Gas-Phase Oxygen Abundances and Radial Metallicity Gradients in the Two Nearby Spiral Galaxies NGC 7793 and NGC 4945

Abstract

Gas-phase abundances in H II regions of two spiral galaxies, NGC 7793 and NGC 4945, have been studied to determine their radial metallicity gradients. We used the strong-line method to derive oxygen abundances from spectra acquired with GMOS-S, the multi-object spectrograph on the 8 m Gemini South telescope. We found that NGC 7793 has a well-defined gas-phase radial oxygen gradient of −0.321 ± 0.112 dex R 25) in the galactocentric range 0.17 < R G /R 25 < 0.82, not dissimilar from gradients calculated with direct abundance methods in galaxies of similar mass and morphology. We also determined a shallow radial oxygen gradient in NGC 4945, −0.253 ± 0.149 dex R 25 1 -(or −0.019 ± 0.011 dex kpc −1 ) for 0.04 < R G / R 25 < 0.51, where the larger relative uncertainty derives mostly from the larger inclination of this galaxy. NGC 7793 and NGC 4945 have been selected for this study because they are similar, in mass and morphology, to M33 and the Milky Way, respectively. Since at zeroth order we expect the radial metallicity gradients to depend on mass and galaxy type, we compared our galaxies in the framework of radial metallicity models best suited for M33 and the Galaxy. We found a good agreement between M33 and NGC 7793, pointing toward similar evolution for the two galaxies. We notice instead differences between NGC 4945 and the radial metallicity gradient model that best fits the Milky Way. We found that these differences are likely related to the presence of an active galactic nucleus combined with a bar in the central regions of NGC 4945, and to its interacting environment.

Star Formation Histories of the LEGUS Spiral Galaxies. I. The Flocculent Spiral NGC 7793∗

“…They find 52 of these sources, and with additional slits on the H ii regions, they estimate a metallicity gradient using strong line calibrations of 12 + log(O/H) = 8.61 ± 0.05 − (0.36 ± 0.10) r/R 25 and a SFR of 0.45 M yr −1 . Another study by Stanghellini et al (2015) analyzed the strong-line oxygen abundances in the H ii regions of the galaxy, finding similar radial metallicity gradients, inside R 25 = 5.24 ± 0.24 arcmin, also in agreement with the pioneering study by Edmunds & Pagel (1984).…”

Metallicity gradients in local Universe galaxies: Time evolution and effects of radial migration

“…However, the slopes of the gradient in the radial region where the two methods overlap are in rough agreement with the gradient from the direct method; the latter method is slightly steeper. As shown by Stanghellini et al (2015), the strong-line abundances can indeed give "first order" constraints to galactic evolutionary models, even if more uncertain than the direct-method abundances. In the case of M31, the best that we can do is to use the slope derived by the larger sample of Sanders et al (2012), thus with a better statistics, rescaling the value of the intercept by the average difference of the two bins in common between Zurita & Bresolin (2012) and Sanders et al (2012).…”

Extinction Maps and Dust-to-gas Ratios in Nearby Galaxies with LEGUS

“…NGC 628 and NGC 7793's large sizes (∼12' and ∼14') and multiple LEGUS pointings provide us with an opportunity to study radial variations within each galaxy. Both galaxies also have published metallicity gradients from Sánchez-Blázquez et al (2014) and Stanghellini et al (2015), providing us a larger sample of metallicities for this study. Later studies will expand upon this sample.…”