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A Dark Matter Deficient Galaxy

Hubble Space Telescope image of NGC2052-DF2

A deep Hubble Space Telescope image of the diffuse dwarf galaxy NGC2052-DF2. Astronomers measured the luminosities of faint stars at the tip of the red giant branch in this galaxy to improve the accuracy of its distance and to conclude that, in agreement with earlier studies, it does appear to be very deficient in dark matter.

NASA/Hubble; Shen et al 2021

The galaxy NGC 1052-DF2 resides in a field of galaxies about sixty-five million light-years away. Its low mass, only about two hundred million solar-masses, makes it a "dwarf" and its size, about fifteen thousand light-years in diameter, places it in the regime of ultra-diffuse galaxies. It is also distinguished by hosting a large population of luminous globular clusters. Two years ago a second, similar faint dwarf galaxy was found near it, and the relative motions of these two galaxies strongly suggest they have very little or no dark matter; for comparison, in the Milky Way (a normal galaxy) the dark matter contains nearly ten times more mass than the stellar matter. Astronomers also noted that, if these mass and motion measurements are accurate, they might be used to test and reject (or confirm) one long-standing alternate theory of gravity to Einstein’s. But first the distance to the galaxy needs to be known more accurately and precisely, since the values for many of the galaxy's properties like motion, luminosity and inferred mass rely on the assumed distance.

CfA astronomer Charlie Conroy was a member of a team of astronomers that determined a new distance for the galaxy. Rather than rely on its velocity /redshift to get the distance, a value that might be confused by its local motion within the group of galaxies, they derived the distance using the brightness of faint red giant stars at the tip of the red giant branch, a standard and well-established method when the stars are bright enough to be accurately studied. As a low-mass star burns through nearly all of its hydrogen and starts to fuse helium in its core, the transition is accompanied by a shrinking in size, a rapid rise in the core temperature, a shift towards a bluer color, and an abrupt decrease in luminosity. This rapid change in brightness is readily apparent in optical and near-infrared wavelengths; the absolute luminosities of those stars can then be determined and, from their apparent luminosities, their distance calculated. The astronomers used forty orbits of the Hubble Space Telescope to measure the brightness of the red giant stars in NGC 1052-DF2 and from that data to determine their distance: 72.7 million light-years plus-or-minus 5%. This new distance confirms the unusual characteristics of the galaxy including its puzzling dearth of dark matter and that its globular clusters are exceptionally luminous (in fact the new distance makes them even more luminous). The result also means the data will not be suitable for tests of the proposed alternative theory of gravity.

Reference: "A Tip of the Red Giant Branch Distance of 22.1±1.2Mpc to the Dark Matter Deficient Galaxy NGC 1052–DF2 from 40 Orbits of Hubble Space Telescope Imaging," Zili Shen, Shany Danieli, Pieter van Dokkum, Roberto Abraham, Jean P. Brodie, Charlie Conroy, Andrew E. Dolphin, Aaron J. Romanowsky, J. M. Diederik Kruijssen, and Dhruba Dutta Chowdhury, The Astrophysical Journal Letters 914, L12, 2021.