Distribution Of Early-type Galaxies Research Articles

The fundamental plane in the hierarchical context

Context. The fundamental plane (FP) relation and the distribution of early-type galaxies (ETGs) in the FP projections cannot be easily explained in the hierarchical framework, where galaxies grow up by merging and as a result of star formation episodes. Aims. We want to show here that both the FP and its projections arise naturally from the combination of the virial theorem (VT) and a new time-dependent relation, describing how luminosity and stellar velocity dispersion change during galaxy evolution. This relation has the form of the Faber-Jackson relation, but a different physical meaning: the new relation is L = L0′(t)σβ(t), where its coefficients L0′ and β are time-dependent and can vary considerably from object to object, at variance with those obtained from the fit of the L − σ plane. Methods. By combining the VT and L = L0′(t)σβ(t) law, we derived an equation for each galaxy that is identical in form to the FP, but with coefficients depending on β. This allowed us to extract the solutions for β as a function of the structural parameters of ETGs and consequently calculate the coefficients of the FP-like equations. Results. We demonstrate that the observed properties of ETGs in the FP and its projections can be understood in terms of variations of β and L0′. These two parameters encrypt the history of galaxy evolution across the cosmic epochs and determine the future aspect of the FP and its projections. In particular, we show that the FP coefficients are simple averages of those in the FP-like equations valid for each galaxy, and that the variations of β naturally explain the distributions of ETGs observed in the FP projections and the direction of the border of the Zone of Exclusion.

Tomography of the Ie-Re and L-Sigma Planes

We have analyzed the distribution of early-type galaxies (ETGs) in the effective surface intensity vs. effective radius (Ie−Re) plane and in the total luminosity vs. central stellar velocity dispersion (L−σ) diagram, with the aim of studying the physical variables that allow the transformation of one space-parameter into the other. We find that the classical Faber–Jackson relation L=L0σα, in which the parameters L0 and α are confined in a small range of possible values, is incompatible with the distribution observed in the Ie−Re plane. The two distributions become mutually consistent only if luminosity is not considered a pure proxy of mass but a variable tightly dependent on the past history of mass assembling and star formation and on the present evolutionary state of the stellar content of a galaxy. The solution comes by considering the L=L0′σβ law proposed by D’Onofrio et al. in 2020, in which both L0′ and β can vary considerably from galaxy to galaxy. We will also show that the data of the Illustris numerical simulation prove the physical foundation of the L=L0′σβ law and confirm the prediction of the Zone of Exclusion (ZoE) originating from the intersection of the virial law with the L=L0′σβ relation. The ZoE is the region in the Ie−Re and Re−Ms diagrams avoided by real galaxies, and the border of which marks the condition of ‘full’ virial equilibrium with no recent significant merger events and no undergoing star formation.

MID-IR ENHANCED GALAXIES IN THE COMA & VIRGO CLUSTERS: LENTICULARS WITH A HIGH STAR FORMATION RATE

We explore the properties of early-type galaxies (ETGs), including ellipticals (E) and lenticulars (S0), in rich environments such as clusters of galaxies (Virgo and Coma). The L_24/L_K distribution of ETGs in both Virgo and Coma clusters shows that some S0s have a much larger L_24/L_K ratio (0.5 to ~2 dex) than the bulk of the ETG population. This could be interpreted as an enhanced star formation rate in these lenticulars. We compare the optical colors of galaxies in these two clusters and investigate the nature of these sources with a large L24/L_K ratio by looking at their spatial distribution within the cluster, by analyzing their optical spectra and by looking at their optical colors compared to late-types. We obtain 10 Coma and 3 Virgo early-type sources with larger L24/L_K ratios than the bulk of their population. We call these sources Mid-Infrared Enhanced Galaxies (MIEGs). In Coma, they are mostly located in the South-West part of the cluster where a substructure is falling onto the main cluster. MIEGs present lower g-r color than the rest of the ETG sample, because of a blue continuum. We interpret the excess L24/L_K ratio as evidence for an enhanced star-formation induced as a consequence of their infall into the main cluster.

Mid-Infrared Enhanced Lenticulars caught in the dusty midst of transformation in the Coma Cluster

AbstractWe explore the properties of early-type galaxies (ETGs) in rich environments such as clusters of galaxies. The L24/LK distribution of ETGs in both Virgo and Coma clusters shows that some lenticulars (S0, 10 in Coma and 3 in Virgo) have a much larger L24/LK ratio (0.5 to ~2 dex) than the bulk of the ETG population. We call these sources Mid-Infrared Enhanced Galaxies (MIEGs). In Coma, they are mostly located in the South-West part of the cluster where a substructure is falling onto the main cluster. MIEGs present lower g-r color than the rest of the ETGs, because of a blue continuum. We interpret the excess L24/LK ratio as evidence for an enhanced star-formation induced as a consequence of their infall into the main cluster.

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

Aims. We aim at finding candidates of potential survivors of high-redshift compact galaxies in SDSS, as targets for more detailed follow-up observations. Methods. From the virial theorem it is expected that for a given mass, compact galaxies have stellar velocity dispersion higher than the mean due to their smaller sizes. Therefore velocity dispersion coupled with size (or mass) is an appropriate method to select relics, independent of the stellar population properties. Based on these consideration we design a set of criteria using distribution of early-type galaxies from SDSS on the log$_{10}$(R$_{0}$)-log$_{10}$($\sigma_{0}$) plane to find the most extreme objects on it. Results. We find 76 galaxies at 0.05 < z < 0.2, which have properties similar to the typical quiescent galaxies at high redshift. We study how well these galaxies fit on well-known local universe relations of early-type galaxies such as the fundamental plane, the red sequence or mass-size relations. As expected from the selection criteria, the candidates are located in an extreme corner of mass-size plane. However, they do not extend as deeply into the so-called zone of exclusion as some of the high-redshift compact galaxies ('red nuggets') found at high redshift, being a factor 2-3 less massive at a given intrinsic scale size. Our candidates are systematically offset from scaling relations of average early-type galaxies, while being in the mass-size range expected for passive evolution of the red nuggets from their high redshift to the present. Conclusions. The 76 selected candidates form a well suited set of objects for further follow-up observations. We argue that selecting a high velocity dispersion is the best way to find analogues of compact high redshift galaxies in the local universe.

The ATLAS3D project – XX. Mass–size and mass–σ distributions of early-type galaxies: bulge fraction drives kinematics, mass-to-light ratio, molecular gas fraction and stellar initial mass function

In the companion Paper XV of this series, we derive accurate total mass-to-light ratios |$(\rm M/L)_{\rm JAM}\approx ({\rm M/L})({\it r}= {R_{\rm e}})$| within a sphere of radius |$r= {R_{\rm e}}$| centred on the galaxy, as well as stellar (M/L)stars (with the dark matter removed) for the volume-limited and nearly mass-selected (stellar mass |$M_\star \gtrsim 6\times 10^9 {\,\mathrm{M}_{\odot }}$|⁠) ATLAS3D sample of 260 early-type galaxies (ETGs, ellipticals Es and lenticulars S0s). Here, we use those parameters to study the two orthogonal projections |$({M_{\rm JAM}}, {\sigma _{\rm e}})$| and |$({M_{\rm JAM}}, {R_{\rm e}^{\rm maj}})$| of the thin Mass Plane (MP) |$({M_{\rm JAM}}, {\sigma _{\rm e}}, {R_{\rm e}^{\rm maj}})$| which describes the distribution of the galaxy population, where |$ {M_{\rm JAM}}\equiv L\times ({\rm M/L})_{\rm JAM}\approx M_\star$|⁠. The distribution of galaxy properties on both projections of the MP is characterized by: (i) the same zone of exclusion (ZOE), which can be transformed from one projection to the other using the scalar virial equation. The ZOE is roughly described by two power laws, joined by a break at a characteristic mass |$ {M_{\rm JAM}}\approx 3\times 10^{10} {\,\mathrm{M}_{\odot }}$|⁠, which corresponds to the minimum Re and maximum stellar density. This results in a break in the mean |$ {M_{\rm JAM}}\text{--} {\sigma _{\rm e}}$| relation with trends |$ {M_{\rm JAM}}\propto \sigma _{\rm e}^{2.3}$| and |$ {M_{\rm JAM}}\propto \sigma _{\rm e}^{4.7}$| at small and large σe, respectively; (ii) a characteristic mass |$ {M_{\rm JAM}}\approx 2\times 10^{11} {\,\mathrm{M}_{\odot }}$| which separates a population dominated by flat fast rotator with discs and spiral galaxies at lower masses, from one dominated by quite round slow rotators at larger masses; (iii) below that mass the distribution of ETGs’ properties on the two projections of the MP tends to be constant along lines of roughly constant σe, or equivalently along lines with |$ {R_{\rm e}^{\rm maj}}\propto {M_{\rm JAM}}$|⁠, respectively (or even better parallel to the ZOE: |$ {R_{\rm e}^{\rm maj}}\propto M_{\rm JAM}^{0.75}$|⁠); (iv) it forms a continuous and parallel sequence with the distribution of spiral galaxies; (v) at even lower masses, the distribution of fast-rotator ETGs and late spirals naturally extends to that of dwarf ETGs (Sph) and dwarf irregulars (Im), respectively. We use dynamical models to analyse our kinematic maps. We show that σe traces the bulge fraction, which appears to be the main driver for the observed trends in the dynamical (M/L)JAM and in indicators of the (M/L)pop of the stellar population like Hβ and colour, as well as in the molecular gas fraction. A similar variation along contours of σe is also observed for the mass normalization of the stellar initial mass function (IMF), which was recently shown to vary systematically within the ETGs’ population. Our preferred relation has the form |$\log _{10} [({\rm M/L})_{\rm stars}/({\rm M/L})_{\rm Salp}]=a+b\times \log _{10}({\sigma _{\rm e}}/130\, {km s^{-1}})$| with a = −0.12 ± 0.01 and b = 0.35 ± 0.06. Unless there are major flaws in all stellar population models, this trend implies a transition of the mean IMF from Kroupa to Salpeter in the interval |$\log _{10}({\sigma _{\rm e}}/{\rm km\, s}^{-1})\approx 1.9\text{--}2.5$| (or |$ {\sigma _{\rm e}}\approx 90\text{--}290$| km s−1), with a smooth variation in between, consistently with what was shown in Cappellari et al. The observed distribution of galaxy properties on the MP provides a clean and novel view for a number of previously reported trends, which constitute special two-dimensional projections of the more general four-dimensional parameters trends on the MP. We interpret it as due to a combination of two main effects: (i) an increase of the bulge fraction, which increases σe, decreases Re, and greatly enhance the likelihood for a galaxy to have its star formation quenched, and (ii) dry merging, increasing galaxy mass and Re by moving galaxies along lines of roughly constant σe (or steeper), while leaving the population nearly unchanged.

2D modelling of the light distribution of early-type galaxies in a volume-limited sample - I. Simulations with artificial data

This paper is the first of two dedicated to the application of a two-dimensional (2D) fit to the light distribution of 73 early-type galaxies belonging to the Virgo and Fornax clusters, a sample of which is 80 per cent complete, volume and magnitude limited down to MB=−17.3, and highly homogeneous. Two empirical models of the surface-brightness distribution of the early-type galaxies have been tested: the first uses a single spheroid represented by the r1/n Sersic law, while the second is characterized by two components, ‘bulge’ and ‘disc’, by means of the (r1/n+exp) laws. The χ2 fitting routine (minuit), used to fit the 2D light distribution of real galaxies, is tested here on artificial galaxy images that were built with structural parameters chosen randomly from a large fixed interval. In this paper we present the properties of the two models, describe the minimization technique, and discuss the results of the tests of the 2D fit obtained from simulated artificial galaxies. By exploiting these tests we were able better to constrain the errors affecting the structural parameters, and the strategies that should be adopted to get the best fit.

2D modelling of the light distribution of early-type galaxies in a volume-limited sample - II. Results for real galaxies

2D modelling of the light distribution of early-type galaxies in a volume-limited sample - II. Results for real galaxies