arXiv subjects

Silvia Pellegrini

Publications and source records attributed to Silvia Pellegrini.

At least 19 recordsLinked to original sources

arXiv2025

Anisotropy ansatz for the Jeans equations: oblate galaxies

In the solution of the Jeans equations for axisymmetric galaxy models the ``$b$-ansatz" is often adopted to prescribe the relation between the vertical and radial components of the velocity dispersion tensor, and close the equations. However, $b$ affects the resulting azimuthal velocity fields quite indirectly, so that the analysis of the model kinematics is usually performed after numerically solving the Jeans equations, a time consuming approach. In a previous work we presented a general method to determine the main properties of the kinematical fields resulting in the $b$-ansatz framework before solving the Jeans equations; results were illustrated by means of disk galaxy models. In this paper we focus more specifically on realistic ellipsoidal galaxy models. It is found that how and where $b$ affects the galaxy kinematical fields is mainly dependent on the flattening of the stellar density distribution, moderately on the presence of a Dark Matter halo, and much less on the specific galaxy density profile. The main trends revealed by the numerical exploration, in particular the fact that more flattened systems can support larger $b$-anisotropy, are explained with the aid of simple ellipsoidal galaxy models, for which most of the analysis can be conducted analytically. The obtained results can be adopted as guidelines for model building and in the interpretation of observational data.

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arXiv2025

X-ray Halos of Early-Type Galaxies with AGN Feedback and Accretion from a Circumgalactic Medium: models and observations

The knowledge of the X-ray properties of the hot gas halos of early-type galaxies has significantly advanced in the past years, for large and homogeneously investigated samples. We compare these results with the X-ray properties of an exploratory set of gas evolution models in realistic early-type galaxies, produced with our high-resolution 2D hydrodynamical code MACER that includes AGN feedback and accretion from a circumgalactic medium. The model X-ray emission and absorption are integrated along the line of sight, to obtain maps of the surface brightness Sigma_X and temperature Tx. The X-ray diagnostics considered are the luminosity and average temperature for the whole galaxy (Lx and ) and within 5 optical effective radii (Lx5 and ), and the circularized profiles Sigma_X(R) and Tx(R). The values for Lx, Lx5, , and compare very well with those observed. The Sigma_X(R) and Tx(R) also present qualitative similarities with those of the representative galaxy NGC5129, and of ETGs with the most commonly observed shape for Tx(R): Sigma_X(R) matches the observed profile over many optical effective radii Re, and Tx(R) reproduces the characteristic bump that peaks at R=(1 - 3)Re. Inside the peak position, Tx(R) declines towards the center, but the explored models are systematically hotter by ~30%; possible explanations for this discrepancy are discussed. Interestingly, Sigma_X(R) and Tx(R) as large as observed outside of R~Re are reproduced only with significant accretion from a circumgalactic medium, highlighting its importance.

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arXiv2024

Anisotropy ansatz for the axisymmetric Jeans equations

The Jeans equations do not form a closed system, and to solve them a parametrization relating the velocity moments is often adopted. For axisymmetric models, a phenomenological choice (the "$b$-ansatz") is widely used for the relation between the vertical ($σ_z^2$) and radial ($σ_R^2$) components of the velocity dispersion tensor, thus breaking their identity present in two-integral systems. However, the way in which the ansatz affects the resulting kinematical fields can be quite complicated, so that the analysis of these fields is usually performed only after numerically computing them. We present here a general procedure to study the properties of the ansatz-dependent fields $\overline{v_φ^2}$, $Δ= \overline{v_φ^2} - σ_z^2$ and $Δ_R = \overline{v_φ^2} - σ_R^2$. Specifically, the effects of the $b$-ansatz can be determined before solving the Jeans equations once the behaviour over the ($R,z$)-plane of three easy-to-build ansatz-independent functions is known. The procedure also constrains the ansatz to exclude unphysical results (as a negative $\overline{v_φ^2}$). The method is illustrated by discussing the cases of three well-known galaxy models: the Miyamoto & Nagai and Satoh disks, and the Binney logarithmic halo, for which the regions and the constraints on the ansatz values can be determined analytically; a two-component (Miyamoto & Nagai plus logarithmic halo) model is also discussed.

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arXiv2023

Jeans modelling of weakly flattened stellar systems

In the homoeoidal expansion, a given ellipsoidally stratified density distribution, and its associated potential, are expanded in the (small) density flattening parameter $η$, and usually truncated at the linear order. The truncated density-potential pair obeys exactly the Poisson equation, and it can be interpreted as the first-order expansion of the original ellipsoidal density-potential pair, or as a new autonomous system. In the first interpretation, in the solutions of the Jeans equations the quadratic terms in $η$ must be discarded (``$η$-linear'' solutions), while in the second (``$η$-quadratic'') all terms are retained. In this work we study the importance of the quadratic terms by using the ellipsoidal Plummer model and the Perfect Ellipsoid, which allow for fully analytical $η$-quadratic solutions. These solutions are then compared with those obtained numerically for the original ellipsoidal models, finding that the $η$-linear models already provide an excellent approximation of the numerical solutions. As an application, the $η$-linear Plummer model (with a central black hole) is used for the phenomenological interpretation of the dynamics of the weakly flattened and rotating globular cluster NGC 4372, confirming that this system cannot be interpreted as an isotropic rotator, a conclusion reached previously with more sophisticated studies.

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arXiv2022

A Parameter Space Exploration of High Resolution Numerically Evolved Early Type Galaxies Including AGN Feedback and Accurate Dynamical Treatment of Stellar Orbits

An extensive exploration of the model parameter space of axisymmetric Early-Type Galaxies (ETGs) hosting a central supermassive Black Hole (SMBH) is conducted by means of high resolution hydrodynamical simulations performed with our code MACER. Global properties such as 1) total SMBH accreted mass, 2) final X-ray luminosity and temperature of the X-ray emitting halos, 3) total amount of new stars formed from the cooling gas, 4) total ejected mass in form of supernovae and AGN feedback induced galactic winds, are obtained as a function of galaxy structure and internal dynamics. In addition to the galactic dark matter halo, the model galaxies are also embedded in a group/cluster dark matter halo; finally cosmological accretion is also included, with amount and time dependence derived from cosmological simulations. Angular momentum conservation leads to the formation of cold HI disks; these disks further evolve under the action of star formation induced by disk instabilities, of the associated mass discharge onto the central SMBH, and of the consequent AGN feedback. At the end of the simulations, the hot (metal enriched) gas mass is roughly $10\%$ the mass in the old stars, with twice as much having been ejected into the intergalactic medium. The cold gas disks are a $\approx$ kpc in size, and the metal rich new stars are in $0.1$ kpc disks. The masses of cold gas and new stars are roughly $0.1\%$ the mass of the old stars. Overall, the final systems appear to reproduce quite successfully the main global properties of real ETGs.

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arXiv2022

The Hot Interstellar Medium

The interstellar medium (ISM) of galaxies very often contains a gas component that reaches the temperature of several million degrees, whose physical and chemical properties can be investigated through imaging and spectroscopy in the X-rays. We review the current knowledge on the origin and retention of the hot ISM in star-forming and early-type galaxies, from a combined theoretical and observational standpoint. As a complex interplay between gravitational processes, environmental effects, and feedback mechanisms contributes to its physical conditions, the hot ISM represents a key diagnostic of the evolution of galaxies.

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arXiv2022

On the polytropic Bondi accretion in two-component galaxy models with a central massive BH

In many investigations involving accretion on a central point mass, ranging from observational studies to cosmological simulations, including semi-analytical modelling, the classical Bondi accretion theory is the standard tool widely adopted. Previous works generalised the theory to include the effects of the gravitational field of the galaxy hosting a central black hole, and of electron scattering in the optically thin limit. Here we apply this extended Bondi problem, in the general polytropic case, to a class of new two-component galaxy models recently presented. In these models, a Jaffe stellar density profile is embedded in a dark matter halo such that the total density distribution follows a $r^{-3}$ profile at large radii; the stellar dynamical quantities can be expressed in a fully analytical way. The hydrodynamical properties of the flow are set by imposing that the gas temperature at infinity is proportional to the virial temperature of the stellar component. The isothermal and adiabatic (monoatomic) cases can be solved analytically, in the other cases we explore the accretion solution numerically. As non-adiabatic accretion inevitably leads to an exchange of heat with the ambient, we also discuss some important thermodynamical properties of the polytropic Bondi accretion, and provide the expressions needed to compute the amount of heat exchanged with the environment, as a function of radius. The results can be useful for the subgrid treatment of accretion in numerical simulations, as well as for the interpretation of observational data.

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arXiv2022

SDSS-IV MaNGA: Integral-field kinematics and stellar population of a sample of galaxies with counter-rotating stellar disks selected from about 4000 galaxies

We present the integral-field kinematics and stellar population properties of 64 galaxies (61 are Early-Type galaxies, ETGs) with Counter-Rotating stellar Disks (CRD) selected from about 4000 galaxies in the MaNGA survey, based on evidence of counter-rotation or two velocity dispersion peaks in the kinematic maps. For 17 CRDs, the counter-rotating components can also be separated spectroscopically. The frequency of CRDs in MaNGA is <5% for ellipticals, <3% for lenticulars and <1% for spirals (at 95% confidence level), consistent with previous estimates. We produced age and metallicity maps, and compared the stellar population properties to those of the general ETGs population. We found that CRDs have similar trends in age and metallicity to ETGs, but are less metallic at low masses, and show flatter age and steeper metallicity gradients, on average. A comparison of the velocity fields of the ionized gas and the stars reveals that in 33 cases the gas corotates with either the inner (15 cases) or outer (18 cases) stellar disk, in 9 cases it is misaligned. In most cases the gas corotates with the younger disk. Evidence of multimodality in the stellar population is found in 31 galaxies, while the 14 youngest and least massive galaxies show ongoing star formation; 14 galaxies, instead, exhibit unimodality, and are the oldest and most massive. As a general result, our work indicates that CRDs form primarily via gas accretion in retrograde rotation with respect to a pre-existing stellar disk.

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arXiv2020

Infra-Red Emission from Cold Gas Dusty Disks in Massive Ellipticals

What is the expected infrared output of elliptical galaxies? Here we report the latest findings obtained in this high time resolution (~10 years) and high spatial resolution (2.5 parsec at center) study. We add a set of grain physics to the MACER code, including (a) dust grains made in passive stellar evolution; (b) dust grain growth due to collision and sticking; (c) grain destruction due to thermal sputtering; (d) dust cooling of hot gas via inelastic collisions; and (e) radiation pressure on dust grains. The code improvements enable us to analyze the effects of dust on metal depletion and AGN obscuration, and also to assess the infrared output of the modeled galaxies. We simulate a representative massive elliptical galaxy of a central stellar velocity dispersion ~ 260 km/s and modest rotation. We find that: (1) the circumnuclear disk (of a size <~ 1 kpc) is dusty in its outer region where most of the metals are in dust grains, while in the inner disk most of the dust grains are destroyed by the AGN irradiation; (2) the dusty disk is optically thick to both the starlight within the disk and the radiation from the central AGN. Thus the AGN will be obscured behind the disk, and the latter is of a covering factor ~ 0.2; (3) the dust infrared emission is mainly due to the AGN irradiation. The median infrared luminosity is ~ 2e44 erg/s, and it can reach >~ 1e46 erg/s during outbursts; (4) the duty cycles of the AGN activities, star formation, and the dust infrared luminosity roughly match observations, e.g., in most of its lifetime, the simulated galaxy is a stereotypical "quiescent" elliptical galaxy with L_{IR} ~ 1e11*L_{solar}, while the star formation rate can exceed 250 M_{solar}/yr during central outbursts.

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arXiv2019

Dynamical models of spheroidal multi-component stellar systems

We present a significantly improved version of our numerical code JASMINE, that can now solve the Jeans equations for axisymmetric models of stellar systems, composed of an arbitrary number of stellar populations, a Dark Matter halo, and a central Black Hole. The stellar components can have different structural (density profile, flattening, mass, scale length), dynamical (rotational support, velocity dispersion anisotropy), and population (age, metallicity, Initial Mass Function, mass-to-light ratio) properties. These models, when combined with observations, will allow to investigate important issues, such as quantifying the systematic effects of IMF variations, of mass-to-light ratio gradients, and of different stellar kinematic components (e.g. counter rotating disks, kinematically decoupled cores) on luminosity-weighted properties. The developed analytical and numerical framework aims at modeling Early-Type Galaxies, but it can also be applied to dwarf Spheroidal galaxies and Globular Clusters.

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arXiv2019

Adding a Suite of Chemical Abundances to the MACER Code for the Evolution of Massive Elliptical Galaxies

We add a suite of chemical abundances to the MACER (Massive AGN Controlled Ellipticals Resolved) 2D code, by solving 12 additional continuity equations for H, He, C, N, O, Ne, Mg, Si, S, Ca, Fe and Ni respectively with sources from AGB stars and supernovae of type Ia and II with metal yields based on standard stellar physics. New stars, formed in Toomre unstable circumnuclear disks (of a size $\lesssim150$ parsec), are assumed to have a top-heavy initial mass function with a power index of 1.65. The metal dilution effects due to cosmic accretion are also included. With the high resolution of few parsecs in central regions, resolved black hole accretion and AGN feedback, we can track the metal enrichment, transportation and dilution throughout the modeled massive elliptical galaxy of velocity dispersion $\sim280$ km/s. We retrieve the chemical composition of the BAL winds launched by the central AGN, synthesize the X-ray features of the hot ISM, and find that (1) the simulated metallicity in the BAL winds could be up to $\sim 8 Z_\odot$, while that of the hot ISM in the host galaxy is $\sim 2.3 Z_\odot$, matching well with SDSS observations of BLR gas; (2) the X-ray emitting hot gas is metal enriched with a typical value $\sim2.5 Z_\odot$; (3) the circumunuclear cold gas disk, where the metals are condensed, further enriched and recycled, plays a critical role in the metal enrichment; (4) the black hole accretion rate $\dot{M}_{\rm BH}$ linearly correlates with the star formation rate $\dot{M}_\star^+$ in the circumnuclear disk, i.e, $\dot{M}_\star^+ \sim 7.7 \dot{M}_{\rm BH}$, but lagged in time by roughly $10^6$ years.

astro-ph.GA↗

arXiv2017

Constraining the Physical State of the Hot Gas Halos in NGC 4649 and NGC 5846

We present results of a joint \textit{Chandra}/\textit{XMM-Newton} analysis of the early-type galaxies NGC 4649 and NGC 5846 aimed at investigating differences between mass profiles derived from X-ray data and those from optical data, to probe the state of the hot ISM in these galaxies. If the hot ISM is at a given radius in hydrostatic equilibrium (HE) the X-ray data can be used to measure the total enclosed mass of the galaxy. Differences from optically-derived mass distributions therefore yield information about departures from HE in the hot halos. The X-ray mass profiles in different angular sectors of NGC 4649 are generally smooth with no significant azimuthal asymmetries within $12$ kpc. Extrapolation of these profiles beyond this scale yields results consistent with the optical estimate. However, in the central region ($r < 3$ kpc) the X-ray data underpredict the enclosed mass, when compared with the optical mass profiles. Consistent with previous results we estimate a non-thermal pressure component accounting for $30\%$ of the gas pressure, likely linked to nuclear activity. In NGC 5846 the X-ray mass profiles show significant azimuthal asymmetries, especially in the NE direction. Comparison with optical mass profiles in this direction suggests significant departures from HE, consistent with bulk gas compression and decompression due to sloshing on $\sim 15$ kpc scales; this effect disappears in the NW direction where the emission is smooth and extended. In this sector we find consistent X-ray and optical mass profiles, suggesting that the hot halo is not responding to strong non-gravitational forces.

astro-ph.GA↗

arXiv2015

X-ray haloes and star formation in early-type galaxies

High resolution 2D hydrodynamical simulations describing the evolution of the hot ISM in axisymmetric two-component models of early-type galaxies well reproduced the observed trends of the X-ray luminosity ($L_\mathrm{x}$) and temperature ($T_\mathrm{x}$) with galaxy shape and rotation, however they also revealed the formation of an exceedingly massive cooled gas disc in rotating systems. In a follow-up of this study, here we investigate the effects of star formation in the disc, including the consequent injection of mass, momentum and energy in the pre-existing interstellar medium. It is found that subsequent generations of stars originate one after the other in the equatorial region; the mean age of the new stars is $> 5$ Gyr, and the adopted recipe for star formation can reproduce the empirical Kennicutt-Schmidt relation. The results of the previous investigation without star formation, concerning $L_\mathrm{x}$ and $T_\mathrm{x}$ of the hot gas, and their trends with galactic shape and rotation, are confirmed. At the same time, the consumption of most of the cold gas disc into new stars leads to more realistic final systems, whose cold gas mass and star formation rate agree well with those observed in the local universe. In particular, our models could explain the observation of kinematically aligned gas in massive, fast-rotating early-type galaxies.

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arXiv2014

The stellar initial mass function of early type galaxies from low to high stellar velocity dispersion: homogeneous analysis of ATLAS$^{\rm 3D}$ and Sloan Lens ACS galaxies

We present an investigation about the shape of the initial mass function (IMF) of early-type galaxies (ETGs), based on a joint lensing and dynamical analysis, and on stellar population synthesis models, for a sample of 55 lens ETGs identified by the Sloan Lens ACS (SLACS) Survey. We construct axisymmetric dynamical models based on the Jeans equations which allow for orbital anisotropy and include a dark matter halo. The models reproduce in detail the observed \textit{HST} photometry and are constrained by the total projected mass within the Einstein radius and the stellar velocity dispersion ($σ$) within the SDSS fibers. Comparing the dynamically-derived stellar mass-to-light ratios $(M_*/L)_{\rm dyn}$, obtained for an assumed halo slope $ρ_{\rm h}\propto r^{-1}$, to the stellar population ones $(M_*/L)_{\rm pop}$, derived from full-spectrum fitting and assuming a Salpeter IMF, we infer the mass normalization of the IMF. Our results confirm the previous analysis by the SLACS team that the mass normalization of the IMF of high $σ$ galaxies is consistent on average with a Salpeter slope. Our study allows for a fully consistent study of the trend between IMF and $σ$ for both the SLACS and \ATLAS samples, which explore quite different $σ$ ranges. The two samples are highly complementary, the first being essentially $σ$ selected, and the latter volume-limited and nearly mass selected. We find that the two samples merge smoothly into a single trend of the form $\logα=(0.38\pm0.04)\times\log(σ_{\rm e}/200\,\mathrm{km~s}^{-1})+(-0.06\pm0.01)$, where $α=(M_*/L)_{\rm dyn}/(M_*/L)_{\rm pop}$ and $σ_{\rm e}$ is the luminosity averaged $σ$ within one effective radius $R_{\rm e}$. This is consistent with a systematic variation of the IMF normalization from Kroupa to Salpeter in the interval $σ_{\rm e}\approx90-270\,\mathrm{km~s}^{-1}$.

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arXiv2014

The effects of galaxy shape and rotation on the X-ray haloes of early-type galaxies - II. Numerical simulations

By means of high resolution 2D hydrodynamical simulations, we study the evolution of the hot ISM for a large set of early-type galaxy models, characterized by various degrees of flattening and internal rotation. The galaxies are described by state-of-the-art axisymmetric two-component models, tailored to reproduce real systems; the dark matter haloes follow the Navarro-Frenk-White or the Einasto profile. The gas is produced by the evolving stars, and heated by Type Ia SNe. We find that, in general, the rotation field of the ISM in rotating galaxies is very similar to that of the stars, with a consequent negligible heating contribution from thermalization of the ordered motions. The relative importance of flattening and rotation in determining the final X-ray luminosity $L_x$ and temperature $T_x$ of the hot haloes is a function of the galactic mass. Flattening and rotation in low mass galaxies favour the establishment of global winds, with the consequent reduction of $L_x$. In medium-to-high mass galaxies, flattening and rotation are not sufficient to induce global winds, however, in the rotating models the nature of the gas flows is deeply affected by conservation of angular momentum, resulting in a reduction of both $L_x$ and $T_x$.

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arXiv2014

AGN Feedback in the Hot Halo of NGC 4649

Using the deepest available $\textit{Chandra}$ observations of NGC 4649 we find strong evidences of cavities, ripples and ring like structures in the hot interstellar medium (ISM) that appear to be morphologically related with the central radio emission. These structures show no significant temperature variations in correspondence with higher pressure regions ($0.5\mbox{kpc}<r<3\mbox{kpc}$). On the same spatial scale, a discrepancy between the mass profiles obtained from stellar dynamic and $\textit{Chandra}$ data represents the telltale evidence of a significant non-thermal pressure component in this hot gas, which is related to the radio jet and lobes. On larger scale we find agreement between the mass profile obtained form $\textit{Chandra}$ data and planetary nebulae and globular cluster dynamics. The nucleus of NGC 4649 appears to be extremely radiatively inefficient, with highly sub-Bondi accretion flow. Consistently with this finding, the jet power evaluated from the observed X-ray cavities implies that a small fraction of the accretion power calculated for the Bondi mass accretion rate emerges as kinetic energy. Comparing the jet power to radio and nuclear X-ray luminosity the observed cavities show similar behavior to those of other giant elliptical galaxies.

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arXiv2014

The effects of stellar dynamics on the X-ray emission of flat early-type galaxies

Observational and numerical studies gave hints that the hot gaseous haloes of ETGs may be sensitive to the galaxy internal kinematics. By using high resolution 2D hydro simulations, and realistic two-component (stars plus dark matter) axisymmetric galaxy models, we study the evolution of the hot haloes in a suite of flat ETGs of fixed mass distribution, but with variable amounts of azimuthal velocity dispersion and rotational support, including the possibility of a counter-rotating inner stellar disc. The hot halo is fed by stellar mass losses and heated by SNIa explosions and thermalization of stellar motions. We measure the value of the thermalization parameter gamma (the ratio between the heating due to the relative velocity between the stellar streaming and the ISM bulk flow, and the heating attainable by complete thermalization of the stellar streaming motions). We find that 1) the X-ray emission and the average temperature are larger in fully velocity dispersion supported systems; 2) 0.1<gamma<0.2 for isotropic rotators (with a trend for being larger for lower dark mass models); 3) systems that are isotropic rotators at large radii with an inner counter-rotating disc, or fully velocity dispersion supported systems with an inner rotating disc, have gamma=1, again with a trend to increase for lower dark mass contents. We also find that the lower X-ray luminosities of isotropic rotators cannot be explained just by their low gamma, but are due to the complicated flow structure, consequence of the angular momentum stored at large radii. X-ray emission weighted temperatures and luminosities nicely match observed values; the X-ray isophotes are boxy in case of significant galaxy rotation. Overall, it is found that rotation has an important role to explain the observational result that more rotationally supported ETGs on average show a lower X-ray emission [abridged].

astro-ph.CO↗

arXiv2013

Fast and Furious: Shock Heated Gas as the Origin of Spatially Resolved Hard X-ray Emission in the Central 5 kpc of the Galaxy Merger NGC 6240

We have obtained a deep, sub-arcsecond resolution X-ray image of the nuclear region of the luminous galaxy merger NGC 6240 with Chandra, which resolves the X-ray emission from the pair of active nuclei and the diffuse hot gas in great detail. We detect extended hard X-ray emission from kT~6 keV (~70 million K) hot gas over a spatial scale of 5 kpc, indicating the presence of fast shocks with velocity of ~2200 km/s. For the first time we obtain the spatial distribution of this highly ionized gas emitting FeXXV, which shows a remarkable correspondence to the large scale morphology of H_2(1-0) S(1) line emission and Hαfilaments. Propagation of fast shocks originated in the starburst driven wind into the ambient dense gas can account for this morphological correspondence. With an observed L(0.5-8 keV)=5.3E+41 erg/s, the diffuse hard X-ray emission is 100 times more luminous than that observed in the classic starburst galaxy M82. Assuming a filling factor of 1% for the 70 MK temperature gas, we estimate its total mass (M_{hot}=1.8E+8 Msun) and thermal energy (E_{th}=6.5E+57 ergs). The total iron mass in the highly ionized plasma is M_{Fe}=4.6E+5 Msun. Both the energetics and the iron mass in the hot gas are consistent with the expected injection from the supernovae explosion during the starburst that is commensurate with its high star formation rate. No evidence for fluorescent Fe I emission is found in the CO filament connecting the two nuclei.

astro-ph.CO↗