Constant Surface Gravity and Density Profile of Dark Matter

The Astrophysical Journal, 2016

Published mass models fitted to galaxy rotation curves are used to study the systematic properties of dark matter (DM) halos in late-type and dwarf spheroidal (dSph) galaxies. Halo parameters are derived by fitting non-singular isothermals to (V 2 −V 2 vis) 1/2 , where V (r) is the observed rotation curve and V vis is the rotation curve of the visible matter. The latter is calculated from the surface brightness assuming that the mass-to-light ratio M/L is constant with radius. "Maximum disk" values of M/L are adjusted to fit as much of the inner rotation curve as possible without making the halo have a hollow core. Rotation curve decomposition becomes impossible fainter than absolute magnitude M B ≃ −14, where V becomes comparable to the velocity dispersion of the gas. To increase the luminosity range further, we include dSph galaxies, which are physically related to spiral and irregular galaxies. Combining the data, we find that DM halos satisfy well defined scaling laws analogous to the "fundamental plane" relations for elliptical galaxies. Halos in less luminous galaxies have smaller core radii r c , higher central densities ρ 0 , and smaller central velocity dispersions σ. Scaling laws provide new and detailed constraints on the nature of DM and on galaxy formation and evolution. Some simple implications include: 1-A single, continuous physical sequence of increasing mass extends from dSph galaxies with M B ≃ −7.6 to Sc I galaxies with M B ≃ −22.4. 2-The high DM densities in dSph galaxies are normal for such tiny galaxies. Since virialized density depends on collapse redshift z coll , ρ 0 ∝ (1 + z coll) 3 , the smallest dwarfs formed at least ∆z coll ≃ 7 earlier than the biggest spirals. 3-The high DM densities of dSphs implies that they are real galaxies formed from primordial density fluctuations. They are not tidal fragments. Tidal dwarfs cannot retain even the low DM densities of their giant-galaxy progenitors. In contrast, dSphs have higher DM densities than do giant-galaxy progenitors. 4-The fact that, as luminosity decreases, dwarf galaxies become much more numerous and also more nearly dominated by DM raises the possibility that there exists a large population of objects that are completely dark. Such objects are a canonical prediction of cold DM theory. If they exist, "empty halos" are likely to be small and dense-that is, darker versions of Draco and UMi. 5-The slopes of the DM parameter correlations provide a measure on galactic mass scales of the slope n of the power spectrum |δ k | 2 ∝ k n of primordial density fluctuations. Our preliminary results not yet corrected for baryonic compression of DM give n ≃ −1.9 ± 0.2. This is consistent with cold DM theory.

Monthly Notices of the Royal Astronomical Society, 2014

We introduce a mass dependent density profile to describe the distribution of dark matter within galaxies, which takes into account the stellar-to-halo mass dependence of the response of dark matter to baryonic processes. The study is based on the analysis of hydrodynamically simulated galaxies from dwarf to Milky Way mass, drawn from the MaGICC project, which have been shown to match a wide range of disk scaling relationships. We find that the best fit parameters of a generic double power-law density profile vary in a systematic manner that depends on the stellar-to-halo mass ratio of each galaxy. Thus, the quantity M ⋆ /M halo constrains the inner (γ) and outer (β) slopes of dark matter density, and the sharpness of transition between the slopes (α), reducing the number of free parameters of the model to two. Due to the tight relation between stellar mass and halo mass, either of these quantities is sufficient to describe the dark matter halo profile including the effects of baryons. The concentration of the haloes in the hydrodynamical simulations is consistent with N-body expectations up to Milky Way mass galaxies, at which mass the haloes become twice as concentrated as compared with pure dark matter runs.

In order to determine as best as possible the nature of the dark matter (DM) particle (mass and decoupling temperature) we compute analytically the DM galaxy properties as the halo density profile, halo radius and surface density and compare them to their observed values. We match the theoretically computed surface density to its observed value in order to obtain: (i) the decreasing of the phasespace density since equilibration till today (ii) the mass of the dark matter particle and the decoupling temperature T d (iii) the kind of the halo density profile (core or cusp). The dark matter particle mass turns to be between 1 and 2 keV and the decoupling temperature T d turns to be above 100 GeV. keV dark matter particles necessarily produce cored density profiles while wimps (m ∼ 100 GeV, T d ∼ 5 GeV) inevitably produce cusped profiles at scales about 0.003 pc. We compute in addition the halo radius r 0 , the halo central density ρ 0 and the halo particle r. m. s. velocity v 2 1/2 halo they all reproduce the observed values within one order of magnitude. These results are independent of the particle physics model and vary very little with the statistics of the dark matter particle. The framework presented here applies to any kind of DM particles: when applied to typical CDM GeV wimps, our results are in agreement with CDM simulations. keV scale DM particles reproduce all observed galaxy magnitudes within one order of magnitude while GeV DM mass particles disagree with observations in up to eleven orders of magnitude.

Monthly Notices of the Royal Astronomical Society, 2012

Using the scalar field dark matter (SFDM) model, it is proposed that galaxies form by condensation of a scalar field (SF) very early in the Universe, forming Bose-Einstein condensate (BEC) drops (i.e. in this model, the haloes of galaxies are gigantic drops of SF). Here, as in the cold dark matter (LCDM) model, large structures form by hierarchy, and thus all the predictions of the LCDM model at large scales are reproduced by the SFDM model. This model predicts that all galaxies must be very similar and must exist for larger redshifts than in the LCDM model. In this paper, we show that BEC dark matter haloes fit the high-resolution rotation curves of a sample of 13 low-surface-brightness galaxies. We compare our fits to those obtained using Navarro-Frenk-White and pseudo-isothermal (PI) profiles. We have found better agreement with the SFDM and PI profiles. The mean value of the logarithmic inner density slopes is α = −0.27 ± 0.18. As a second result, we find a natural way to define the core radius with the advantage of being model-independent. Using this new definition in the BEC density profile, we find that the recent observation of the constant dark matter central surface density can be reproduced. We conclude that, in light of the difficulties that the standard model is currently facing, the SFDM model could be a worthy alternative to enable us to continue exploring further.

Symposium - International Astronomical Union, 2004

After looking at the difference in the mass distribution between massive spiral and dwarf irregular (dIrr) and low surface brightness (LSB) galaxies, the central Dark Matter (DM) concentration (flat vs cuspy) in dwarf and LSB galaxies, derived from observations, will be examined. We will then present what kind of observational constraints can be put on the total mass and total extent of DM halos from the studies of individual galaxies, small groups,…

Mon Notic Roy Astron Soc, 2004

We investigate in detail the mass distribution obtained by means of high resolution rotation curves of 25 galaxies of different morphological types. The dark matter contribution to the circular rotation velocity is well-described by resorting to a dark component whose density shows an inner core, i.e. a central constant density region. We find a very strong correlation between the core radius size $R_C$ and the stellar exponential scale length $R_D$: $R_C \simeq 13 (\frac {R_D} {5 {\rm kpc}})^{1.05} {\rm kpc} $, and between $R_C$ and the galaxy dynamical mass at this distance, $M_{dyn}(R_C)$. These relationships would not be expected if the core radii were the product of an incorrect decomposition procedure, or the biased result of wrong or misunderstood observational data. The very strong correlation between the dark and luminous scale lengths found here seems to hold also for different Hubble types and opens new scenarios for the nature of the dark matter in galaxies.

2003

We review the most recent evidence for the amazing properties of the density distribution of the dark matter around spiral galaxies. Their rotation curves, coadded according to the galaxy luminosity, conform to an Universal profile which can be represented as the sum of an exponential thin disk term plus a spherical halo term with a flat density core. From dwarfs to giants, these halos feature a constant density region of size r 0 and core density ρ 0 related by ρ 0 = 4.5 × 10-2(r 0/kpc)-2/3M⊙pc-3. At the highest masses ρ 0 decreases exponentially with r 0, revealing a lack of objects with disk masses > 1011M⊙ and central densities > 1.5 × 10-2 (r 0/kpc)-3M⊙pc-3 implying a maximum mass of ≈ 2 × 1012M⊙ for a dark halo hosting a stellar disk. The fine structure of dark matter halos is obtained from the kinematics of a number of suitable low-luminosity disk galaxies. The halo circular velocity increases linearly with radius out to the edge of the stellar disk, implying a constant dark halo density over the entire disk region. The properties of halos around normal spirals provide substantial evidence of a discrepancy between the mass distributions predicted in the Cold Dark Matter scenario and those actually detected around galaxies.

2010

Abstract We examine the density profiles of dark matter halos by analyzing data from the LasDamas (Large Suite of Dark Matter Simulations) project. LasDamas consists of a large suite of cosmological N-body simulations that follow the evolution of dark matter in the universe. The aim of LasDamas is to obtain adequate resolution in many large boxes, resulting in a huge volume appropriate for statistical studies of galaxies and halos.

The Astrophysical Journal, 2003

We measure the average gravitational shear profile of 6 massive clusters (M vir ∼ 10 15 M ⊙ ) at z = 0.3 out to a radius ∼ 2h −1 Mpc. The measurements are fitted to a generalized NFW-like halo model ρ(r) with an arbitrary r → 0 slope α. The data are well fitted by such a model with a central cusp with α ∼ 0.9 − 1.6 (68% confidence interval). For the standard-NFW case α = 1.0, we find a concentration parameter c vir that is consistent with recent predictions from high-resolution CDM N-body simulations. Our data are also well fitted by an isothermal sphere model with a softened core. For this model, our 1σ upper limit for the core radius corresponds to a limit σ ⋆ ≤ 0.1cm 2 g −1 on the elastic collision cross-section in a self-interacting dark matter model.

Monthly Notices of The Royal Astronomical Society, 2005

Using six high resolution dissipationless simulations with a varying box size in a flat LCDM universe, we study the mass and redshift dependence of dark matter halo shapes for M_vir = 9.0e11 - 2.0e14, over the redshift range z=0-3, and for two values of sigma_8=0.75 and 0.9. Remarkably, we find that the redshift, mass, and sigma_8 dependence of the mean smallest-to-largest axis ratio of halos is well described by the simple power-law relation <s> = (0.54 +- 0.02)(M_vir/M_*)^(-0.050 +- 0.003), where s is measured at 0.3 R_vir and the z and sigma_8 dependences are governed by the characteristic nonlinear mass, M_*=M_*(z,sigma_8). We find that the scatter about the mean s is well described by a Gaussian with sigma ~ 0.1, for all masses and redshifts. We compare our results to a variety of previous works on halo shapes and find that reported differences between studies are primarily explained by differences in their methodologies. We address the evolutionary aspects of individual halo shapes by following the shapes of the halos through ~100 snapshots in time. We determine the formation scalefactor a_c as defined by Wechsler et al. (2002) and find that it can be related to the halo shape at z = 0 and its evolution over time.

In spiral galaxies, we explain their non-Keplerian rotation curves (RCs) by means of a non-luminous component embedding their stellar-gaseous disks. Understanding the detailed properties of this component (labelled Dark Matter, DM) is one of the most pressing issues of Cosmology. We investigate the recent relationship (claimed by Walker et al. 2010, hereafter W+10) between $r$, the galaxy radial coordinate, and $V_h(r)$, the dark halo contribution to the circular velocity at $r$, {\it a}) in the framework of the Universal Rotation Curve (URC) paradigm and directly {\it b}) by means of the kinematics of a large sample of DM dominated spirals. We find a general agreement between the W+10 claim, the distribution of DM emerging from the URC and that inferred in the (low luminosity) objects of our sample. We show that such a phenomenology, linking the spiral's luminosity, radii and circular velocities, implies an evident inconsistency with (naive) predictions in the $\Lambda$ Cold Dark Matter ($\Lambda$CDM) scenario.

Monthly Notices of The Royal Astronomical Society, 2004

We investigate in detail the mass distribution obtained by means of high-resolution rotation curves of 25 galaxies of different morphological types. The dark matter contribution to the circular rotation velocity is well-described by resorting to a dark component, the density of which shows an inner core, i.e. a central constant density region. We find a very strong correlation between the core radius size RC and the stellar exponential scalelength RD: RC≃ 13 [RD/(5 kpc)]1.05 kpc, and between RC and the galaxy dynamical mass at this distance, Mdyn(RC). These relationships would not be expected if the core radii were the product of an incorrect decomposition procedure, or the biased result of wrong or misunderstood observational data. The very strong correlation between the dark and luminous scalelengths found here seems to hold also for different Hubble types and opens new scenarios for the nature of the dark matter in galaxies.

Universe

The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales of galaxies. Indeed, N-body simulations predict some galaxy properties that are at odds with the observations. These discrepancies are primarily related to the dark-matter distribution in the innermost regions of the halos of galaxies and to the dynamical properties of dwarf galaxies. They may have three different origins: (1) the baryonic physics affecting galaxy formation is still poorly understood and it is thus not properly included in the model; (2) the actual properties of dark matter differs from those of the conventional cold dark matter; (3) the theory of gravity departs from General Relativity. Solving these discrepancies is a rapidly evolving research field. We illustrate some...

Monthly Notices of the Royal Astronomical Society, 2001

One of the predictions of the standard cold dark matter model is that dark haloes have centrally divergent density profiles. An extensive body of rotation curve observations of dwarf and low surface brightness galaxies shows the dark haloes of those systems to be characterized by soft constant density central cores. Several physical processes have been proposed to produce soft cores in dark haloes, each one with different scaling properties. With the aim of discriminating among them we have examined the rotation curves of dark matter dominated dwarf and low surface brightness galaxies and the inner mass profiles of two clusters of galaxies lacking a central cD galaxy and with evidence of soft cores in the centre. The core radii and central densities of these haloes scale in a well defined manner with the depth of their potential wells, as measured through the maximum circular velocity. As a result of our analysis we identify self-interacting cold dark matter as a viable solution to the core problem, where a non-singular isothermal core is formed in the halo center surrounded by a Navarro, Frenk, & White profile in the outer parts. We show that this particular physical situation predicts core radii in agreement with observations. Furthermore, using the observed scalings, we derive an expression for the minimum cross section (σ) which has an explicit dependence with the halo dispersion velocity (v). If m x is the mass of the dark matter particle: σ/m x ≈ 4 10 −25 (100 kms −1 /v) cm 2 /Gev.

Monthly Notices of the Royal Astronomical Society, 2014

We investigate the black hole (BH) scaling relation in galaxies using a model in which the galaxy halo and central BH are a self-gravitating sphere of dark matter (DM) with an isotropic, adiabatic equation of state. The equipotential where the escape velocity approaches the speed of light defines the horizon of the BH. We find that the BH mass (m•) depends on the DM entropy, when the effective thermal degrees of freedom (F) are specified. Relations between BH and galaxy properties arise naturally, with the BH mass and DM velocity dispersion following m• ∝ σF/2 (for global mean density set by external cosmogony). Imposing observationally derived constraints on F provides insight into the microphysics of DM. Given that DM velocities and stellar velocities are comparable, the empirical correlation between m• and stellar velocity dispersions σ⋆ implies that 7 ≲ F < 10. A link between m• and globular cluster properties also arises because the halo potential binds the globular cluster swarm at large radii. Interestingly, for F > 6 the dense dark envelope surrounding the BH approaches the mean density of the BH itself, while the outer halo can show a nearly uniform kpc-scale core resembling those observed in galaxies.