Octavio Valenzuela

WHERE ARE THE MISSING GALACTIC SATELLITES

According to the hierarchical clustering scenario, galaxies are assembled by merging and accretion of numerous satellites of di†erent sizes and masses. This ongoing process is not 100% efficient in destroying all of the accreted satellites, as evidenced by the satellites of our Galaxy and of M31. Using published data, we have compiled the circular velocity distribution function (VDF) of galaxy satellites in the (V circ ) Local Group. We Ðnd that within the volumes of radius of 570 kpc (400 h~1 kpc assuming the Hubble constant1 h \ 0.7) centered on the Milky Way and Andromeda, the average VDF is roughly approx- imated as km s~1)~1.4B0.4 h3 Mpc~3 for in the range B10E70 km s~1. n( ( V circ ) B 55 ^ 11(V circ /10 V circ The observed VDF is compared with results of high-resolution cosmological simulations. We Ðnd that the VDF in models is very di†erent from the observed one : km s~1)~2.75 h3 n( ( V circ ) B 1200(V circ /10 Mpc~3. Cosmological models thus predict that a halo the size of our Galaxy should have about 50 dark matter satellites with circular velocity greater than 20 km s~1 and mass greater than 3 ) 108 within M _ a 570 kpc radius. This number is signiÐcantly higher than the approximately dozen satellites actually observed around our Galaxy. The di†erence is even larger if we consider the abundance of satellites in simulated galaxy groups similar to the Local Group. The models predict D300 satellites inside a 1.5 Mpc radius, while only D40 satellites are observed in the Local Group. The observed and predicted VDFs cross at B50 km s~1, indicating that the predicted abundance of satellites with km s~1 V circ Z 50 is in reasonably good agreement with observations. We conclude, therefore, that unless a large fraction of the Local Group satellites has been missed in observations, there is a dramatic discrepancy between observations and hierarchical models, regardless of the model parameters. We discuss several possible explanations for this discrepancy including identiÐcation of some satellites with the high-velocity clouds observed in the Local Group and the existence of dark satellites that failed to accrete gas and form stars either because of the expulsion of gas in the supernovae-driven winds or because of gas heating by the intergalactic ionizing background. Subject headings : cosmology : theory E galaxies : clusters : general E galaxies : interactions E Galaxy : formation E Local Group E methods : numerical

Forming disc galaxies in ΛCDM simulations

We used fully cosmological, high resolution N-body + SPH simulations to follow the formation of disk galaxies with rotational velocities between 135 and 270 km/sec in a ΛCDM universe. The simulations include gas cooling, star formation, the effects of a uniform UV background and a physically motivated description of feedback from supernovae. The host dark matter halos have a spin and last major merger redshift typical of galaxy sized halos as measured in recent large scale N–Body simulations. The simulated galaxies form rotationally supported disks with realistic exponential scale lengths and fall on both the I-band and baryonic Tully Fisher relations. An extended stellar disk forms inside the Milky Way sized halo immediately after the last major merger. The combination of UV background and SN feedback drastically reduces the number of visible satellites orbiting inside a Milky Way sized halo, bringing it in fair agreement with observations. Our simulations predict that the average age of a primary galaxy’s stellar population decreases with mass, because feedback delays star formation in less massive galaxies. Galaxies have stellar masses and current star formation rates as a function of total mass that are in good agreement with observational data. We discuss how both high mass and force resolution and a realistic description of star formation and feedback are important ingredients to match the observed properties of galaxies.

Substructure and Halo Density Profiles in a Warm Dark Matter Cosmology

We performed a series of high-resolution collisionless N-body simulations designed to study the substructure of Milky Way-size galactic halos (host halos) and the density profiles of halos in a warm dark matter (WDM) scenario with a nonvanishing cosmological constant. The virial masses of the host halos range from 3.5 × 1012 to 1.7 × 1012 h-1 M☉, and they have more than 105 particles each. A key feature of the WDM power spectrum is the free-streaming length Rf,WDM, which fixes an additional parameter for the model of structure formation. We analyze the substructure of host halos using three Rf,WDM values: 0.2, 0.1, and 0.05 Mpc, and compare results to the predictions of the cold dark matter (CDM) model. We find that guest halos (satellites) do form in the WDM scenario, but are more easily destroyed by dynamical friction and tidal disruption than their counterparts in a CDM model. The small number of guest halos that we find in the WDM models with respect to the CDM one is the result of a lower guest halo accretion and a higher satellite destruction rate. These two phenomena operate almost with the same intensity in delivering a reduced number of guest halos at z = 0. For the model with Rf,WDM = 0.1 Mpc, the number of accreted small halos is a factor of 2.5 below that of the CDM model, while the fraction of destroyed satellites is almost twice as large as that of the CDM model. The larger the Rf,WDM value, the greater the size of these two effects and the smaller the abundance of satellites. Under the assumption that each guest halo hosts a luminous galaxy, we find that the observed circular velocity function of satellites around the Milky Way and Andromeda is well described by the Rf,WDM = 0.1 Mpc WDM model. In the Rf,WDM = 0.1-0.2 Mpc models, the surviving guest halos at z = 0, whose masses are in the range Mh ≈ 109-1011 h-1 M☉, have an average concentration parameter c1/5 = r(Mh)/r(Mh/5), which is approximately twice as small as that of the corresponding CDM guest halos. This difference very likely produces the higher satellite destruction rate found in the WDM models. The density profile of host halos is well described by the Navarro, Frenk, & White (NFW) fit, whereas guest halos show a wide variety of density profiles. A tendency to form shallow cores is not evident; the profiles, however, are limited by a poor mass resolution in the innermost regions where shallow cores could be expected.

Formation and Structure of Halos in a Warm Dark Matter Cosmology

Using high-resolution cosmological N-body simulations, we study how the density profiles of dark matter halos are affected by the filtering of the density power spectrum below a given scale length and by the introduction of a thermal velocity dispersion. In the warm dark matter (WDM) scenario, both the free-streaming scale, Rf, and the velocity dispersion, v, are determined by the mass, mW, of the WDM particle. We found that v is too small to affect the density profiles of WDM halos. Down to the resolution attained in our simulations (~0.01 virial radii), there is not any significant difference in the density profiles and concentrations of halos obtained in simulations with and without the inclusion of v. Resolved soft cores appear only when we artificially increase the thermal velocity dispersion to a value that is much higher than v. We show that the size of soft cores in a monolithic collapse is related to the tangential velocity dispersion. The density profiles of the studied halos with masses down to ~0.01 the filtering mass Mf can be described by the Navarro-Frenk-White shape; soft cores are not formed. Nevertheless, the concentrations of these halos are lower than those of the CDM counterparts and are approximately independent of mass. The cosmogony of halos with masses Mf is not hierarchical: they form through monolithic collapse and by fragmentation of larger structures. The formation epoch of these halos is slightly later than that of halos with masses ≈Mf. The lower concentrations of WDM halos with respect to their CDM counterparts can be accounted for by their late formation epoch. Overall, our results point to a series of advantages of a WDM model over the CDM one. In addition to solving the substructure problem, a WDM model with Rf ~ 0.16 Mpc (mW ≈ 0.75 keV; flat cosmology with ΩΛ = h = 0.7) also predicts concentrations, a Tully-Fisher relation, and formation epochs for small halos, which seems to be in better agreement with observations than CDM predictions.

Secular bar formation in galaxies with a significant amount of dark matter

Using high resolution N-body simulations of stellar disks embedded in cosmologically motivated dark matter halos, we study the evolution of bars and the transfer of angular momentum between halos and bars. We find that dynamical friction results in some transfer of angular momentum to the halo, but the effect is much smaller than previously found in low resolution simulations and is incompatible with early analytical estimates. After 5 Gyrs of evolution the stellar component loses only 5% – 7% of its initial angular momentum. Mass and force resolutions are crucial for the modeling of bar dynamics. In low resolution (300 – 500 pc) simulations we find that the bar slows down and angular momentum is lost relatively fast. In simulations with millions of particles reaching a resolution of 20-40 pc, the pattern speed may not change over billions of years. Our high resolution models produce bars which are fast rotators, where the ratio of the corotation radius to the bar major semi-axis lies in the range R = 1.2 1.7, marginally compatible with observational results. In contrast to many previous simulations, we find that bars are relatively short. As in many observed cases, the bar major semi-axis is close to the exponential length of the disk. The transfer of angular momentum between inner and outer parts of the disk plays a very important role in the secular evolution of the disk and the bar. The bar formation increases the exponential length of the disk by a factor of 1.2 -1.5. The transfer substantially increases the stellar mass in the center of the galaxy and decreases the dark matter-to-baryons ratio. As the result, the central 2 kpc region is always strongly dominated by the baryonic component. At intermediate (3 – 10 kpc) scales the disk is sub-dominant. These models demonstrate that the efficiency of angular momentum transfer to the dark matter has been greatly overestimated. More realistic models produce bar structure in striking agreement with observational results.

Is there evidence for flat cores in the halos of dwarf galaxies? The case of NGC 3109 and NGC 6822

Two well-studied dwarf galaxies, NGC 3109 and NGC 6822, present some of the strongest observational support for a flat core at the center of galactic dark matter (DM) halos. We use detailed, cosmologically motivated numerical models to investigate the systematic effects and the accuracy of recovering parameters of the galaxies. Some of our models match the observed structure of the two galaxies remarkably well. Our analysis shows that the rotation curves of these two galaxies are instead quite compatible with their DM halos having steep cuspy density profiles. The rotation curves in our models are measured using standard observational techniques, projecting velocities along the line of sight of an imaginary observer and performing a tilted-ring analysis. The models reproduce the rotation curves of both galaxies and the disk surface brightness profiles, as well as the profile of isophotal ellipticity and position angle. The models are centrally dominated by baryons; however, the DM component is globally dominant. The simulated disk mass is marginally consistent with a stellar mass-to-light ratio, in agreement with the observed colors and the detected gaseous mass. We show that noncircular motions, combined with gas pressure support and projection effects, result in a large underestimation of the circular velocity in the central ~1 kpc region, creating the illusion of a constant-density core. Although the systematic effects mentioned above are stronger in barred systems, they are also present in axisymmetric disks. Our results strongly suggest that there is no contradiction between the observed rotation curves in dwarf galaxies and the cuspy central DM density profiles predicted by cold dark matter models.

The Rotation Curves of Dwarf Galaxies: A Problem for Cold Dark Matter?

We address the issue of accuracy in recovering density profiles from observations of rotation curves of galaxies. We observe and analyze our models in much the same way as observers do the real galaxies. Our models include stellar disks, disks with bars, and small bulges. We find that the tilted-ring model analysis produces an underestimate of the central rotational velocity. In some cases the galaxy halo density profile seems to have a flat core, while in reality it does not. We identify three effects that explain the systematic biases: inclination, small bulge, and bar. Inclination effects are due to the finite thickness of the disk, bar, or bulge. Admixture of a nonrotating bulge component reduces the rotational velocity. A small (200-500 pc) bulge may be overlooked, leading to systematic bias even on relatively large (~1 kpc) distances. In the case of a disk with a bar, the underestimate of the circular velocity is larger because of a combination of noncircular motions and random velocities. The effect of the bar depends on the angle that the bar makes with the line of sight. Signatures of bars can be difficult to detect in the surface brightness profiles of the model galaxies. The variations of inclination angle and isophote position angle with radius are more reliable indicators of bar presence than the surface brightness profiles. The systematic biases in the central ~1 kpc of galaxies are not large. Each effect separately gives typically a few km s-1 error, but the effects add up. In some cases the error in circular velocity was a factor of 2, but typically we get about a 20% effect. The result is the false inference that the density profile of the halo flattens in the central parts. Our observations of real galaxies show that for a large fraction of galaxies the velocity of gas rotation (as measured by emission lines) is very close to the rotation of the stellar component (as measured by absorption lines). This implies that the systematic effects discussed in this paper are also applicable both for the stars and emission-line gas.

Dwarf Dark Matter Halos

We study properties of dark matter halos at high redshifts z ¼ 2-10 for a vast range of masses with the emphasis on dwarf halos with masses of 10 7 -10 9 h � 1 M� . We find that the density profiles of relaxed dwarf halos are well fitted by the Navarro, Frenk, & White (NFW) profile and do not hav ec ores. We compute the halo mass function and the halo spin parameter distribution and find that the former is very well reproduced by the Sheth & Tormen model, while the latter is well fitted by a lognormal distribution with k0 ¼ 0:042 andk ¼ 0: 63. We estimate the distribution of concentrations for halos in a mass range that covers 6 orders of magnitude, from 10 7 to 10 13 h � 1 M� , and find that the data are well reproduced by the model of Bullock et al. The extrapolation of our results to z ¼ 0 predicts that present-day isolated dwarf halos should have a very large median concentration of � 35. We measure the subhalo circular velocity functions for halos with masses that range from 4:6 ;10 9 to 10 13 h � 1 Mand find that they are similar when normalized to the circular velocity of the parent halo. Dwarf halos studied in this paper are many orders of magnitude smaller than well-studied cluster- and Milky Way-sized halos. Yet, in all respects the dwarfs are just downscaled versions of the large halos. They are cuspy and, as expected, more concentrated. They have the same spin parameter distribution and follow the same mass function that was measured for large halos. Subject headingg cosmology: theory — dark matter — galaxies: formation — galaxies: halos — methods: numerical

Structure and Subhalo Population of Halos in a Self-interacting Dark Matter Cosmology

A series of high-resolution numerical simulations were performed to study the structure and substructure of Milky Way-sized (MW-sized) and cluster-sized halos in a ΛCDM cosmology with self-interacting (SI) dark matter particles. The cross section per unit of particle mass has the form σDM = σ0(1/v100)α, where σ0 is a constant in units of cm2 g-1 and v100 is the relative velocity in units of 100 km s-1. Different values for σ0 with α = 0 or 1 were used. For small values of σDM = const (0.5, α = 0), the core density of the halos at z = 0 is typically higher at a given mass for lower values of σ0 or, at a given σ0, for lower masses. For values of σ0 as high as 3.0, both cluster- and MW-sized halos may undergo the gravothermal catastrophe before z = 0. The core expansion occurs in a stable regime because the heat capacity C is positive in the center. After the maximum expansion, the isothermal core is hotter than the periphery and C < 0. Then the gravothermal catastrophe is triggered. The instability onset can be delayed by both the dynamical heating of the halo by major mergers and the interaction of cool particles with the hot environment of a host halo. When α = 1, the core density of cluster- and MW-sized halos is similar. Using σDM = 0.5-1.0(1/v100), our predictions agree with the central densities and the core scaling laws of halos inferred from the observations of both dwarf and low surface brightness galaxies and clusters of galaxies. Regarding the cumulative vmax function of subhalos within MW-sized halos, when (σ0, α) = (0.1, 0.0), (0.5, 0.0), or (0.5, 1.0) it agrees roughly with observations (luminous satellites) for vmax 30 km s-1, while at vmax = 20 km s-1 the functions are already a factor of 5-8 higher, similar to the CDM predictions. For (σ0, α) = (1.0, 1.0), this function lies above the corresponding CDM function. The structure and number of subhalos are affected by the scattering properties of the host halo rather than by those of the subhalos. The halos with SI have more specific angular momentum at a given mass shell and are rounder than their CDM counterparts. However, the angular momentum excess with regard to CDM is small. We conclude that the introduction of SI particles with σDM 1/v100 may remedy the cuspy core problem of the CDM cosmogony, at the same time keeping a subhalo population similar to that of the CDM halos.

SDSS-IV MaNGA : a distinct mass distribution explored in slow-rotating early-type galaxies.

We study the radial acceleration relation (RAR) for early-type galaxies (ETGs) in the SDSS MaNGA MPL5 data set. The complete ETG sample show a slightly offset RAR from the relation reported by McGaugh et al. (2016) at the low-acceleration end; we find that the deviation is due to the fact that the slow rotators show a systematically higher acceleration relation than the McGaughs RAR, while the fast rotators show a consistent acceleration relation to McGaughs RAR. There is a 1σ significant difference between the acceleration relations of the fast and slow rotators, suggesting that the acceleration relation correlates with the galactic spins, and that the slow rotators may have a different mass distribution compared with fast rotators and late-type galaxies. We suspect that the acceleration relation deviation of slow rotators may be attributed to more galaxy merger events, which would disrupt the original spins and correlated distributions of baryons and dark matter orbits in galaxies.