# Selected Publications

highlights from recent projects; abstracts as published

### Tidal destruction in a low mass galaxy environment: the discovery of tidal tails around DDO 44

We report the discovery of a $>1$ degree ($\sim$50 kpc) long stellar tidal stream emanating from the dwarf galaxy DDO 44, a likely satellite of Local Volume galaxy NGC 2403 located $\sim$70 kpc in projection from its companion. NGC 2403 is a roughly Large Magellanic Cloud stellar-mass galaxy 3 Mpc away, residing at the outer limits of the M 81 group. We are mapping a large region around NGC 2403 as part of our MADCASH (Magellanic Analogs’ Dwarf Companions and Stellar Halos) survey, reaching point source depths (90% completeness) of (g, i) = (26.5, 26.2). Density maps of old, metal-poor RGB stars reveal tidal streams extending on two sides of DDO 44, with the streams directed toward NGC 2403. We estimate total luminosities of the original DDO 44 system (dwarf and streams combined) to be $M_{i, \rm{tot}} = −13.4$ and $M_{g, \rm{tot}} = −12.6$, with $\sim$25−30% of the luminosity in the streams. Analogs of roughly LMC-mass hosts with massive tidally disrupting satellites are rare in the Illustris simulations, especially at large separations such as that of DDO 44. The few analogs that are present in the models suggest that even low-mass hosts can efficiently quench their massive satellites.
In The Astrophysical Journal, 2019

### Chemical Abundances of Hydrostatic and Explosive Alpha-elements in Sagittarius Stream Stars

We analyze chemical abundances of stars in the Sagittarius (Sgr) tidal stream using high-resolution Gemini+GRACES spectra of 42 members of the highest surface-brightness portions of both the trailing and leading arms. Targets were chosen using a 2MASS+WISE color-color selection, combined with the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) radial velocities. In this Letter, we analyze [Fe/H] and $\alpha$-elements produced by both hydrostatic (O, Mg) and explosive (Si, Ca, Ti) nucleosynthetic processes. The average [Fe/H] for our Sgr stream stars is lower than that for stars in the Sgr core, and stars in the trailing and leading arms show systematic differences in [Fe/H]. Both hydrostatic and explosive elements are depleted relative to Milky Way (MW) disk and halo stars, with a larger gap between the MW trend and Sgr stars for the hydrostatic elements. Chemical abundances of Sgr stream stars show similar patterns to those measured in the core of the Sgr dSph. We explore the ratio of hydrostatic to explosive $\alpha$-elements $\left [\alpha_{\rm h/ex}\right ]$ (which we refer to as the HEx ratio). Our observed HEx ratio trends for Sgr debris are deficient relative to MW stars. Via simple chemical evolution modeling, we show that these HEx ratio patterns are consistent with a Sgr IMF that lacks the most massive stars. This study provides a link between the chemical properties in the intact Sgr core and the significant portion of the Sgr system’s luminosity that is estimated to currently reside in the streams.
In The Astrophysical Journal Letters, 2018

### First Results from the MADCASH Survey: A Faint Dwarf Galaxy Companion to the Low-mass Spiral Galaxy NGC 2403 at 3.2 Mpc

We report the discovery of the faintest known dwarf galaxy satellite of a Large Magellanic Cloud (LMC) stellar-mass host beyond the Local Group (LG), based on deep imaging with Subaru/Hyper Suprime-Cam. Magellanic Analog Dwarf Companions And Stellar Halos (MADCASH) J074238+652501-dw lies $\sim$35 kpc in projection from NGC 2403, a dwarf spiral galaxy at D$\approx$3.2 Mpc. This new dwarf has $M_g=-7.4\pm0.4$ and a half-light radius of 168$\pm$70 pc, at the calculated distance of 3.39$\pm$0.41 Mpc. The color-magnitude diagram reveals no evidence of young stellar populations, suggesting that MADCASH J074238+652501-dw is an old, metal-poor dwarf similar to low-luminosity dwarfs in the LG. The lack of either detected HI gas ($M_{\rm HI} / L_{\rm V} < 0.69 M_\odot / L_\odot$, based on Green Bank Telescope observations) or GALEX NUV/FUV flux enhancement is consistent with a lack of young stars. This is the first result from the MADCASH survey, which is conducting a census of the stellar substructure and faint satellites in the halos of Local Volume LMC analogs via resolved stellar populations. Models predict a total of $\sim4-10$ satellites at least as massive as MADCASH J074238+652501-dw around a host with the mass of NGC 2403, with 2-3 within our field of view, slightly more than the one such satellite observed in our footprint.
In The Astrophysical Journal Letters, 2016

### Tidal Streams in the Local Group and Beyond

From the publisher: This volume is written by leading scientists in the field, who review the current state of our knowledge of tidal streams in the Milky Way, the Andromeda galaxy, and in other nearby galaxies. The cosmological origins of dwarf galaxies and the physical processes by which they are tidally disrupted into streams and incorporated into galaxy halos are discussed. The techniques that have been used to identify tidal streams are presented, and will be useful to researchers who would like to find substructures in the next generation of optical sky surveys, including Pan-STARRS and LSST. The methods that are currently under development to constrain both large scale distribution of dark matter in the Milky Way and the (small scale) lumpiness of the dark matter distribution are also explained. The authors also provide motivation for future spectroscopic surveys of Milky Way halo stars, which will aid both in the identification of tidal streams and the constraint of dark matter properties.This volume is aimed at graduate students who are beginning this field of research, but is also a resource for researchers who study tidal streams and related fields. In addition to presenting the physical processes by which tidal streams are created, it also reviews the current state of the observations and the progress towards utilizing these observations to constrain the distribution of dark matter in the Milky Way. The book will introduce anyone with a background in astrophysics to the field of tidal streams.
Astrophysics and Space Science Library, Volume 420, 2016

### Substructure in Bulk Velocities of Milky Way Disk Stars

We find that Galactic disk stars near the anticenter exhibit velocity asymmetries in both the Galactocentric radial and vertical components across the midplane as well as azimuthally. These findings are based on Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic velocities for a sample of ~400,000 F-type stars, combined with proper motions from the PPMXL catalog for which we have derived corrections to the zero points based in part on spectroscopically discovered galaxies and QSOs from LAMOST. In the region within 2 kpc outside the Sun’s radius and ±2 kpc from the Galactic midplane, we show that stars above the plane exhibit net outward radial motions with downward vertical velocities, while stars below the plane have roughly the opposite behavior. We discuss this in the context of other recent findings, and conclude that we are likely seeing the signature of vertical disturbances to the disk due to an external perturbation.
In The Astrophysical Journal Letters, 2013

# Recent Publications

. Tidal destruction in a low mass galaxy environment: the discovery of tidal tails around DDO 44. In The Astrophysical Journal, 2019.

. ΛCDM predictions for the satellite population of M33. In Monthly Notices of the Royal Astronomical Society, 2018.

. Boötes III is a Disrupting Dwarf Galaxy Associated with the Styx Stellar Stream. In The Astrophysical Journal, 2018.

. A Deeper Look at the New Milky Way Satellites: Sagittarius II, Reticulum II, Phoenix II, and Tucana III. In The Astrophysical Journal, 2018.

. 3D Asymmetrical motions of the Galactic outer disc with LAMOST K giant stars. In Monthly Notices of the Royal Astronomical Society, 2018.

. Chemical Abundances of Hydrostatic and Explosive Alpha-elements in Sagittarius Stream Stars. In The Astrophysical Journal Letters, 2018.

. Discovery of Distant RR Lyrae Stars in the Milky Way Using DECam. In The Astrophysical Journal, 2018.

. Mapping the Milky Way with LAMOST - II. The stellar halo. In Monthly Notices of the Royal Astronomical Society, 2018.

. Deep Subaru Hyper Suprime-Cam Observations of Milky Way Satellites Columba I and Triangulum II. In The Astronomical Journal, 2017.

. A Map of the Local Velocity Substructure in the Milky Way Disk. In The Astrophysical Journal, 2017.

. The predicted luminous satellite populations around SMC- and LMC-mass galaxies - a missing satellite problem around the LMC?. In Monthly Notices of the Royal Astronomical Society, 2017.

. Serendipitous Discovery of RR Lyrae Stars in the Leo V Ultra-faint Galaxy. In The Astrophysical Journal Letters, 2017.

. Red Runaways II: Low-mass Hills Stars in SDSS Stripe 82. In The Astrophysical Journal, 2016.

. First Results from the MADCASH Survey: A Faint Dwarf Galaxy Companion to the Low-mass Spiral Galaxy NGC 2403 at 3.2 Mpc. In The Astrophysical Journal Letters, 2016.

. Characterizing the SHARDS of Disrupted Milky Way Satellites with LAMOST. In The Astrophysical Journal, 2016.

. Tidal Streams in the Local Group and Beyond. Astrophysics and Space Science Library, Volume 420, 2016.

. Estimation of Distances to Stars with Stellar Parameters from LAMOST. In The Astronomical Journal, 2015.

. Substructure in Bulk Velocities of Milky Way Disk Stars. In The Astrophysical Journal Letters, 2013.

. The Origin of the Virgo Stellar Substructure. In The Astrophysical Journal, 2012.

. An algorithm for preferential selection of spectroscopic targets in LEGUE. In Research in Astronomy and Astrophysics, 2012.

# Recent Posts

### What do we learn by studying the outer Milky Way halo?

In my previous post I discussed our project to find some of the most distant stars in the outskirts of our Milky Way galaxy. (The illustration above shows the fields on the sky that we have observed so far as part of our survey.) In this post, I’ll explain more about why we want to find these distant Milky Way halo stars. Mass of the Milky Way We don’t even know the mass of our Galaxy better than about a factor of two.

### Studying the outer Milky Way halo with distant RR Lyrae variable stars

Hi, folks! In this, the first blog post on my site, I’ll explain a bit about one of my research projects that I’m especially excited about lately. I’ll aim to keep it at a level that is understandable for “non-experts,” but inevitably there will be some astro jargon that creeps in… -Jeff The sparsely-populated outermost regions of our Milky Way galaxy are poorly studied. In part, this is because stars of a given type become fainter the more distant they are from us – think about a flashlight: if I stand a few feet away from you, it will look pretty bright (you could read a book by its light), while if I’m a block away, it will just look like a small point of light.

# Teaching Experience

Courses taught:

• “Intro to Astrophysics”, Earlham College (Fall 2015)
• “Physics of Music”, Earlham College (Spring 2015)
• “General/Analytical Physics Labs”, Earlham College (Fall 2014)
• “Intro to Astronomy & Astrophysics”, Rensselaer Polytechnic Institute (Fall 2012)

Teaching Assistantships:

• “Intro to Astronomical Observation” (head TA for all sections), “Intro to Stars, Galaxies, and the Universe”, and “Archaeo-Astronomy”, University of Virginia (2002-2004)