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The Draco Dwarf Spheroidal Galaxy in the First Year of DESI Data

J. Ding, C. Rockosi, Ting S. Li, S. E. Koposov, A. H. Riley, W. Wang, A. P. Cooper, N. Kizhuprakkat, M. Lambert, G. E. Medina, N. Sandford, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, A. de la Macorra, P. Doel, J. E. Forero-Romero, E. Gaztanaga, S. Gontcho A Gontcho, G. Gutierrez, J. Guy, M. Ishak, R. Kehoe, T. Kisner, A. Kremin, O. Lahav, M. Landriau, L. Le Guillou, A. Meisner, R. Miquel, J. Moustakas, F. Prada, I. Perez-Rafols, G. Rossi, E. Sanchez, M. Schubnell, J. Silber, D. Sprayberry, G. Tarle, B. A. Weaver, R. Zhou

TL;DR

Using year-1 DESI data and SV observations, the Draco dwarf spheroidal galaxy is re-examined to map its kinematics, chemistry, and possible extended structure. A two-component Gaussian mixture model incorporating line-of-sight velocity, metallicity, and Gaia DR3 proper motions identifies 155 high-prob Draco members (p>0.9) and yields a mean heliocentric velocity of -290.62 ± 0.80 km/s with a dispersion of 9.57^{+0.66}_{-0.62} km/s and a mean [Fe/H] of -2.10 ± 0.04. The analysis reveals a metallicity gradient that steepens inside the half-light radius and flattens beyond it; eight high-probable members lie outside the King tidal radius, four of which are new, not preferentially aligned with Draco's orbit. The mean surface brightness within the annulus from 48.1 to 81 arcmin is 34.02 mag arcsec^-2, and the results are consistent with previous Draco studies while extending constraints to the galaxy's outskirts.

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 {\it Gaia} DR3 proper motions. We find a mean line of sight velocity of $ -290.62\pm0.80$ km s$^{-1}$ with dispersion = $9.57^{+0.66}_{-0.62}$ km s$^{-1}$ and mean metallicity $\rm{[Fe/H]}$ = $-2.10\pm0.04$, 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 extra-tidal stars are not preferentially aligned along the orbit of Draco. We compute an average surface brightness of 34.02 mag $\rm arcsec^{-2}$ within an elliptical annulus from the King tidal radius of 48.1 arcmin to 81 arcmin.

The Draco Dwarf Spheroidal Galaxy in the First Year of DESI Data

TL;DR

Using year-1 DESI data and SV observations, the Draco dwarf spheroidal galaxy is re-examined to map its kinematics, chemistry, and possible extended structure. A two-component Gaussian mixture model incorporating line-of-sight velocity, metallicity, and Gaia DR3 proper motions identifies 155 high-prob Draco members (p>0.9) and yields a mean heliocentric velocity of -290.62 ± 0.80 km/s with a dispersion of 9.57^{+0.66}_{-0.62} km/s and a mean [Fe/H] of -2.10 ± 0.04. The analysis reveals a metallicity gradient that steepens inside the half-light radius and flattens beyond it; eight high-probable members lie outside the King tidal radius, four of which are new, not preferentially aligned with Draco's orbit. The mean surface brightness within the annulus from 48.1 to 81 arcmin is 34.02 mag arcsec^-2, and the results are consistent with previous Draco studies while extending constraints to the galaxy's outskirts.

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 {\it Gaia} DR3 proper motions. We find a mean line of sight velocity of km s with dispersion = km s and mean metallicity = , 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 extra-tidal stars are not preferentially aligned along the orbit of Draco. We compute an average surface brightness of 34.02 mag within an elliptical annulus from the King tidal radius of 48.1 arcmin to 81 arcmin.

Paper Structure

This paper contains 4 sections, 5 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Data
  3. Color-Magnitude Selection
  4. Methods

Figures (2)

  • Figure 1: Color-magnitude diagram of our DESI Draco sample. The region inside the dashed lines is the color selection for the Draco candidates sample from the DESI DR1 Stellar Catalog data. For the color-magnitude cuts used in the selection, we use an old, metal-poor ([Fe/H] = $-$1.5 dex) Dartmouth isochrone with an age = 10 Gyr Dotter2008 and select stars within $\pm$0.39 mag of the Isochrone $g-r$ value. We use Legacy survey Dey2019 photometry of the M92 globular cluster to define a region for the horizontal branch, since that is not included in the isochrone. The data sample after the color cut is labeled in red triangle, and the final selected data sample (see §\ref{['2.1']}) is labeled in blue circle.
  • Figure 2: The probability distribution for Draco members identified with the GMM. From this probability distribution, we define the high probability members to be $p$$>$ 0.9. There are 155 stars that pass this criterion.