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Chemical Evolution of the Inner 2 Degrees of the Milky Way Bulge: [α/Fe] Trends and Metallicity Gradients

Ryde, Nils LU ; Schultheis, M.; Grieco, V.; Matteucci, F.; Rich, R. M. and Uttenthaler, S. (2016) In Astronomical Journal 151(1).
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 alpha-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 [alpha/Fe] element trends versus metallicity, and by trying to derive the metallicity gradient in the b < 2 degrees region. [alpha/Fe] and metallicities have been... (More)
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 alpha-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 [alpha/Fe] element trends versus metallicity, and by trying to derive the metallicity gradient in the b < 2 degrees region. [alpha/Fe] and metallicities have been determined by spectral synthesis of 2 mu m spectra of 28 M-giants in the Bulge, lying along the southern minor axis at (l, b) = (0, 0), (0, -1 degrees), and (0, -2 degrees). 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 [alpha/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 [alpha/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. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Galaxy: bulge, Galaxy: stellar content, Galaxy: structure, infrared:, stars, stars: abundances, stars: fundamental parameters
in
Astronomical Journal
volume
151
issue
1
publisher
The American Astronomical Society
external identifiers
  • wos:000368250900001
  • scopus:84954478220
ISSN
1538-3881
DOI
10.3847/0004-6256/151/1/1
language
English
LU publication?
yes
id
d32a96a7-655a-4e64-bf8b-1648d4948c81 (old id 8747889)
date added to LUP
2016-02-23 10:29:20
date last changed
2017-10-01 03:06:29
@article{d32a96a7-655a-4e64-bf8b-1648d4948c81,
  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 alpha-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 [alpha/Fe] element trends versus metallicity, and by trying to derive the metallicity gradient in the b &lt; 2 degrees region. [alpha/Fe] and metallicities have been determined by spectral synthesis of 2 mu m spectra of 28 M-giants in the Bulge, lying along the southern minor axis at (l, b) = (0, 0), (0, -1 degrees), and (0, -2 degrees). 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 [alpha/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 [alpha/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 &lt; [Fe/H] &lt; +0.3, with a lower dispersion in the GC: -0.2 &lt; [Fe/H] &lt; +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.},
  articleno    = {1},
  author       = {Ryde, Nils and Schultheis, M. and Grieco, V. and Matteucci, F. and Rich, R. M. and Uttenthaler, S.},
  issn         = {1538-3881},
  keyword      = {Galaxy: bulge,Galaxy: stellar content,Galaxy: structure,infrared:,stars,stars: abundances,stars: fundamental parameters},
  language     = {eng},
  number       = {1},
  publisher    = {The American Astronomical Society},
  series       = {Astronomical Journal},
  title        = {Chemical Evolution of the Inner 2 Degrees of the Milky Way Bulge: [α/Fe] Trends and Metallicity Gradients},
  url          = {http://dx.doi.org/10.3847/0004-6256/151/1/1},
  volume       = {151},
  year         = {2016},
}