The chemical properties of the Milky Way’s on-bar and off-bar regions: evidence for inhomogeneous star formation history in the bulge. (arXiv:2008.03320v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Lian1_J/0/1/0/all/0/1">Jianhui Lian1</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zasowski_G/0/1/0/all/0/1">Gail Zasowski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hasselquist_S/0/1/0/all/0/1">Sten Hasselquist</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Neumann_J/0/1/0/all/0/1">Justus Neumann</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Majewski_S/0/1/0/all/0/1">Steven R. Majewski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cohen_R/0/1/0/all/0/1">Roger E. Cohen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fernandez_Trincado_J/0/1/0/all/0/1">José G. Fernández-Trincado</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lane_R/0/1/0/all/0/1">Richard R. Lane</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Longa_Pena_P/0/1/0/all/0/1">Peélope Longa-Peña</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Roman_Lopes_A/0/1/0/all/0/1">Alexandre Roman-Lopes</a>
Numerous studies of integrated starlight, stellar counts, and kinematics have
confirmed that the Milky Way is a barred galaxy. However, far fewer studies
have investigated the bar’s stellar population properties, which carry valuable
independent information regarding the bar’s formation history. Here we conduct
a detailed analysis of chemical abundance distributions ([Fe/H] and [Mg/Fe]) in
the on-bar and off-bar regions to study the azimuthal variation of star
formation history (SFH) in the inner Galaxy. We find that the on-bar and
off-bar stars at Galactocentric radii 3 $< r_{rm GC}<$ 5 kpc have remarkably
consistent [Fe/H] and [Mg/Fe] distribution functions and [Mg/Fe]–[Fe/H]
relation, suggesting a common SFH shared by the long bar and the disc. In
contrast, the bar and disc at smaller radii (2 $< r_{rm GC} <$ 3 kpc) show
noticeable differences, with relatively more very metal-rich ([Fe/H]~0.4) stars
but fewer solar abundance stars in the bar. Given the three-phase star
formation history proposed for the inner Galaxy in Lian et al. (2020b), these
differences could be explained by the off-bar disc having experienced either a
faster early quenching process or recent metal-poor gas accretion. Vertical
variations of the abundance distributions at small $r_{rm GC}$ suggest a wider
vertical distribution of low-$alpha$ stars in the bar, which may serve as
chemical evidence for vertical heating through the bar buckling process. The
lack of such vertical variations outside the bulge may then suggest a lack of
vertical heating in the long bar.
Numerous studies of integrated starlight, stellar counts, and kinematics have
confirmed that the Milky Way is a barred galaxy. However, far fewer studies
have investigated the bar’s stellar population properties, which carry valuable
independent information regarding the bar’s formation history. Here we conduct
a detailed analysis of chemical abundance distributions ([Fe/H] and [Mg/Fe]) in
the on-bar and off-bar regions to study the azimuthal variation of star
formation history (SFH) in the inner Galaxy. We find that the on-bar and
off-bar stars at Galactocentric radii 3 $< r_{rm GC}<$ 5 kpc have remarkably
consistent [Fe/H] and [Mg/Fe] distribution functions and [Mg/Fe]–[Fe/H]
relation, suggesting a common SFH shared by the long bar and the disc. In
contrast, the bar and disc at smaller radii (2 $< r_{rm GC} <$ 3 kpc) show
noticeable differences, with relatively more very metal-rich ([Fe/H]~0.4) stars
but fewer solar abundance stars in the bar. Given the three-phase star
formation history proposed for the inner Galaxy in Lian et al. (2020b), these
differences could be explained by the off-bar disc having experienced either a
faster early quenching process or recent metal-poor gas accretion. Vertical
variations of the abundance distributions at small $r_{rm GC}$ suggest a wider
vertical distribution of low-$alpha$ stars in the bar, which may serve as
chemical evidence for vertical heating through the bar buckling process. The
lack of such vertical variations outside the bulge may then suggest a lack of
vertical heating in the long bar.
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