Cores Of Dark Halos Research Articles

Building stellar bulges and halo cores from massive clumps observed in the DYNAMO-HST sample

ABSTRACT We present N-body simulations of the process of bulge formation in disc galaxies due to inward migration of massive stellar clumps. The process is accompanied by dark halo heating, with a quasi-isothermal core replacing the initial central density cusp, transforming an initially dark matter dominated central region into a baryon dominated one. The characteristics of the clumps are chosen to be compatible with low redshift observations of stellar clumps in DYNAMO-HST galaxies, which may be relatively long lived in terms of being robust against internal starburst-instigated disruption. We thus test for disruption due to tidal stripping using different clump internal radial profiles; Plummer, Hernquist, and Jaffe, in ascending order of see per central density profile. Our calculations predict that in order for clump migration to be effective in building galactic bulges and dark halo cores, steeply increasing central clump profiles, or a less massive or less concentrated haloes, are preferred. The dependence on such factors may contribute to the diversity in observed total mass distributions and resulting rotation curves in galaxies. When the process is most efficient, a ‘bulge-halo conspiracy’, with a singular isothermal total density akin to that observed bright galaxies, results.

THE STRUCTURE AND DARK HALO CORE PROPERTIES OF DWARF SPHEROIDAL GALAXIES

The structure and dark matter halo core properties of dwarf spheroidal galaxies (dSphs) are investigated. A double-isothermal (DIS) model of an isothermal, non selfgravitating stellar system, in gravitational equilibrium and embedded in an isothermal dark halo core provides an excellent fit to the various observed stellar surface density distributions �∗(r). Despite its constant velocity dispersion, the stellar system can be well characterised by King profiles (King 1966) with a broad distribution of concentration parameters c = log(r∗,t/r∗,c), with r∗,t and r∗,c the stellar tidal and core radius, respectively. The DIS model confirms the suggestion of Kormendy & Freeman (2014) that the core scale length of the stellar system, defined as a∗ = −(dln�∗/dr 2 ) −1/2 , is sensitive to the central dark matter density �0. In contrast to single-component systems, r∗,t however does not trace the tidal radius of the galaxy but the core radius rc of its dark matter halo. c is therefore sensitive to the ratio �∗/�0 with �∗ and �0 the stellar and dark matter velocity dispersion, respectively. Simple empirical relationships are derived that allow to calculate the dark halo core parameters �0, rc and �0, given the observable quantities �∗, a∗ and c. The DIS model is applied to the Milky Way’s dSphs. Their halo velocity dispersions lie in a narrow range of 10 km/s ≤ �0 ≤ 18 km/s with halo core radii of 280 pc ≤ rc ≤ 1.3 kpc and rc ≈ 2a∗. All dSphs follow closely the same universal scaling relations h�0rci ≡ �0 × rc = 75 +8545 M⊙ pc −2 and � 2 0 ×r −1 c = 0.45 +0.51 −0.27 (km/s) 2 pc −1 that characterise the cores of more massive galaxies over a range of 18 magnitudes in blue magnitude MB. For given h�0rcithe core mass is a strong function of core radius, Mc ∼ r 2 . Inside a fixed radius ru, with ru the logarithmic mean of the dSph’s core radii, the total mass Mu = 2.17h�0rcir 2 u is however roughly constant. Outliers with smaller masses are expected for dSphs with core radii that are much larger or smaller than ru. For the Milky Way’s dSphs we find ru = 400 ± 100 pc and Mu = 2.6 ± 1.4 × 10 7 M⊙, in agreement with Strigari et al. (2008). Due to their small rc, the core densities of the Galaxy’s dSphs are very higher, with �0 = 0.03 - 0.3 M⊙ pc −3 . The dSphs would have to be on galactic orbits with pericenters smaller than a few kpc in order for their stellar systems to be affected by Galactic tides which is very unlikely. dSphs should therefore be tidally undisturbed. Observational evidence for tidal effects might then provide a serious challenge for the cold dark matter scenario.

The structure of spiral galaxies: radial profiles in stellar mass-to-light ratio and the dark matter distribution

<i>Context. <i/>The colour and metallicity gradients observed in spiral galaxies suggest that the mass-to-light ratio (/<i>L<i/>) of the stellar disc is a function of radius. This is indeed predicted by chemo-photometric models of galactic discs.<i>Aims. <i/>We investigate the distribution of luminous and dark matter in spiral galaxies, taking into account the radial dependence of the stellar /<i>L<i/>, which is usually assumed to be constant in studies of the mass structure.<i>Methods. <i/>From earlier chemo-photometric models and in agreement with the observed radial profiles of galaxy colours, we derive the typical average /<i>L<i/> profile of the stellar discs of spiral galaxies. We computed the corresponding variable mass-to-light (VML) stellar surface density profile and then the VML disc contribution to the circular velocity. We used the latter, combined with a well-studied dark matter velocity profile, to mass model co-added rotation curves.<i>Results. <i/>By investigating rotation curves in the framework of VML stellar discs, we confirm the scenario obtained with the constant /<i>L<i/> assumption: a dark matter halo with a shallow core, an inner baryon-dominated region, and a larger proportion of dark matter in smaller objects. However, the resulting size of the the dark halo core and of the inner baryon dominance region are somewhat smaller. The stronger role that VML discs have in the innermost regions is important for constraining the galaxy mass structure in both <i>Λ<i/> Cold Dark Matter and MOND scenarios.

THE BLACK HOLE MASS, STELLAR MASS-TO-LIGHT RATIO, AND DARK HALO IN M87

We model the dynamical structure of M87 (NGC4486) using high spatial resolution long-slit observations of stellar light in the central regions, two-dimensional stellar light kinematics out to half of the effective radius, and globular cluster velocities out to eight effective radii. We simultaneously fit for four parameters: black hole mass, dark halo core radius, dark halo circular velocity, and stellar mass-to-light (M/L) ratio. We find a black hole mass of 6.4(±0.5) × 109 M☉ (the uncertainty is 68% confidence marginalized over the other parameters). The stellar M/LV = 6.3 ± 0.8. The best-fit dark halo core radius is 14 ± 2 kpc, assuming a cored logarithmic potential. The best-fit dark halo circular velocity is 715 ± 15 km s−1. Our black hole mass is over a factor of 2 larger than previous stellar dynamical measures, and our derived stellar M/L ratio is two times lower than previous dynamical measures. When we do not include a dark halo, we measure a black hole mass and stellar M/L ratio that is consistent with previous measures, implying that the major difference is in the model assumptions. The stellar M/L ratio from our models is very similar to that derived from stellar population models of M87. The reason for the difference in the black hole mass is because we allow the M/L ratio to change with radius. The dark halo is degenerate with the stellar M/L ratio, which is subsequently degenerate with the black hole mass. We argue that dynamical models of galaxies that do not include the contribution from a dark halo may produce a biased result for the black hole mass. This bias is especially large for a galaxy with a shallow light profile such as M87, and may not be as severe in galaxies with steeper light profiles unless they have a large stellar population change with radius.

Bounds on the Mass and Abundance of Dark Compact Objects and Black Holes in Dwarf Spheroidal Galaxy Halos

We establish new dynamical constraints on the mass and abundance of compact objects in the halo of dwarf spheroidal (dSph) galaxies. In order to preserve kinematically cold the second peak of the Ursa Minor (UMi) dSph against gravitational scattering, we place upper limits on the density of compact objects as a function of their assumed mass. The mass of the dark matter constituents cannot be larger than 103 M? at a halo density in UMi's core of 0.35 M? pc-3. This constraint rules out a scenario in which dark halo cores are formed by two-body relaxation processes. Our bounds on the fraction of dark matter in compact objects with masses 3 ? 103 M? improve those based on dynamical arguments in the Galactic halo. In particular, objects with masses ~105 M? can comprise no more than a halo mass fraction ~0.01. Better determinations of the velocity dispersion of old overdense regions in dSph galaxies may result in more stringent constraints on the mass of halo objects. For illustration, if the preliminary value of 0.5 km s-1 for the secondary peak of UMi is confirmed, compact objects with masses above ~100 M? could be excluded from comprising all its dark matter halo.

The Effect of Central Baryonic Cores in Dark Halos on the Evaluation of Strong Lensing Probabilities

We present an estimate of the strong lensing probability by dark halos, with emphasis on the role of the baryonic matter arising purely from radiative cooling. We treat the contribution of the cooled baryons optimistically with all the cooled baryons confined within a central core, and including no feedback process from stellar evolution. Our two-component model provides a strong lensing probability that is in good agreement with the observed distribution of multiple images of quasars, provided that the cooled baryons are deposited within a spherical region of radius of 0.1 times the virial radius and follow an isothermal profile. It is pointed out that strong lensing may be used as an additional probe of baryon physics in dark halos though this may meanwhile complicate the test of the inner density profiles of dark matter in halos using the observed strong lensing probability.

Galaxy Formation in Triaxial Halos: Black Hole–Bulge–Dark Halo Correlation

The masses of supermassive black holes (SBHs) show correlations with bulge properties in disk and elliptical galaxies. We study the formation of galactic structure within flat-core, mildly triaxial halos and show that these correlations can be understood within the framework of a baryonic component modifying the orbital structure in the underlying potential. In particular, we find that terminal properties of bulges and their central SBHs are constrained by the destruction of box orbits in the harmonic cores of dark halos and the emergence of progressively less eccentric loop orbits there. SBH masses, M•, should exhibit a tighter correlation with bulge velocity dispersions, σB, than with bulge masses, MB, in accord with observations, if there is a significant scatter in the MH-σH relation for the halo. In the context of this model the observed M•-σB relation implies that halos should exhibit a Faber-Jackson type relationship between their masses and velocity dispersions. The most important prediction of our model is that halo properties determine the bulge and SBH parameters. The model also has important implications for galactic morphology and the process of disk formation.

REEXAMINING THE UNIVERSALITY OF THE BURKERT PROFILE ON CLUSTER SCALES: DARK MATTER CORES

The Burkert profile is a competing candidate for the analytic approximation of virialized dark halos especially when dark matter particles have a finite cross-section for elastic collisions. In this paper we reexamine its universality in massive systems, using an ensemble of 45 nearby X-ray clusters and 20 distant X-ray/lensing clusters. Despite the fact that this empirical profile turns out a great success on galactic scales and also reproduces approximately the X-ray observed surface brightness profiles of clusters, the dark matter cores of clusters predicted by the Burkert profile are too large to be reconciled with the strong gravitational lensing measurements. Specifically, the typical dark halo cores of clusters represented by the Burkert profile are about 0.2 Mpc, and only a small fraction (~1/4) of clusters can have compact cores smaller than 0.1 Mpc. This will constitute a critical challenge to the Burkert profile as a universal dark matter density law over entire mass ranges.

Dark matter halo cores in hierarchical clustering theories

In hierarchical clustering theories, smaller masses generally collapse earlier than larger masses and so are denser on the average. The core of a small mass halo could be dense enough to resist disruption and survive undigested, when it gets incorporated into a bigger object, and determine the halo structure in the inner regions. We examine in this talk the possible consequences of this idea in determining the structure of dark halo cores, by considering, both simple scaling arguments, and a novel fluid approach to self-similar collapse solutions for the dark matter phase space density.

Cluster infall with friction

A completely dissipationless N-body numerical simulation of the formation of a small galactic cluster is reported. Galaxy tracers are identified as the high-density regions of the galaxy-scale halos present prior to cluster formation. The largest cluster in the simulation is measured and found to have b(vr) of about 0.8 in the radial velocity dispersion with a galaxy population in better accord with cluster observations than the dark matter distribution. Comparison of the energy history of infalling material shows that the dense cores of dark halos, where galaxies are located, lose energy approximately as expected for dynamical friction terminated by tidal disruption. 40 refs.

Cosmological velocity bias

view Abstract Citations (87) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cosmological Velocity Bias Carlberg, R. G. ; Couchman, H. M. P. ; Thomas, P. A. Abstract Gravitationally bound objects, such as galaxies, that collapse from a cosmological background can exhibit both a correlation bias and, as shown here, a significant bias in their peculiar velocity dispersion with respect to the density field. The correlation bias, bxi_, begins as a statistical enhancement that dwindles as relatively less overdense regions collapse. Ultimately, merging can diminish the correlation to less than that in the density field. Pairwise center-of-mass peculiar velocities of collapsed regions are reduced below that of the density field to a small extent by the removal of their internal velocity dispersion and by the tendency of galaxies to form preferentially near the bottom of group and cluster potential wells. More importantly, dynamical friction slows the objects and their associated halos as dynamical ciustering develops, leading to a signiflcant velocity dispersion bias, b_v_, with respect to the density field. The detailed simulations of a cold dark matter cosmology reported bere indicate that together these two kinds of bias can reconcile a galaxy correlation length of 5h^-1^ Mpc and a peculiar velocity dispersion of approximately 300 km s^-1^ with an {OMEGA} = 1 cosmology, for a range of correlation biases. Values of bxi_ ~ 1 and b_v_ ~ 0.5 are favored using additional normalizing data. For the adopted star formation algorithm, the galaxies in the simulation are found to have properties similar to those of the dense cores of dark halos. Publication: The Astrophysical Journal Pub Date: April 1990 DOI: 10.1086/185686 Bibcode: 1990ApJ...352L..29C Keywords: Cosmology; Gravitational Collapse; Radial Velocity; Cooling Flows (Astrophysics); Star Formation; Velocity Distribution; Astrophysics; COSMOLOGY; GALAXIES: CLUSTERING; GRAVITATION full text sources ADS |