A search for the most massive galaxies - II. Structure, environment and formation

Abstract

We study a sample of 43 early-type galaxies, selected from the Sloan Digital Sky Survey (SDSS) because they appeared to have velocity dispersions of σ ≥ 350 km s −1 . High-resolution photometry in the SDSS i passband using the High-Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope shows that just less than half of the sample is made up of superpositions of two or three galaxies, so the reported velocity dispersion is incorrect. The other half of the sample is made up of single objects with genuinely large velocity dispersions. None of these objects has σ larger than 426 ± 30 km s −1 . These objects define rather different size-, mass-and density-luminosity relations than the bulk of the early-type galaxy population; for their luminosities, they are the smallest, most massive and densest galaxies in the Universe. Although the slopes of the scaling relations they define are rather different from those of the bulk of the population, they lie approximately parallel to those of the bulk at fixed σ . This suggests that these objects are simply the large-σ extremes of the early-type population; they are not otherwise unusual. These objects appear to be of two distinct types. The less luminous (M r > −23) objects are rather flattened, and their properties suggest some amount of rotational support. While this may complicate interpretation of the SDSS velocity dispersion estimate, and hence estimates of their dynamical mass and density, we argue that these objects are extremely dense for their luminosities, suggesting merger histories with abnormally large amounts of gaseous dissipation. The more luminous objects (M r < −23) tend to be round and to lie in or at the centres of clusters. Their circular isophotes, large velocity dispersions and environments are consistent with the hypothesis that they are brightest cluster galaxies (BCGs). Models in which BCGs form from predominantly radial mergers having little angular momentum predict that they should be prolate. If viewed along the major axis, such objects would appear to have abnormally large velocity dispersions for their sizes, and to be abnormally round for their luminosities. This is true of the objects in our sample, once we account for the fact that the most luminous galaxies (M r < −23.5), and BCGs, become slightly less round with increasing luminosity. Thus, the shapes of the most luminous galaxies suggest that they formed from radial mergers, and the shapes of the most luminous objects in our large-σ sample suggest that they are the densest of these objects, viewed along the major axis.

Stellar Populations of the Most Massive Galaxies

“…These systems are located either in high or low-density environments and show a variety of small surface-brightness structure. Using archival HST/ACS images and Gemini/GMOS spectroscopy, we will explore the photometric and spectroscopic properties of these galaxies.These massive galaxies define steeper size, mass, and mass density-luminosity relations than the bulk of the SDSS early-type galaxy population (Bernardi et al 2008). These results are consistent with dry mergers contributing importantly to the mass budget of massive galaxies.…”

“…These massive galaxies define steeper size, mass, and mass density-luminosity relations than the bulk of the SDSS early-type galaxy population (Bernardi et al 2008). These results are consistent with dry mergers contributing importantly to the mass budget of massive galaxies.…”

Dozens of compact and high velocity-dispersion, early-type galaxies in Sloan Digital Sky Survey

“…Since b19 was first analysed in Bernardi et al (2008), we have cross-matched our candidate sample with their list of 43 massive early-type galaxies. Although they also selected their sample by high central velocity dispersion, the only other galaxy in common with our candidate sample, aside from b19, is b17, which has the internal ID 3.…”

“…Our goal is to define a sample to be used for follow-up observations to determine whether there is a systematic variation in the initial mass function, such as the bottom-heavy initial mass function of b19 (Läsker et al 2013), and if they host over-massive central black hole such as the one in NGC 1277 (van den Bosch et al 2012). Furthermore, follow-up observation will also be required to clean the sample from high central velocity dispersion galaxies that are superpositions of two or more galaxies, which is a known issue with this kind of galaxies in SDSS (Bernardi et al 2008).…”

“…The starting point of our investigation is [BHF2008] 19, which is the galaxy with ID number 19 on the list of the highest dispersion galaxies in SDSS by Bernardi et al (2008). It is a very compact and massive early-type galaxy with a size of R e = 2.17 kpc and mass of 10 11 M .…”