upper panels) and from RLs (lower panels). Apart from the data for the H ii regions of the MCs, we include results for M33 (Toribio San Cipriano et al 2016), MW Esteban et al 2004Esteban et al , 2013, M101 ) and star-forming dwarf galaxies (López- Sánchez et al 2007;Esteban et al 2014). In the right-panels, we computed the mean of ADF(O 2+ ) values for different metallicity bins of the O/H ratio determined either from CELs or RLs.…”

“…In the last years, this line has been accessible in Galactic and extragalactic H ii regions thanks to the arrival of large aperture ground-based optical telescopes (e.g. Peimbert et al 1986Esteban et al 2002Esteban et al , 2009Esteban et al , 2014López-Sánchez et al 2007;Toribio San Cipriano et al 2016).…”

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

“…We supplement this sample with recent observations of extragalactic H ii regions from Croxall et al (2016) and Toribio San Cipriano et al (2016), bringing the total sample size to 1052. While all of the H ii regions in this sample have measurements of at least one auroral line, some of these auroral-line measurements have low S/N or are otherwise unreliable.…”