NGC 55: a disc galaxy with flat abundance gradients

Abstract

We present new spectroscopic observations obtained with GMOS@Gemini-S of a sample of 25 H ii regions located in NGC 55, a late-type galaxy in the nearby Sculptor group. We derive physical conditions and chemical composition through the T e -method for 18 H ii regions, and strong-line abundances for 22 H ii regions. We provide abundances of He, O, N, Ne, S, Ar, finding a substantially homogenous composition in the ionised gas of the disc of NGC 55, with no trace of radial gradients. The oxygen abundances, both derived with T e -and strong-line methods, have similar mean values and similarly small dispersions: 12+log(O/H)=8.13±0.18 dex with the former and 12+log(O/H)=8.17±0.13 dex with the latter. The average metallicities and the flat gradients agree with previous studies of smaller samples of H ii regions and there is a qualitative agreement with the blue supergiant radial gradient as well. We investigate the origin of such flat gradients comparing NGC 55 with NGC 300, its companion galaxy, which is also twin of NGC 55 in terms of mass and luminosity. We suggest that the differences in the metal distributions in the two galaxies might be related to the differences in their K-band surface density profile. The flatter profile of NGC 55 probably causes in this galaxy higher infall/outflow rates than in similar galaxies. This likely provokes a strong mixing of gas and a re-distribution of metals.

Changes: The Past, Present, and Future of the Nearby Dwarf Galaxy NGC 55∗ †

“…The Hα and [NII]6584 lines were both fit with two Gaussians having the same width, and the results indicate that [NII] contributes ∼ 6% of the total signal in the central regions of NGC 55, with a ±1% dispersion. This is consistent with line strengths measured in NGC 55 HII regions by Magrini et al (2017) -Mean spectra of the three angular intervals centered on the minor axis of NGC 55 that are marked in the lower panel of Figure 2. Wavelengths near Hα and the Ca triplet are shown.…”

Kinematics and physical properties of the nearby galaxy NGC 4656 and its TDG candidate

“…In addition, on this plot we have included the oxygen distribution for NGC 55, which is a nearby galaxy that displays tidal features and similar characteristics than NGC 4656 (NGC 55 is located at 2.34 Mpc, with a total mass of 2.0×10 10 M and R 25 =11 kpc, Westmeier et al 2013;Kudritzki et al 2016). In order to analyze its oxygen distribution, we used the abundances that were recently published by Magrini et al (2017), who used the N2 calibrator proposed by Marino et al (2013). Finally, projected distances for this system were re-calculated by using the positions given by Magrini et al (2017) and the method described in §3.3.…”

“…In order to analyze its oxygen distribution, we used the abundances that were recently published by Magrini et al (2017), who used the N2 calibrator proposed by Marino et al (2013). Finally, projected distances for this system were re-calculated by using the positions given by Magrini et al (2017) and the method described in §3.3. Then, the gradient for NGC 55 is represented by magenta asterisks and a dashed-dotted magenta line.…”

“…The green dashed line represents the oxygen abundance gradient estimated with the N2 calibrator(Marino et al 2013) for the NGC 4656 regions obtained byZasov et al (2017) (green circles). For comparison, the dotted-dashed magenta line represents the oxygen abundance gradient estimated with the N2 calibrator(Marino et al 2013) for the NGC 55 regions obtained byMagrini et al (2017) (asterisks magenta).…”

What do planetary nebulae and H II regions reveal about the chemical evolution of nearby dwarf galaxies?

“…Concerning the dwarf galaxy metallicities (Z), the above characteristics also imply that for the star-forming dIrr the best Z indicator is the O/H (oxygen abundance) derived from the H ii regions. A few selected recent examples in the literature are: Berg et al (2016); Pilyugin & Grebel (2016); Magrini et al (2017); Flores-Durán et al (2017); Kumari et al (2018)). Although some gas has been found in the central regions of dSph (e.g., in NGC 185, Gonçalves et al 2012), these galaxies are dominated by the old stellar 1 2 Denise R. Gonçalves populations.…”