GeminiFocus April 2018 | Page 12

brightest galaxies in smaller groups. Figure 2. The slope η of the velocity dispersion profile is plotted against the central velocity dispersion σ 0 for individual galaxies in multiple different samples of galaxies (indicated in the legend). The blue points represent brightest galaxies in groups (BGGs) of high (square) and low (circles) density, while the green, red, and yellow points represent brightest cluster galaxies (BCGs) in various samples of galaxy clusters. The grey points indicate generic “early-type galaxies” (ETGs). The central dispersion σ 0 is larger for massive galaxies because their stars move more rapidly in the stronger gravitational fields of such galaxies. The dispersion profile η is negative if the stars farther from the galaxy center move more slowly, and it is positive if the stars move rapidly at larger distances. Thus, massive BCGs tend to have rising profiles, with the stars increasing their velocities at larger distances from the galaxy center. 10 information about the mass of a galaxy and how that mass is distributed. In a recent study, astronomers used the GMOS instruments at both Gemini North and South to measure the stellar velocity dispersions (a measure of the mean random stellar speed) and velocity dispersion pro- files (its variation with galactocentric radius) in a sample of 32 massive elliptical galaxies, each of which is the brightest member with- in a large cluster of galaxies. Such brightest cluster galaxies (BCGs) tend to reside near the centers of their respective clusters, and therefore they are generally embedded within very extended distributions of both light and dark matter. The sample of BCGs in this study included some of the most mas- sive galaxies in the Universe out to a dis- tance of five billion light years. The researchers found a surprising variety in the shapes of the velocity dispersion pro- files for the BCGs, with a large fraction of them showing rising dispersion profiles. This means that the stars within these galaxies are moving faster at larger galactocentric distances in response to an increasing gravi- tational force. In comparison, rising velocity profiles are much rarer in other massive el- lipticals that are not BCGs, including many GeminiFocus “You would naively think that massive el- liptical galaxies are a homogeneous, well- behaved class of ob- jects, but the most massive beasts, those in the centers of groups and clusters, continue to surprise us,” said Ilani Loubser, an astrono- mer at North-West Uni- versity in South Africa and the lead author of the study. She also noted, “The quality, and the wealth of information we can measure from the GMOS spectra (even in poor weath- er!), is remarkable.” The study also found that the slopes of the velocity dispersion profiles correlate with the galaxy luminosity (Figure 2), in the sense that the increase in the speed of the stars was greater in brighter BCGs, as well as in bright- est group galaxies. Whether the full diversity in the observed velocity dispersion profiles is consistent with standard models for the growth of massive galaxies is not yet clear. The researchers present their results as a challenge for detailed cosmological simula- tions. The work has been accepted for publication in Monthly Notices of the Royal Astronomical Society, and a preprint is available online. An Ultra-diffuse Galaxy Devoid of Dark Matter In most galaxies, the stars are like a luminous frosting on the predominant mass of dark matter whose gravity holds the whole con- fection together. Both the most luminous gi- ants and the faintest dwarfs have especially large fractions of dark matter, outweighing April 2018