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Massive Cluster Galaxies Move in Unexpected Ways

May 3, 2018

Figure 1. MS0440+02 galaxy cluster. The central galaxy is a multi-component BCG formed by six bright elliptical spheroids, all at the same redshift. This is a color composite GMOS South image (g, r, i) of the clusters. The size of the image is 2.6 x 2.6 arcmin2 (N up, E left). Credit: R. Carrasco (Gemini Observatory/AURA) and Tomás Verdugo (UNAM).

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Figure 2. The slope η of the velocity dispersion profile is plotted against the central velocity dispersion σ0 for galaxies in multiple different samples. 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 slope η is negative if the velocity dispersion decreases with radius and positive if it rises. Thus, massive BCGs tend to have rising profiles, with the stellar velocities responding to the cluster potential at larger radii. [Reproduced from Loubser et al. 2018, MNRAS, in press.]

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Astronomers using data from both of the Gemini Multi-Object Spectrographs (GMOS - North and South) measured the motions of stars within a sample of 32 massive elliptical cluster galaxies and found the stellar motions inconsistent with these galaxies’ solitary cousins.

The galaxies chosen are known as brightest cluster galaxies (BCGs) because they are the brightest members of large galaxy clusters. The international team of astronomers obtained Gemini spectra to find the relative velocities of stars within each galaxy and then determine the central stellar velocity dispersions and radial dispersion profiles for each galaxy. “This is similar to what we see in our own Solar System with the different velocities of the planets around the Sun,” said John Blakeslee, Gemini Observatory's Head of Science. “We use the planets’ velocities to determine our Solar System’s mass distribution and it is also how we know the Sun’s mass accurately.”

The researchers discovered a surprising variety in the shapes of the velocity dispersion profiles for the BCGs, with a large fraction showing rising dispersion profiles (Figure 2). A rising velocity dispersion profile means that the stars within these galaxies are moving faster as you look further from the galaxy’s core in response to an increasing gravitational force. In comparison, rising velocity dispersion profiles are much rarer in other massive ellipticals that are not BCGs, including many brightest galaxies in groups (BGGs).

“You would naively think that massive elliptical galaxies are a homogeneous, well-behaved class of objects, but the most massive beasts, those in the centers of groups and clusters, continue to surprise us,” said Ilani Loubser, an astronomer at North-West University in South Africa and the lead author of the study, which has been accepted for publication in Monthly Notices of the Royal Astronomical Society. She also noted, “The quality, and the wealth of information we can measure from the GMOS spectra (even in poor weather), is remarkable!”

BCGs tend to reside near the centers of their respective clusters, and are therefore generally embedded within extended distributions of both light and dark matter. The sample of BCGs in this study included some of the most massive known galaxies in the Universe out to a distance of about 3.2 billion light years (z ~ 0.3).

The study also found that the slopes of the velocity dispersion profiles correlate with the galaxy luminosity, in the sense that the increase in the speed of the stars is greater in brighter BCGs, as well as BGGs. 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. More detailed comparisons with velocity dispersion profiles in cosmological simulations are needed.

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