arXiv subjects

Carlo Nipoti

Publications and source records attributed to Carlo Nipoti.

At least 73 records · Page 4Linked to original sources

arXiv2013

The Density Profiles of Massive, Relaxed Galaxy Clusters. I. The Total Density Over Three Decades in Radius

Clusters of galaxies are excellent locations to probe the distribution of baryons and dark matter (DM) over a wide range of scales. We study a sample of seven massive, relaxed galaxy clusters with centrally-located brightest cluster galaxies (BCGs) at z=0.2-0.3. Using the observational tools of strong and weak gravitational lensing, combined with resolved stellar kinematics within the BCG, we measure the total radial density profile, comprising both dark and baryonic matter, over scales of ~3-3000 kpc. Lensing-derived mass profiles typically agree with independent X-ray estimates within ~15%, suggesting that departures from hydrostatic equilibrium are small and that the clusters in our sample (except A383) are not strongly elongated along the line of sight. The inner logarithmic slope gamma_tot of the total density profile measured over r/r200=0.003-0.03, where rho_tot ~ r^(-gamma_tot), is found to be nearly universal, with a mean = 1.16 +- 0.05 (random) +0.05-0.07 (systematic) and an intrinsic scatter of < 0.13 (68% confidence). This is further supported by the very homogeneous shape of the observed velocity dispersion profiles, obtained via Keck spectroscopy, which are mutually consistent after a simple scaling. Remarkably, this slope agrees closely with numerical simulations that contain only dark matter, despite the significant contribution of stellar mass on the scales we probe. The Navarro-Frenk-White profile characteristic of collisionless cold dark matter is a better description of the total mass density at radii >~ 5-10 kpc than that of dark matter alone. Hydrodynamical simulations that include baryons, cooling, and feedback currently provide a poorer match. We discuss the significance of our findings for understanding the assembly of BCGs and cluster cores, particularly the influence of baryons on the inner DM halo. [abridged]

astro-ph.CO↗

arXiv2012

Do globular clusters possess Dark Matter halos? A case study in NGC 2419

We use recently published measurements of the kinematics, surface brightness and stellar mass-to-light ratio of the globular cluster NGC 2419 to examine the possibility that this Galactic halo satellite is embedded in a low-mass dark matter halo. NGC 2419 is a promising target for such a study, since its extreme Galactocentric distance and large mass would have greatly facilitated the retention of dark matter. A Markov-Chain Monte Carlo approach is used to investigate composite dynamical models containing a stellar and a dark matter component. We find that it is unlikely that a significant amount of dark matter (less than approx. 6% of the luminous mass inside the tidal limit of the cluster) can be present if the stars follow an anisotropic Michie model and the dark matter a double power law model. However, we find that more general models, derived using a new technique we have developed to compute non-parametric solutions to the spherical Jeans equation, suggest the presence of a significant dark matter fraction (approximately twice the stellar mass). Thus the presence of a dark matter halo around NGC 2419 cannot be fully ruled out at present, yet any dark matter within the 10 arcmin visible extent of the cluster must be highly concentrated and cannot exceed 1.1x10^6 Solar masses (99% confidence), in stark contrast to expectations for a plausible progenitor halo of this structure.

astro-ph.CO↗

arXiv2012

Thermal stability of a weakly magnetized rotating plasma

The thermal stability of a weakly magnetized, rotating, stratified, optically thin plasma is studied by means of linear-perturbation analysis. We derive dispersion relations and criteria for stability against axisymmetric perturbations that generalize previous results on either non-rotating or unmagnetized fluids. The implications for the hot atmospheres of galaxies and galaxy clusters are discussed.

astro-ph.GA↗

arXiv2011

Polytropic model fits to the globular cluster NGC 2419 in Modified Newtonian Dynamics

We present an analysis of the globular cluster NGC 2419, using a polytropic model in Modified Newtonian Dynamics (MOND) to reproduce recently published high quality data of the structure and kinematics of the system. We show that a specific MOND polytropic model of NGC 2419 suggested by a previous study can be completely ruled out by the data. Furthermore, the highest likelihood fit polytrope in MOND is a substantially worse model (by a factor of approximately 5000) than a Newtonian Michie model studied in an earlier contribution by Ibata et al. (2011). We conclude that the structure and dynamics of NGC 2419 favor Newtonian dynamics and do indeed challenge the MOND theory.

astro-ph.CO↗

arXiv2011

Dry mergers and the size evolution of early-type galaxies

Dry (i.e. dissipationless) merging has been proposed as the main driver of the observed size evolution of early-type galaxies (ETGs). The actual role of this mechanism is questioned by the tightness of the local stellar mass-size relation of ETGs. Combining this observed scaling law with simple merging models, which should bracket cosmologically motivated merging histories, we draw the following conclusions: 1) local massive ETGs can have assembled at most ~45% of their stellar mass via dry mergers; 2) extreme fine tuning is required for this to be the case.

astro-ph.GA↗

arXiv2011

Radial-orbit instability in modified Newtonian dynamics

The stability of radially anisotropic spherical stellar systems in modified Newtonian dynamics (MOND) is explored by means of numerical simulations performed with the N-body code N-MODY. We find that Osipkov-Merritt MOND models require for stability larger minimum anisotropy radius than equivalent Newtonian systems (ENSs) with dark matter, and also than purely baryonic Newtonian models with the same density profile. The maximum value for stability of the Fridman-Polyachenko-Shukhman parameter in MOND models is lower than in ENSs, but higher than in Newtonian models with no dark matter. We conclude that MOND systems are substantially more prone to radial-orbit instability than ENSs with dark matter, while they are able to support a larger amount of kinetic energy stored in radial orbits than purely baryonic Newtonian systems. An explanation of these results is attempted, and their relevance to the MOND interpretation of the observed kinematics of globular clusters, dwarf spheroidal and elliptical galaxies is briefly discussed.

astro-ph.GA↗

arXiv2010

Thermal instability in rotating galactic coronae

The thermal stability of rotating, stratified, unmagnetized atmospheres is studied by means of linear-perturbation analysis, finding stability, overstability or instability, depending on the properties of the gas distribution, but also on the nature of the perturbations. In the relevant case of distributions with outward-increasing specific entropy and angular momentum, axisymmetric perturbations grow exponentially, unless their wavelength is short enough that they are damped by thermal conduction; non-axisymmetric perturbations typically undergo overstable oscillations in the limit of zero conductivity, but are effectively stabilized by thermal conduction, provided rotation is differential. To the extent that the studied models are representative of the poorly constrained hot atmospheres of disc galaxies, these results imply that blob-like, cool overdensities are unlikely to grow in galactic coronae, suggesting an external origin for the high-velocity clouds of the Milky Way.

astro-ph.GA↗

arXiv2010

Are galactic coronae thermally unstable?

A substantial fraction of the baryons of disk galaxies like the Milky Way is expected to reside in coronae of gas at the virial temperature. This is the only realistic reservoir of gas available to feed star formation in the disks. The way this feeding occurs depends crucially on whether galactic coronae can fragment into cool clouds via thermal instability. Here I summarize arguments suggesting that galactic coronae are not prone to thermal instability, and I briefly discuss the implications for galaxy-formation models and for the origin of the high-velocity clouds of the Milky Way.

astro-ph.GA↗

arXiv2009

Secular evolution of galaxies and galaxy clusters in decaying dark matter cosmology

If the dark matter sector in the universe is composed by metastable particles, galaxies and galaxy clusters are expected to undergo significant secular evolution from high to low redshift. We show that the decay of dark matter, with a lifetime compatible with cosmological constraints, can be at the origin of the observed evolution of the Tully-Fisher relation of disk galaxies and alleviate the problem of the size-evolution of elliptical galaxies, while being consistent with the current observational constraints on the gas fraction of clusters of galaxies.

astro-ph.CO↗

arXiv2009

Cool gas accretion, thermal evaporation and quenching of star formation in elliptical galaxies

The most evident features of colour-magnitude diagrams of galaxies are the red sequence of quiescent galaxies, extending up to the brightest elliptical galaxies, and the blue cloud of star-forming galaxies, which is truncated at a luminosity L~L*. The truncation of the blue cloud indicates that in the most massive systems star formation must be quenched. For this to happen the virial-temperature galactic gas must be kept hot and any accreted cold gas must be heated. The elimination of accreted cold gas can be due to thermal evaporation by the hot interstellar medium, which in turn is prevented from cooling by feedback from active galactic nuclei.

astro-ph.GA↗

arXiv2009

Dry mergers and the formation of early-type galaxies: constraints from lensing and dynamics

Dissipationless (gas-free or "dry") mergers have been suggested to play a major role in the formation and evolution of early-type galaxies, particularly in growing their mass and size without altering their stellar populations. We perform a new test of the dry merger hypothesis by comparing N-body simulations of realistic systems to empirical constraints provided by recent studies of lens early-type galaxies. We find that major and minor dry mergers: i) preserve the nearly isothermal structure of early-type galaxies within the observed scatter; ii) do not change more than the observed scatter the ratio between total mass M and "virial" mass R_e*sigma/2G (where R_e is the half-light radius and sigma the projected velocity dispersion); iii) increase strongly galaxy sizes [as M^(0.85+/-0.17)] and weakly velocity dispersions [as M^(0.06+/-0.08)] with mass, thus moving galaxies away from the local observed M-R_e and M-sigma relations; iv) introduce substantial scatter in the M-R_e and M-sigma relations. Our findings imply that, unless there is a high degree of fine tuning of the mix of progenitors and types of interactions, present-day massive early-type galaxies cannot have assembled more than ~50% of their mass, and increased their size by more than a factor ~1.8, via dry merging.

astro-ph.GA↗

arXiv2009

Cusp-core dichotomy of elliptical galaxies: the role of thermal evaporation

There are two families of luminous elliptical galaxies: cusp galaxies, with steep central surface-brightness profiles, and core galaxies, whose surface-brightness profiles have flat central cores. Thermal evaporation of accreted cold gas by the hot interstellar medium may be at the origin of this cusp-core dichotomy: in less massive (hot-gas poor) galaxies central cores are likely to be refilled by central starbursts following cold gas infall, while in more massive (hot-gas rich) galaxies most cold gas is eliminated and central cores survive. This scenario is consistent with the observation that cusp and core galaxies differ systematically in terms of optical luminosity, X-ray gas content, age of the central stellar population, and properties of the active galactic nucleus.

astro-ph.GA↗

arXiv2009

Crash-testing the CAULDRON code for joint lensing and dynamics analysis of early-type galaxies

We apply the joint lensing and dynamics code for the analysis of early-type galaxies, CAULDRON, to a rotating N-body stellar system with dark matter halo which significantly violates the two major assumptions of the method, i.e. axial symmetry supported by a two-integral distribution function. The goal is to study how CAULDRON performs in an extreme case, and to determine which galaxy properties can still be robustly recovered. Three data sets, corresponding to orthogonal lines of sight, are generated from the N-body system and analysed with the identical procedure followed in the study of real lens galaxies, adopting an axisymmetric power-law total density distribution. We find that several global properties of the N-body system are recovered with remarkable accuracy, despite the fact that the adopted power-law model is too simple to account for the lack of symmetry of the true density distribution. In particular, the logarithmic slope of the total density distribution is robustly recovered to within less than 10 per cent (with the exception of the ill-constrained very inner regions), the inferred angle-averaged radial profile of the total mass closely follows the true distribution, and the dark matter fraction of the system (inside the effective radius) is correctly determined within ~ 10 per cent of the total mass. Unless the line of sight direction is almost parallel to the total angular momentum vector of the system, reliably recovered quantities also include the angular momentum, the V/sigma ratio, and the anisotropy parameter delta. We conclude that the CAULDRON code can be safely and effectively applied to real early-type lens galaxies, providing reliable information also for systems that depart significantly from the method's assumptions.

astro-ph↗

arXiv2008

N-body simulations in modified Newtonian dynamics

We describe some results obtained with N-MODY, a code for N-body simulations of collisionless stellar systems in modified Newtonian dynamics (MOND). We found that a few fundamental dynamical processes are profoundly different in MOND and in Newtonian gravity with dark matter. In particular, violent relaxation, phase mixing and galaxy merging take significantly longer in MOND than in Newtonian gravity, while dynamical friction is more effective in a MOND system than in an equivalent Newtonian system with dark matter.

astro-ph↗

arXiv2008

N-MODY: a code for collisionless N-body simulations in modified Newtonian dynamics

We describe the numerical code N-MODY, a parallel particle-mesh code for collisionless N-body simulations in modified Newtonian dynamics (MOND). N-MODY is based on a numerical potential solver in spherical coordinates that solves the non-linear MOND field equation, and is ideally suited to simulate isolated stellar systems. N-MODY can be used also to compute the MOND potential of arbitrary static density distributions. A few applications of N-MODY indicate that some astrophysically relevant dynamical processes are profoundly different in MOND and in Newtonian gravity with dark matter.

astro-ph↗

arXiv2007

The role of thermal evaporation in galaxy formation

In colour-magnitude diagrams most galaxies fall in either the ``blue cloud'' or the ``red sequence'', with the red sequence extending to significantly brighter magnitudes than the blue cloud. The bright-end of the red sequence comprises elliptical galaxies (Es) with boxy isophotes and luminosity profiles with shallow central cores, while fainter Es have disky isophotes and power-law inner surface-brightness (SB) profiles. An analysis of published data reveals that the centres of galaxies with power-law central SB profiles have younger stellar populations than the centres of cored galaxies. We argue that thermal evaporation of cold gas by virial-temperature gas plays an important role in determining these phenomena. In less massive galaxies, thermal evaporation is not very efficient, so significant amounts of cold gas can reach the galaxy centre and fill a central core with newly formed stars, consistent with the young stellar ages of the cusps of Es with power-law SB profiles. In more massive galaxies, cold gas is evaporated within a dynamical time, so star formation is inhibited, and a core in the stellar density profile produced by dissipationless dynamics cannot be refilled. The different observed properties of AGN in higher-mass and lower-mass ellipticals are also explained because in the former the central black holes invariably accrete hot gas, while in the latter they typically accrete cold gas. An important consequence of our results is that at the present time there cannot be blue, star-forming galaxies in the most massive galactic halos, consistent with the observed truncation of the blue cloud at L*. [abridged]

astro-ph↗

arXiv2007

Galaxy merging in MOND

We present the results of N-body simulations of dissipationless galaxy merging in Modified Newtonian Dynamics (MOND). For comparison, we also studied Newtonian merging between galaxies embedded in dark matter halos, with internal dynamics equivalent to the MOND systems. We found that the merging timescales are significantly longer in MOND than in Newtonian gravity with dark matter, suggesting that observational evidence of rapid merging could be difficult to explain in MOND. However, when two galaxies eventually merge, the MOND merging end-product is hardly distinguishable from the final stellar distribution of an equivalent Newtonian merger with dark matter.

astro-ph↗

arXiv2007

Vertical dynamics of disk galaxies in MOND

We investigate the possibility of discriminating between Modified Newtonian Dynamics (MOND) and Newtonian gravity with dark matter, by studying the vertical dynamics of disk galaxies. We consider models with the same circular velocity in the equatorial plane (purely baryonic disks in MOND and the same disks in Newtonian gravity embedded in spherical dark matter haloes), and we construct their intrinsic and projected kinematical fields by solving the Jeans equations under the assumption of a two-integral distribution function. We found that the vertical velocity dispersion of deep-MOND disks can be much larger than in the equivalent spherical Newtonian models. However, in the more realistic case of high-surface density disks this effect is significantly reduced, casting doubts on the possibility of discriminating between MOND and Newtonian gravity with dark matter by using current observations.

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