CHEMICAL EVOLUTION OF THE INNER 2 DEGREES OF THE MILKY WAY BULGE: [α/Fe] TRENDS AND METALLICITY GRADIENTS

Abstract

The structure, formation, and evolution of the Milky Way bulge is a matter of debate. Important diagnostics for discriminating between models of bulge formation and evolution include α-abundance trends with metallicity, and spatial abundance and metallicity gradients. Due to the severe optical extinction in the inner Bulge region, only a few detailed investigations of this region have been performed. Here we aim at investigating the inner 2 degrees of the Bulge (projected galactocentric distance of approximately 300 pc), rarely investigated before, by observing the [α/Fe] element trends versus metallicity, and by trying to derive the metallicity gradient in the b < 2°region.[α/Fe] and metallicities have been determined by spectral synthesis of 2 μm spectra of 28 M-giants in the Bulge, lying along the southern minor axis at (l, b) = (0, 0), (0, −1°), and (0, −2°). These were observed with the CRIRES spectrometer at the Very Large Telescope, (VLT) at high spectral resolution. Low-resolution K-band spectra, observed with the ISAAC spectrometer at the VLT, are used to determine the effective temperature of the stars. We present the first connection between the Galactic center (GC) and the Bulge using similar stars, high spectral resolution, and analysis techniques. The [α/Fe] trends in all our three fields show a large similarity among each other and with trends further out in the Bulge. All point to a rapid star formation episode in the Bulge. We find that there is a lack of an [α/Fe] gradient in the Bulge all the way into the center, suggesting a homogeneous Bulge when it comes to the enrichment process and star formation history. We find a large range of metallicities from −1.2 < [Fe/H] < +0.3, with a lower dispersion in the GC: −0.2 < [Fe/H] < +0.3. The derived metallicities of the stars in the three fields get, in the mean, progressively higher the closer to the Galactic plane they lie. We could interpret this as a continuation of the metallicity gradient established further out in the Bulge, but due to the low number of stars and possible selection effects, more data of the same sort as presented here is necessary to conclude on the inner metallicity gradient from our data alone. Our results firmly argue for the center being in the context of the Bulge rather than very distinct.

Tracing the evolution of the Galactic bulge with chemodynamical modelling of alpha-elements

“…A further important conclusion, reached by Johnson et al (2013), Ryde et al (2016) and McWilliam (2016), is that these alpha-elements show the same behaviour in all different parts of the Galactic bulge so far studied.…”

“…Gonzalez et al (2015) and Ryde et al (2016). d) As regards the IMF, as stated in Sect. 2, our adopted IMF assumes the usual Salpeter index x IMF = 1.35 and extends to 100 M , whereas some chemical evolution models by other authors assume a different IMF.…”

“…6 and 7 show published magnesium abundances from Gonzalez et al (2011), Johnson et al (2014 and Alves-Brito et al (2010) that revise the same stars as in Meléndez et al (2008) and Ryde et al (2016) for red giants, from Hill et al (2011) and Gonzalez et al (2015) for Red Clump (RC) stars and from Bensby et al (2013) for microlensed dwarfs. The data are compared with models with ν SF = 3, 1 and 0.5 Gyr −1 .…”

The Galactic Bulge

“…In particular, the abundances of alpha-elements Mg, Si, Ca, Ti, given by Alves-Brito et al (2010), Gonzalez et al (2011), Gonzalez et al (2015, Bensby et al (2013), Johnson et al (2014), and Ryde et al (2016), are enhanced in all fields, showing essentially no difference between different fields. The same applies to Oxygen but there is a larger spread among different authors.…”

Temperatures and metallicities of M giants in the Galactic bulge from low-resolutionK-band spectra

“…They were then able to obtain accurate detailed chemical abundances of nine M giant stars close to the Galactic centre using the CRIRES high-resolution spectrograph. This technique was extended to latitudes at b = −1 • and b = −2 • (Ryde et al 2016). Ramirez et al (1997) and Ramírez et al (2000) studied the behaviour of the 12 CO band head situated at 2.3 µm with low-resolution K-band spectra (R ∼ 2000−4000) and found for M giants a remarkably tight relation between the equivalent Based on observations collected at the European Southern Observatory, Chile, program number 089.B-0312B.…”