arXiv subjects

X. Hernandez

Publications and source records attributed to X. Hernandez.

At least 55 records · Page 3Linked to original sources

arXiv2013

Isothermal distributions in MONDian gravity as a simple unifying explanation for the ubiquitous $ρ\propto r^{-3}$ density profiles in tenuous stellar halos

That the stellar halo of the Milky Way has a density profile which to first approximation satisfies $ρ\propto r^{-3}$ has been known for a long time. More recently, it has become clear that M31 also has such an extended stellar halo, which approximately follows the same radial scaling. Studies of distant galaxies have revealed the same phenomenology. Also, we now know that the density profiles of the globular cluster systems of our Galaxy and Andromeda to first approximation follow $ρ\propto r^{-3}$, $Σ\propto R^{-2}$ in projection. Recently, diffuse populations of stars have been detected spherically surrounding a number of Galactic globular clusters, extending much beyond the Newtonian tidal radii, often without showing any evidence of tidal features. Within the standard Newtonian and GR scenario, numerous and diverse particular explanations have been suggested, individually tailored to each of the different classes of systems described above. Here we show that in a MONDian gravity scenario, any isothermal tenuous halo of tracer particles forming a small perturbation surrounding a spherically symmetric mass distribution, will have an equilibrium configuration which to first approximation satisfies a $ρ\propto r^{-3}$ scaling.

astro-ph.GA↗

arXiv2013

The massive elliptical galaxy NGC 4649 from the perspective of extended gravity

Elliptical galaxies are systems where dark matter is usually less necessary to explain observed dynamics than in the case of spiral galaxies, however there are some instances where Newtonian gravity and the observable mass are insufficient to explain their observed structure and kinematics. Such is the case of NGC 4649, a massive elliptical galaxy in the Virgo cluster for which recent studies report a high fraction of dark matter, 0.78 at $4R_e$. However this galaxy has been studied within the MOND hypothesis, where a good agreement with the observed values of velocity dispersion is found. Using a MONDian gravity force law, here we model this galaxy as a self-consistent gravitational equilibrium dynamical system. This force law reproduces the MOND phenomenology in the $a a_{0}$. Within the MONDian $a<a_{0}$ scales, centrifugal equilibrium or dispersion velocities become independent of radius, and show a direct proportionality to the fourth root of the total baryonic mass, $V^{4}\propto(M G a_{0})$. We find that the recent detailed observations of the surface brightness profile and the velocity dispersion profile for this galaxy are consistent with the phenomenology expected in MONDian theories of modified gravity, without the need of invoking the presence of any hypothetical dark matter.

astro-ph.CO↗

arXiv2012

Flattened velocity dispersion profiles in Globular Clusters: Newtonian tides or modified gravity?

Over the past couple of years, a number of observational studies have confirmed the flattening of the radial velocity dispersion profiles for stars in various nearby globular clusters. As the projected radial coordinate is increased, a radius appears beyond which, the measured velocity dispersion ceases to drop and settles at a fixed value, $σ_{\infty}$. Under Newtonian gravity, this is explained by invoking tidal heating from the overall Milky Way potential on the outer, more loosely bound stars, of the globular clusters in question. From the point of view of modified gravity theories, such an outer flattening is expected on crossing the critical acceleration threshold $a_{0}$, beyond which, a transition to MONDian dynamics is expected, were equilibrium velocities cease to be a function of distance. In this paper we attempt to sort out between the above competing explanations, by looking at their plausibility in terms of an strictly empirical approach. We determine Newtonian tidal radii using masses accurately calculated through stellar population modelling, and hence independent of any dynamical assumptions, distances, size and orbital determinations for a sample of 16 globular clusters. We show that their Newtonian tidal radii at perigalacticon are generally larger that the radii at which the flattening in the velocity dispersion profiles occurs, by large factors of 4, on average. While this point makes the Newtonian tidal explanation suspect, it is found that the radii at which the flattening is observed on average correlate with the radii where the $a_{0}$ threshold is crossed, and that $σ_{\infty}$ values scale with the fourth root of the total masses, all features predicted under modified gravity theories.

astro-ph.GA↗

arXiv2012

The spin of late-type galaxies at redshifts z < 1.2

We study the evolution of the galactic spin using data of high redshift galaxies in the fields of the Great Observatories Origins Deep Survey (GOODS). Through simple dynamical considerations we estimate the spin for the disc galaxies in our sample and find that its distribution is consistent with that found for nearby galaxies. Defining a dimensionless angular momentum parameter for the disc component of the galaxies ($λ_{d}$), we do not find signs of evolution in the redshift range $0.4 \leq z \leq 1.2$. We find that the mass and environmental dependence of the spin of our high redshift galaxies are similar to that of low-$z$ galaxies; showing a strong dependence on mass, in the sense that low-mass systems present higher $λ_{d}$ values than high-mass galaxies, with no significant dependence on the environmental density. These results lead us to conclude that, although individual disc galaxies might occasionally suffer from strong evolution, they evolve in such a way that the overall spin distribution of the galactic population remains constant from $z\sim1$ to the present epoch.

astro-ph.CO↗

arXiv2012

Wide binaries as a critical test for Gravity theories

Assuming Newton's gravity and GR to be valid at all scales leads to the dark matter hypothesis as a requirement demanded by the observed dynamics and measured baryonic content at galactic and extragalactic scales. Alternatively, modified gravity scenarios where a change of regime appears at acceleration scales $a<a_{0}$ have been proposed. This modified regime at $a<a_{0}$ will generically be characterised by equilibrium velocities which become independent of distance. Here we identify a critical test in this debate and we propose its application to samples of wide binary stars. Since for $1 M_{\odot}$ systems the acceleration drops below $a_{0}$ at scales of around 7000 AU, a statistical survey of wide binaries with relative velocities and separations reaching $10^{4}$ AU and beyond should prove useful to the above debate. We apply the proposed test to the best currently available data. Results show a constant upper limit to the relative velocities in wide binaries which is independent of separation for over three orders of magnitude, in analogy with galactic flat rotation curves in the same $a<a_{0}$ acceleration regime. Our results are suggestive of a breakdown of Kepler's third law beyond $a \approx a_{0}$ scales, in accordance with generic predictions of modified gravity theories designed not to require any dark matter at galactic scales and beyond.

astro-ph.GA↗

arXiv2012

A Phase Space Diagram for Gravity

In modified theories of gravity including a critical acceleration scale, $a_{0}$, a critical length scale, $r_{M}=(GM/a_{0})^{1/2}$, will naturally arise, with the transition from the Newtonian to the dark matter mimicking regime occurring for systems larger than $r_{M}$. This adds a second critical scale to gravity, in addition to the one introduced by the criterion $v < c$ of the Shwarzschild radius, $r_{S}=2GM/c^{2}$. The distinct dependencies of the two above length scales give rise to non-trivial phenomenology in the (mass, length) plane for astrophysical structures, which we explore here. Surprisingly, extrapolation to atomic scales suggests gravity should be at the dark matter mimicking regime there.

astro-ph.CO↗

arXiv2012

The outskirts of globular clusters as modified gravity probes

In the context of theories of gravity modified to account for the observed dynamics of galactic systems without the need to invoke the existence of dark matter, a prediction often appears regarding low acceleration systems: wherever $a$ falls below $a_{0}$ one should expect a transition from the classical to the modified gravity regime.This modified gravity regime will be characterised by equilibrium velocities which become independent of distance, and which scale with the fourth root of the total baryonic mass, $V^{4} \propto M$. The two above conditions are the well known flat rotation curves and Tully-Fisher relations of the galactic regime. Recently however, a similar phenomenology has been hinted at, at the outskirts of Galactic globular clusters, precisely in the region where $a<a_{0}$. Radial profiles of the projected velocity dispersion have been observed to stop decreasing along Keplerian expectations, and to level off at constant values beyond the radii where $a<a_{0}$. We have constructed gravitational equilibrium dynamical models for a number of globular clusters for which the above gravitational anomaly has been reported, using a modified Newtonian force law which yields equilibrium velocities equivalent to MOND. We find models can be easily constructed having an inner Newtonian region and an outer modified gravity regime, which reproduce all observational constraints, surface brightness profiles, total masses and line of sight velocity dispersion profiles. Through the use of detailed single stellar population models tuned individually to each of the globular clusters in question, we derive estimates of the total masses for these systems. Interestingly, we find that the asymptotic values of the velocity dispersion profiles are consistent with scaling with the fourth root of the total masses, as expected under modified gravity scenarios.

astro-ph.CO↗

arXiv2012

Non-relativistic Extended Gravity and its applications across different astrophysical scales

Using dimensional analysis techniques we present an extension of Newton's gravitational theory built under the assumption that Milgrom's acceleration constant is a fundamental quantity of nature. The gravitational force converges to Newton's gravity and to a MOND-like description in two different mass and length regimes. It is shown that a modification on the force sector (and not in the dynamical one as MOND does) is more convenient and can reproduce and predict different phenomena usually ascribed to dark matter at the non-relativistic level.

astro-ph.GA↗

arXiv2012

Wide binaries as a critical test of Classical Gravity

Modified gravity scenarios where a change of regime appears at acceleration scales $a<a_{0}$ have been proposed. Since for $1 M_{\odot}$ systems the acceleration drops below $a_{0}$ at scales of around 7000 AU, a statistical survey of wide binaries with relative velocities and separations reaching $10^{4}$ AU and beyond should prove useful to the above debate. We apply the proposed test to the best currently available data. Results show a constant upper limit to the relative velocities in wide binaries which is independent of separation for over three orders of magnitude, in analogy with galactic flat rotation curves in the same $a<a_{0}$ acceleration regime. Our results are suggestive of a breakdown of Kepler's third law beyond $a \approx a_{0}$ scales, in accordance with generic predictions of modified gravity theories designed not to require any dark matter at galactic scales and beyond.

astro-ph.GA↗

arXiv2011

Probabilistic distributions of $M/L$ values for ultra-faint dwarf spheroidal galaxies: stochastic samplings of the IMF

As the total mass of stellar populations of fixed IMF, age and metallicity tends to infinity, resulting intrinsic $M/L$ values quickly converge to fixed numbers associated to the particulars of the stars in question. When going to very small stellar populations, the stochastic sampling of the IMF will yield an inherent spread for the $M/L$ values, which will become probabilistic quantities. For the recently discovered ultra-faint dwarf spheroidal galaxies, with total luminosities dropping below 10^3 L_V/L_o, it is important to asses the amplitude of the probabilistic spread in inherent $M/L$ values. The total baryonic masses of these systems are usually estimated from their luminosities, assuming fixed, deterministic $M/L$ values, suitable for the infinite population limit. These total baryonic masses are crucial for the testing of structure formation scenarios, as the local ultra-faint dwarf spheroidals represent the most extreme galactic scales known. Also subject to reliable $M/L$ values, is the use of these systems as possible discriminants between dark matter and modified gravity theories. By simulating large collections of stellar populations, each consisting of a particular collection of individual stars, we compute statistical distributions for the resulting $M/L$ values. For total numbers of stars in the range of what is observed for the ultra-faint dwarf spheroidals, inherent $M/L$ values of stellar populations can be expected to vary by factors of upwards of 3, interestingly, systematically skewed towards higher values than what corresponds to the infinite population limit. This can explain part of the spread in reported baryonic masses for these systems, which also appear as shifted systematically towards high dark to baryonic mass ratios at fixed stellar velocity dispersions.

astro-ph.GA↗

arXiv2011

A perturbative analysis of Quasi-Radial density waves in galactic disks

The theoretical understanding of density waves in disk galaxies starts from the classical WKB perturbative analysis of tight-winding perturbations, the key assumption being that the potential due to the density wave is approximately radial. The above has served as a valuable guide in aiding the understanding of both simulated and observed galaxies, in spite of a number of caveats being present. The observed spiral or bar patterns in real galaxies are frequently only marginally consistent with the tight-winding assumption, often in fact, outright inconsistent. Here we derive a complementary formulation to the problem, by treating quasi-radial density waves under simplified assumptions in the linear regime. We assume that the potential due to the density wave is approximately tangential, and derive the corresponding dispersion relation of the problem. We obtain an instability criterion for the onset of quasi-radial density waves, which allows a clear understanding of the increased stability of the higher order modes, which appear at progressively larger radii, as often seen in real galaxies. The theory naturally yields a range of pattern speeds for these arms which appears constrained by the condition $Ω_{p}<Ω_{0} \pm κ/m$. For the central regions of galaxies where solid body rotation curves might apply, we find weak bars in the oscillatory regime with various pattern speeds, including counter rotating ones, and a prediction for $Ω_{p}$ to increase towards the centre, as seen in the rapidly rotating bars within bars of some numerical simulations. We complement this study with detailed numerical simulations of galactic disks and careful Fourier analysis of the emergent perturbations, which support the theory presented.

astro-ph.GA↗

arXiv2011

The galactic spin of AGN galaxies

Using an extensive sample of galaxies selected from the Sloan Digital Sky Survey Data Release 5, we compare the angular momentum distribution of active galactic nucleus (AGN) with non-AGN hosting late-type galaxies. To this end we characterize galactic spin through the dimensionless angular momentum parameter $λ$, which we estimate approximately through simple dynamical considerations. Using a volume limited sample, we find a considerable difference when comparing the empirical distributions of $λ$ for AGNs and non-AGN galaxies, the AGNs showing typically low $λ$ values and associated dispersions, while non-AGNs present higher $λ$ values and a broader distribution. A more striking difference is found when looking at $λ$ distributions in thin $M_{r}$ cuts, while the spin of non-AGN galaxies presents an anti-correlation with $M_{r}$, with bright (massive) galaxies having low spins, AGN host galaxies present uniform values of $λ$ at all magnitudes, a behavior probably imposed by the fact that most late-type AGN galaxies present a narrow range in color, with a typical constant $λ$ value. We also find that the fraction of AGN hosting galaxies in our sample strongly depends on galactic spin, increasing dramatically for decreasing $λ$. For AGN host galaxies we compute the mass of their supermassive black holes and find that this value tends to be higher for low spin galaxies, even at fixed luminosity, a result that could account, to a certain extent, for the spread on the luminosity black-hole mass relation.

astro-ph.CO↗

arXiv2011

Quantifying galactic morphological transformations in the cluster environment

We study the effects of the cluster environment on galactic morphology by defining a dimensionless angular momentum parameter $λ_{d}$, to obtain a quantitative and objective measure of galaxy type. The use of this physical parameter allows us to take the study of morphological transformations in clusters beyond the measurements of merely qualitative parameters, e.g. S/E ratios, to a more physical footing. To this end, we employ an extensive Sloan Digital Sky Survey sample (Data Release 7), with galaxies associated with Abell galaxy clusters. The sample contains 121 relaxed Abell clusters and over 51,000 individual galaxies, which guarantees a thorough statistical coverage over a wide range of physical parameters. We find that the median $λ_{d}$ value tends to decrease as we approach the cluster center, with different dependences according to the mass of the galaxies and the hosting cluster; low and intermediate mass galaxies showing a strong dependence, while massive galaxies seems to show, at all radii, low $λ_{d}$ values. By analysing trends in $λ_{d}$ as functions of the nearest neighbour environment, clustercentric radius and velocity dispersion of clusters, we can identify clearly the leading physical processes at work. We find that in massive clusters ($σ>700$ km/s), the interaction with the cluster central region dominates, whilst in smaller clusters galaxy-galaxy interactions are chiefly responsible for driving galactic morphological transformations.

astro-ph.CO↗

arXiv2010

A natural approach to extended Newtonian gravity: tests and predictions across astrophysical scales

In the pursuit of a general formulation for a modified gravitational theory at the non-relativistic level and as an alternative to the dark matter hypothesis, we construct a model valid over a wide variety of astrophysical scales. Through the inclusion of Milgrom's acceleration constant into a gravitational theory, we show that very general formulas can be constructed for the acceleration felt by a particle. Dimensional analysis shows that this inclusion naturally leads to the appearance of a mass-length scale in gravity, breaking its scale invariance. A particular form of the modified gravitational force is constructed and tested for consistency with observations over a wide range of astrophysical environments, from solar system to extragalactic scales. We show that over any limited range of physical parameters, which define a specific class of astrophysical objects, the dispersion velocity of a system must be a power law of its mass and size. These powers appear linked together through a natural constraint relation of the theory. This yields a generalised gravitational equilibrium relation valid for all astrophysical systems. A general scheme for treating spherical symmetrical density distributions is presented, which in particular shows that the fundamental plane of elliptical galaxies, the Newtonian virial equilibrium, the Tully-Fisher and the Faber-Jackson relations, as well as the scalings observed in local dwarf spheroidal galaxies, are nothing but particular cases of that relation when applied to the appropriate mass-length scales. We discuss the implications of this approach for a modified theory of gravity and emphasise the advantages of working with the force, instead of altering Newton's second law of motion, in the formulation of a gravitational theory.

astro-ph.GA↗

arXiv2010

Understanding local Dwarf Spheroidals and their scaling relations under MOdified Newtonian Dynamics

We use a specific form of the interpolation function in the MOND formalism, which optimally accounts for the internal structure of dwarf spheroidal (dSph) galaxies, to explore the consequences it has on the scaling relations seen in these systems.}} {The particular form of the interpolation function we used leads to a law of gravity that does not degrade the good fit of the MOND proposal on galactic scales, and in fact, slightly improves the accordance with observations at dSph scales. This formalism yields a good description of gravitational phenomena without the need of invoking any still undetected and hypothetically dominant dark matter, in the weak field regime probed by local dSph galaxies.} {Isothermal equilibrium density profiles then yield projected surface density profiles for the local dSph galaxies in very good agreement with observational determinations, for values of the relevant parameters as inferred from recent observations of these Galactic satellites. The observed scaling relations for these systems are also naturally accounted for within the proposed scheme, including a previously unrecognised correlation of the inferred mass-to-light ratios of local dSph's with the ages of their stellar populations, which is natural in modified gravity schemes in the absence of dark matter.} {The results shed some light on the form that the MOND interpolating function may have in the most challenging regime, which occurs at moderate accelerations and intermediate mass--weighted lengths.

astro-ph.GA↗

arXiv2010

An upper limit to the central density of dark matter haloes from consistency with the presence of massive central black holes

We study the growth rates of massive black holes in the centres of galaxies from accretion of dark matter from their surrounding haloes. By considering only the accretion due to dark matter particles on orbits unbound to the central black hole, we obtain a firm lower limit to the resulting accretion rate. We find that a runaway accretion regime occurs on a timescale which depends on the three characteristic parameters of the problem: the initial mass of the black hole, and the volume density and velocity dispersion of the dark matter particles in its vicinity. An analytical treatment of the accretion rate yields results implying that for the largest black hole masses inferred from QSO studies ($>10^{9} M_{\odot}$), the runaway regime would be reached on time scales which are shorter than the lifetimes of the haloes in question for central dark matter densities in excess of $250 M_{\odot}$pc$^{-3}$. Since reaching runaway accretion would strongly distort the host dark matter halo, the inferences of QSO black holes in this mass range lead to an upper limit on the central dark matter densities of their host haloes of $ρ_{0} < 250 M_{\odot} $pc$^{-3}$. This limit scales inversely with the assumed central black hole mass. However, thinking of dark matter profiles as universal across galactic populations, as cosmological studies imply, we obtain a firm upper limit for the central density of dark matter in such structures.

astro-ph.CO↗

arXiv2009

Clues on the origin of galactic angular momentum from looking at galaxy pairs

We search for correlations between the spin in pairs of spiral galaxies, to study if the angular momentum gain for each galaxy was the result of tidal torques imprint by the same tidal field. To perform our study we made use of a sample of galaxy pairs identified using the Sloan Digital Sky Survey. We find a weak, but statistically significant correlation between the spin magnitude of neighbouring galaxies, but no clear alignment between their orientation. We show that events such as interactions with close neighbours play an important role in the value of the spin for the final configuration, as we find these interactions tend to reduce the value of the $λ$ spin parameter of late-type galaxies considerably, with dependence on the morphology of the neighbour. This implies that the original tidal field for each pair could have been similar, but the redistribution of angular momentum at later stages of evolution is important.

astro-ph.GA↗

arXiv2009

Origin and evolution of galactic spin from looking at galaxy pairs

To study if the angular momentum gain for each member of a galaxy pair was the result of tidal torques imprinted by the same tidal field, we search for correlations between the spin in pairs of spiral galaxies identified using the Sloan Digital Sky Survey. We find a weak, but statistically significant correlation between the spin magnitude of neighbouring galaxies. We show that events such as close interactions with neighbours play an important role in the value of the spin for the final configuration, as we find these interactions tend to reduce the value of the $λ$ spin parameter of late-type galaxies considerably. This implies that the original tidal field for each pair could have been similar, but the redistribution of angular momentum at later stages of evolution is important

astro-ph.CO↗