Stellar Metallicities and Elemental Abundance Ratios of z~1.4 Massive Quiescent Galaxies. (arXiv:1907.04327v1 [astro-ph.GA])

Stellar Metallicities and Elemental Abundance Ratios of z~1.4 Massive Quiescent Galaxies. (arXiv:1907.04327v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kriek_M/0/1/0/all/0/1">Mariska Kriek</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Price_S/0/1/0/all/0/1">Sedona H. Price</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Conroy_C/0/1/0/all/0/1">Charlie Conroy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Suess_K/0/1/0/all/0/1">Katherine Suess</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mowla_L/0/1/0/all/0/1">Lamiya Mowla</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pasha_I/0/1/0/all/0/1">Imad Pasha</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bezanson_R/0/1/0/all/0/1">Rachel Bezanson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dokkum_P/0/1/0/all/0/1">Pieter van Dokkum</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Barro_G/0/1/0/all/0/1">Guillermo Barro</a>

The chemical composition of galaxies has been measured out to z~4. However,
nearly all studies beyond z~0.7 are based on strong-line emission from HII
regions within star-forming galaxies. Measuring the chemical composition of
distant quiescent galaxies is extremely challenging, as the required stellar
absorption features are faint and shifted to near-infrared wavelengths. Here,
we present ultra-deep rest-frame optical spectra of five massive quiescent
galaxies at z~1.4, all of which show numerous stellar absorption lines. We
derive the abundance ratios [Mg/Fe] and [Fe/H] for three out of five galaxies;
the remaining two galaxies have too young luminosity-weighted ages to yield
robust measurements. Similar to lower-redshift findings, [Mg/Fe] appears
positively correlated with stellar mass, while [Fe/H] is approximately constant
with mass. These results may imply that the stellar mass-metallicity relation
was already in place at z~1.4. While the [Mg/Fe]-mass relation at z~1.4 is
consistent with the z<0.7 relation, [Fe/H] at z~1.4 is ~0.2 dex lower than at z<0.7. With a [Mg/Fe] of 0.44(+0.08,-0.07) the most massive galaxy may be more alpha-enhanced than similar-mass galaxies at lower redshift, but the offset is less significant than the [Mg/Fe] of 0.6 previously found for a massive galaxy at z=2.1. Nonetheless, these results combined may suggest that [Mg/Fe] in the most massive galaxies decreases over time, possibly by accreting low-mass, less alpha-enhanced galaxies. A larger galaxy sample is needed to confirm this scenario. Finally, the abundance ratios indicate short star-formation timescales of 0.2-1.0 Gyr.

The chemical composition of galaxies has been measured out to z~4. However,
nearly all studies beyond z~0.7 are based on strong-line emission from HII
regions within star-forming galaxies. Measuring the chemical composition of
distant quiescent galaxies is extremely challenging, as the required stellar
absorption features are faint and shifted to near-infrared wavelengths. Here,
we present ultra-deep rest-frame optical spectra of five massive quiescent
galaxies at z~1.4, all of which show numerous stellar absorption lines. We
derive the abundance ratios [Mg/Fe] and [Fe/H] for three out of five galaxies;
the remaining two galaxies have too young luminosity-weighted ages to yield
robust measurements. Similar to lower-redshift findings, [Mg/Fe] appears
positively correlated with stellar mass, while [Fe/H] is approximately constant
with mass. These results may imply that the stellar mass-metallicity relation
was already in place at z~1.4. While the [Mg/Fe]-mass relation at z~1.4 is
consistent with the z<0.7 relation, [Fe/H] at z~1.4 is ~0.2 dex lower than at
z<0.7. With a [Mg/Fe] of 0.44(+0.08,-0.07) the most massive galaxy may be more
alpha-enhanced than similar-mass galaxies at lower redshift, but the offset is
less significant than the [Mg/Fe] of 0.6 previously found for a massive galaxy
at z=2.1. Nonetheless, these results combined may suggest that [Mg/Fe] in the
most massive galaxies decreases over time, possibly by accreting low-mass, less
alpha-enhanced galaxies. A larger galaxy sample is needed to confirm this
scenario. Finally, the abundance ratios indicate short star-formation
timescales of 0.2-1.0 Gyr.

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