arXiv subjects

C. Nipoti

Publications and source records attributed to C. Nipoti.

54 records · Page 3Linked to original sources

arXiv2011

Galactic fountains and the rotation of disc-galaxy coronae

In galaxies like the Milky Way, cold (~ 10^4 K) gas ejected from the disc by stellar activity (the so-called galactic-fountain gas) is expected to interact with the virial-temperature (~ 10^6 K) gas of the corona. The associated transfer of momentum between cold and hot gas has important consequences for the dynamics of both gas phases. We quantify the effects of such an interaction using hydrodynamical simulations of cold clouds travelling through a hot medium at different relative velocities. Our main finding is that there is a velocity threshold between clouds and corona, of about 75 km/s, below which the hot gas ceases to absorb momentum from the cold clouds. It follows that in a disc galaxy like the Milky Way a static corona would be rapidly accelerated: the corona is expected to rotate and to lag, in the inner regions, by ~ 80-120 km/s with respect to the cold disc. We also show how the existence of this velocity threshold can explain the observed kinematics of the cold extra-planar gas.

astro-ph.GA↗

arXiv2010

The mode of gas accretion onto star-forming galaxies

It is argued that galaxies like ours sustain their star formation by transferring gas from an extensive corona to the star-forming disc. The transfer is effected by the galactic fountain -- cool clouds that are shot up from the plane to kiloparsec heights above the plane. The Kelvin-Helmholtz instability strips gas from these clouds. If the pressure and the the metallicity of the corona are high enough, the stripped gas causes a similar mass of coronal gas to condense in the cloud's wake. Hydrodynamical simulations of cloud-corona interaction are presented. These confirm the existence of a critical ablation rate above which the corona is condensed, and imply that for the likely parameters of the Galactic corona this rate lies near the actual ablation rate of clouds. In external galaxies trails of HI behind individual clouds will not be detectable, although the integrated emission from all such trails should be significant. Parts of the trails of the clouds that make up the Galaxy's fountain should be observable and may account for features in targeted 21-cm observations of individual high-velocity clouds and surveys of Galactic HI emission. Taken in conjunction with the known decline in the availability of cold infall with increasing cosmic time and halo mass, the proposed mechanism offers a promising explanation of the division of galaxies between the blue cloud to the red sequence in the colour-luminosity plane.

astro-ph.GA↗

arXiv2010

Galactic fountains and gas accretion

Star-forming disc galaxies such as the Milky Way need to accrete $\gsim$ 1 $M_{\odot}$ of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation.

astro-ph.GA↗

arXiv2009

Can dry merging explain the size evolution of early-type galaxies?

The characteristic size of early-type galaxies (ETGs) of given stellar mass is observed to increase significantly with cosmic time, from redshift z>2 to the present. A popular explanation for this size evolution is that ETGs grow through dissipationless ("dry") mergers, thus becoming less compact. Combining N-body simulations with up-to-date scaling relations of local ETGs, we show that such an explanation is problematic, because dry mergers do not decrease the galaxy stellar-mass surface-density enough to explain the observed size evolution, and also introduce substantial scatter in the scaling relations. Based on our set of simulations, we estimate that major and minor dry mergers increase half-light radius and projected velocity dispersion with stellar mass (M) as M^(1.09+/-0.29) and M^(0.07+/-0.11), respectively. This implies that: 1) if the high-z ETGs are indeed as dense as estimated, they cannot evolve into present-day ETGs via dry mergers; 2) present-day ETGs cannot have assembled more than ~45% of their stellar mass via dry mergers. Alternatively, dry mergers could be reconciled with the observations if there was extreme fine tuning between merger history and galaxy properties, at variance with our assumptions. Full cosmological simulations will be needed to evaluate whether this fine-tuned solution is acceptable.

astro-ph.GA↗

arXiv2009

Stationary models for the extra-planar gas in disc galaxies

The kinematics of the extra-planar neutral and ionised gas in disc galaxies shows a systematic decline of the rotational velocity with height from the plane (vertical gradient). This feature is not expected for a barotropic gas, whilst it is well reproduced by baroclinic fluid homogeneous models. The problem with the latter is that they require gas temperatures (above $10^5$ K) much higher than the temperatures of the cold and warm components of the extra-planar gas layer. In this paper, we attempt to overcome this problem by describing the extra-planar gas as a system of gas clouds obeying the Jeans equations. In particular, we consider models having the observed extra-planar gas distribution and gravitational potential of the disc galaxy NGC 891: for each model we construct pseudo-data cubes and we compare them with the HI data cube of NGC 891. In all cases the rotational velocity gradients are in qualitative agreement with the observations, but the synthetic and the observed data cubes of NGC 891 show systematic differences that cannot be accommodated by any of the explored models. We conclude that the extra-planar gas in disc galaxies cannot be satisfactorily described by a stationary Jeans-like system of gas clouds.

astro-ph.GA↗

arXiv2009

Globular clusters in modified Newtonian dynamics: velocity-dispersion profiles from self-consistent models

We test the modified Newtonian dynamics (MOND) theory with the velocity-dispersion profiles of Galactic globular clusters populating the outermost region of the Milky Way halo, where the Galactic acceleration is lower than the characteristic MOND acceleration a_0. For this purpose, we constructed self-consistent, spherical models of stellar systems in MOND, which are the analogues of the Newtonian King models. The models are spatially limited, reproduce well the surface-brightness profiles of globular clusters, and have velocity-dispersion profiles that differ remarkably in shape from the corresponding Newtonian models. We present dynamical models of six globular clusters, which can be used to efficiently test MOND with the available observing facilities. A comparison with recent spectroscopic data obtained for NGC2419 suggests that the kinematics of this cluster might be hard to explain in MOND.

astro-ph.GA↗

arXiv2009

Do high-velocity clouds form by thermal instability?

We examine the proposal that the HI "high-velocity" clouds (HVCs) surrounding the Milky Way and other disc galaxies form by condensation of the hot galactic corona via thermal instability. Under the assumption that the galactic corona is well represented by a non-rotating, stratified atmosphere, we find that for this formation mechanism to work the corona must have an almost perfectly flat entropy profile. In all other cases the growth of thermal perturbations is suppressed by a combination of buoyancy and thermal conduction. Even if the entropy profile were nearly flat, cold clouds with sizes smaller than 10 kpc could form in the corona of the Milky Way only at radii larger than 100 kpc, in contradiction with the determined distances of the largest HVC complexes. Clouds with sizes of a few kpc can form in the inner halo only in low-mass systems. We conclude that unless even slow rotation qualitatively changes the dynamics of a corona, thermal instability is unlikely to be a viable mechanism for formation of cold clouds around disc galaxies.

astro-ph.GA↗

arXiv2009

Strong Gravitational Lensing as a Probe of Gravity, Dark-Matter and Super-Massive Black Holes

Whereas considerable effort has been afforded in understanding the properties of galaxies, a full physical picture, connecting their baryonic and dark-matter content, super-massive black holes, and (metric) theories of gravity, is still ill-defined. Strong gravitational lensing furnishes a powerful method to probe gravity in the central regions of galaxies. It can (1) provide a unique detection-channel of dark-matter substructure beyond the local galaxy group, (2) constrain dark-matter physics, complementary to direct-detection experiments, as well as metric theories of gravity, (3) probe central super-massive black holes, and (4) provide crucial insight into galaxy formation processes from the dark matter point of view, independently of the nature and state of dark matter. To seriously address the above questions, a considerable increase in the number of strong gravitational-lens systems is required. In the timeframe 2010-2020, a staged approach with radio (e.g. EVLA, e-MERLIN, LOFAR, SKA phase-I) and optical (e.g. LSST and JDEM) instruments can provide 10^(2-4) new lenses, and up to 10^(4-6) new lens systems from SKA/LSST/JDEM all-sky surveys around ~2020. Follow-up imaging of (radio) lenses is necessary with moderate ground/space-based optical-IR telescopes and with 30-50m telescopes for spectroscopy (e.g. TMT, GMT, ELT). To answer these fundamental questions through strong gravitational lensing, a strong investment in large radio and optical-IR facilities is therefore critical in the coming decade. In particular, only large-scale radio lens surveys (e.g. with SKA) provide the large numbers of high-resolution and high-fidelity images of lenses needed for SMBH and flux-ratio anomaly studies.

astro-ph.CO↗

arXiv2008

Mass distribution and orbital anisotropy of early-type galaxies: constraints from the Mass Plane

Massive early-type galaxies are observed to lie on the Mass Plane (MP), a two-dimensional manifold in the space of effective radius R_e, projected mass M_p (measured via strong gravitational lensing) and projected velocity dispersion sigma within R_e/2. The MP is less `tilted' than the Fundamental Plane, and the two have comparable associated scatter. This means that c_e2=2*G*M_p/(R_e*sigma^2) is a nearly universal constant in the range sigma=175-400 km/s. This finding can be used to constrain the mass distribution and internal dynamics of early-type galaxies. We find that a relatively wide class of spherical galaxy models has values of c_e2 in the observed range, because c_e2 is not very strongly sensitive to the mass distribution and orbital anisotropy. If the total mass distribution is isothermal, a broad range of stellar luminosity profile and anisotropy is consistent with the observations, while NFW dark-matter halos require more fine tuning of the stellar mass fraction, luminosity profile and anisotropy. If future data can cover a broader range of masses, the MP could be seen to be tilted and the value of any such tilt would provide a discriminant between models for the total mass-density profile of the galaxies. [Abridged]

astro-ph↗

arXiv2008

Dynamical friction in modified Newtonian dynamics

We have tested a previous analytical estimate of the dynamical friction timescale in Modified Newtonian Dynamics (MOND) with fully non-linear N-body simulations. The simulations confirm that the dynamical friction timescale is significantly shorter in MOND than in equivalent Newtonian systems, i.e. systems with the same phase-space distribution of baryons and additional dark matter. An apparent conflict between this result and the long timescales determined for bars to slow and mergers to be completed in previous N-body simulations of MOND systems is explained. The confirmation of the short dynamical-friction timescale in MOND underlines the challenge that the Fornax dwarf spheroidal poses to the viability of MOND.

astro-ph↗

arXiv2007

Phase mixing in MOND

Dissipationless collapses in Modified Newtonian Dynamics (MOND) have been studied by using our MOND particle-mesh N-body code, finding that the projected density profiles of the final virialized systems are well described by Sersic profiles with index m<4 (down to m~2 for a deep-MOND collapse). The simulations provided also strong evidence that phase mixing is much less effective in MOND than in Newtonian gravity. Here we describe "ad hoc" numerical simulations with the force angular components frozen to zero, thus producing radial collapses. Our previous findings are confirmed, indicating that possible differences in radial orbit instability under Newtonian and MOND gravity are not relevant in the present context.

astro-ph↗

arXiv2007

Dissipationless collapses in MOND

Dissipationless collapses in Modified Newtonian Dynamics (MOND) are studied by using a new particle-mesh N-body code based on our numerical MOND potential solver. We found that low surface-density end-products have shallower inner density profile, flatter radial velocity-dispersion profile, and more radially anisotropic orbital distribution than high surface-density end-products. The projected density profiles of the final virialized systems are well described by Sersic profiles with index m~4, down to m~2 for a deep-MOND collapse. Consistently with observations of elliptical galaxies, the MOND end-products, if interpreted in the context of Newtonian gravity, would appear to have little or no dark matter within the effective radius. However, we found impossible (under the assumption of constant mass-to-light ratio) to simultaneously place the resulting systems on the observed Kormendy, Faber-Jackson and Fundamental Plane relations of elliptical galaxies. Finally, the simulations provide strong evidence that phase mixing is less effective in MOND than in Newtonian gravity.

astro-ph↗

arXiv2006

Dissipationless collapse, weak homology and central cores of elliptical galaxies

By means of high-resolution N-body simulations we revisited the dissipationless collapse scenario for galaxy formation. We considered both single-component collapses and collapses of a cold stellar distribution in a live dark matter halo. Single-component collapses lead to stellar systems whose projected profiles are fitted very well by the Sersic R^(1/m) law with 3.6 < m < 8. The stellar end-products of collapses in a dark matter halo are still well described by the R^(1/m) law, but with 1.9 < m < 12, where the lowest m values are obtained when the halo is dominant. In all the explored cases the profiles at small radii deviate from their global best-fit R^(1/m) model, being significantly flatter. The break-radius values are comparable with those measured in `core' elliptical galaxies, and are directly related to the coldness of the initial conditions. The dissipationless collapse of initially cold stellar distributions in pre-existing dark matter haloes may thus have a role in determining the observed weak homology of elliptical galaxies.

astro-ph↗

arXiv2005

Axisymmetric and triaxial MOND density-potential pairs

We present a simple method, based on the deformation of spherically symmetric potentials, to construct explicit axisymmetric and triaxial MOND density-potential pairs. General guidelines to the choice of suitable deformations, so that the resulting density distribution is nowhere negative, are presented. This flexible method offers for the first time the possibility to study the MOND gravitational field for sufficiently general and realistic density distributions without resorting to sophisticated numerical codes. The technique is illustrated by constructing the MOND density-potential pair for a triaxial galaxy model that, in the absence of deformation, reduces to the Hernquist sphere. Such analytical solutions are also relevant to test and validate numerical codes. Here we present a new numerical potential solver designed to solve the MOND field equation for arbitrary density distributions: the code is tested with excellent results against the analytic MOND triaxial Hernquist model and the MOND razor-thin Kuzmin disk, and a simple application is finally presented.

astro-ph↗

arXiv2003

Brightest cluster galaxy formation in the cluster C0037-2522: flattening of the dark matter cusp

The X-ray cluster C0337-2522 at redshift $z=0.59$ hosts in its core a group of five elliptical galaxies. Using N-body simulations we show that a multiple merging event among the five galaxies is expected to take place in the next few Gyrs, forming a central brightest cluster galaxy. We also find indications that dynamical friction heating associated with this event is likely to modify the central slope of the cluster dark matter density profile.

astro-ph↗

arXiv2003

Galactic cannibalism in the galaxy cluster C0337-2522 at z=0.59

According to the galactic cannibalism model, cD galaxies are formed in the center of galaxy clusters by merging of massive galaxies and accretion of smaller stellar systems: however, observational examples of the initial phases of this process are lacking. We have identified a strong candidate for this early stage of cD galaxy formation: a group of five elliptical galaxies in the core of the X-ray cluster C0337-2522 at redshift z=0.59. With the aid of numerical simulations, in which the galaxies are represented by N-body systems, we study their dynamical evolution up to z=0; the cluster dark matter distribution is also described as a N-body system. We find that a multiple merging event in the considered group of galaxies will take place before z=0 and that the merger remnant preserves the Fundamental Plane and the Faber-Jackson relations, while its behavior with respect to the Mbh-sigma relation is quite sensitive to the details of black hole merging [abridged].

astro-ph↗

arXiv2003

Galaxy merging, the Fundamental Plane of elliptical galaxies, and the Mbh-sigma relation

We explore the effects of dissipationless merging on the Fundamental Plane of elliptical galaxies by using a N-body code based on a new, high performance numerical scheme (Dehnen 2002). We investigate the extreme cases of galaxy growth by equal mass merging and accretion of small stellar systems; in a subset of simulations we also consider the presence of dark matter halos around the merging galaxies. Curiously, we found that the Fundamental Plane is preserved by major merging, while in the accretion scenario its edge--on thickness is only marginally reproduced, with substantial thickening in the case of merging with low angular momentum. We also found that both the Faber-Jackson and Kormendy relations are not reproduced by the simulations. Finally, we discuss the implications of our results for the origin of the Mbh-sigma and Magorrian relations. We found that dissipationless merging is unable to reproduce the Mbh-sigma relation, if Mbhs add linearly (while the Magorrian relation is nicely reproduced); on the contrary a black hole merging with substantial emission of gravitational waves reproduces the Mbh-sigma relation but fails at reproducing the Magorrian relation. [Abridged]

astro-ph↗

arXiv2002

A parallel implementation of a new fast algorithm for N-body simulations

A new, momentum preserving fast Poisson solver for N-body systems sharing effective O(N) computational complexity, has been recently developed by Dehnen (2000, 2002). We have implemented the proposed algorithms in a Fortran-90 code, and parallelized it by a domain decomposition using the MPI routines. The code has been applied to intensive numerical investigations of galaxy mergers, in particular focusing on the possible origin of some of the observed scaling relations of elliptical galaxies. We found that the Fundamental Plane is preserved by an equal mass merging hierarchy, while it is not in a scenario where galaxies grow by accretion of smaller stellar systems. In addition, both the Faber-Jackson and Kormendy relations are not reproduced by our simulations.

astro-ph↗