The structural and dynamical properties of compact elliptical galaxies

Abstract

Dedicated photometric and spectroscopic surveys have provided unambiguous evidence for a strong stellar mass-size evolution of galaxies within the last 10 Gyr. The likely progenitors of today's most massive galaxies are remarkably small, disky, passive and have already assembled much of their stellar mass at redshift z=2. An indepth analysis of these objects, however, is currently not feasible due to the lack of high-quality, spatially-resolved photometric and spectroscopic data. In this paper, we present a sample of nearby compact elliptical galaxies (CEGs), which bear resemblance to the massive and quiescent galaxy population at earlier times. Hubble Space Telescope (HST ) and wide-field integral field unit (IFU) data have been obtained, and are used to constrain orbit-based dynamical models and stellar population synthesis (SPS) fits, to unravel their structural and dynamical properties. We first show that our galaxies are outliers in the present-day stellar mass-size relation. They are, however, consistent with the mass-size relation of compact, massive and quiescent galaxies at redshift z=2. The compact sizes of our nearby galaxies imply high central stellar mass surface densities, which are also in agreement with the massive galaxy population at higher redshift, hinting at strong dissipational processes during their formation. Corroborating evidence for a largely passive evolution within the last 10 Gyr is provided by their orbital distribution as well as their stellar populations, which are difficult to reconcile with a very active (major) merging history. This all supports that we can use nearby CEGs as local analogues of the high-redshift, massive and quiescent galaxy population, thus providing additional constraints for models of galaxy formation and evolution.

On the formation mechanisms of compact elliptical galaxies

“…Such objects have been nicknamed 'relic galaxies', but so far they have been found in the nearby Universe with masses of ∼10 11 M ⊙ (e.g. Trujillo et al 2014;Ferré-Mateu et al 2015;Ferré-Mateu et al 2017;Yıldırım et al 2017). The fact that this galaxy has no host but is in a cluster, further reinforces this idea, as such relics seem to prefer the centers of clusters (e.g.…”

Galaxy structural analysis with the curvature of the brightness profile

“…However, the structure of this galaxy is not trivial to be resolved. Yıldırım et al (2017) comment that the PA twist in the centre of the galaxy may imply the existence of a bar and a dust disk. Examining κ in Fig.…”

“…(i) galaxies with complex structure, e.g. bars, rings, arms, disks, which are notably seem: NGC 1211 (Buta 2013), NGC 0936 ( (Tarenghi et al 1994;Prieto et al 2001); (iii) galaxies with well known structure in shape, for example ellipticals: NGC 472, NGC 1270 (Yıldırım et al 2017).…”

“…NGC 1270 is classified as an elliptical galaxy (de Vaucouleurs et al 1991) and further as a compact elliptical galaxy (Yıldırım et al 2017). The curvature κ(R) is displayed in Fig.…”

“…NGC 472 is a compact elliptical galaxy (Yıldırım et al 2017). Similar to NGC 1270, it does show some perturbations in the inner regions -the two local peaks are attributed to inhomogeneities -but the shape of κ follows an overall shape resembling a pure component, which is smooth and decreases from inner to outer regions.…”