Young main-sequence (MS) stars and red-giant branch (RGB) stars around M81, M82, and NGC 3077

Astronomers using the Subaru Telescope’s Hyper Suprime-Cam prime-focus camera recently observed the nearby large spiral galaxy M81, together with its two brightest neighbors, M82 and NGC3077. The results of their observations are deep, super wide-field images of the galaxies and their populations of young stars. As part of a Galactic Archaeology study, the team discovered that the spatial distribution of the young stars around these galaxies follows very closely that of their distribution of neutral hydrogen.

Young main-sequence (MS) stars and red-giant branch (RGB) stars around M81, M82, and NGC 3077. Left: yellow is brighter stars, and blue is fainter stars. Right: color-coded for the metallicity, namely yellow is metal rich, blue is metal poor. Solid line shows the R25 radius of the galaxy measured in the visible light. Source: NAOJ

The left panel shows the spatial distribution of young MS stars and core helium-burning stars, which are color-coded according to their i-band luminosity. Bright stars are mainly located in the inner disk of M81, while most of the young stars in outlying concentrations are fainter than i=24 mag and have similar luminosity distributions as that of the stellar stream between M81 and NGC 3077. They are between 30-160 million years old. The study indicates the ages of stars in these tidal features are synchronized to each other, and that these systems were produced by recent tidal interactions between M81, M82 and NGC 3077.

The distribution of RGB stars in the right panel shows that the extended stellar halos of the three main galaxies overlap each other, and that the outer regions of M82 and NGC 3077 are highly perturbed. This is likely a consequence of the recent gravitational encounter.

The color of each point in the figure is a rough proxy for metallicity. The RGB stars in M82’s outer halo have significantly bluer colors, showing that they are more metal-poor than those in M81, the NGC 3077 halos and the inner halo of M82. The satellite galaxies, KDG 61, BK5N, and IKN cannot be seen in the maps of young stars, but appear as over-densities of old populations in the bottom panel. This implies they are not the product of the recent interaction between M81, M82 and NGC 3077.

Their first results from the M81 study with Suprime-Cam on Subaru Telescope were released in March 2010.

Pseudo-color images from HSC observation which contains M81, M82, and NGC 3077. Diameter of the FOV is 1.5 degrees. Bottom-left: close-up of M81. Bottom-center: further close-up of M81 showing the spiral arm. Bottom-right: color composite of the images used for the analysis

Source: Subaru Telescope