Publication Search Results

ID: 159424
Type: article
Authors: Bonaca, Ana; Naidu, Rohan P.; Conroy, Charlie; Caldwell, Nelson; Cargile, Phillip A.; Han, Jiwon Jesse; Johnson, Benjamin D.; Kruijssen, J. M. Diederik; Myeong, G. C.; Speagle, Joshua S.; Ting, Yuan-Sen; Zaritsky, Dennis
Abstract: The origins of most stellar streams in the Milky Way are unknown. With improved proper motions provided by Gaia EDR3, we show that the orbits of 23 Galactic stellar streams are highly clustered in orbital phase space. Based on their energies and angular momenta, most streams in our sample can plausibly be associated with a specific (disrupted) dwarf galaxy host that brought them into the Milky Way. For eight streams we also identify likely globular cluster progenitors (four of these associations are reported here for the first time). Some of these stream progenitors are surprisingly far apart, displaced from their tidal debris by a few to tens of degrees. We identify stellar streams that appear spatially distinct, but whose similar orbits indicate they likely originate from the same progenitor. If confirmed as physical discontinuities, they will provide strong constraints on the mass loss from the progenitor. The nearly universal ex situ origin of existing stellar streams makes them valuable tracers of galaxy mergers and dynamical friction within the Galactic halo. Their phase-space clustering can be leveraged to construct a precise global map of dark matter in the Milky Way, while their internal structure may hold clues to the small-scale structure of dark matter in their original host galaxies.

ID: 159615
Type: article
Authors: Carter, Courtney; Conroy, Charlie; Zaritsky, Dennis; Ting, Yuan-Sen; Bonaca, Ana; Naidu, Rohan P.; Johnson, Benjamin D.; Cargile, Phillip A.; Caldwell, Nelson; Speagle, Joshua; Han, Jiwon Jesse
Abstract: Ancient, very metal-poor (VMP) stars offer a window into the earliest epochs of galaxy formation and assembly. We combine data from the H3 Spectroscopic Survey and Gaia to measure metallicities, abundances of α elements, stellar ages, and orbital properties of a sample of 482 VMP ([Fe/H] 70% of VMP stars near the disk are on prograde orbits and this fraction increases toward lower metallicities. This result is unexpected if metal-poor stars are predominantly accreted from many small systems with no preferred orientation, as such a scenario would imply a mostly isotropic distribution. Furthermore, we find there is some evidence for higher fractions of prograde orbits among stars with lower [α/Fe]. Isochrone-based ages for main-sequence turn-off stars reveal that these VMP stars are uniformly old (≍12 Gyr) irrespective of the α abundance and metallicity, suggesting that the metal-poor population was not born from the same well-mixed gas disk. We speculate that the VMP population has a heterogeneous origin, including both in situ formation in the ancient disk and accretion from a satellite with the same direction of rotation as the ancient disk at early times. Our precisely measured ages for these VMP stars on prograde orbits show that the Galaxy has had a relatively quiescent merging history over most of cosmic time, and implies the angular momentum alignment of the Galaxy has been in place for at least 12 Gyr.

ID: 157675
Type: article
Authors: Bonaca, Ana; Conroy, Charlie; Cargile, Phillip A.; Naidu, Rohan P.; Johnson, Benjamin D.; Zaritsky, Dennis; Ting, Yuan-Sen; Caldwell, Nelson; Han, Jiwon Jesse; van Dokkum, Pieter
Abstract: The archeological record of stars in the Milky Way opens a uniquely detailed window into the early formation and assembly of galaxies. Here we use 11,000 main-sequence turn-off stars with well-measured ages, $[\mathrm{Fe}/{\rm{H}}]$ , $[\alpha /\mathrm{Fe}]$ , and orbits from the H3 Survey and Gaia to time the major events in the early Galaxy. Located beyond the Galactic plane, $1\lesssim | Z| /\mathrm{kpc}\lesssim 4$ , this sample contains three chemically distinct groups: a low-metallicity population, and low-α and high-α groups at higher metallicity. The age and orbit distributions of these populations show that (1) the high-α group, which includes both disk stars and the in situ halo, has a star formation history independent of eccentricity that abruptly truncated 8.3 ± 0.1 Gyr ago (z ≃ 1); (2) the low-metallicity population, which we identify as the accreted stellar halo, is on eccentric orbits and its star formation truncated $10.2{.}_{-0.1}^{+0.2}$ Gyr ago (z ≃ 2); (3) the low-α population is primarily on low-eccentricity orbits and the bulk of its stars formed less than 8 Gyr ago. These results suggest a scenario in which the Milky Way accreted a satellite galaxy at z ≍ 2 that merged with the early disk by z ≍ 1. This merger truncated star formation in the early high-α disk and perturbed a fraction of that disk onto halo-like orbits. The merger enabled the formation of a chemically distinct, low-α disk at z ≲ 1. The lack of any stars on halo-like orbits at younger ages indicates that this event was the last significant disturbance to the Milky Way disk.

ID: 157303
Type: article
Authors: Bonaca, Ana; Conroy, Charlie; Hogg, David W.; Cargile, Phillip A.; Caldwell, Nelson; Naidu, Rohan P.; Price-Whelan, Adrian M.; Speagle, Joshua S.; Johnson, Benjamin D.
Abstract: The 100° long thin stellar stream in the Milky Way halo, GD-1, has an ensemble of features that may be due to dynamical interactions. Using high-resolution MMT/Hectochelle spectroscopy we show that a spur of GD-1-like stars outside of the main stream are kinematically and chemically consistent with the main stream. In the spur, as in the main stream, GD-1 has a low intrinsic radial velocity dispersion, σ_{V_r} ≲ 1 km s^-1, is metal-poor, [Fe/H] ≈ -2.3, and has little intrinsic spread in the [Fe/H] and [α/Fe] abundances, which point to a common globular cluster progenitor. At a fixed location along the stream, the median radial velocity offset between the spur and the main stream is smaller than 0.5 km s^-1, comparable to the measurement uncertainty. A flyby of a massive, compact object can change orbits of stars in a stellar stream and produce features like the spur observed in GD-1. In this scenario, the radial velocity of the GD-1 spur relative to the stream constrains the orbit of the perturber and its current on-sky position to ≈5000 deg². The family of acceptable perturber orbits overlaps the stellar and dark-matter debris of the Sagittarius dwarf galaxy in present-day position and velocity. This suggests that GD-1 may have been perturbed by a globular cluster or an extremely compact dark-matter subhalo formerly associated with Sagittarius.

ID: 156872
Type: article
Authors: Caldwell, Nelson; Bonaca, Ana; Price-Whelan, Adrian M.; Sesar, Branimir; Walker, Matthew G.
Abstract: We present new kinematic data for the Ophiuchus stellar stream. Spectra have been taken of member candidates at the MMT telescope using Hectospec, Hectochelle, and Binospec, which provide more than 1800 new velocities. Combined with proper-motion measurements of stars in the field by the Gaia-DR2 catalog, we have derived stream membership probabilities, resulting in the detection of more than 200 likely members. These data show the stream extends to more than three times the length shown in the discovery data. A spur to the main stream is also detected. The high-resolution spectra allow us to resolve the stellar velocity dispersion, found to be 1.6 ± 0.3 km s^-1.

ID: 157673
Type: article
Authors: Cargile, Phillip A.; Conroy, Charlie; Johnson, Benjamin D.; Ting, Yuan-Sen; Bonaca, Ana; Dotter, Aaron; Speagle, Joshua S.
Abstract: We present MINESweeper, a tool to measure stellar parameters by jointly fitting observed spectra and broadband photometry to model isochrones and spectral libraries. This approach enables the measurement of spectrophotometric distances, in addition to stellar parameters such as T_eff, $\mathrm{log}g$, [Fe/H], [α/Fe], and radial velocity. MINESweeper employs a Bayesian framework and can easily incorporate a variety of priors, including Gaia parallaxes. Mock data are fit in order to demonstrate how the precision of derived parameters depends on evolutionary phase and signal-to-noise ratio. We then fit a selection of data in order to validate the model outputs. Fits to a variety of benchmark stars including Procyon, Arcturus, and the Sun result in derived stellar parameters that are in good agreement with the literature. We then fit combined spectra and photometry of stars in the open and globular clusters M92, M13, M3, M107, M71, and M67. Derived distances, [Fe/H], [α/Fe], and $\mathrm{log}g$-T_eff relations are in overall good agreement with literature values, although there are trends between metallicity and $\mathrm{log}g$ within clusters that point to systematic uncertainties at the ≍0.1 dex level. Finally, we fit a large sample of stars from the H3 Spectroscopic Survey in which high-quality Gaia parallaxes are also available. These stars are fit without the Gaia parallaxes so that the geometric parallaxes can serve as an independent test of the spectrophotometric distances. Comparison between the two reveals good agreement within their formal uncertainties after accounting for the Gaia zero-point uncertainties.

ID: 157674
Type: article
Authors: Naidu, Rohan P.; Conroy, Charlie; Bonaca, Ana; Johnson, Benjamin D.; Ting, Yuan-Sen; Caldwell, Nelson; Zaritsky, Dennis; Cargile, Phillip A.
Abstract: In the ΛCDM paradigm, the Galactic stellar halo is predicted to harbor the accreted debris of smaller systems. To identify these systems, the H3 Spectroscopic Survey, combined with Gaia, is gathering 6D phase-space and chemical information in the distant Galaxy. Here we present a comprehensive inventory of structure within 50 kpc from the Galactic center using a sample of 5684 giants at $| b| \gt 40^\circ $ and $| Z| \gt 2\,\mathrm{kpc}$ . We identify known structures including the high-α disk, the in situ halo (disk stars heated to eccentric orbits), Sagittarius (Sgr), Gaia-Sausage-Enceladus (GSE), the Helmi Streams, Sequoia, and Thamnos. Additionally, we identify the following new structures: (i) Aleph ([Fe/H] = -0.5), a low-eccentricity structure that rises a surprising 10 kpc off the plane, (ii) and (iii) Arjuna ([Fe/H] = -1.2) and I'itoi ([Fe/H] 80% of the halo is built by two massive (M_⋆ ∼ 10⁸-10⁹M_☉) accreted dwarfs: GSE ([Fe/H] = -1.2) within 25 kpc and Sgr ([Fe/H] = -1.0) beyond 25 kpc. This explains the relatively high overall metallicity of the halo ([Fe/H] ≍ -1.2). We attribute ≳95% of the sample to one of the listed structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk.

ID: 158763
Type: article
Authors: Zaritsky, Dennis; Conroy, Charlie; Naidu, Rohan P.; Cargile, Phillip A.; Putman, Mary; Besla, Gurtina; Bonaca, Ana; Caldwell, Nelson; Han, Jiwon Jesse; Johnson, Benjamin D.; Speagle, Joshua S.; Ting, Yuan-Sen
Abstract: We report the discovery of 15 stars in the H3 survey that lie, in projection, near the tip of the trailing gaseous Magellanic Stream (MS). The stars have Galactocentric velocities ^-1, Galactocentric distances of ?40 to 80 kpc (increasing along the MS), and [Fe/H] consistent with that of stars in the Small Magellanic Cloud. These 15 stars comprise 94% (15 of 16) of the H3 observed stars to date that have R_GAL > 37.5 kpc, -350 km s^-1 _GSR ^-1, and are not associated with the Sagittarius Stream. They represent a unique portion of the Milky Way's outer halo phase space distribution function and confirm that unrelaxed structure is detectable even at radii where H3 includes only a few hundred stars. Due to their statistical excess, their close association with the MS and HI compact clouds in the same region, both in position and velocity space, and their plausible correspondence with tidal debris in a published simulation, we identify these stars as debris of past Magellanic Cloud encounters. These stars are evidence for a stellar component of the tidal debris field far from the Clouds themselves and provide unique constraints on the interaction.

ID: 156233
Type: article
Authors: Zaritsky, Dennis; Conroy, Charlie; Zhang, Huanian; Naidu, Rohan P.; Bonaca, Ana; Caldwell, Nelson; Cargile, Phillip A.; Johnson, Benjamin D.
Abstract: The timing argument provides a lower limit on the mass of the Milky Way. Using a sample of 32 stars at R > 60 kpc drawn from the H3 Spectroscopic Survey and mock catalogs created from published numerical simulations, we find that M₂₀₀ > 0.91 × 10¹² M_☉ with 90% confidence. We recommend using this limit to refine the allowed prior mass range in more complex and sophisticated statistical treatments of Milky Way dynamics. The use of such a prior would have significantly reduced many previously published uncertainty ranges. Our analysis suggests that the most likely value of M₂₀₀ is ≈1.5 × 10¹² M_☉, but establishing this as the Milky Way mass requires a larger sample of outer halo stars and a more complete analysis of the inner halo stars in H3. The imminent growth in the sample of outer halo stars due to ongoing and planned surveys will make this possible.

ID: 154407
Type: article
Authors: Conroy, Charlie; Bonaca, Ana; Cargile, Phillip; Johnson, Benjamin D.; Caldwell, Nelson; Naidu, Rohan P.; Zaritsky, Dennis; Fabricant, Daniel; Moran, Sean; Rhee, Jaehyon; Szentgyorgyi, Andrew; Berlind, Perry; Calkins, Michael L.; Kattner, ShiAnne; Ly, Chun
Abstract: Modern theories of galaxy formation predict that the Galactic stellar halo was hierarchically assembled from the accretion and disruption of smaller systems. This hierarchical assembly is expected to produce a high degree of structure in the combined phase and chemistry space; this structure should provide a relatively direct probe of the accretion history of our Galaxy. Revealing this structure requires precise 3D positions (including distances), 3D velocities, and chemistry for large samples of stars. The Gaia satellite is delivering proper motions and parallaxes for >1 billion stars to G ≈ 20. However, radial velocities and metallicities will only be available to G ≈ 15, which is insufficient to probe the outer stellar halo (≳10 kpc). Moreover, parallaxes will not be precise enough to deliver high-quality distances for stars beyond ̃10 kpc. Identifying accreted systems throughout the stellar halo therefore requires a large ground-based spectroscopic survey to complement Gaia. Here we provide an overview of the H3 Stellar Spectroscopic Survey, which will deliver precise stellar parameters and spectrophotometric distances for ≈200,000 stars to r = 18. Spectra are obtained with the Hectochelle instrument at the MMT, which is configured for the H3 Survey to deliver resolution R ≈ 23,000 spectra covering the wavelength range 5150-5300 Å. The survey is optimized for stellar halo science and therefore focuses on high Galactic latitude fields (| b| > 30^\circ ), sparsely sampling 15,000 sq. degrees. Targets are selected on the basis of Gaia parallaxes, enabling very efficient selection of bona fide halo stars. The survey began in the fall of 2017 and has collected 88,000 spectra to-date. All of the data, including the derived stellar parameters, will eventually be made publicly available via the survey website: h3s urvey.rc.fas.harvard.edu.

ID: 154515
Type: article
Authors: Conroy, Charlie; Naidu, Rohan P.; Zaritsky, Dennis; Bonaca, Ana; Cargile, Phillip; Johnson, Benjamin D.; Caldwell, Nelson
Abstract: The Galactic stellar halo is predicted to have formed at least partially from the tidal disruption of accreted dwarf galaxies. This assembly history should be detectable in the orbital and chemical properties of stars. The H3 Survey is obtaining spectra for 200,000 stars and, when combined with Gaia data, is providing detailed orbital and chemical properties of Galactic halo stars. Unlike previous surveys of the halo, the H3 target selection is based solely on magnitude and Gaia parallax the survey therefore provides a nearly unbiased view of the entire stellar halo at high latitudes. In this paper we present the distribution of stellar metallicities as a function of Galactocentric distance and orbital properties for a sample of 4232 kinematically selected halo giants to 100 kpc. The stellar halo is relatively metal- rich, =-1.2, and there is no discernible metallicity gradient over the range 6 =-1.2, and there is no discernible metallicity gradient over the range 6 _{gal gal gal} > 30 kpc, respectively. The Sagittarius stream dominates the metallicity distribution at 20─40 kpc for stars on prograde orbits. The Gaia─Enceladus merger remnant dominates the metallicity distribution for radial orbits to ≈30 kpc. Metal-poor stars with [Fe/H] < −2 are a small population of the halo at all distances and orbital categories. We associate the "in situ" stellar halo with stars displaying thick disk chemistry on halo-like orbits; such stars are confined to | z| < 10 {kpc}. The majority of the stellar halo is resolved into discrete features in chemical─orbital space, suggesting that the bulk of the stellar halo formed from the accretion and tidal disruption of dwarf galaxies. The relatively high metallicity of the halo derived in this work is a consequence of the unbiased selection function of halo stars and, in combination with the recent upward revision of the total stellar halo mass, implies a Galactic halo metallicity that is typical for its mass.

ID: 147906
Type: article
Authors: Conroy, Charlie; Bonaca, Ana; Naidu, Rohan P.; Eisenstein, Daniel J.; Johnson, Benjamin D.; Dotter, Aaron; Finkbeiner, Douglas P.
Abstract: We present a color-based method for identifying red giants based on the Pan-STARRS grz and WISE W1 and W2 photometry. We utilize a subsample of bright stars with precise parallaxes from Gaia's second data release (DR2) to verify that the color-based selection reliably separates dwarfs from giants. The selection is conservative in the sense that contamination is small (&ap;30%) but not all giants are included (the selection primarily identifies K giants). The color-based selection can be applied to stars brighter than W1 &ap; 16, more than two magnitudes fainter than techniques relying on shallower 2MASS photometry. Many streams and clouds are visible in the resulting sky maps, especially when binned by Gaia DR2 proper motions, including the Sagittarius stream, the Hercules--Aquila Cloud, the Eastern Banded Structure, Monoceros, and the Virgo Overdensity. In addition to the characterization of new and known stellar streams, we expect that this method for selecting red giants will enable detailed analysis of the diffuse stellar halo to distances exceeding 100 kpc.