arXiv subjects

Heidi Jo Newberg

Publications and source records attributed to Heidi Jo Newberg.

At least 19 recordsLinked to original sources

arXiv2025

Estimate of Current Mass of the Large Magellanic Cloud from the Orphan-Chenab Tidal Stream

By fitting the tilt in the path of the Orphan-Chenab Stream (OCS), we conclude that the current mass of the Large Magellanic Cloud (LMC) within 30 kpc is $4.7$-$5.1 \times 10^{10}$ M$_\odot$. We note that the tidal radius of the LMC of this mass is 16.9 kpc, indicating that our measured mass approximates the current bound mass of the LMC. Previous measurements of the LMC mass based on fitting the observed path of the OCS through the Milky Way (MW) halo reported the total mass of the LMC. We show that because the closest approach of the LMC to the OCS, where the gravitational perturbation of the stream path is the highest, is about 20 kpc, the mass of the LMC outside of 30 kpc is not constrained and depends entirely on the assumed radial profile at large radius. Our best-fit total mass varies between $4.5 \times 10^{10}$ and $2.2 \times 10^{11}$ M$_\odot$ or more, depending on the presumed radial profile of the LMC. We also show that previous measurements of the mass of the LMC that used a particle-spray method to simulate the path of the OCS suffered from systematic error because they assumed that all particles were stripped from the dwarf galaxy at the tidal radius; N-body simulations show that particles are actually released from a range of distances from the center of mass of the OCS. In contrast, the choice of MW potential has little effect on the estimated LMC mass from the OCS.

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arXiv2025

The Dispersion Leverage Coronagraph (DLC): A nulling coronagraph for use on primary objective grating telescopes

We present the Dispersion Leverage Coronagraph (DLC), a novel variation of the Achromatic Interfero Coronagraph (AIC) that is designed for optical systems featuring large, dispersive primary objective gratings. DLC was originally designed for the Diffractive Interfero Coronagraph Exoplanet Resolver (DICER), a notional 20m class infrared space telescope utilizing the enhanced one-dimensional angular resolution of large diffraction gratings in order to discover and characterize near-Earth exoplanets. Here we develop the theoretical foundation for DLC, and apply it to DICER as an example use case. We derive important properties of the DLC system including focal plane transmission maps, stellar leakage, residual optical path difference tolerance, and pointing error/jitter considerations. Ultimately, we found that DLC effectively nulls an on-axis target across the entire spectrum in the focal plane, allowing for 2D/lambda diffraction-limited imaging. It requires asymmetrical fine-guidance tolerances on pointing error/jitter. We work through a benchmark DICER design, explaining the need for a second disperser to reduce background from Zodiacal light, and showing that it could plausibly find and characterize approximately 4 nearby, habitable exoplanets around Sun-like stars in a seven year mission; about 30% of the habitable exoplanets within 8 pc were found in our simulation. The DLC may be useful for any application requiring extremely high resolution, close-companion spectroscopy.

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arXiv2024

The Debris of the "Last Major Merger" is Dynamically Young

The Milky Way's (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the "last major merger." Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor collided with the MW proto-disk 8-11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the MW disk within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space, because the morphology of debris depends on how long it has had to phase mix. The recently-identified phase-space folds in Gaia DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations at late times. Roughly 20\% of the stars in the prograde local stellar halo are associated with the observed caustics. Based on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1--2 Gyr ago. We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data 1--2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the "last major merger" did not collide with the MW proto-disk at early times, as is thought for the GSE, but instead collided with the MW disk within the last few Gyr, consistent with the body of work surrounding the VRM.

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arXiv2022

Primary Objective Grating Telescopy: Optical Properties and Feasibility of Applications

We develop the theoretical foundation for primary objective grating (POG) telescopy. In recent years, a wide range of telescope designs that collect the light over a large grating and focus it with a secondary receiving optic that is placed at grazing exodus have been proposed by Thomas D. Ditto, and are sometimes referred to as Dittoscopes. Applications include discovery and characterization of exoplanets, discovery of near-Earth asteroids, and spectroscopic surveys of the sky. These telescopes would have small aerial mass, and therefore provide a path forward to launch large telescopes into space. Because this series of telescope designs departs from traditional telescope designs, it has been difficult to evaluate which applications are most advantageous for this design. Here, we define a new figure of merit, the "modified etendue," that characterizes the photon collection capability of a POG. It is demonstrated that the diffraction limit for observations is determined by the length of the grating. We evaluate the effects of atmospheric seeing for ground-based applications and the disambiguation of position vs. wavelength in the focal plane using a second dispersing element. Finally, some strategies for fully reaping the benefits of POG optical characteristics are discussed.

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arXiv2022

A Swing of the Pendulum: The Chemodynamics of the Local Stellar Halo Indicate Contributions from Several Radial Merger Events

We find that the chemical abundances and dynamics of APOGEE and GALAH stars in the local stellar halo are inconsistent with a scenario in which the inner halo is primarily composed of debris from a single, massive, ancient merger event, as has been proposed to explain the Gaia-Enceladus/Gaia Sausage (GSE) structure. The data contains trends of chemical composition with energy which are opposite to expectations for a single massive, ancient merger event, and multiple chemical evolution paths with distinct dynamics are present. We use a Bayesian Gaussian mixture model regression algorithm to characterize the local stellar halo, and find that the data is best fit by a model with four components. We interpret these components as the VRM, Cronus, Nereus, and Thamnos; however, Nereus and Thamnos likely represent more than one accretion event because the chemical abundance distributions of their member stars contain many peaks. Although the Cronus and Thamnos components have different dynamics, their chemical abundances suggest they may be related. We show that the distinct low and high alpha halo populations from Nissen & Schuster (2010) are explained by VRM and Cronus stars, as well as some in-situ stars. Because the local stellar halo contains multiple substructures, different popular methods of selecting GSE stars will actually select different mixtures of these substructures, which may change the apparent chemodynamic properties of the selected stars. We also find that the Splash stars in the solar region are shifted to higher v_phi and slightly lower [Fe/H] than previously reported.

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arXiv2022

60 candidate high-velocity stars originating from the Sagittarius dwarf spheroidal galaxy in Gaia EDR3

Using proper motions from Gaia Early Data Release 3 (Gaia EDR 3) and radial velocities from several surveys, we identify 60 candidate high-velocity stars with total velocity greater than 75\% escape velocity that probably origin from Sagittarius dwarf spheroidal galaxy (Sgr) by orbital analysis. Sgr's gravity has little effect on the results and the Large Magellanic Cloud's gravity has non-negligible effect on only a few stars. The closest approach of these stars to the Sgr occurs when the Sgr passed its pericenter ($\sim$ 38.2 Myr ago), which suggest they were tidally stripped from the Sgr. The positions of these stars in the HR diagram and the chemical properties of 19 of them with available [Fe/H] are similar with the Sgr stream member stars. This is consistent with the assumption of their accretion origin. Two of the 60 are hypervelocity stars, which may also be produced by Hills mechanism.

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arXiv2022

The Local Stellar Halo is Not Dominated by a Single Radial Merger Event

We use halo dwarf stars with photometrically determined metallicities that are located within 2 kpc of the Sun to identify local halo substructure. The kinematic properties of these stars do not indicate a single, dominant radial merger event (RME). The retrograde Virgo Radial Merger (VRM) component has [Fe/H] = -1.7. A second, non-rotating RME component we name Nereus is identified with [Fe/H] = -2.1 and has similar energy as the VRM. A possible third RME we name Cronus is identified that is co-rotating with the disk, has lower energy than the VRM, and has [Fe/H] = -1.2. We identify the Nyx Stream in the data. In addition to these substructures, we observe metal-poor halo stars ([Fe/H] ~ -2.0 and $σ_v$ ~ 180 km s$^{-1}$) and a disk/Splash component with lower rotational velocity than the disk and lower metallicity than typically associated with the Splash. An additional excess of halo stars with low velocity and metallicity of [Fe/H] = -1.5 could be associated with the shell of a lower energy RME or indicate that lower energy halo stars have higher metallicity. Stars which comprise the "Gaia Sausage" velocity structure are a combination of the components identified in this work.

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arXiv2022

Estimate of the Mass and Radial Profile of the Orphan-Chenab Stream's Dwarf Galaxy Progenitor Using MilkyWay@home

We fit the mass and radial profile of the Orphan-Chenab Stream's (OCS) dwarf galaxy progenitor by using turnoff stars in the Sloan Digital Sky Survey (SDSS) and the Dark Energy Camera (DEC) to constrain N-body simulations of the OCS progenitor falling into the Milky Way on the 1.5 PetaFLOPS MilkyWay@home distributed supercomputer. We infer the internal structure of the OCS's progenitor under the assumption that it was a spherically symmetric dwarf galaxy comprised of a stellar system embedded in an extended dark matter halo. We optimize the evolution time, the baryonic and dark matter scale radii, and the baryonic and dark matter masses of the progenitor using a differential evolution algorithm. The likelihood score for each set of parameters is determined by comparing the simulated tidal stream to the angular distribution of OCS stars observed in the sky. We fit the total mass of the OCS's progenitor to ($2.0\pm0.3$) $\times 10^7 M_\odot$ with a mass-to-light ratio of $γ=73.5\pm10.6$ and ($1.1\pm0.2$)$\times10^6M_{\odot}$ within 300 pc of its center. Within the progenitor's half-light radius, we estimate total a mass of ($4.0\pm1.0$)$\times10^5M_{\odot}$. We also fit the current sky position of the progenitor's remnant to be $(α,δ)=((166.0\pm0.9)^\circ,(-11.1\pm2.5)^\circ)$ and show that it is gravitationally unbound at the present time. The measured progenitor mass is on the low end of previous measurements, and if confirmed lowers the mass range of ultrafaint dwarf galaxies. Our optimization assumes a fixed Milky Way potential, OCS orbit, and radial profile for the progenitor, ignoring the impact of the Large Magellanic Cloud.

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arXiv2021

Dynamically Produced Moving Groups in Interacting Simulations

We show that Smoothed Particle Hydrodynamics (SPH) simulations of dwarf galaxies interacting with a Milky Way-like disk produce moving groups in the simulated stellar disk. We analyze three different simulations: one that includes dwarf galaxies that mimic the Large Magellanic Cloud, Small Magellanic Cloud and the Sagittarius dwarf spheroidal; another with a dwarf galaxy that orbits nearly in the plane of the Milky Way disk; and a null case that does not include a dwarf galaxy interaction. We present a new algorithm to find large moving groups in the $V_R, V_ϕ$ plane in an automated fashion that allows us to compare velocity sub-structure in different simulations, at different locations, and at different times. We find that there are significantly more moving groups formed in the interacting simulations than in the isolated simulation. A number of dwarf galaxies are known to orbit the Milky Way, with at least one known to have had a close pericenter approach. Our analysis of simulations here indicates that dwarf galaxies like those orbiting our Galaxy produce large moving groups in the disk. Our analysis also suggests that some of the moving groups in the Milky Way may have formed due to dynamical interactions with perturbing dwarf satellites. The groups identified in the simulations by our algorithm have similar properties to those found in the Milky Way, including similar fractions of the total stellar population included in the groups, as well as similar average velocities and velocity dispersions.

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arXiv2021

Element abundance analysis of the metal-rich stellar halo and high-velocitythick disk in the galaxy

Based on the second Gaia data release (DR2) and APOGEE (DR16) spectroscopic surveys, wedefined two kinds of star sample: high-velocity thick disk (HVTD) with $vϕ>90km/s$ and metal-richstellar halo (MRSH) with $vϕ<90km/s$. Due to high resolution spectra data from APOGEE (DR16),we can analyze accurately the element abundance distribution of HVTD and MRSH. These elementsabundance constituted a multidimensional data space, and we introduced an algorithm method forprocessing multi-dimensional data to give the result of dimensionality reduction clustering. Accordingto chemical properties analysis, we derived that some HVTD stars could origin from the thin disk,and some MRSH stars from dwarf galaxies, but those stars which have similar chemical abundancecharacteristics in both sample may form in-situ.

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arXiv2021

Introduction to Tidal Streams

Dwarf galaxies that come too close to larger galaxies suffer tidal disruption; the differential gravitational force between one side of the galaxy and the other serves to rip the stars from the dwarf galaxy so that they instead orbit the larger galaxy. This process produces "tidal streams" of stars, which can be found in the stellar halo of the Milky Way, as well as in halos of other galaxies. This chapter provides a general introduction to tidal streams, including the mechanism through which the streams are created, the history of how they were discovered, and the observational techniques by which they can be detected. In addition, their use in unraveling galaxy formation history and the distribution of dark matter in galaxies is discussed, as is the interaction between these dwarf galaxy satellites and the disk of the larger galaxy.

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arXiv2021

Mapping Milky Way Halo Substructure using Stars in the Extended Blue Tail of the Horizontal Branch

Although Blue Horizontal Branch (BHB) stars are commonly used to trace halo substructure, the stars bluer than (g-r)<-0.3 are ignored due to the difficulty in determining their absolute magnitudes. The blue extention of the horizontal branch (HBX) includes BHB tail stars and Extreme Horizontal Branch (EHB) stars. We present a method for identifying HBX stars in the field, using spectra and photometry from the Sloan Digital Sky Survey Data Release 14 (SDSS DR14). We derive an estimate for the absolute magnitudes of BHB tail and EHB stars as a function of color, and use this relationship to calculate distances. We identify an overdensity of HBX stars that appears to trace the northern end of the Hercules-Aquila Cloud (HAC). We identify three stars that are likely part of a tidal stream, but this is not enough stars to explain the observed overdensity. Combining SDSS data with Gaia DR2 proper motions allows us to show that the orbits of the majority of the HBX stars in the overdensity are on high eccentricity orbits similar to those in the Virgo Radial Merger/Gaia-Enceladus/Gaia Sausage structure, and that the overdensity of high eccentricity orbits extends all the way to the Virgo Overdensity. We use stellar kinematics to separate the HBX stars into disk stars andhalo stars. The halo stars are primarily on highly eccentric (radial) orbits. The fraction of HBX stars that are EHBs is highest in the disk population and lowest in the low eccentricity halo stars.

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arXiv2021

An Algorithm for Reconstructing the Orphan Stream Progenitor with MilkyWay@home Volunteer Computing

We have developed a method for estimating the properties of the progenitor dwarf galaxy from the tidal stream of stars that were ripped from it as it fell into the Milky Way. In particular, we show that the mass and radial profile of a progenitor dwarf galaxy evolved along the orbit of the Orphan Stream, including the stellar and dark matter components, can be reconstructed from the distribution of stars in the tidal stream it produced. We use MilkyWay@home, a PetaFLOPS-scale distributed supercomputer, to optimize our dwarf galaxy parameters until we arrive at best-fit parameters. The algorithm fits the dark matter mass, dark matter radius, stellar mass, radial profile of stars, and orbital time. The parameters are recovered even though the dark matter component extends well past the half light radius of the dwarf galaxy progenitor, proving that we are able to extract information about the dark matter halos of dwarf galaxies from the tidal debris. Our simulations assumed that the Milky Way potential, dwarf galaxy orbit, and the form of the density model for the dwarf galaxy were known exactly; more work is required to evaluate the sources of systematic error in fitting real data. This method can be used to estimate the dark matter content in dwarf galaxies without the assumption of virial equilibrium that is required to estimate the mass using line-of-sight velocities. This demonstration is a first step towards building an infrastructure that will fit the Milky Way potential using multiple tidal streams.

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arXiv2020

Two substructures in the nearby stellar halo found in Gaia and RAVE

We use the second Gaia data release (Gaia DR2), combined with RAVE spectroscopic surveys, to identify the substructures in the nearby stellar halo. We select 3,845 halo stars kinematically and chemically, and determine their density distribution in energy and angular momentum space. To select the substructures from overdensities, we reshuffle the velocities and estimate their significance. Two statistically significant substructures, GR-1 and GR-2, are identified. GR-1 has a high binding energy and small $z$-angular momentum. GR-2 is metal-rich but retrograde. They are both new substructure, and may be accretion debris of dwarf galaxies.

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arXiv2020

Existence of the Metal-Rich Stellar Halo and High-velocity Thick Disk in the Galaxy

Based on the second Gaia data release (DR2), combined with the LAMOST and APOGEE spectroscopic surveys, we study the kinematics and metallicity distribution of the high-velocity stars that have a relative speed of at least 220 ${\rm km\ s^{-1}}$ with respect to the local standard of rest in the Galaxy. The rotational velocity distribution of the high-velocity stars with [Fe/H] $>-1.0$ dex can be well described by a two-Gaussian model, with peaks at $V_ϕ\sim +164.2\pm0.7$ and $V_ϕ\sim +3.0\pm0.3$ ${\rm km\ s^{-1}}$, associated with the thick disk and halo, respectively. This implies that there should exist a high-velocity thick disk (HVTD) and a metal-rich stellar halo (MRSH) in the Galaxy. The HVTD stars have the same position as the halo in the Toomre diagram and but show the same rotational velocity and metallicity as the canonical thick disk. The MRSH stars have basically the same rotational velocity, orbital eccentricity, and position in the Lindblad and Toomre diagram as the canonical halo stars, but they are more metal-rich. Furthermore, the metallicity distribution function (MDF) of our sample stars are well fitted by a four-Gaussian model, associated with the outer-halo, inner-halo, MRSH, and HVTD, respectively. Chemical and kinematic properties and age imply that the MRSH and HVTD stars may form in situ.

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arXiv2020

The Milky Way's Shell Structure Reveals the Time of a Radial Collision

We identify shell structures in the Milky Way for the first time. We find 2 shells in the Virgo Overdensity (VOD) region and 2 shells in the Hercules Aquila Cloud (HAC) region using Sloan Digital Sky Survey, Gaia, and LAMOST data. These shell stars are a subset of the substructure previously identified as the Virgo Radial Merger (VRM). Timing arguments for these shells indicate that their progenitor dwarf galaxy passed through the Galactic center 2.7 +/- 0.2 Gyr ago. Based on the time of collision, it is also possible that the VRM is related to the phenomenon that created phase-space spirals in the vertical motion of the disk and/or the Splash, and could have caused a burst of star formation in the inner disk. We analyze phase mixing in a collection of radial merger N-body simulations, and find that shell structure similar to that observed in Milky Way data disappears by 5 Gyr after collision with the Galactic center. The method used to calculate the merger time of the VRM was able to reliably recover the correct merger times for these simulations. Previous work supports the idea that the VRM and the Gaia Sausage/Gaia-Enceladus Merger are the same. However, the Gaia Sausage is widely believed to be 8--11 Gyr old. The disparate ages could be reconciled if the larger age is associated with an infall time when the progenitor crossed the virial radius; we do not constrain the time at which the progenitor became bound to the Milky Way. Alternatively, the Gaia Sausage could be younger than previously thought.

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arXiv2019

New nearby hypervelocity stars and their spatial distribution from Gaia DR2

Base on about 4,500 large tangential velocity ($V_\mathrm{tan}>0.75V_\mathrm{esc}$) with high-precision proper motions and $5σ$ parallaxes in Gaia DR2 5D information derived from parallax and proper motion, we identify more than 600 high velocity stars with $50\%$ unbound probability. Of these, 28 nearby (less than 6 kpc) late-type Hypervelocity stars (HVSs) with over $99\%$ possibility of unbound are discovered. In order to search for the unbound stars from the full Gaia DR2 6D phase space information derived from parallax, proper motion and radial velocity, we also identify 28 stars from the total velocity ($V_\mathrm{gc}>0.75V_\mathrm{esc}$) that have probabilities greater than $50\%$ of being unbound from the Galaxy. Of these, only three have a nearly $99\%$ probabilities of being unbound. On the whole HVSs subsample, there is 12 sources reported by other surveys. We study the spatial distribution of angular positions and angular separation of HVSs. We find the unbound HVSs are spatially anisotropic that is most significant in the Galactic longitude at more than $3σ$ level, and lower unbound probability HVSs are systematically more isotropic. The spatial distribution can reflect the origin of HVSs and we discuss the possible origin link with the anisotropy.

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arXiv2019

The Virgo Overdensity Explained

We suggest that the Virgo Overdensity (VOD) of stars in the stellar halo is the result of a radial dwarf galaxy merger that we call the Virgo Radial Merger. Because the dwarf galaxy passed very near to the Galactic center, the debris has a large range of energies but nearly zero $L_z$ angular momentum. The debris appears to extend from 5 to 50 kpc from the Sun in the Virgo region. We connect different parts of this merger debris to the Perpendicular and Parallel Streams (the Virgo Stellar Stream is associated with either or both of these streams), the Hercules-Aquila Cloud (HAC), and possibly the Eridanus Phoenix Overdensity (EriPhe). This radial merger can explain the majority of the observed moving groups of RR Lyrae and blue horizontal branch stars that have previously been identified in Virgo. This merger also produces debris in the Solar neighborhood similar to that identified as the Gaia-Enceladus or it Gaia-sausage merger. Orbits are provided for components of the Virgo Radial Merger progenitor and for debris that appears to be related to the Cocytos Stream, which was also recovered in the Virgo region.

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