The birth of the Milky Way: the in-situ halo and early thick disk as uncovered by accurate stellar ages with Gaia. (arXiv:1901.02900v1 [astro-ph.GA])

The birth of the Milky Way: the in-situ halo and early thick disk as uncovered by accurate stellar ages with Gaia. (arXiv:1901.02900v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gallart_C/0/1/0/all/0/1">Carme Gallart</a> (1,2), <a href="http://arxiv.org/find/astro-ph/1/au:+Bernard_E/0/1/0/all/0/1">Edouard J. Bernard</a> (3), <a href="http://arxiv.org/find/astro-ph/1/au:+Brook_C/0/1/0/all/0/1">Chris B. Brook</a> (1,2), <a href="http://arxiv.org/find/astro-ph/1/au:+Ruiz_Lara_T/0/1/0/all/0/1">Tomas Ruiz-Lara</a> (1,2), <a href="http://arxiv.org/find/astro-ph/1/au:+Cassisi_S/0/1/0/all/0/1">Santi Cassisi</a> (4,5), <a href="http://arxiv.org/find/astro-ph/1/au:+Hill_V/0/1/0/all/0/1">Vanessa Hill</a> (3), <a href="http://arxiv.org/find/astro-ph/1/au:+Monelli_M/0/1/0/all/0/1">Matteo Monelli</a> (1,2) ((1) Instituto de Astrofisica de Canarias, Spain, (2) Departamento de Astrofisica, Universidad de La Laguna, Spain, (3) Observatoire de la Cote d&#x27;Azur, CNRS, France (4) INAF-Astronomical Observatory of Abruzzo, Italy, (5) INFN, Sezione di Pisa, Italy)

As Milky Way-like galaxies form, smaller stellar systems accrete onto the
main galactic progenitor, assembling a kinematically hot, spheroidal-like
stellar halo surrounding the galactic disc. There is extensive observational
evidence of this merging process both in the Milky Way and in external
galaxies. However, unambiguously identifying the oldest stellar population
formed in the Milky Way main progenitor has remained elusive, as a good
fraction of observed stars in our Galactic halo have characteristics pointing
to an external origin. In this paper, we unveil the old stellar population that
can be associated with the first star formation events in the Milky Way. We
identify this population with the red sequence of the enigmatic, double
sequenced colour-magnitude diagram (CMD) of the kinematically hot, halo stars
discovered in the second data release of the Gaia mission. This discovery
results from the novel, robust determination of stellar age distributions in
the thick disc and halo Gaia CMDs, using the well established technique of CMD
modelling, and the comparison with state-of-the-art cosmological simulations of
galaxy formation. The age distributions unambiguously show that the stars in
the blue and red sequences of the halo CMD are equally old, and older than the
bulk of thick disc stars. This evidence allows us to identify the red sequence
as the ‘in-situ’ halo stars, formed within the seed progenitor of our Milky
Way. The in-situ halo is naturally more metal rich than the accreted halo
population originating in the lower-mass galaxy Gaia-Enceladus whose stars form
most of the blue sequence. These findings provide a crystal clear picture of
the early sequence of events that gave rise to the complex Milky Way structure,
and have far reaching implications for understanding the formation of disc
galaxies in general.

As Milky Way-like galaxies form, smaller stellar systems accrete onto the
main galactic progenitor, assembling a kinematically hot, spheroidal-like
stellar halo surrounding the galactic disc. There is extensive observational
evidence of this merging process both in the Milky Way and in external
galaxies. However, unambiguously identifying the oldest stellar population
formed in the Milky Way main progenitor has remained elusive, as a good
fraction of observed stars in our Galactic halo have characteristics pointing
to an external origin. In this paper, we unveil the old stellar population that
can be associated with the first star formation events in the Milky Way. We
identify this population with the red sequence of the enigmatic, double
sequenced colour-magnitude diagram (CMD) of the kinematically hot, halo stars
discovered in the second data release of the Gaia mission. This discovery
results from the novel, robust determination of stellar age distributions in
the thick disc and halo Gaia CMDs, using the well established technique of CMD
modelling, and the comparison with state-of-the-art cosmological simulations of
galaxy formation. The age distributions unambiguously show that the stars in
the blue and red sequences of the halo CMD are equally old, and older than the
bulk of thick disc stars. This evidence allows us to identify the red sequence
as the ‘in-situ’ halo stars, formed within the seed progenitor of our Milky
Way. The in-situ halo is naturally more metal rich than the accreted halo
population originating in the lower-mass galaxy Gaia-Enceladus whose stars form
most of the blue sequence. These findings provide a crystal clear picture of
the early sequence of events that gave rise to the complex Milky Way structure,
and have far reaching implications for understanding the formation of disc
galaxies in general.

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