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O. Valenzuela

Publications and source records attributed to O. Valenzuela.

At least 37 records · Page 2Linked to original sources
arXiv2014

Searching for tidal tails around $ω$ Centauri using RR Lyrae Stars

We present a survey for RR Lyrae stars in an area of 50 deg$^2$ around the globular cluster $ω$ Centauri, aimed to detect debris material from the alleged progenitor galaxy of the cluster. We detected 48 RR Lyrae stars of which only 11 have been previously reported. Ten among the eleven previously known stars were found inside the tidal radius of the cluster. The rest were located outside the tidal radius up to distances of $\sim 6$ degrees from the center of the cluster. Several of those stars are located at distances similar to that of $ω$ Centauri. We investigated the probability that those stars may have been stripped off the cluster by studying their properties (mean periods), calculating the expected halo/thick disk population of RR Lyrae stars in this part of the sky, analyzing the radial velocity of a sub-sample of the RR Lyrae stars, and finally, studying the probable orbits of this sub-sample around the Galaxy. None of these investigations support the scenario that there is significant tidal debris around $ω$ Centauri, confirming previous studies in the region. It is puzzling that tidal debris have been found elsewhere but not near the cluster itself.

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arXiv2014

Simulations of isolated dwarf galaxies formed in dark matter halos with different mass assembly histories

We present high-resolution N-body/hydrodynamics simulations of dwarf galaxies formed in isolated CDM halos with the same virial mass, Mv~2.5x10^10 Msun at z=0, in order to (1) study the mass assembly histories (MAHs) of the halo, stars, and gas components, and (2) explore the effects of the halo MAHs on the stellar/baryonic assembly of the simulated dwarfs and on their z~0 properties. Overall, the simulated dwarfs are roughly consistent with observations. Our main results are: a) The stellar-to-halo mass ratio is ~0.01 and remains roughly constant since z~1 (the stellar MAHs follow closely the halo MAHs), with a smaller value at higher z's for those halos that assemble their mass later. b) The evolution of the galaxy gas fraction, fg, is episodic and higher, most of the time, than the stellar fraction. When fg decreases (increases), the gas fraction in the halo typically increases (decreases), showing that the SN driven outflows play an important role in regulating the gas fractions -and hence the SFR- of the dwarfs. However, in most cases, an important fraction of the gas escapes the virial radius, Rv; at z=4 the total baryon fraction inside Rv is 1.5-2 times smaller than the universal one, while at z=0 is 2-6 times smaller, with the earlier assembled halos ejecting more gas. c) The SF histories are episodic with changes in the SFRs of factors 2-10 on average. d) Although the dwarfs formed in late assembled halos show more extended SF histories, their z~0 SFRs are still below the ones measured for local isolated dwarfs. e) The effects of baryons on Mv are such that at almost any time Mv is 10-20% smaller than the corresponding Mv obtained in pure N-body simulations. Our results suggest that rather than increasing the strength of the SN-driven outflows, processes that reduce the SF efficiency even more will help to solve the potential issues faced by the CDM-based simulations of dwarfs.

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arXiv2013

Non-Axisymmetric Structure in the Satellite Dwarf Galaxy NGC2976: Implications for its Dark/Bright Mass Distribution and Evolution

We present the result of an extensive search for non-axisymmetric structures in the dwarf satellite galaxy of M81: NGC 2976, using multiwavelength archival observations. The galaxy is known to present kinematic evidence for a bysimmetric distortion, anyhow the stellar bar presence is controversial, due to the possible interpretation of NGC 2976 presenting an elliptical disk triggered by a prolate dark matter halo. We applied diagnostics in order to detect stellar bars or spiral arms. The m=2 fourier phase has a jump around 60 arcsecs consistent with a central bar and bisymmetric arms. The CO, 3.6 $μ$ surface brightness and the dust lanes are consistent with a gas rich central bar and possibly with gaseous spiral arms. The kinematic jumps related with the dust lanes suggest that the bar perturbation in the disk kinematics is non-negligible and the reported non-circular motions, the central gas excess and the nuclear X-ray source (AGN/Starburst) might be produced by the central bar. SPH simulations of disks inside triaxial dark halos suggest that the two symmetric spots at 130 arcsecs and the narrow arms may be produced by gas at turning points in an elliptical disk, alternatively the potential ellipticity can be originated by tidally induced strong stellar bar/arms. We conclude that, the gas rich bar and spiral arms triggered by tidal stirring, and primordial halo triaxiality, can explain most of the non-circular motions, mass redistribution and nuclear activity. Distinguishing between them requires detailed modeling of environmental effects. An analysis similar to this may reveal such kind of structures in other nearby dwarf satellite galaxies, and if confirmed, the same evolutionary scenario will be applicable to them. This implies biases constraining their dark matter distribution and also making comparison against theoretical predictions for isolated galaxies.

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arXiv2013

Clumpy Disc and Bulge Formation

We present a set of hydrodynamical/Nbody controlled simulations of isolated gas rich galaxies that self-consistently include SN feedback and a detailed chemical evolution model, both tested in cosmological simulations. The initial conditions are motivated by the observed star forming galaxies at z ~ 2-3. We find that the presence of a multiphase interstellar media in our models promotes the growth of disc instability favouring the formation of clumps which in general, are not easily disrupted on timescales compared to the migration time. We show that stellar clumps migrate towards the central region and contribute to form a classical-like bulge with a Sersic index, n > 2. Our physically-motivated Supernova feedback has a mild influence on clump survival and evolution, partially limiting the mass growth of clumps as the energy released per Supernova event is increased, with the consequent flattening of the bulge profile. This regulation does not prevent the building of a classical-like bulge even for the most energetic feedback tested. Our Supernova feedback model is able to establish a self-regulated star formation, producing mass-loaded outflows and stellar age spreads comparable to observations. We find that the bulge formation by clumps may coexit with other channels of bulge assembly such as bar and mergers. Our results suggest that galactic bulges could be interpreted as composite systems with structural components and stellar populations storing archaeological information of the dynamical history of their galaxy.

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arXiv2011

Understanding the spiral structure of the Milky Way using the local kinematic groups

We study the spiral arm influence on the solar neighbourhood stellar kinematics. As the nature of the Milky Way (MW) spiral arms is not completely determined, we study two models: the Tight-Winding Approximation (TWA) model, which represents a local approximation, and a model with self-consistent material arms named PERLAS. This is a mass distribution with more abrupt gravitational forces. We perform test particle simulations after tuning the two models to the observational range for the MW spiral arm properties. We explore the effects of the arm properties and find that a significant region of the allowed parameter space favours the appearance of kinematic groups. The velocity distribution is mostly sensitive to the relative spiral arm phase and pattern speed. In all cases the arms induce strong kinematic imprints for pattern speeds around 17 km/s/kpc (close to the 4:1 inner resonance) but no substructure is induced close to corotation. The groups change significantly if one moves only ~0.6 kpc in galactocentric radius, but ~2 kpc in azimuth. The appearance time of each group is different, ranging from 0 to more than 1 Gyr. Recent spiral arms can produce strong kinematic structures. The stellar response to the two potential models is significantly different near the Sun, both in density and kinematics. The PERLAS model triggers more substructure for a larger range of pattern speed values. The kinematic groups can be used to reduce the current uncertainty about the MW spiral structure and to test whether this follows the TWA. However, groups such as the observed ones in the solar vicinity can be reproduced by different parameter combinations. Data from velocity distributions at larger distances are needed for a definitive constraint.

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arXiv2011

The specific star formation rate and stellar mass fraction of low-mass central galaxies in cosmological simulations

(Abridged) By means of high-resolution cosmological simulations in the context of the LCDM scenario, the specific star formation rate (SSFR=SFR/Ms, Ms is the stellar mass)--Ms and stellar mass fraction (Fs=Ms/Mh, Mh is the halo mass)--Ms relations of low-mass galaxies (2.5< Mh/10^10 Msun <50 at redshift z=0) at different epochs are predicted. The Hydrodynamics ART code was used and some variations of the sub-grid parameters were explored. Most of simulated galaxies, specially those with the highest resolutions, have significant disk components and their structural and dynamical properties are in reasonable agreement with observations of sub-M* field galaxies. However, the SSFRs are 5-10 times smaller than the averages of several (compiled and homogenized here) observational determinations for field blue/star-forming galaxies at z<0.3 (at low masses, most of observed field galaxies are actually blue/star-forming). This inconsistency seems to remain even at z~1.5 though less drastic. The Fs of simulated galaxies increases with Mh as semi-empirical inferences show, but in absolute values the former are ~5-10 times larger than the latter at z=0; this difference increases probably to larger factors at z~1-1.5. The inconsistencies reported here imply that simulated low-mass galaxies (0.2<Ms/10^9 Msun <30 at z=0) assembled their stellar masses much earlier than observations suggest. This confirms the predictions previously found by means of LCDM-based models of disk galaxy formation and evolution for isolated low-mass galaxies (Firmani & Avila-Reese 2010), and highlight that our implementation of astrophysics into simulations and models are still lacking vital ingredients.

astro-ph.CO↗
arXiv2011

The Bulgeless Seyfert/LINER Galaxy NGC 3367: Disk, Bar, Lopsidedness and Environment

NGC3367 is a nearby isolated active galaxy that shows a radio jet, a strong bar and evidence of lopsidedness. We present a quantitative analysis of the stellar and gaseous structure of the galaxy disk and a search for evidence of recent interaction based on new UBVRI Halpha and JHK images and on archival Halpha Fabry-Perot and HI VLA data. From a coupled 1D/2D GALFIT bulge/bar/disk decomposition an (B/D ~ 0.07-0.1) exponential pseudobulge is inferred in all the observed bands. A NIR estimate of the bar strength = 0.44 places NGC 3367 bar among the strongest ones. The asymmetry properties were studied using (1) optical and NIR CAS indexes (2) the stellar (NIR) and gaseous (Halpha, HI) A_1 Fourier mode amplitudes and (3) the HI integrated profile and HI mean intensity distribution. While the average stellar component shows asymmetry values close to the average found in the Local Universe for isolated galaxies, the young stellar component and gas values are largely decoupled showing significantly larger A_1 mode amplitudes suggesting that the gas has been recently perturbed. Our search for (1) faint stellar structures in the outer regions (up to u_R ~ 26 mag arcsec^{-2}), (2) (Halpha) star-forming satellite galaxies and (3) regions with different colors (stellar populations) along the disk all failed. Such an absence is interpreted using recent numerical simulations to constrain a tidal event with an LMC like galaxy to some dynamical times in the past or to a current very low mass, gas rich accretion. We conclude that a cold accretion mode (gas and small/dark galaxies) may be responsible of the nuclear activity and peculiar (young stars and gas) morphology regardless of the highly isolated environment. Black hole growth in bulgeless galaxies may be triggered by cosmic smooth mass accretion.

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arXiv2009

Stellar Kinematic Constraints on Galactic Structure Models Revisited: Bar and Spiral Arm Resonances

We study the phase space available to the local stellar distribution using a Galactic potential consistent with several recent observational constraints. We find that the induced phase space structure has several observable consequences. The spiral arm contribution to the kinematic structure in the solar neighborhood may be as important as the one produced by the Galactic bar. We suggest that some of the stellar kinematic groups in the solar neighborhood, like the Hercules structure and the kinematic branches, can be created by the dynamical resonances of self-gravitating spiral arms and not exclusively by the Galactic bar. A structure coincident with the Arcturus kinematic group is developed when a hot stellar disk population is considered, which introduces a new perspective on the interpretation of its extragalactic origin. A bar-related resonant mechanism can modify this kinematic structure.We show that particles in the dark matter disk-like structure predicted by recent LCDM galaxy formation experiments, with similar kinematics to the thick disk, are affected by the same resonances, developing phase space structures or dark kinematic groups that are independent of the Galaxy assembly history and substructure abundance. We discuss the possibility of using the stellar phase space groups as constraints to non-axisymmetric models of the Milky Way structure.

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arXiv2008

Dynamics of barred galaxies: effects of disk height

We study dynamics of bars in models of disk galaxies embeded in realistic dark matter halos. We find that disk thickness plays an important, if not dominant, role in the evolution and structure of the bars. We also make extensive numerical tests of different N-body codes used to study bar dynamics. Models with thick disks typically used in this type of modeling (height-to-length ratio hz/Rd=0.2) produce slowly rotating, and very long, bars. In contrast, more realistic thin disks with the same parameters as in our Galaxy (hz/Rd= 0.1) produce bars with normal length Rbar approx R_d, which rotate quickly with the ratio of the corotation radius to the bar radius 1.2-1.4 compatible with observations. Bars in these models do not show a tendency to slow down, and may lose as little as 2-3 percent of their angular momentum due to dynamical friction with the dark matter over cosmological time. We attribute the differences between the models to a combined effect of high phase-space density and smaller Jeans mass in the thin disk models, which result in the formation of a dense central bulge. Special attention is paid to numerical effects such as the accuracy of orbital integration, force and mass resolution. Using three N-body codes -- Gadget, ART, and Pkdgrav -- we find that numerical effects are very important and, if not carefully treated, may produce incorrect and misleading results. Once the simulations are performed with sufficiently small time-steps and with adequate force and mass resolution, all the codes produce nearly the same results: we do not find any systematic deviations between the results obtained with TREE codes (Gadget and Pkdgrav) and with the Adaptive-Mesh-Refinement (ART) code.

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arXiv2007

On the Structure of Dark Matter Halos at the Damping Scale of the Power Spectrum with and without Relict Velocities

We report a series of high-resolution cosmological N-body simulations designed to explore the formation and properties of dark matter halos with masses close to the damping scale of the primordial power spectrum of density fluctuations. We further investigate the effect that the addition of a random component, v_rms, into the particle velocity field has on the structure of halos. We adopted as a fiducial model the Lambda Warm Dark Matter cosmology with a non-thermal sterile neutrino mass of 0.5 keV. The filtering mass corresponds then to M_f = 2.6x10^12 M_sun/h. Halos of masses close to M_f were simulated with several million of particles. The results show that, on one hand, the inner density slope of these halos (at radii <~0.02 the virial radius Rvir) is systematically steeper than the one corresponding to the NFW fit or to the CDM counterpart. On the other hand, the overall density profile (radii larger than 0.02Rvir) is less curved and less concentrated than the NFW fit, with an outer slope shallower than -3. For simulations with v_rms, the inner halo density profiles flatten significantly at radii smaller than 2-3 kpc/h (<~0.010-0.015Rvir). A constant density core is not detected in our simulations, with the exception of one halo for which the flat core radius is ~1 kpc/h. Nevertheless, if ``cored'' density profiles are used to fit the halo profiles, the inferred core radii are ~0.1-0.8 kpc/h, in rough agreement with theoretical predictions based on phase-space constrains, and on dynamical models of warm gravitational collapse. A reduction of v_rms by a factor of 3 produces a modest decrease in core radii, less than a factor of 1.5. We discuss the extension of our results into several contexts, for example, to the structure of the cold DM micro-halos at the damping scale of this model.

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arXiv2006

Forming Disk Galaxies in Lambda CDM 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 Lambda CDM 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.

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arXiv2006

Bars and Cold Dark Matter Halos

The central part of a dark matter halo reacts to the presence and evolution of a bar. Not only does the halo absorb angular momentum from the disk, it can also be compressed and have its shape modified. We study these issues in a series of cosmologically motivated, highly resolved N-body simulations of barred galaxies run under different initial conditions. In all models we find that the inner halo's central density increases. We model this density increase using the standard adiabatic approximation and the modified formula by Gnedin et al. and find that halo mass profiles are better reproduced by this latter. In models with a strong bar, the dark matter in the central region forms a bar-like structure (``dark matter bar''), which rotates together with the normal bar formed by the stellar component (``stellar bar''). The minor-to-major axial ratio of a halo bar changes with radius with a typical value 0.7 in the central disk region. DM bar amplitude is mostly a function of the stellar bar strength. Models in which the bar amplitude increases or stays roughly constant with time, initially large (40%-60%) misalignment between the halo and disk bars quickly decreases with time as the bar grows. The halo bar is nearly aligned with the stellar bar (~10 degrees lag for the halo) after ~2 Gyr. The torque, which the halo bar exerts on the stellar bar, can serve as a mechanism to regulate the angular momentum transfer from the disk to the halo.

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arXiv2005

Secular evolution of galactic discs: constraints on phase-space density

It was argued in the past that bulges of galaxies cannot be formed through collisionless secular evolution because that would violate constraints on the phase-space density: the phase-space density in bulges is several times larger than in the inner parts of discs. We show that these arguments against secular evolution are not correct. Observations give estimates of the coarsely grained phase-space densities of galaxies, f'=rho_s/(sigma_R sigma_phi sigma_z), where rho_s is stellar density and sigma_R, sigma_phi, sigma_z are the radial, tangential, and vertical rms velocities of stars. Using high-resolution N-body simulations, we study the evolution of f' in discs of Galaxy-size models. During the secular evolution, the discs, which are embedded in live CDM haloes, form a bar and then a thick, dynamically hot, central mass concentration. In the course of evolution f' declines at all radii, not just in the central region. However, the decline is different in different parts of the disc. In the inner disc, f'(R) develops a valley with a minimum around the end of the central mass concentration. The final result is that the values of f' in the central regions are significantly larger than those in the inner disc. The minimum, which gets deeper with time, seems to be due to a large phase mixing produced by the outer bar. We find that the shape and the amplitude of f'(R) for different simulations agree qualitatively with the observed f'(R) in our Galaxy. Curiously enough, the fact that the coarsely grained phase-space density of the bulge is significantly larger than the one of the inner disc turns out to be an argument in favor of secular formation of bulges, not against it.

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arXiv2004

Dwarf Dark Matter Halos

We study properties of dark matter halos at high redshifts z=2-10 for a vast range of masses with the emphasis on dwarf halos with masses 10^7-10^9 Msun/h. We find that the density profiles of relaxed dwarf halos are well fitted by the NFW profile and do not have cores. 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 lambda_0 = 0.042 and sigma_lambda = 0.63. We estimate the distribution of concentrations for halos in mass range that covers six orders of magnitude from 10^7 Msun/h to 10^13} Msun/h, and find that the data are well reproduced by the model of Bullock et al. The extrapolation of our results to z = 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 x 10^9 Msun/h to 10^13 Msun/h and 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 down-scaled 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.

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arXiv2003

The structure of halos in Self-Interacting Cold Dark Matter models

High-resolution numerical simulations were performed to study the structure and substructure of Milky Way- and cluster-sized halos in a LCDM cosmology with self-interacting (SI) dark particles, where the particle cross section, +AFw-sig, is assumed constant or inversely proportional to the relative velocity. We conclude that the cuspy halo problem at galaxy scales of the LCDM cosmogony can be solved in the latter case. In this case, the inner density of galaxy-cluster halos is only slightly affected by the SI, resulting in agreement with observational constraints. At the same time, the subhalo population in galaxy and cluster SI-CDM halos remains roughly similar to that seen on collisionless CDM halos.

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arXiv2002

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

We study the structure of Milky Way (MW)- and cluster-sized halos in a Lambda Cold Dark Matter (CDM) cosmology with self-interacting (SI) dark particles. The cross section per unit of particle mass has the form sigma = sig_0(1/v_100)^alpha, where sig_0 is a constant in units of cm^2/gr and v_100 is the relative velocity in units of 100 km/s. Different values for sigma with alpha= 0 or 1 were used. For small values of sigma = const. (sig_0<0.5), the core density of the halos at z=0 is typically higher at a given mass for lower values of sig_0 or, at a given sig_0, for lower masses. For values of sig_0 as high as 3.0, the halos may undergo the gravothermal catastrophe before z=0. When alpha = 1, the core density of cluster- and MW-sized halos is similar. Using sigma = 0.5-1.0x(1/v_100), our predictions agree with the central densities and the core scaling laws of halos both inferred from the observations of dwarf and LSB galaxies and clusters of galaxies. The cumulative Vmax-functions of subhalos in MW-sized halos with (sig_0,alpha) = (0.1,0.0), (0.5,0.0) and (0.5,1.0) agree roughly with observations (luminous satellites) for Vmax > 30 km/s, while at Vmax = 20 km/s the functions are a factor 5-8 higher, similar to the CDM predictions. The halos with SI have slightly more specific angular momentum at a given mass shell and are rounder than their CDM counterparts. We conclude that the introduction of SI particles with sigma \propto 1/v_100 may remedy the cuspy core problem of the CDM cosmogony, while the subhalo population number remains similar to that of the CDM halos.

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arXiv2001

Formation and structure of halos in a warm dark matter cosmology

(Abridged) 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, R_f, and the velocity dispersion, v_w, are determined by the mass m_w of the WDM particle. We find that v_w is too small to affect the density profiles of WDM halos. Down to the resolution attained in our simulations, there is not any significant difference in the density profiles and concentrations of halos obtained in simulations with and without the inclusion of v_w. The density profiles of halos with masses down to ~0.01 the filtering mass M_f can be described by the NFW shape; significant 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 lower concentrations of WDM halos with respect to their CDM counterparts can be accounted for 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 R_f~0.16 Mpc (m_w~0.75 kev; flat cosmology with Omega_L=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, relative to CDM predictions.

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