Stellar Metallicity of the Extended Disk and Distance of the Spiral Galaxy NGC 3621

Abstract

Low resolution (∼ 4.5Å) ESO VLT/FORS spectra of blue supergiant stars are analyzed to determine stellar metallicities (based on elements such as iron, titanium, magnesium) in the extended disk of the spiral galaxy NGC 3621. Mildly subsolar metallicity (-0.30 dex) is found for the outer objects beyond 7 kpc independent of galactocentric radius and compatible with the absence of a metallicity gradient confirming the results of a recent investigation of interstellar medium H II region gas oxygen abundances. The stellar metallicities are slightly higher than those from the H II regions when based on measurements of the weak forbidden auroral oxygen line at 4363Å but lower than the ones obtained with the R 23 strong line method. It is shown that the present level of metallicity in the extended disk cannot be the result of chemical evolution over the age of the disk with the present rate of in situ star formation. Additional mechanisms must be involved. In addition to metallicity, stellar effective temperatures, gravities, interstellar reddening, and bolometric magnitudes are determined. After application of individual reddening corrections for each target the flux-weighted gravityluminosity relationship of blue supergiant stars is used to obtain a distance modulus of 29.07 ± 0.09 mag (distance D = 6.52 ± 0.28 Mpc). This new distance is discussed in relation to Cepheid and tip of the red giant branch distances.

Metallicities in the Outer Regions of Spiral Galaxies

“…Equation (4) can also be used to estimate that, given the low star formation rates measured in the outer disks of spiral galaxies and the large H I content, the timescale necessary to reach the observed metal enrichment can be longer than the star formation timescale within an inside-out scenario for galaxy growth or even longer than a Hubble time, reinforcing the notion that the metallicities measured in outer disks exceed the values attainable by in situ star formation alone (see also Eq. 3 in Kudritzki et al 2014 for an alternative calculation based on the closed box model, and leading to the same conclusion).…”

“…This seems to be consistent with the shallow overall abundance gradient, both in dex kpc −1 and normalized to the isophotal radius, which is possibly the consequence of galaxy interactions. It is also worth pointing out that the abundance break observed in NGC 3621 by Bresolin, Kennicutt and Ryan-Weber (2012) has been confirmed by the independent spectroscopic analysis of five blue supergiant stars, straddling the isophotal radius, by Kudritzki et al (2014). The stellar metallicities are intermediate between the nebular metallicities determined from the N2 and R23 diagnostics.…”

Red Supergiants as Cosmic Abundance Probes: The Magellanic Clouds

“…Correct interpretation of their spectra is made possible by sophisticated model atmospheres, and so such work is free of the systematic effects that hamper studies of H II regions such as unknown temperature structures and large-scale inhomogeneities. A small sample of nearby galaxies has been studied using blue supergiants (BSGs; e.g., Kudritzki et al 2012Kudritzki et al , 2013Kudritzki et al , 2014, and it was shown in the case of NGC 300 that the BSG results were in excellent agreement with those of auroral line H II region measurements (Bresolin et al 2009).…”

The flux-weighted gravity-luminosity relationship of blue supergiant stars as a constraint for stellar evolution

“…With absolute visual magnitudes M V −9.5 mag, a single BSG can be as bright as a globular cluster or a dwarf galaxy. Because of their brightness, they are ideal objects for studying the chemical composition of the young stellar population in galaxies far beyond the local group and for determining extragalactic distances using the tools of quantitative stellar spectral analysis (see, for instance, Kudritzki et al 2014, and references therein). At the same time, BSGs are also extremely useful for constraining models of stellar evolution either through investigation of their chemical surface composition, revealing the presence of rotationally or convectively induced mixing (see, e.g., Gies & Lambert 1992;Hunter et al 2008b,a;Przybilla et al 2010;Urbaneja et al 2011;Firnstein & Przybilla 2012;McEvoy et al 2015;Maeder et al 2014), through the number ratios of BSG, RSG, and MS stars (Dohm-Palmer & Skillman 2002;Eggenberger et al 2002), or through study of their pulsational properties (Saio et al 2013).…”

“…In recent years, this new method has been successfully applied to galaxies out to a distance of 7 Mpc (Urbaneja et al 2008;U et al 2009;Kudritzki et al 2012Kudritzki et al , 2013Kudritzki et al , 2014Hosek et al 2014).…”