Evolution Of Elliptical Galaxies Research Articles

Optical Morphologies of Millijansky Radio Galaxies Observed by theHubble Space Telescopeand in the Very Large Array FIRST Survey

We report on a statistical study of the 51 radio galaxies at the millijansky flux level from the Faint Images of the Radio Sky at Twenty centimeters, including their optical morphologies and structure obtained with the Hubble Space Telescope. Our optical imaging is significantly deeper (~2 mag) than previous studies with the superior angular resolution of space-based imaging. We that find 8/51 (16%) of the radio sources have no optically identifiable counterpart to AB~24 mag. For the remaining 43 sources, only 25 are sufficiently resolved in the HST images to reliably assign a visual classification: 15 (60%) are elliptical galaxies, 2 (8%) are late-type spiral galaxies, 1 (4%) is an S0, 3 (12%) are point-like objects (quasars), and 4 (16%) are merger systems. We find a similar distribution of optical types with measurements of the Sersic index. The optical magnitude distribution of these galaxies peaks at I~20.7+-0.5 AB mag, which is ~3 mag brighter than the depth of our typical HST field and is thus not due to the WFPC2 detection limit. This supports the luminosity-dependent density evolutionary model, where the majority of faint radio galaxies typically have L*-optical luminosities and a median redshift of z~0.8 with a relatively abrupt redshift cut-off at z>~2. We discuss our results in the context of the evolution of elliptical galaxies and active galactic nuclei.

The Dramatic Size Evolution of Elliptical Galaxies and the Quasar Feedback

Observations have evidenced that passively evolving massive galaxies at high redshift are much more compact than local galaxies with the same stellar mass. We argue that the observed strong evolution in size is directly related to the quasar feedback, which removes huge amounts of cold gas from the central regions in a Salpeter time, inducing an expansion of the stellar distribution. The new equilibrium configuration, with a size increased by a factor >~ 3, is attained after ~ 40 dynamical times, corresponding to ~ 2 Gyr. This means that massive galaxies observed at z >~ 1 will settle on the Fundamental Plane by z ~ 0.8-1. In less massive galaxies (M_star ~ 1 Gyr, the evolution for smaller galaxies is more difficult to perceive. Significant evolution of velocity dispersion is predicted for both small and large galaxies.

The rate of secular evolution in elliptical galaxies with central masses

We study a series of N-body simulations representing elliptical galaxies with central masses. Starting from two different systems with smooth centres, which have initially a triaxial configuration and are in equilibrium, we insert to them central masses of various values. Immediately after such an insertion, a system presents a high fraction of particles moving in chaotic orbits, a fact causing a secular evolution towards a new equilibrium state. The chaotic orbits responsible for the secular evolution are identified. Their typical Lypaunov exponents are found to scale with the central mass as a power law L∝ms with s close to 1/2. The requirements for an effective secular evolution within a Hubble time are examined. These requirements are quantified by introducing a quantity called effective chaotic momentum ⁠. This quantity is found to correlate well with the rate of the systems' secular evolution. In particular, we find that when falls below a threshold value (0.004 in our N-body units), a system does no longer exhibit significant secular evolution.

The fractional virial potential energy in two-component systems

Two-component systems are conceived as macrogases, and the related equation of state is expressed using the virial theorem for subsystems, under the restriction of homeoidally striated density profiles. Explicit calculations are performed for a useful reference case and a few cases of astrophysical interest, both with and without truncation radius. Shallower density profiles are found to yield an equation of state, ? = ?(y, m), characterized (for assigned values of the fractional mass, m = Mj /Mi ) by the occurrence of two extremum points, a minimum and a maximum, as found in an earlier attempt. Steeper density profiles produce a similar equation of state, which implies that a special value of m is related to a critical curve where the above mentioned extremum points reduce to a single horizontal inflexion point, and curves below the critical one show no extremum points. The similarity of the isofractional mass curves to van der Waals' isothermal curves, suggests the possibility of a phase transition in a bell-shaped region of the (Oy?) plane, where the fractional truncation radius along a selected direction is y = Rj /Ri , and the fractional virial potential energy is ? = (Eji )vir /(Eij )vir . Further investigation is devoted to mass distributions described by Hernquist (1990) density profiles, for which an additional relation can be used to represent a sample of N = 16 elliptical galaxies (EGs) on the (Oy?) plane. Even if the evolution of elliptical galaxies and their hosting dark matter (DM) haloes, in the light of the model, has been characterized by equal fractional mass, m, and equal scaled truncation radius, or concentration, ?u = Ru /r? , u = i, j, still it cannot be considered as strictly homologous, due to different values of fractional truncation radii, y, or fractional scaling radii, y? = r? /r? , deduced from sample objects.

High resolution X-ray spectroscopy and chemical evolution in elliptical galaxies

We discuss the elemental composition in the hot ISM of elliptical galaxies derived from new and recent X-ray spectral analysis in the context of new phenomenological models of their chemical evolution. Star formation histories, the IMF, the astrophysics of supernovae, and the nature of galactic winds impact the metal content and relative abundances in the hot ISM. We evaluate how X-ray spectroscopy may be utilized to deconstruct how elliptical galaxies, and the stellar populations that compose them, form and evolve, with an emphasis on present and future high resolution spectroscopic analysis.

Model colour-magnitude diagrams of elliptical galaxies

Context. Formation and evolution of elliptical galaxies are still matters of debate. Knowledge of their star formation histories (SFHs) is essential to understand the formation mechanism. Colour-magnitude (C-M) diagrams of elliptical galaxies should provide one of the most important clues to resolve the SFHs of the elliptical galaxies. Aims. The aim of this study is to illustrate what information can be obtained from C-M diagrams of elliptical galaxies and what differences appear in theoretical C-M diagrams of different formation scenarios. Methods. Using a C-M diagram simulator which consistently solves SFH and chemical evolution, we present and discuss six examples of model C-M diagrams. The C-M diagrams can be considered a first approach to the theoretical C-M diagrams of elliptical galaxies of which formation is described as the monolithic collapse and the hierarchical scenarios. Results. We show that there are three main differences in the C-M diagrams between the two formation scenarios; the magnitude of main sequence turnoff (MSTO), horizontal branch (HB) morphology, and colour distribution of red giant branch (RGB) stars. Since a later epoch of the final star formation is assumed in the hierarchical model, the magnitude of the MSTO is brighter than that of the monolithic collapse. The C-M diagram predicted by the hierarchical scenario shows well-defined blue HB. On the other hand, the C-M diagram shows few blue HB stars in the monolithic collapse case. The colour distributions predicted by the former have bluer peaks than those by the latter. We find that this corresponds to a higher frequency of metal-poor stars in the former model and metal-poor populations are born in the progenitor galaxies. In addition to the model C-M diagrams, we discuss metallicity distributions and α/Fe abundance patterns predicted by the two scenarios.

The Importance of Dry and Wet Merging on the Formation and Evolution of Elliptical Galaxies

With the aid of a simple yet robust approach we investigate the influence of dissipationless and dissipative merging on galaxy structure, and the consequent effects on the scaling laws followed by elliptical galaxies. Our results suggest that ellipticals cannot be originated by parabolic merging of low mass spheroids only, even in presence of substantial gas dissipation. However, we also found that scaling laws such as the Faber-Jackson, Kormendy, Fundamental Plane, and the Mbh -sigma relations, when considered over the whole mass range spanned by ellipticals in the local universe, are robust against merging. We conclude that galaxy scaling laws, possibly established at high redshift by the fast collapse in pre-existing dark matter halos of gas rich and clumpy stellar distributions, are compatible with a (small) number of galaxy mergers at lower redshift.

Slow evolution of elliptical galaxies induced by dynamical friction

Context.Dynamical friction is expected to play an important role in a variety of astrophysical contexts, yet we still lack a quantitative understanding of this basic mechanism and of its effects. In fact, numerical simulations have shown that in inhomogeneous systems the classical idealized description, given by Chandrasekhar (1943), may have severe limitations, possibly related to the global nature of the processes that are involved.

GEMS: The destiny of Blue Spheroidal Galaxies

One of the key predictions of hierarchical galaxy formation models is that a significant fraction of elliptical galaxies form in late merging events. One of the most important diagnostics of such an assembly is the existence of blue spheroidal galaxies, which have spheroid-dominated morphologies and blue colors indicating recent star formation, as an intermediate step in the evolution of elliptical galaxies.

K-band Evolution of Elliptical Galaxies in the Cluster Abell 2390 at z=0.23

AbstractWe investigate the near-infrared K-band evolution of early-type galaxies in the cluster of galaxies Abell2390 at z = 0.23. Using the Omega-Prime camera at the 3.5-m Calar Alto telescope deep imaging (texp = 53 min) over a 6' × 6' field has been obtained. The measured K-band magnitudes of 28 galaxies are combined with the spectroscopic and morphological data of Fritz et al. (2005) to construct the Faber-Jackson and Fundamental Plane relations in the NIR. By comparing our distant galaxies to a local sample of cluster ellipticals (Pahre 1999), we find on average a mild luminosity evolution for both scaling relations (ΔMK ~ 0.6 − 0.7 mag) compatible with passive evolution of the stellar populations.

Non Adiabatic Evolution of Elliptical Galaxies by Dynamical Friction

AbstractMany astrophysical problems, ranging from structure formation in cosmology to dynamics of elliptical galaxies, refer to slow processes of evolution of essentially collisionless self-gravitating systems. To determine the relevant quasi-equilibrium configuration at time t from given initial conditions, such slow evolution is often approximated in terms of adiabatic evolution. Here we focus on the slow process of evolution induced by dynamical friction of a spherical stellar system (the host galaxy) on a minority component of “satellites” (distributed in a spherical shell), to determine to what extent an adiabatic description might be applied.

Slow evolution of elliptical galaxies induced by dynamical friction

Context. Many astrophysical problems, ranging from structure formation in cosmology to dynamics of elliptical galaxies, refer to slow processes of evolution of essentially collisionless self-gra vitating systems. In order to determine the relevant quasi- equilibrium configuration at time t from given initial conditions, it is often argued that such s low evolution may be approximated in terms of adiabatic evolution, for the calculation of which effi cient semi‐analytical techniques are available. Aims. Here we focus on the slow process of evolution, induced by dynamical friction of a host stellar system on a minority component of “satellites”, that we have investigated in a previous paper, to determine to what extent an adiabatic description might be applied. Methods. The study is realized by comparing directly N‐body simulations of the stellar system evolution (in two significantly di fferent models) from initial to final conditions in a controlled numerical en vironment. Results. We demonstrate that for the examined process the adiabatic description is going to provide incorrect answers, not only quantitatively, but also qualitatively. The two classes of models considered exhibit generally similar trends in evolution, with one exception noted in relation to the evolution of the total density profile. Conclusions. This simple conclusion should be taken as a warning against the indiscriminate use of adiabatic growth prescriptions in studies of structure of galaxies.

Photochemical evolution of elliptical galaxies - II. The impact of merging-induced starbursts

The effects of late gas accretion episodes and subsequent merger-induced starbursts on the photochemical evolution of elliptical galaxies are studied and compared to the picture of galaxy formation occurring at high redshift with a unique and intense starburst modulated by a very short infall, as suggested by Pipino and Matteucci in Paper I. By means of the comparison with the colour-magnitude relations (CMRs) and the [〈Mg/Fe〉V]–σ relation observed in ellipticals, we conclude that either bursts involving a gas mass comparable to the mass already transformed into stars during the first episode of star formation (SF) and occurring at any redshift, or bursts occurring at low redshift (i.e. z⩽ 0.2) and with a large range of accreted mass, are ruled out. These models fail in matching the above relations even if the initial infalling hypothesis is relaxed, and the galaxies form either by means of more complicated SF histories or by means of the classical monolithic model. On the other hand, galaxies accreting a small amount of gas at high redshift (i.e. z⩾ 3) produce a spread in the model results, with respect to the best model of Paper I, which is consistent with the observational scatter of the CMRs, although there is only marginal agreement with the [〈Mg/Fe〉V]–σ relation. Therefore, only small perturbations to the standard scenario seem to be allowed. We stress that the strongest constraints to galaxy-formation mechanisms are represented by the chemical abundances, whereas the colours can be reproduced under several different hypotheses.

The Lack of Structural and Dynamical Evolution of Elliptical Galaxies since z ~ 1.5: Clues from Self-Consistent Hydrodynamic Simulations

We present results of a study on the evolution of the parameters characterizing the structure and dynamics of the relaxed elliptical-like objects (ELOs) identified at z=0, z=1 and z=1.5 in a set of hydrodynamical, self-consistent simulations operating in the context of a concordance cosmological model. The values of the stellar mass, the stellar half-mass radius and the stellar mean-square velocity have been measured in each ELO and found to populate, at any z, a flattened ellipsoid close to a plane (the dynamical plane, DP). Our simulations indicate that, at the intermediate zs considered, individual ELOs evolve, increasing the values of these parameters as a consequence of on-going mass assembly, but, nevertheless, their DP is roughly preserved within its scatter, in agreement with observations of the Fundamental Plane of ellipticals at different zs. We briefly discuss how this lack of significant dynamical and structural evolution in ELO samples arises, in terms of the two different phases operating in the mass aggregation history of their dark matter halos. According with our simulations, most dissipation involved in ELO formation takes place at the early violent phase, causing the stellar mass, the stellar half-mass radius and the stellar mean-square velocity parameters to settle down to the DP, and, moreover, the transformation of most of the available gas into stars. In the subsequent slow phase, ELO stellar mass growth preferentially occurs through non-dissipative processes, so that the DP is preserved and the ELO star formation rate considerably decreases. These results hint, for the first time, to a possible way of explaining, in the context of cosmological simulations, different apparently paradoxical observational results on ellipticals.

A Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies

We report on the discovery of a new correlation between global parameters of the hot interstellar gas in elliptical galaxies. We reanalyze archival Chandra data for 30 normal early-type systems, removing the contributions of resolved and unresolved point sources to reveal the X-ray morphology of the hot gas. We determine the half-light radius, R_X, and the mean surface brightness, I_X, from the gas surface brightness profiles. A spectral analysis determines the temperature, T_X, of the gas within 3 optical effective radii. We find that the galaxies lie on an X-ray Gas Fundamental Plane (XGFP) of the form T_X ~ R_X^{0.28} I_X^{0.22}. This is close to, but distinct from, a simple luminosity-temperature relation. The intrinsic width of the XGFP is only 0.07dex, nearly identical to that of the stellar (optical) fundamental plane (SFP). This is surprising since X-ray gas masses are typically ~10^{-2} of the stellar masses. We show that the XGFP is not a simple consequence of the virial theorem or hydrostatic equilibrium, and that it is essentially independent of the SFP. The XGFP thus represents a genuinely new constraint on the hydrodynamical evolution of elliptical galaxies.

The elliptical galaxy colour-magnitude relation as a discriminant between the monolithic and merger paradigms

The colour-magnitude relation (CMR) of cluster elliptical galaxies has been widely used to constrain their star formation histories (SFHs) and to discriminate between the monolithic collapse and merger paradigms of elliptical galaxy formation. We use a �CDM hierarchical merger model of galaxy formation to investigate the existence and redshift evolution of the elliptical galaxy CMR in the merger paradigm. We show that the star formation history (SFH) of cluster ellipticals predicted by the model is quasimonolithic, with only � 10 percent of the total stellar mass forming after z � 1. The quasi-monolithic SFH results in a predicted CMR that agrees well with its observed counterpart in the redshift range 0 < z < 1.27. We use our analysis to argue that the elliptical-only CMR can be used to constrain the SFHs of present-day cluster ellipticals only if we believe a priori in the monolithic collapse model. It is not a meaningful tool for constraining the SFH in the merger paradigm, since a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star forming systems at higher redshift, which cannot be ignored when deriving the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between the two competing theories of elliptical galaxy evolution.

Self-regulated active galactic nuclei heating in elliptical galaxies

Abstract We study the effect of active galactic nuclei (AGN) heating on the chemodynamical evolution of elliptical galaxies and their X-ray and optical properties using high-resolution Λ-dominated cold dark matter cosmological simulations. Our model considers an AGN as being ‘active’ when a convergent gas inflow condition exists within the nucleus of a galaxy; otherwise, the AGN is assumed to remain dormant. This induces a self-regulated activity for the AGN, the result of which leads to a stable hot corona and the suppression of significant late-time star formation – characteristics not encountered in traditional chemodynamical models of ellipticals. These properties of our AGN heating model lead to a system consistent with both the X-ray and optical properties of comparable elliptical galaxies.

The effects of Population III stars on the chemical and photometrical evolution of ellipticals

We have studied the effects of an hypothetical initial generation containing very massive stars (M > 100 Msun, pair-creation SNe) on the chemical and photometric evolution of elliptical galaxies. To this purpose, we have computed the evolution of a typical elliptical galaxy with luminous mass of the order of 10^11 Msun and adopted chemical evolution models already tested to reproduce the main features of ellipticals. We have tested several sets of yields for very massive zero-metallicity stars: these stars should produce quite different amounts of heavy elements than lower mass stars. We found that the effects of population III stars on the chemical enrichment is negligible if only one or two generations of such stars occurred, whereas they produce quite different results from the standard models if they continuously formed for a period not shorter than 0.1 Gyr. In this case, the results are at variance with the main observational constraints of ellipticals such as the average [ ] ratio in stars and the integrated colors. Therefore, we conclude that if population III stars ever existed they must have been present for a very short period of time and their effects on the following evolution of the parent galaxy must have been negligible. This effect is minimum if a more realistic model with initial infall of gas rather than the classic monolithic model is adopted. Ultimately, we conclude that there is no need to invoke a generation of very massive stars in ellipticals to explain their chemical and photometric properties.

THE ACCRETION RATES–LUMINOSITY RELATIONS AND EVOLUTIONARY SEQUENCE OF FSRQs, BL Lac OBJECTS AND RADIO GALAXIES

A sample of 39 blazars with well-established rapid variability timescale and bolometric luminosity has been compiled from literature. Based on the assumption that central supermassive black holes (SMBHs) are Kerr black holes, the upper limits of SMBHs were estimated. The masses ranged from 107.2M⊙ to 109.4M⊙, showing a distribution of three subclasses: massive flat spectra radio quasars (FSRQs) and smaller mass BL Lacs, occupy separate regions, while medium mass FSRQ and BL Lac object bridge the gap. In addition, we complied a sample of radio galaxies including 9 sources for which their black holes masses have been derived from the M BH -σ correlation and their optical and infrared data have been well observed. We find that the intrinsic accretion rates are quite different between FSRQs, BL Lacs and the radio galaxies. The diagram of the intrinsic accretion rate–luminosity relations shows that FSRQs occur in the earlier, high luminosity, violent phase of galactic evolution sequence, while BL Lacs occur in the low luminosity, transition phase between quasars and radio galaxies, and the radio galaxies occur in the late stage of the elliptical galaxy evolution sequence. Our theoretical results prove that the evolutionary track of the elliptical galaxy evolutionary sequence is from FSRQs to BL Lacs and then to the radio galaxies. The evolution diagram of blazars derived in this paper seems to be similar to Hertzsprung–Russell diagram of star evolution.

Evolution of Elliptical Galaxies at $z \gtrsim 1$ Revealed from a Large, Multicolor Sample of Extremely Red Objects

Abstract We studied the evolution of elliptical galaxies at $z \gtrsim 1$ using a sample of 247 Extremely Red Objects (EROs) with $R-K_\mathrm{s} \ge 3.35$ (AB) and $K_\mathrm{s} \le 22.1$ (AB) constructed from $BVR \, i'z'JHK_\mathrm{s}$ multicolor data of a $114 \,\mathrm{arcmin}^2$ area in the Subaru/XMM-Newton Deep Field taken with the Subaru Telescope and the UH 2.2 m telescope. By fitting template spectra of old galaxies (OGs) and young, dusty starbursts (DSs) to the multicolor data, we classified EROs into these two classes and estimated their redshifts. We found that 58% of the EROs in our sample belong to the OG class and that these OGs have a wide range of colors at any redshift. We derived rest-frame $B$-band luminosity functions of OGs in our sample at $z=1 \hbox{-} 1.5$ and $1.5 \hbox{-} 2.5$, and found that the number density of galaxies obeying passive evolution drops at $z \gtrsim 1.5$ by a factor of three or more. The spatial correlation length of OGs is estimated to be $r_0 \gtrsim 8 \,h^{-1} \,\mathrm{Mpc}$, which is comparable to, or larger than, those of present-day early-type galaxies of similar luminosities. We also discussed properties of DSs in our data.