The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as ...the cosmological constant problem, the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. ΛCDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the ΛDCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the ΛCDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe.
ABSTRACT
Recently, the discovery of the radial acceleration relation (RAR) in galaxies has been regarded as an indirect support of alternative theories of gravity such as modified Newtonian dynamics ...(MOND) and modified gravity. This relation indicates a tight correlation between dynamical mass and baryonic mass in galaxies with different sizes and morphology. However, if the RAR relation is scale independent and could be explained by alternative theories of gravity, this relation should be universal and true for galaxy clusters as well. In this article, by using the X-ray data of a sample of galaxy clusters, we investigate if there exists any tight correlation between dynamical mass and baryonic mass in galaxy clusters, assuming hot gas mass distribution almost representing baryonic distribution and that the galaxy clusters are virialized. We show that the resulting RAR of 52 non-cool-core galaxy clusters scatters in a large parameter space, possibly due to our simplifying assumptions and unclear matter content in galaxy clusters. This might indicate that the RAR is unlikely to be universal and scale independent.
The present paper extends a previous study of Del Popolo to clusters of galaxies, considering how baryon-dark matter (DM) interplay shapes the density profiles of dwarf galaxies. Cluster density ...profiles are determined taking into account dynamical friction, random and ordered angular momentum and the response of DM haloes to condensation of baryons. We find that haloes containing only DM are characterized by Einasto's profiles, and that profiles flatten with increasing content of baryons and increasing values of random angular momentum. The analytical results obtained in the first part of the paper are applied to well-studied clusters, the inner profiles of which have slopes flatter than Navarro, Frenk & White (NFW) predictions (A611, A383) or are characterized by profiles in agreement with the NFW model (MACS J1423.8+2404, RXJ1133). By using an independently measured baryonic fraction and a typical spin parameter value λ≃ 0.03 and adjusting the random angular momentum, we re-obtain the mass and density profiles of the quoted clusters. Finally, we show that the baryonic mass inside ≃10 kpc, M
b, in, is correlated with the total mass of the clusters as
.
In this paper, we study how the dark matter density profiles of dwarf galaxies in the mass range 108-1010 M⊙ are modified by the interaction of the dwarf galaxy with neighbouring structures, and by ...the changing baryon fraction in dwarf galaxies. With this aim, and referring to an earlier paper by Del Popolo, we determine the density profiles of the dwarf galaxies, taking into account the effect of tidal interaction with neighbouring structures, the effects of ordered and random angular momentum, dynamical friction, the response of dark matter haloes to the condensation of baryons and the effects produced by the presence of baryons. As already shown in the earlier paper, the slope of the density profile of inner haloes flattens with decreasing halo mass, and the profile is well approximated by a Burkert profile. We thus treat the angular momentum generated by tidal torques and the baryon fraction as a parameter in order to understand how the latter influences the density profiles. The analysis shows that dwarf galaxies that have suffered a smaller tidal torque (and consequently have smaller angular momentum) are characterized by steeper profiles with respect to dwarf galaxies subject to higher torque. Similarly, dwarf galaxies that have a smaller baryon fraction also have steeper profiles than those that have a larger baryon fraction. When tidal torquing is shut down and baryons are not present, the density profile is very well approximated by an Einasto profile, similarly to dwarf galaxies obtained in dissipationless N-body simulations. Then, we apply the result of the previous analysis to the dark matter halo rotation curves of three different dwarfs: NGC 2976, which is known to have a flat inner core; NGC 5949, which has a profile intermediate between a cored and a cuspy one; and NGC 5963, which has a cuspy profile. After calculating the baryon fraction, which is ≃0.1 for the three galaxies, we fitted the rotation curves, changing the value of the angular momentum. NGC 2976 has a higher value of ordered angular momentum (λ≃ 0.04) than NGC 5949 (λ≃ 0.025). For NGC 5963, a very steep profile can be obtained with a low value of λ (λ≃ 0.02) and also by decreasing the value of the random angular momentum. For NGC 2976, the tidal interaction with M81 could also have influenced the inner part of the density profile. Finally, we show how the inner density profile correlates with the tidal index for dwarf and low surface brightness galaxies given by Karachentsev et al.
We study the cusp/core problem using a secondary infall model that takes into account the effect of ordered and random angular momentum, dynamical friction, and baryons adiabatic contraction (AC). ...The model is applied to structures on galactic scales (normal and dwarfs spiral galaxies) and on clusters of galaxies scales. Our analysis suggest that angular momentum and dynamical friction are able, on galactic scales, to overcome the competing effect of AC eliminating the cusp. The slope of density profile of inner halos flattens with decreasing halo mass and the profile is well approximated by a Burkert's profile. In order to obtain the Navarro-Frenk-White (NFW) profile, starting from the profiles obtained from our model, the magnitude of angular momentum and dynamical friction must be reduced with respect to the values predicted by the model itself. The rotation curves of four lower sideband galaxies from Gentile et al. are compared to the rotation curves obtained by the model in the present paper obtaining a good fit to the observational data. The time evolution of the density profile of a galaxy of 108-109 M shows that after a transient steepening, due to the AC, the density profile flattens to alpha 0. On cluster scales we observe a similar evolution of the dark matter (DM) density profile but in this case the density profile slope flattens to alpha 0.6 for a cluster of 1014 M . The total mass profile, differently from that of DM, shows a central cusp well fitted by an NFW model.
ABSTRACT
In the past few decades, some studies pointed out that magnetic field might affect the rotation curves in galaxies. However, the impact is relatively small compared with the effects of dark ...matter and the baryonic components. In this letter, we revisit the impact of magnetic field on the rotation curve of our Galaxy. We show that the inner Galactic rotation curve could be affected significantly by the magnetic field. The addition of the inner bulge component, which has been proposed previously to account for the inner rotation curve data, is not necessary. The magnetic field contribution can fully account for the excess of the inner rotation velocity between 5 to 50 pc from the Galactic Centre. Our analysis can also constrain the azimuthal component of the central regular magnetic field strength to $B_0 \sim 50-60\, \mu$G, which is consistent with the observed range.
In this article, we test the conclusion of the universality of the dark matter (DM) halo surface density μ0D = ρ0
r
0. According to our study, the dispersion of values of μ0D is twice as high as that ...found previously by other authors. We conclude that the DM surface density and its Newtonian acceleration are not constant but correlate with the luminosity, morphological type, (B − V)0 colour index and content of neutral hydrogen. These DM parameters are higher for more luminous systems of early types with red colour and low gas content. We also found that the correlation of DM parameters with colour index appears to be the manifestation of a stronger relation between DM halo mass and the colour of a galaxy. This finding is in agreement with cosmological simulations. These results leave little room for the recently claimed universality of DM column density. We also found that isolated galaxies in our sample (contained in the Analysis of the interstellar Medium of Isolated GAlaxies (AMIGA) catalogue) do not differ significantly in their value of μ0D from the entire sample. Thus, since the AMIGA catalogue gives a sample of galaxies that have not interacted with a significant mass neighbour in the past 3 Gyr, the difference between systems with low and high values of μ0D is not related to merging events during this period of time.
We study the effect of modifying the equation of state parameter (
) of dark energy on the results of the previous study 1 (hereafter Paper II), in which dark energy was assumed to be a cosmological ...constant. As a first step, solving the equation of motion, we found a relation between mass,
, and the turn-around radius,
. Then, we obtained a relationship between the velocity,
, and radius,
. The relation was fitted to data of the groups used in Paper II to obtain the Hubble parameter, and the mass,
of the groups. We observe a significant reduction of the mass,
, comparing Paper II results (for which
) and the case
, while the Hubble constant increases going from
to
.
On the universality of density profiles Del Popolo, A.
Monthly Notices of the Royal Astronomical Society,
November 2010, Letnik:
408, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The secondary infall model (described in a previous paper by this author), which takes into account the effect of dynamical friction, ordered and random angular momentum, baryon adiabatic contraction ...and dark matter (DM) baryon interplay, is used to study how the inner slopes of relaxed ΛCDM haloes with and without baryons (baryons+DM and pure DM) depend on redshift and on halo mass. This method is applied to structures on galactic scales and clusters of galaxies scales. It is found that the inner logarithmic density slope, α≡ d log ρ/d log r, of DM haloes with baryons has a significant dependence on halo mass and redshift, with slopes ranging from α≃ 0 for dwarf galaxies to α≃ 0.4 for objects of M≃ 1013 M⊙ and α≃ 0.94 for M≃ 1015 M⊙ clusters of galaxies. Structure slopes increase with increasing redshift, and this trend reduces going from galaxies to clusters. In the case of density profiles constituted just of DM, the mass and redshift dependence of the slope are very slight. In this last case, the analysis of Merritt et al. is used. They compared N-body density profiles with various parametric models, finding systematic variation in the profile shape with halo mass. This last analysis suggests that the galaxy-sized haloes obtained with this model have a different shape parameter (i.e. a different mass distribution) than the cluster-sized haloes obtained with the same model. The results of the present paper argue against universality of density profiles constituted by DM and baryons and confirm the claims of a systematic variation in profile shape with halo mass, for DM haloes.
In the present paper, we compare the predictions of two well known mechanisms considered able to solve the cusp/core problem (a. supernova feedback; b. baryonic clumps-DM interaction) by comparing ...their theoretical predictions to recent observations of the inner slopes of galaxies with masses ranging from dSphs to normal spirals. We compare the
α
-
V
rot
and the
α
-
M
∗
relationships, predicted by the two models with high resolution data coming from Adams et al. (Astrophys. J. 789, 63,
2014
), Simon et al. (Astrophys. J. 621, 757,
2005
), LITTLE THINGS (Oh et al. in Astron. J. 149, 180,
2015
), THINGS dwarves (Oh et al. in Astron. J. 141, 193,
2011a
; Oh et al. in Astron. J. 142, 224,
2011b
), THINGS spirals (Oh et al. in Astron. J. 149, 180,
2015
), Sculptor, Fornax and the Milky Way. The comparison of the theoretical predictions with the complete set of data shows that the two models perform similarly, while when we restrict the analysis to a smaller subsample of higher quality, we show that the method presented in this paper (baryonic clumps-DM interaction) performs better than the one based on supernova feedback. We also show that, contrarily to the first model prediction, dSphs of small mass could have cored profiles. This means that observations of cored inner profiles in dSphs having a stellar mass
<
10
6
M
⊙
not necessarily imply problems for the
Λ
CDM model.
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