arXiv subjects

Andreia Carrillo

Publications and source records attributed to Andreia Carrillo.

At least 19 recordsLinked to original sources

arXiv2026

The Milky Way's Missing In-Situ Halo

The Milky Way's stellar halo preserves a record of its mass assembly history, encoding accretion events in its structural properties. Among the most prominent of these features is a strong break in the halo density profile at $\approx$20-30 kpc, long attributed to the apocenter pile-up of stars from the Gaia-Enceladus/Sausage merger. However, whether this interpretation is consistent with state-of-the-art cosmological simulations remains unclear. In this work, we compare the Milky Way's measured stellar halo density profile between galactocentric radii of $1\unicode{x2013}100$ kpc to those of Milky Way-mass galaxies from the FIRE-2 and Auriga cosmological zoom-in simulation suites, spanning a total of 24 simulated galaxies. We find that simulated halo profiles are significantly steeper than Milky Way measurements within $\approx$15 kpc, and that profile breaks are rare in simulations and never as strong as the Milky Way's. Of the three galaxies with statistically significant breaks, only one exhibits a break attributable to apocenter pile-up of an accreted merger remnant. Decomposing the simulated profiles, we find that the accreted halo profiles are broadly consistent with Milky Way measurements, while the in-situ halo dominates within $\approx$20 kpc and drives the discrepancy. These results raise a compelling question: where is the Milky Way's in-situ halo? The origin of this tension may reflect systematic biases in current halo measurements, an overproduction of early spheroidal star formation in cosmological simulations, or more fundamental differences in the underlying physics (e.g., dark matter) governing the assembly and structure of the inner Galaxy.

astro-ph.GA↗

arXiv2026

Stirring Things Up: Bar-induced substructures in the stellar halo of a cosmological Milky Way analogue

The stellar halo of the Milky Way contains the remnants of past accretion events, which could be detectable as substructures in the classical integrals of motion space, such as energy and angular momentum (E-Lz). However, our galaxy also contains a non-axisymmetric stellar bar, which traps stars in resonant orbits, leading to substructures in phase-space. Using a high-resolution magneto-hydrodynamic cosmological zoom-in simulation of a Milky Way analogue, we explore the connection between the bar and the accreted stellar halo. We find that the bar induces prominent substructures, or "ridges", in E-Lz, caused by the resonances. The most pronounced of these is caused by the corotation and the retrograde 1:1 resonances, with weaker ridges visible due to the prograde 1:1 and outer Lindblad resonance. The ridges are present across much of the stellar halo, with variations in radius due to the morphology of different orbital families. We explore the scattering of orbits at the resonances, finding that stars trapped at the 1:1 retrograde resonance become more circularised and have more negative angular momentum. Additionally, stars can move between the corotation and retrograde 1:1 families, thus alternating between prograde and retrograde motion. Due to these scatterings and the pre-existing metallicity gradients in the accreted population, the bar-induced substructures have distinct metallicities compared to stars in the surrounding phase-space. Our results suggest the need for caution when searching the Milky Way stellar halo for accreted substructures in both integral of motions and chemical spaces, since these can be induced by internal perturbations.

astro-ph.GA↗

arXiv2025

From order to chaos: the blurred out metallicity gradient of the Gaia-Enceladus/Sausage progenitor

The powerful combination of Gaia with other Milky Way large survey data has ushered in a deeper understanding of the assembly history of our Galaxy, which is marked by the accretion of Gaia-Enceladus/Sausage (GES). As a step towards reconstructing this significant merger, we examine the existence and destruction of its stellar metallicity gradient. We investigate 8 GES-like progenitors from the Auriga simulations and find that all have negative metallicity gradients at infall with a range of -0.09 to -0.03 dex/kpc against radius and -1.99 to -0.41 dex/$\rm 10^{-5} km^{2}s^{-2}$ against the stellar orbital energy. These gradients get blurred and become shallower when measured at $z=0$ in the Milky Way-like host. The percentage change in the radial metallicity gradient is consistently high (78-98\%), while the percentage change in the energy space varies much more (9-91\%). We also find that the most massive progenitors show the smallest changes in their energy metallicity gradients. At the same present-day galactocentric radius, lower metallicity stars originate from the outskirts of the GES progenitor. Similarly, at fixed metallicity, stars at higher galactocentric radii tend to originate from the GES outskirts. We find that the GES stellar mass, total mass, infall time, and the present-day Milky Way total mass are correlated with the percentage change in metallicity gradient, both in radius and in energy space. It is therefore vital to constrain these properties further to pin down the infall metallicity gradient of the GES progenitor and understand the onset of such ordered chemistry at cosmic noon.

astro-ph.GA↗

arXiv2025

Nearby stellar substructures in the Galactic halo from DESI Milky Way Survey Year 1 Data Release

We report five nearby ($d_{\mathrm{helio}} < 5$ kpc) stellar substructures in the Galactic halo from a subset of 138,661 stars in the Dark Energy Spectroscopic Instrument (DESI) Milky Way Survey Year 1 Data Release. With an unsupervised clustering algorithm, HDBSCAN*, these substructures are independently identified in Integrals of Motion ($E_{\mathrm{tot}}$, $L_{\mathrm z}$, $\log{J_r}$, $\log{J_z}$) space and Galactocentric cylindrical velocity space ($V_{R}$, $V_{\phi}$, $V_{z}$). We associate all identified clusters with known nearby substructures (Helmi streams, M18-Cand10/MMH-1, Sequoia, Antaeus, and ED-2) previously reported in various studies. With metallicities precisely measured by DESI, we confirm that the Helmi streams, M18-Cand10, and ED-2 are chemically distinct from local halo stars. We have characterised the chemodynamic properties of each dynamic group, including their metallicity dispersions, to associate them with their progenitor types (globular cluster or dwarf galaxy). Our approach for searching substructures with HDBSCAN* reliably detects real substructures in the Galactic halo, suggesting that applying the same method can lead to the discovery of new substructures in future DESI data. With more stars from future DESI data releases and improved astrometry from the upcoming Gaia Data Release 4, we will have a more detailed blueprint of the Galactic halo, offering a significant improvement in our understanding of the formation and evolutionary history of the Milky Way Galaxy.

astro-ph.GA↗

arXiv2025

The Cocytos Stream: A Disrupted Globular Cluster from our Last Major Merger?

The census of stellar streams and dwarf galaxies in the Milky Way provides direct constraints on galaxy formation models and the nature of dark matter. The DESI Milky Way survey (with a footprint of 14,000$~deg{^2}$ and a depth of $r<19$ mag) delivers the largest sample of distant metal-poor stars compared to previous optical fiber-fed spectroscopic surveys. This makes DESI an ideal survey to search for previously undetected streams and dwarf galaxies. We present a detailed characterization of the Cocytos stream, which was re-discovered using a clustering analysis with a catalog of giants in the DESI year 3 data, supplemented with Magellan/MagE spectroscopy. Our analysis reveals a relatively metal-rich ([Fe/H]$=-1.3$) and thick stream (width$=1.5^\circ$) at a heliocentric distance of $\approx 25$ kpc, with an internal velocity dispersion of 6.5-9 km s$^{-1}$. The stream's metallicity, radial orbit, and proximity to the Virgo stellar overdensities suggest that it is most likely a disrupted globular cluster that came in with the Gaia-Enceladus merger. We also confirm its association with the Pyxis globular cluster. Our result showcases the ability of wide-field spectroscopic surveys to kinematically discover faint disrupted dwarfs and clusters, enabling constraints on the dark matter distribution in the Milky Way.

astro-ph.GA↗

arXiv2024

Exploring the interaction between the MW and LMC with a large sample of blue horizontal branch stars from the DESI survey

The Large Magellanic Cloud (LMC) is a Milky Way (MW) satellite that is massive enough to gravitationally attract the MW disc and inner halo, causing significant motion of the inner MW with respect to the outer halo. In this work, we probe this interaction by constructing a sample of 9,866 blue horizontal branch (BHB) stars with radial velocities from the DESI spectroscopic survey out to 120 kpc from the Galactic centre. This is the largest spectroscopic set of BHB stars in the literature to date, and it contains four times more stars with Galactocentric distances beyond 50 kpc than previous BHB catalogues. Using the DESI BHB sample combined with SDSS BHBs, we measure the bulk radial velocity of stars in the outer halo and observe that the velocity in the Southern Galactic hemisphere is different by 3.7$\sigma$ from the North. Modelling the projected velocity field shows that its dipole component is directed at a point 22 degrees away from the LMC along its orbit, which we interpret as the travel direction of the inner MW. The velocity field includes a monopole term that is -24 km/s, which we refer to as compression velocity. This velocity is significantly larger than predicted by the current models of the MW and LMC interaction. This work uses DESI data from its first two years of observations, but we expect that with upcoming DESI data releases, the sample of BHB stars will increase and our ability to measure the MW-LMC interaction will improve significantly.

astro-ph.GA↗

arXiv2024

The frequency of metal-enrichment of cool helium-atmosphere white dwarfs using the DESI Early Data Release

There is overwhelming evidence that white dwarfs host planetary systems; revealed by the presence, disruption, and accretion of planetary bodies. A lower limit on the frequency of white dwarfs that host planetary material has been estimated to be roughly 25-50 per cent; inferred from the ongoing or recent accretion of metals onto both hydrogen-atmosphere and warm helium-atmosphere white dwarfs. Now with the unbiased sample of white dwarfs observed by the Dark Energy Spectroscopic Instrument (DESI) survey in their Early Data Release (EDR), we have determined the frequency of metal-enrichment around cool-helium atmosphere white dwarfs as 21 $\pm$ 3 per cent using a sample of 234 systems. This value is in good agreement with values determined from previous studies. With the current samples we cannot distinguish whether the frequency of planetary accretion varies with system age or host-star mass, but the DESI data release 1 will contain roughly an order of magnitude more white dwarfs than DESI EDR and will allow these parameters to be investigated.

astro-ph.EP↗

arXiv2023

The individual abundance distributions of disc stars across birth radii in GALAH

Individual abundances in the Milky Way disc record stellar birth properties (e.g. age, birth radius ($R_{\rm birth}$)) and capture the diversity of the star-forming environments over time. Assuming an analytical relationship between ([Fe/H], [$α$/Fe]) and $R_{\rm birth}$, we examine the distributions of individual abundances [X/Fe] of elements C, O, Mg, Si, Ca ($α$), Al (odd-z), Mn (iron-peak), Y, and Ba (neutron-capture) for stars in the Milky Way. We want to understand how these elements might differentiate environments across the disc. We assign tracks of $R_{\rm birth}$ in the [$α$/Fe] vs. [Fe/H] plane as informed by expectations from simulations for $\sim 59,000$ GALAH stars in the solar neighborhood ($R\sim7-9$ kpc) which also have inferred ages. Our formalism for $R_{\rm birth}$ shows that older stars ($\sim$10 Gyrs) have a $R_{\rm birth}$ distribution with smaller mean values (i.e., $\bar{R}_{\mbox{birth}}$$\sim5\pm0.8$ kpc) compared to younger stars ($\sim6$ Gyrs; $\bar{R}_{\mbox{birth}}$$\sim10\pm1.5$ kpc), for a given [Fe/H], consistent with inside-out growth. The $α$-, odd-z, and iron-peak element abundances decrease as a function of $R_{\rm birth}$, whereas the neutron-capture abundances increase. The $R_{\rm birth}$-[Fe/H] gradient we measure is steeper compared to the present-day gradient (-0.067 dex/kpc vs -0.058 dex/kpc), which we also find true for $R_{\rm birth}$-[X/Fe] gradients. These results (i) showcase the feasibility of relating the birth radius of stars to their element abundances, (ii) the abundance gradients across $R_{\rm birth}$ are steeper than those over current radius, and (iii) offer an observational comparison to expectations on element abundance distributions from hydrodynamical simulations.

astro-ph.GA↗

arXiv2023

NANCY: Next-generation All-sky Near-infrared Community surveY

The Nancy Grace Roman Space Telescope is capable of delivering an unprecedented all-sky, high-spatial resolution, multi-epoch infrared map to the astronomical community. This opportunity arises in the midst of numerous ground- and space-based surveys that will provide extensive spectroscopy and imaging together covering the entire sky (such as Rubin/LSST, Euclid, UNIONS, SPHEREx, DESI, SDSS-V, GALAH, 4MOST, WEAVE, MOONS, PFS, UVEX, NEO Surveyor, etc.). Roman can uniquely provide uniform high-spatial-resolution (~0.1 arcsec) imaging over the entire sky, vastly expanding the science reach and precision of all of these near-term and future surveys. This imaging will not only enhance other surveys, but also facilitate completely new science. By imaging the full sky over two epochs, Roman can measure the proper motions for stars across the entire Milky Way, probing 100 times fainter than Gaia out to the very edge of the Galaxy. Here, we propose NANCY: a completely public, all-sky survey that will create a high-value legacy dataset benefiting innumerable ongoing and forthcoming studies of the universe. NANCY is a pure expression of Roman's potential: it images the entire sky, at high spatial resolution, in a broad infrared bandpass that collects as many photons as possible. The majority of all ongoing astronomical surveys would benefit from incorporating observations of NANCY into their analyses, whether these surveys focus on nearby stars, the Milky Way, near-field cosmology, or the broader universe.

astro-ph.IM↗

arXiv2023

Can we really pick and choose? Benchmarking various selections of Gaia Enceladus/Sausage stars in observations with simulations

Large spectroscopic surveys plus Gaia astrometry have shown us that the inner stellar halo of the Galaxy is dominated by the debris of Gaia Enceladus/Sausage (GES). With the richness of data at hand, there are a myriad of ways these accreted stars have been selected. We investigate these GES selections and their effects on the inferred progenitor properties using data constructed from APOGEE and Gaia. We explore selections made in eccentricity, energy-angular momentum (E-Lz), radial action-angular momentum (Jr-Lz), action diamond, and [Mg/Mn]-[Al/Fe] in the observations, selecting between 144 and 1,279 GES stars with varying contamination from in-situ and other accreted stars. We also use the Auriga cosmological hydrodynamic simulations to benchmark the different GES dynamical selections. Applying the same observational GES cuts to nine Auriga galaxies with a GES, we find that the Jr-Lz method is best for sample purity and the eccentricity method for completeness. Given the average metallicity of GES (-1.28 < [Fe/H] < -1.18), we use the $z=0$ mass-metallicity relationship to find an average $\rm M_{\star}$ of $\sim 4 \times 10^{8}$ $\rm M_{\odot}$. We adopt a similar procedure and derive $\rm M_{\star}$ for the GES-like systems in Auriga and find that the eccentricity method overestimates the true $\rm M_{\star}$ by $\sim2.6\times$ while E-Lz underestimates by $\sim0.7\times$. Lastly, we estimate the total mass of GES to be $\rm 10^{10.5 - 11.1}~M_{\odot}$ using the relationship between the metallicity gradient and the GES-to-in-situ energy ratio. In the end, we cannot just `pick and choose' how we select GES stars, and instead should be motivated by the science question.

astro-ph.GA↗

arXiv2023

Overview of the DESI Milky Way Survey

We describe the Milky Way Survey (MWS) that will be undertaken with the Dark Energy Spectroscopic Instrument (DESI) on the Mayall 4m telescope at the Kitt Peak National Observatory. Over the next 5 yr DESI MWS will observe approximately seven million stars at Galactic latitudes |b|>20 degrees, with an inclusive target selection scheme focused on the thick disk and stellar halo. MWS will also include several high-completeness samples of rare stellar types, including white dwarfs, low-mass stars within 100pc of the Sun, and horizontal branch stars. We summarize the potential of DESI to advance understanding of Galactic structure and stellar evolution. We introduce the final definitions of the main MWS target classes and estimate the number of stars in each class that will be observed. We describe our pipelines for deriving radial velocities, atmospheric parameters, and chemical abundances. We use ~500,000 spectra of unique stellar targets from the DESI Survey Validation program (SV) to demonstrate that our pipelines can measure radial velocities to ~1 km/s and [Fe/H] accurate to ~0.2 dex for typical stars in our main sample. We find the stellar parameter distributions from ~100 sq. deg of SV observations with >90% completeness on our main sample are in good agreement with expectations from mock catalogs and previous surveys.

astro-ph.GA↗

arXiv2022

Snowmass2021 Cosmic Frontier White Paper: Prospects for obtaining Dark Matter Constraints with DESI

Despite efforts over several decades, direct-detection experiments have not yet led to the discovery of the dark matter (DM) particle. This has led to increasing interest in alternatives to the Lambda CDM (LCDM) paradigm and alternative DM scenarios (including fuzzy DM, warm DM, self-interacting DM, etc.). In many of these scenarios, DM particles cannot be detected directly and constraints on their properties can ONLY be arrived at using astrophysical observations. The Dark Energy Spectroscopic Instrument (DESI) is currently one of the most powerful instruments for wide-field surveys. The synergy of DESI with ESA's Gaia satellite and future observing facilities will yield datasets of unprecedented size and coverage that will enable constraints on DM over a wide range of physical and mass scales and across redshifts. DESI will obtain spectra of the Lyman-alpha forest out to z~5 by detecting about 1 million QSO spectra that will put constraints on clustering of the low-density intergalactic gas and DM halos at high redshift. DESI will obtain radial velocities of 10 million stars in the Milky Way (MW) and Local Group satellites enabling us to constrain their global DM distributions, as well as the DM distribution on smaller scales. The paradigm of cosmological structure formation has been extensively tested with simulations. However, the majority of simulations to date have focused on collisionless CDM. Simulations with alternatives to CDM have recently been gaining ground but are still in their infancy. While there are numerous publicly available large-box and zoom-in simulations in the LCDM framework, there are no comparable publicly available WDM, SIDM, FDM simulations. DOE support for a public simulation suite will enable a more cohesive community effort to compare observations from DESI (and other surveys) with numerical predictions and will greatly impact DM science.

astro-ph.CO↗

arXiv2022

The relationship between age, metallicity, and abundances for disk stars in a simulated Milky Way galaxy

Observations of the Milky Way's low-$α$ disk show that at fixed metallicity, [Fe/H], several element abundance, [X/Fe], correlate with age, with unique slopes and small scatters around the age-[X/Fe] relations. In this study, we turn to simulations to explore the age-[X/Fe] relations for the elements C, N, O, Mg, Si, S, and Ca that are traced in a FIRE-2 cosmological zoom-in simulation of a Milky Way-like galaxy, m12i, and understand what physical conditions give rise to the observed age-[X/Fe] trends. We first explore the distributions of mono-age populations in their birth and current locations, [Fe/H], and [X/Fe], and find evidence for inside-out radial growth for stars with ages < 7 Gyr. We then examine the age-[X/Fe] relations across m12i's disk and find that the direction of the trends agree with observations, apart from C, O, and Ca, with remarkably small intrinsic scatters, $σ_{int}$ (0.01-0.04 dex). This $σ_{int}$ measured in the simulations is also metallicity-dependent, with $σ_{int}$ $\approx$ 0.025 dex at [Fe/H]=-0.25 dex versus $σ_{int}$ $\approx$ 0.015 dex at [Fe/H]=0 dex, and a similar metallicity dependence is seen in the GALAH survey for the elements in common. Additionally, we find that $σ_{int}$ is higher in the inner galaxy, where stars are older and formed in less chemically-homogeneous environments. The age-[X/Fe] relations and the small scatter around them indicate that simulations capture similar chemical enrichment variance as observed in the Milky Way, arising from stars sharing similar element abundances at a given birth place and time.

astro-ph.GA↗

arXiv2022

The detailed chemical abundance patterns of accreted halo stars from the optical to infrared

Understanding the assembly of our Galaxy requires us to also characterize the systems that helped build it. In this work, we accomplish this by exploring the chemistry of accreted halo stars from the Gaia-Enceladus/Gaia-Sausage (GES) selected in the infrared from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16. We use high resolution optical spectra for 62 GES stars to measure abundances in 20 elements spanning the $α$, Fe-peak, light, odd-Z, and notably, the neutron-capture groups of elements to understand their trends in the context of and in contrast to the Milky Way and other stellar populations. Using these derived abundances we find that the optical and the infrared abundances agree to within 0.15 dex except for O, Co, Na, Cu, and Ce. These stars have enhanced neutron-capture abundance trends compared to the Milky Way, and their [Eu/Mg] and neutron-capture abundance ratios (e.g., [Y/Eu], [Ba/Eu], [Zr/Ba], [La/Ba], and [Nd/Ba]) point to r-process enhancement and a delay in s-process enrichment. Their [$α$/Fe] trend is lower than the Milky Way trend for [Fe/H]$>$-1.5 dex, similar to previous studies of GES stars and consistent with the picture that these stars formed in a system with a lower rate of star formation. This is further supported by their depleted abundances in Ni, Na, and Cu abundances, again, similar to previous studies of low-$α$ stars with accreted origins.

astro-ph.SR↗

arXiv2021

The Stars of the HETDEX Survey. I. Radial Velocities and Metal-Poor Stars from Low-Resolution Stellar Spectra

The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) is an unbiased, massively multiplexed spectroscopic survey, designed to measure the expansion history of the universe through low-resolution ($R\sim750$) spectra of Lyman-Alpha Emitters. In its search for these galaxies, HETDEX will also observe a few 10$^{5}$ stars. In this paper, we present the first stellar value-added catalog within the internal second data release of the HETDEX Survey (HDR2). The new catalog contains 120,571 low-resolution spectra for 98,736 unique stars between $10 < G < 22$ spread across the HETDEX footprint at relatively high ($b\sim60^\circ$) Galactic latitudes. With these spectra, we measure radial velocities (RVs) for $\sim$42,000 unique FGK-type stars in the catalog and show that the HETDEX spectra are sufficient to constrain these RVs with a 1$σ$ precision of 28.0 km/s and bias of 3.5 km/s with respect to the LAMOST surveys and 1$σ$ precision of 27.5 km/s and bias of 14.0 km/s compared to the SEGUE survey. Since these RVs are for faint ($G\geq16$) stars, they will be complementary to Gaia. Using t-Distributed Stochastic Neighbor Embedding (t-SNE), we also demonstrate that the HETDEX spectra can be used to determine a star's T${\rm{eff}}$, and log g and its [Fe/H]. With the t-SNE projection of the FGK-type stars with HETDEX spectra we also identify 416 new candidate metal-poor ([Fe/H] $< -1$~dex) stars for future study. These encouraging results illustrate the utility of future low-resolution stellar spectroscopic surveys.

astro-ph.GA↗

arXiv2020

A Giant Planet Candidate Transiting a White Dwarf

Astronomers have discovered thousands of planets outside the solar system, most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star, but more distant planets can survive this phase and remain in orbit around the white dwarf. Some white dwarfs show evidence for rocky material floating in their atmospheres, in warm debris disks, or orbiting very closely, which has been interpreted as the debris of rocky planets that were scattered inward and tidally disrupted. Recently, the discovery of a gaseous debris disk with a composition similar to ice giant planets demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether the planets can survive the journey. So far, the detection of intact planets in close orbits around white dwarfs has remained elusive. Here, we report the discovery of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95% confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red-giant phase and shrinks due to friction. In this case, though, the low mass and relatively long orbital period of the planet candidate make common-envelope evolution less likely. Instead, the WD 1856+534 system seems to demonstrate that giant planets can be scattered into tight orbits without being tidally disrupted, and motivates searches for smaller transiting planets around white dwarfs.

astro-ph.EP↗

arXiv2020

The VIRUS-P Exploration of Nearby Galaxies (VENGA): The stellar populations and assembly of NGC 2903's bulge, bar, and outer disc

We study the stellar populations and assembly of the nearby spiral galaxy NGC 2903's bulge, bar, and outer disc using the VIRUS-P Exploration of Nearby Galaxies IFS survey. We observe NGC 2903 with a spatial resolution of 185 pc using the Mitchell Spectrograph's 4.25 arcsec fibres at the 2.7 Harlan J. Smith telescope. Bulge-bar-disc decomposition on the 2MASS Ks-band image of NGC 2903 shows that it has ~6%, 6%, and 88%, of its stellar mass in the bulge, bar, and outer disc, respectively, and its bulge has a low Sersic index of ~0.27, suggestive of a disky bulge. We perform stellar population synthesis and find that the outer disc has 46% of its mass in stars >5 Gyr, 48% in stars between 1 and 5 Gyr, and <10% in younger stars. Its stellar bar has 65% of its mass in ages 1-5 Gyr and has metallicities similar to the outer disc, suggestive of the evolutionary picture where the bar forms from disc material. Its bulge is mainly composed of old high-metallicity stars though it also has a small fraction of young stars. We find enhanced metallicity in the spiral arms and central region, tracing areas of high star formation as seen in the Halpha map. These results are consistent with the idea that galaxies of low bulge-to-total mass ratio and low bulge Sersic index like NGC 2903 has not had a recent major merger event, but has instead grown mostly through minor mergers and secular processes.

astro-ph.GA↗

arXiv2019

Identical or fraternal twins? : The chemical homogeneity of wide binaries from Gaia DR2

One of the high-level goals of Galactic archaeology is chemical tagging of stars across the Milky Way to piece together its assembly history. For this to work, stars born together must be uniquely chemically homogeneous. Wide binary systems are an important laboratory to test this underlying assumption. Here we present the detailed chemical abundance patterns of 50 stars across 25 wide binary systems comprised of main-sequence stars of similar spectral type identified in Gaia DR2 with the aim of quantifying their level of chemical homogeneity. Using high-resolution spectra obtained with McDonald Observatory, we derive stellar atmospheric parameters and precise detailed chemical abundances for light/odd-Z (Li, C, Na, Al, Sc, V, Cu), $α$ (Mg, Si, Ca), Fe-peak (Ti, Cr, Mn, Fe, Co, Ni, Zn), and neutron capture (Sr, Y, Zr, Ba, La, Nd, Eu) elements. Results indicate that 80% (20 pairs) of the systems are homogeneous in [Fe/H] at levels below 0.02 dex. These systems are also chemically homogeneous in all elemental abundances studied, with offsets and dispersions consistent with measurement uncertainties. We also find that wide binary systems are far more chemically homogeneous than random pairings of field stars of similar spectral type. These results indicate that wide binary systems tend to be chemically homogeneous but in some cases they can differ in their detailed elemental abundances at a level of [X/H] ~ 0.10 dex, overall implying chemical tagging in broad strokes can work.

astro-ph.SR↗