About

Constance Mary Rockosi is an Associate Professor of Astronomy and Astrophysics at UC Santa Cruz and an Assistant Astronomer at UCO/Lick Observatory. Her research focuses on studying the Milky Way galaxy to understand how and why it evolved to its current state. By examining our Galaxy, which is a fairly typical spiral galaxy, she aims to gain insights into the evolution of other spiral galaxies and interpret observations of galaxies forming at high redshift. Her work addresses fundamental questions about the Galaxy's growth through merging and accretion of smaller systems, investigating whether the Galaxy grew by accreting gas that later formed stars or by accreting stars formed elsewhere. She also studies the timing and impact of major accretion events and the distribution of different stellar populations within the Galaxy, including the metal-poor stellar halo, thick disk populations, and the old thin disk. Additionally, she explores the role and distribution of dark matter in the Galaxy by analyzing the motions of distant stars, using data from large-area surveys such as the Sloan Digital Sky Survey (SDSS) and its extension SEGUE, which map the structure and stellar populations of the Milky Way. Rockosi contributed to the design and construction of the SDSS imaging camera during her graduate studies and has experience with various spectrographs used in astronomical observations. She holds a B.S.E. in Electrical Engineering from Princeton University and a Ph.D. in Astronomy and Astrophysics from the University of Chicago. Prior to her current position, she was a Hubble Postdoctoral Fellow at the University of Washington's Seattle Astronomy Department.

Research topics

• Astronomy
• Physics
• Astrophysics
• Computer Science
• Quantum mechanics
• Particle physics
• Optics

Selected publications

• Constraining the Milky Way Halo Accretion History With Simulated Stellar Halos: Designing the HALO7D-X Survey

  ArXiv.org · 2025-07-07

  articleOpen access

  We present the design for HALO7D-X, a survey of the stellar halo to investigate the accretion history of the Milky Way. The survey will use a combination of Hubble Space Telescope (HST) and Gaia data for sky position and proper motions of faint stars (18<G<21.5 mag), while line-of-sight velocity, distance, [Fe/H], and [alpha/Fe] will be measured using follow-up Keck spectroscopy. The survey will cover 30 lines of sight, made up of multiple HST archival fields and optimized for Keck DEIMOS spectroscopy. We use mock survey observations of the Bullock and Johnston stellar halo simulations to investigate the sensitivity of HALO7D-X to constrain the basic parameters of the accretion history of our Galaxy's stellar halo. We find that we are sensitive to the mass distribution and accretion timeline of the stellar halo progenitors, but not their orbital circularity. We find that the simulated halos fall into three different groups based on the similarities in their distributions of the observable dimensions of our survey. These groups are also distinct from each other in the mass distribution and accretion timeline of their progenitor satellites, showing that by using similarities in our observables among halos, we are able to identify similarities in their accretion histories. With HALO7D-X we will compare real Milky Way data with simulated halos and use this connection between observables and progenitor mass and accretion timeline to learn about the formation of our Galaxy's stellar halo.

  PublisherOA PDF

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

  ArXiv.org · 2025-04-16

  articleOpen access

  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 $r&lt;19$ 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.

  PublisherOA PDFDOI

• Constraining the Milky Way Halo Accretion History with Simulated Stellar Halos: Designing the HALO7D-X Survey

  The Astrophysical Journal · 2025-07-30 · 1 citations

  articleOpen access

  Abstract We present the design for HALO7D-X, a survey of the stellar halo to investigate the accretion history of the Milky Way. The survey will use a combination of Hubble Space Telescope (HST) and Gaia data for sky position and proper motions of faint stars (18 &lt; G &lt; 21.5 mag), while line-of-sight velocity, distance, [Fe/H], and [ α /Fe] will be measured using follow-up Keck spectroscopy. The survey will cover 30 lines of sight, made up of multiple HST archival fields and optimized for Keck DEIMOS spectroscopy. We use mock survey observations of the Bullock and Johnston stellar halo simulations to investigate the sensitivity of HALO7D-X to constrain the basic parameters of the accretion history of our Galaxy’s stellar halo. We find that we are sensitive to the mass distribution and accretion timeline of the stellar halo progenitors, but not their orbital circularity. We find that the simulated halos fall into three different groups based on the similarities in their distributions of the observable dimensions of our survey. These groups are also distinct from each other in the mass distribution and accretion timeline of their progenitor satellites, showing that by using similarities in our observables among halos, we are able to identify similarities in their accretion histories. With HALO7D-X, we will compare real Milky Way data with simulated halos and use this connection between observables and progenitor mass and accretion timeline to learn about the formation of our Galaxy’s stellar halo.

  PublisherDOI

• The Draco Dwarf Spheroidal Galaxy in the First Year of Dark Energy Spectroscopic Instrument Data

  The Astrophysical Journal · 2025-11-20

  articleOpen access

  Abstract We investigate the spatial distribution, kinematics, and metallicity of stars in the Draco dwarf spheroidal galaxy using data from the Dark Energy Spectroscopic Instrument (DESI). We identify 155 high-probability members of Draco using line-of-sight velocity and metallicity information derived from DESI spectroscopy along with Gaia Data Release 3 proper motions. We find a mean line-of-sight velocity of −290.62 ± 0.80 km s −1 with dispersion = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>9.5</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.62</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.66</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km s −1 and mean metallicity [Fe/H] = −2.10 ± 0.04, consistent with previous results. We also find that Draco has a steep metallicity gradient within the half-light radius, and a metallicity gradient that flattens beyond the half-light radius. We identify eight high-probability members outside the King tidal radius, four of which we identify for the first time. These extratidal stars are not preferentially aligned along the orbit of Draco. We compute an average surface brightness of 34.02 mag <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">arcsec</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> within an elliptical annulus from the King tidal radius of 48 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mi>.</mml:mi> <mml:mi>′</mml:mi> </mml:mover> </mml:math> 1–81′.

  PublisherDOI

• The Dark Matter Content of Milky Way Dwarf Spheroidal Galaxies: Draco, Sextans, and Ursa Minor

  The Astrophysical Journal · 2025-11-10 · 2 citations

  articleOpen accessCorresponding

  Abstract The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI) has so far observed three classical dwarf spheroidal galaxies (dSphs): Draco, Sextans, and Ursa Minor. Based on the observed line-of-sight velocities and metallicities of their member stars, we apply the axisymmetric Jeans Anisotropic Multi-Gaussian Expansion modeling (JAM) approach to recover their inner dark matter distributions. In particular, both the traditional single-population Jeans model and the multiple population chemodynamical model are adopted. With the chemodynamical model, we divide member stars of each dSph into metal-rich and metal-poor populations. The metal-rich populations are more centrally concentrated and dynamically colder, featuring lower velocity dispersion profiles than the metal-poor populations. We find a diversity of the inner density slopes γ of dark matter halos, with the best constraints by the single-population or chemodynamical models consistent with each other. The inner density slopes are <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.7</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.35</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.34</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.2</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.12</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.22</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.3</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.16</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.20</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> for Draco, Sextans, and Ursa Minor, respectively. We also present the measured astrophysical J and D factors of the three dSphs. Our results indicate that the study of the dark matter content of dSphs through stellar kinematics is still subject to uncertainties behind both the methodology and the observed data, through comparisons with previous measurements and datasets.

  PublisherDOI

• DESI 2024 VI: cosmological constraints from the measurements of baryon acoustic oscillations

  arXiv (Cornell University) · 2025 · 897 citations

  • Physics
  • Astrophysics
  • Particle physics

  Abstract We present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Lyman- α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI), to be released in the DESI Data Release 1. DESI BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1 &lt; z &lt; 4.2. To mitigate confirmation bias, a blind analysis was implemented to measure the BAO scales. DESI BAO data alone are consistent with the standard flat ΛCDM cosmological model with a matter density Ω m =0.295±0.015. Paired with a baryon density prior from Big Bang Nucleosynthesis and the robustly measured acoustic angular scale from the cosmic microwave background (CMB), DESI requires H 0 =(68.52±0.62) km s -1 Mpc -1 . In conjunction with CMB anisotropies from Planck and CMB lensing data from Planck and ACT, we find Ω m =0.307± 0.005 and H 0 =(67.97±0.38) km s -1 Mpc -1 . Extending the baseline model with a constant dark energy equation of state parameter w , DESI BAO alone require w =-0.99 +0.15 -0.13 . In models with a time-varying dark energy equation of state parametrised by w 0 and w a , combinations of DESI with CMB or with type Ia supernovae (SN Ia) individually prefer w 0 &gt; -1 and w a &lt; 0. This preference is 2.6 σ for the DESI+CMB combination, and persists or grows when SN Ia are added in, giving results discrepant with the ΛCDM model at the 2.5 σ , 3.5 σ or 3.9 σ levels for the addition of the Pantheon+, Union3, or DES-SN5YR supernova datasets respectively. For the flat ΛCDM model with the sum of neutrino mass ∑ m ν free, combining the DESI and CMB data yields an upper limit ∑ m ν &lt; 0.072 (0.113) eV at 95% confidence for a ∑ m ν &gt; 0 (∑ m ν &gt; 0.059) eV prior. These neutrino-mass constraints are substantially relaxed if the background dynamics are allowed to deviate from flat ΛCDM.

  Publisher

• Data-driven Discovery of Diffuse Interstellar Bands with APOGEE Spectra

  The Astrophysical Journal · 2024-04-01 · 4 citations

  articleOpen access

  Abstract Data-driven models of stellar spectra are useful tools to study nonstellar information, such as the diffuse interstellar bands (DIBs) caused by intervening interstellar material. Using ∼55,000 spectra of ∼17,000 red clump stars from the APOGEE DR16 data set, we create second-order polynomial models of the continuum-normalized flux as a function of stellar parameters ( T eff , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>g</mml:mi> </mml:math> , [Fe/H], [ α /Fe], and age). The model and data show good agreement within uncertainties across the APOGEE wavelength range, although many regions reveal residuals that are not in the stellar rest-frame. We show that many of these residual features—having average extrema at the level of ∼3% in stellar flux on average—can be attributed to incompletely removed spectral lines from the Earth’s atmosphere and DIBs from the interstellar medium (ISM). After removing most of the remaining contamination from Earth’s sky, we identify 84 absorption features not seen in unreddened sightlights that have &lt;50% probability of being noise artifacts—with 25 of these features having &lt;5% probability of being noise artifacts—including all 10 previously known DIBs in the APOGEE wavelength range. Because many of these features occur in the wavelength windows that APOGEE uses to measure chemical abundances, note that characterization and removal of this nonstellar contamination establish an important step in reaching the precision required for chemical tagging experiments. Proper characterization of these features will benefit Galactic ISM science and the currently ongoing Milky Way Mapper program of Sloan Digital Sky Survey V, which relies on the APOGEE spectrograph.

  PublisherOA PDFDOI

• Asteroseismology of the Nearby K Dwarf σ Draconis Using the Keck Planet Finder and TESS

  The Astrophysical Journal · 2024-10-30 · 13 citations

  articleOpen access

  Abstract Asteroseismology of dwarf stars cooler than the Sun is very challenging owing to the low amplitudes and rapid timescales of oscillations. Here we present the asteroseismic detection of solar-like oscillations at 4-minute timescales ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>ν</mml:mi> <mml:mi>max</mml:mi> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>4300</mml:mn> </mml:math> μ Hz) in the nearby K dwarf σ Draconis using extreme-precision Doppler velocity observations from the Keck Planet Finder and 20 s cadence photometry from NASA’s Transiting Exoplanet Survey Satellite. The star is the coolest dwarf star to date with both velocity and luminosity observations of solar-like oscillations, having amplitudes of 5.9 ± 0.8 cm s −1 and 0.8 ± 0.2 ppm, respectively. These measured values are in excellent agreement with established luminosity−velocity amplitude relations for oscillations and provide further evidence that mode amplitudes for stars with T eff &lt; 5500 K diminish in scale following an ( L / M ) 1.5 relation. By modeling the star’s oscillation frequencies from photometric data, we measure an asteroseismic age of 4.5 ± 0.9 (ran) ± 1.2 (sys) Gyr. The observations demonstrate the capability of next-generation spectrographs and precise space-based photometry to extend observational asteroseismology to nearby cool dwarfs, which are benchmarks for stellar astrophysics and prime targets for directly imaging planets using future space-based telescopes.

  PublisherOA PDFDOI

• Innovations and advances in instrumentation at the W. M. Keck Observatory, vol. III

  2024-07-19

  articleOpen access

  Since the start of science operations in 1993, the twin 10-meter W. M. Keck Observatory (WMKO) telescopes have continued to maximize their scientific impact to produce transformative discoveries that keep the U.S. observing community on the frontiers of astronomical research. Upgraded capabilities and new instrumentation are provided though collaborative partnerships primarily with the Caltech and University of California instrument development teams and through additional collaborations with the University of Notre Dame, the University of Hawaii, Swinburne University of Technology, industry, and other organizations. This paper summarizes the status and performance of observatory infrastructure projects, technology upgrades, and new additions to the suite of observatory instrumentation. We also provide a status of instrumentation projects in early and advanced stages of development that will achieve the goals and objectives summarized in the 2023 Keck Observatory strategic plan. Developed in collaboration with the WMKO science community, the Keck strategic plan sets our sites on 2035 and meets goals identified in the Astro2020 Decadal Survey.

  PublisherOA PDFDOI

• Overview of the Fiber System for the Dark Energy Spectroscopic Instrument

  The Astronomical Journal · 2024-11-08 · 53 citations

  articleOpen access

  Abstract The Dark Energy Spectroscopic Instrument (DESI) is a revolutionary instrument designed for precise measurements of cosmic distances and the investigation of dark energy. DESI utilizes 5000 optical fibers to simultaneously measure the spectra of distant objects and aims to measure 40 million galaxies and quasars in a 5 yr survey. One of the critical challenges to DESI’s success was ensuring that the fiber system was not only highly efficient but also delivered a highly stable beam enabling more reliable sky subtraction for measurements of faint objects. We achieved this stability by minimizing the stress on the fiber system during the manufacture and operation of the telescope and fiber positioning robots. We installed the DESI fiber system on the 4 m Mayall telescope with ≥99% of fibers intact, and the instrument has delivered superb optical performance throughout the initial years of the DESI survey, including ≥90% average throughput when injected with a focal ratio of ∼ f /3.9 as delivered by the primary focus corrector, excluding fiber absorption losses. The design of DESI required multiple innovations to achieve these requirements, such as cleaved fibers bonded with a UV-curing epoxy to glass ferrules in the focal plane and fusion splicing instead of physical connectors. In this paper, we describe the development, delivery, and installation of the fiber system, the innovations that made the state-of-the-art performance possible, and the key lessons learned that could benefit future projects.

  PublisherDOI

Recent grants

• Collaborative Research: Towards the Edge of the Milky Way. The First Contiguous Spectroscopic Survey of the True Outer Halo.

  NSF · $196k · 2012–2017

Frequent coauthors

• Timothy C. Beers

  116 shared

• Donald G. York

  93 shared

• B. Yanny

  92 shared

• David J. Schlegel

  92 shared

• G. R. Knapp

  80 shared

• James E. Gunn

  79 shared

• Young Sun Lee

  University of Notre Dame

  77 shared

• Carlos Allende Prieto

  77 shared

Education

• Ph.D., Astronomy

  University of California, Santa Cruz

  1990

• M.S., Astronomy

  University of California, Santa Cruz

  1985

• B.A., Physics

  University of California, Santa Cruz

  1982