From Gemini Observatory: “Massive Cluster Galaxies Move in Unexpected Ways”


Gemini Observatory
From Gemini Observatory

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).

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.]

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.

Gemini Observatory GMOS on Gemini South

Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet


Gemini/North telescope at Maunakea, Hawaii, USA,4,207 m (13,802 ft) above sea level

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.

See the full article here .

Please help promote STEM in your local schools.


Stem Education Coalition

Gemini/North telescope at Maunakea, Hawaii, USA,4,207 m (13,802 ft) above sea level

Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet


Gemini’s mission is to advance our knowledge of the Universe by providing the international Gemini Community with forefront access to the entire sky.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai’i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in six partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country’s contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, and the Brazilian Ministério da Ciência, Tecnologia e Inovação. The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.