Draco Dwarf

Lore & Background

The Draco Dwarf was discovered by Albert George Wilson of Lowell Observatory in 1954 on photographic plates of the National Geographic Society's Palomar Observatory Sky Survey. In 1961, Walter Baade and Henrietta H. Swope studied the galaxy and discovered over 260 variables, with 138 in the cluster's center; all but five were RR Lyrae variables. From this work a distance modulus of 19.55 was derived, implying a distance of 81 kpc, with a modern estimate of 80 ± 10 kpc. Its size is around 830 ± 100 × 570 ± 70 pc. Paul W. Hodge analyzed the distribution of its stars in 1964 and concluded its ellipticity was 0.29 ± 0.04.

The galaxy contains primarily an old population of stars, with 75% to 90% of its stars formed more than ~10 Gyr ago, followed by a low rate of formation and a small burst around 2–3 Gyr ago. It has a single Gaussian metallicity distribution with an average of [Fe/H] = −1.74 dex and a standard deviation of 0.24 dex, with a small tail of metal-rich stars. The central region shows a concentration of more metal-rich stars, with red horizontal branch stars more centrally concentrated than blue horizontal branch stars. The Draco Dwarf contains many red giant branch stars, five identified carbon stars, and four likely asymptotic giant branch stars. It has insignificant amounts of interstellar matter and is basically dust free.

Radial velocity measurements of individual stars have revealed a large internal velocity dispersion, giving a mass-to-luminosity ratio of up to 440 M☉/L☉, suggesting large amounts of dark matter. The hypothesis that large velocity dispersions could be explained as tidal dwarfs is not supported by the galaxy's narrow horizontal branch width, leaving dark matter as the explanation. As of 2007, the Draco Dwarf was considered the most dark matter dominated object known. At large radii, the radial velocity dispersion exhibits strange behavior, possibly due to more than one stellar population, suggesting further study is needed. In 2024, a group using the Hubble Space Telescope measured proper motions with 18 years of data, making Draco the first dwarf galaxy to have its 3D velocity dispersion profile radially resolved, showing its dark matter distribution is in better agreement with the LCDM model, helping to alleviate the cusp-core problem.

Reader's Guide

The Draco Dwarf holds significant importance in astrophysics as a laboratory for dark matter studies. Its large internal velocity dispersion, yielding a mass-to-luminosity ratio of up to 440 M☉/L☉, indicates that dark matter dominates its mass. The narrow horizontal branch width rules out tidal disruption as an explanation for the high velocity dispersion, reinforcing the dark matter interpretation. As of 2007, it was considered the most dark matter dominated object known. The 2024 Hubble Space Telescope study, using 18 years of data, provided the first radially resolved 3D velocity dispersion profile for a dwarf galaxy, showing that Draco's dark matter distribution aligns with the LCDM model and helps address the cusp-core problem. The galaxy's old stellar population, low metallicity, and lack of interstellar matter make it a pristine system for studying early galaxy formation and the properties of dark matter on small scales. Its status as a faint Milky Way satellite, with an absolute magnitude of −8.6 and luminosity of only 2×10^5 L☉, underscores its value for understanding the faintest galaxies and their role in cosmological models.

Did You Know?

• It contains over 260 variable stars, of which 133 in the cluster's center are RR Lyrae variables.
• The galaxy has a mass-to-luminosity ratio of up to 440 M☉/L☉, suggesting it is dominated by dark matter.