In the current hierarchical paradigm of galaxy formation, early-type galaxies assemble their masses with time. Their formation entails two phases: an initial one of strong star formation is followed by an extensive growth through accretion and mergers of smaller satellites, that causes a dramatic increase of their sizes towards low redshifts.
Numerical cosmological simulations predict that the mergers which build up the halos of massive galaxies at the centre of clusters and groups involve satellites with mass ratios 1:5. As massive galaxies typically have a larger metal-content, one would expect that the light in these halos comes mostly from stars at about half-solar metallicity and thus with redder optical colours.
It comes as a surprise that the outermost halos of the nearby massive galaxies are bluer instead, indicating that the satellite galaxy progenitors where these stars were born had to be either overall very young in age (hence star-forming) or very small in mass.
Using data obtained with the Planetary Nebula Spectrograph (PN.S) on the William Herschel Telescope (WHT), J. Hartke, M. Arnaboldi and collaborators pursued an original investigation to constrain the mass of the satellite progenitors of the stars in the halos and determine a proxy for their dynamical age by measuring the motions along the lines of sight of hundreds of stars that are in a particular stage of their evolution: the Planetary Nebulae (PNe).
PNe are the late stages of Sun-like stars whose [OIII] 5007 Å emission is relatively strong. Such 'green' monochromatic light makes them similar to beacons whose motions are easy to measure. Previous studies have shown that the distribution of the PN velocities is a fair sampling of that of the parent stars. For this study, Harke et al. used the PN velocities in the halo of the brightest galaxy in the Virgo cluster, NGC 4472 or M49.
By combining accurate velocities from the PN.S instrument with magnitudes measured during an imaging campaign at the Subaru telescope, Hartke et al. identified distinct components in the magnitude-velocity plane of the entire PN sample. Instead of a uniform stellar population floating in the gravitational potential of the galaxy, astronomers identified three populations, one associated with the smooth halo of M49, a sub-component of bright planetary nebulae associate with the recent accretion of a dwarf galaxy (VCC 1249), and a population of stars associated with the intra-group light.
As shown in the following figure, the distinct velocity-dispersion profiles are plotted from the whole, bright and faint subsamples. The combination of precise photometry from SuprimeCam on the Subaru telescope with the accurate velocities measured with the PN.S instrument on the WHT made it possible to identify these populations with confidence.