Metal-Poor Stars at the Heart of the Milky Way

Motivation and Goals

When exploring the oldest stars in the Milky Way, there are two major components that are well-studied:

the old disk (aka the thick disk, or alpha-enhanced disk). Our recent exploration has found that we can trace stars back to ages of 13 Gyrs, but that the [M/H] distribution extends from [M/H] ~ -1 for the oldest stars, to [M/H] ~ 0 for the youngest. Which stars are responsible to enhance the gas of the old disk to [M/H] = -1 is not known:
the stellar halo, which has been parsed by recent Gaia- and/or spectroscopy enabled analyses into a set of components of components that reflect the ancient merger history. Their age - [M/H] distribution and abundance patterns generally imply an ex situ origin. The most prominent component dominating the inner halo (<15 kpc) is the GSE ("Gaia-Sausage-Enceladus"), stars on radial orbits with apo-centers ranging from 10kpc to 100 kpc, GSE stars reflect a merger with the proto-Milky Way about 11 kpc ago.
Age-[Fe/H] distribution from Xiang&Rix2022, showing that in samples at the soral radius, the old disk stats at 13 Gyrs and [Fe/H] ~ -1; the (ex-situ?) halo stars have a different enrichment track.

However, we know little about ancient, metal poor stars (say, [M/H] < -1) at the heart of the Milky Way, say, within 1-5 kpc of the Galactic Center. We should expect stars in the heart of the MW to be a mix of

in-situ stars that reflect this earliest star formation in the MW's main potential well. Why "earliest"? Because it presumably did not take long at the heart of the MW to enrich birth gas to [M/H] >= -1. Such stars are very much expected to exist from cosmological simulations. Whether they would be best dubbed as "innermost halo", or as "innermost old/thick disk" would depend on their angular momentum and terminology preferences.
stars that spend much of their life at far greater Galactocentric radii, but are currently passing through the center. I.e. we should expect GSE stars near their peri-centers.

Selection Approach

Much of past research has focussed on finding halo stars away from the Galactic disk, as this mitigates the detrimental effects of dust extinction and of the sample contamination by more metal-rich stars. [Notable exceptions: early efforts with the sky mapper.]

Here we want to explore whether the Gaia DR3 BP/RP spectra afford a clean and efficient selection of metal-poor stars in the inner galaxy. We do this by selecting RGB/RC stars towards the Galactic center (+-30 degrees) and deriving data-driven revised [M/H] estimates by

deriving narrow-band filter fluxes (Stromgren and similar; a la "the Galaxy in your favourite colors") -- Vedant Chandra
deriving [M/H] estimates via xgboost in training with the APOGEE DR17 data set .

The initial query was:

SELECT source_id,ra,dec,phot_g_mean_mag,phot_bp_mean_mag,bp_rp,parallax,parallax_error

FROM gaiadr3.gaia_source

WHERE

parallax < 100.*power(10.,0.2*(0.9 - (phot_g_mean_mag - 1.5*(bp_rp-1.))))

and

parallax < 1.

and

abs(b)<30 and (l<30 or l > 330.)

and

bp_rp between 1.0 and 3.5

and

phot_bp_mean_mag < 15.5

where the 1st condition selects for stars with M(de-reddened) < 0.9, i.e. eliminates the MS; the color cut is designed to eliminate stars bluer than the blue edge of the unreddened, metal-poor RC; and -- importantly -- phot_bp_mean_mag < 15.5 implies that the BP/Rp spectra should have good S/N in the blue. This query returns 2.1M stars.

The features used in the [M/H] estimate by xgboost are <...> and the training set is the full APOGEE DR17 RC/RGB set.

The distribution of these stars in the B-R and Stromgren-m1 space is shown below, where the "streak" in the bottom-left corner is reddened hot stars, that we excise before training (they are not well represented in the training set).

Verification

The upshot is that it looks like we can select low-[M/H] samples cleanly and effectively. Here are a few plots that illustrate this:

Cross-/Self-Validation of [M/H] estimates (X-axis) against presumed ground-truth (APOGEE; Y-axis). The figure shows that a) for [M/H] > -0.9 one gets robust and precise [M/H] estimates (whose precision could presumably be improved). More pertinent for the questions at hand is that the selection below [M/H] = -0.9 works robustly, albeit with larger scatter. But stars stars selected to have [M/H] < -1 are at [M/H] < -1.

Resulting [M/H]-distribution of the overall 2.1M star sample of the "inner galaxy". This deserves more analysis, but the metal-poor end looks very plausible, and not swamped by false positives. In particular there are 10x fewer stars at [M/H] = -2 than at [M/H] = -1, as expected for naive closed-box models.

The next 3 panels show the on-sky distribution of the metal-rich sample ([M/H] > -0.4), an intermediate sample ( -0.9 < [M/H] < -0.4 ), and the metal-poor sample ( [M/H] < -0.9). The latter sample is (in projection) very much concentrated towards the Galactic center.

What gives credence to the fact that our metal-poor selection picks up stars in the innermost galaxy is the parallax distribution: the metal-poor stars (green) are peaked at 8.2 kpc (blue line0; the metal-rich stars (blue) are seen throughout the disk; intermediate [M/H] stars are in orange.

HWR note to self; the flags were set wrong for the plots; the sample is too small.

Results: what do we see?

The first result is indeed the previous figure: there is a population of (~20k) stars that are metal-poor ([M/H] < 0.9) and very centrally concentrated: there is a metal-poor (and presumably ancient) heart of the MW.

That immediately brings up the question of "on what orbits do these low-[M/H] stars move"? Are they confined to the inner MW, or just passing through.

In general about 3/4 of the sample have RVS velocities. The low-[M/H] sample is in the most crowded part and has only 1/3 RVS velocities. Fron those (Price-Whelan) we calculate orbits.

[Note distance uncertainties are tricky, and deserve attention]

This is what the apocenter -- eccentricity plane looks like:

The plot above excites HWR: the vast majority of stars have a broad eccentricity distribution from 0.2 to 0., and remain confined to the inner ~5 kpc. They are not just passing through. At high eccentricities there is a tail of stars with apocenters of 10kpc to >100 kpc, as expected e.g. for members of GSE, "just passing through" the center. [Need to check ho much of that is in the literature]

{Selection effects matter}

This plot overlays in orange the [M/H]-distribution of all stars on highly eccentric orbits (ecc>0.9). One can think of them as kinematically-selected GSE members. If they were GSE members, the metal rich tail would seem puzzling. What's going on?

It gets potentially more interesting if we plot for the highly eccentric orbits, ecc>0.85, the distribution [M/H] vs r_{apo}:

One sees:

One interpretation is: at r_apo > 10kpc the [M/H]-distribution is quite GSE-like; for tightly bound orbits the [M/H] is higher: somet6hing different, or the more-metal-rich ex-core of GSE?