Javiera Guedes

INSIDE OUT AND UPSIDE DOWN: TRACING THE ASSEMBLY OF A SIMULATED DISK GALAXY USING MONO-AGE STELLAR POPULATIONS

We analyze the present day structure and assembly history of a high-resolution hydrodynamic simulation of the formation of a Milky-Way-(MW)-like disk galaxy, from the Eris simulation suite, dissecting it into cohorts of stars formed at different epochs of cosmic history. At z = 0, stars with t form 3 are quickly scattered into rounded, kinematically hot configurations. The oldest disk cohorts form in structures that are radially compact and relatively thick, while subsequent cohorts form in progressively larger, thinner, colder configurations from gas with increasing levels of rotational support. The disk thus forms inside out in a radial sense and upside down in a vertical sense. Secular heating and radial migration influence the final state of each age cohort, but the changes they produce are small compared to the trends established at formation. The predicted correlations of stellar age with spatial and kinematic structure are in good qualitative agreement with the correlations observed for mono-abundance stellar populations in the MW.

Formation and Detectability of Terrestrial Planets around α Centauri B

We simulate the formation of planetary systems around α Centauri B. The N-body accretionary evolution of a Σ ∝ r−1 disk populated with 400-900 lunar-mass protoplanets is followed for 200 Myr. All simulations lead to the formation of multiple-planet systems with at least one planet in the 1-2 M⊕ mass range at 0.5-1.5 AU. We examine the detectability of our simulated planetary systems by generating synthetic radial velocity observations including noise based on the radial velocity residuals to the recently published three planet fit to the nearby K0 V star HD 69830. Using these synthetic observations, we find that we can reliably detect a 1.8 M⊕ planet in the habitable zone of α Centauri B after only 3 years of high cadence observations. We also find that the planet is detectable even if the radial velocity precision is 3 m s−1, as long as the noise spectrum is white. Our results show that the greatest uncertainty in our ability to detect rocky planets in the α Centauri system is the unknown magnitude of ultralow frequency stellar noise.

The distribution of dark matter in the Milky Way's disk

We present an analysis of the effects of dissipational baryonic physics on the local dark matter (DM) distribution at the location of the Sun, with an emphasis on the consequences for direct detection experiments. Our work is based on a comparative analysis of two cosmological simulations with identical initial conditions of a Milky Way halo, one of which (Eris) is a full hydrodynamic simulation and the other (ErisDark) is a DM-only one. We find that in Eris two distinct processes lead to a 30% enhancement of DM in the disk plane at the location of the Sun: the accretion and disruption of satellites resulting in a DM component with net angular momentum, and the contraction of baryons pulling the DM into the disk plane without forcing it to co-rotate. Owing to its particularly quiescent merger history for dark halos of Milky Way mass, the co-rotating dark disk in Eris is less massive than what has been suggested by previous work, contributing only 9% of the local DM density. Yet, since the simulation results in a realistic Milky Way analog galaxy, its DM halo provides a plausible alternative to the Maxwellian standard halo model (SHM) commonly used in direct detection analyses. The speed distribution in Eris is broadened and shifted to higher speeds, compared to its DM-only twin simulation ErisDark. At high speeds f(v) falls more steeply in Eris than in ErisDark or the SHM, easing the tension between recent results from the CDMS-II and XENON100 experiments. The non-Maxwellian aspects of f(v) are still present, but much less pronounced in Eris than in the DM-only runs. The weak dark disk increases the time-averaged scattering rate by only a few percent at low recoil energies. On the high velocity tail, however, the increase in typical speeds due to baryonic contraction results in strongly enhanced mean scattering rates compared to ErisDark, although they are still suppressed compared to the SHM. Similar trends are seen regarding the amplitude of the annual modulation, while the modulated fraction is increased compared to the SHM and decreased compared to ErisDark.

CANDELS Visual Classifications: Scheme, Data Release, and First Results

We have undertaken an ambitious program to visually classify all galaxies in the five CANDELS fields down to H <24.5 involving the dedicated efforts of 65 individual classifiers. Once completed, we expect to have detailed morphological classifications for over 50,000 galaxies spanning 0 < z < 4 over all the fields. Here, we present our detailed visual classification scheme, which was designed to cover a wide range of CANDELS science goals. This scheme includes the basic Hubble sequence types, but also includes a detailed look at mergers and interactions, the clumpiness of galaxies, k-corrections, and a variety of other structural properties. In this paper, we focus on the first field to be completed - GOODS-S, which has been classified at various depths. The wide area coverage spanning the full field (wide+deep+ERS) includes 7634 galaxies that have been classified by at least three different people. In the deep area of the field, 2534 galaxies have been classified by at least five different people at three different depths. With this paper, we release to the public all of the visual classifications in GOODS-S along with the Perl/Tk GUI that we developed to classify galaxies. We present our initial results here, including an analysis of our internal consistency and comparisons among multiple classifiers as well as a comparison to the Sersic index. We find that the level of agreement among classifiers is quite good and depends on both the galaxy magnitude and the galaxy type, with disks showing the highest level of agreement and irregulars the lowest. A comparison of our classifications with the Sersic index and restframe colors shows a clear separation between disk and spheroid populations. Finally, we explore morphological k-corrections between the V-band and H-band observations and find that a small fraction (84 galaxies in total) are classified as being very different between these two bands. These galaxies typically have very clumpy and extended morphology or are very faint in the V-band.

An Off-center Density Peak in the Milky Way's Dark Matter Halo?

We show that the position of the central dark matter density peak may be expected to dier from the dynamical center of the Galaxy by several hundred parsec. In Eris, a high resolution cosmological hydrodynamics simulation of a realistic Milky-Way-analog disk galaxy, this oset is 300 - 400 pc ( 3 gravitational softening lengths) after z = 1. In its dissipationless dark-matter-only twin simulation ErisDark, as well as in the Via Lactea II and GHalo simulations, the oset remains below one softening length for most of its evolution. The growth of the DM oset coincides with a attening of the central DM density prole in Eris inwards of 1 kpc, and the direction from the dynamical center to the point of maximum DM density is correlated with the orientation of the stellar bar, suggesting a barhalo interaction as a possible explanation. A dark matter density oset of several hundred parsec greatly aects expectations of the dark matter annihilation signals from the Galactic Center. It may also support a dark matter annihilation interpretation of recent reports by Weniger (2012) and Su & Finkbeiner (2012) of highly signicant 130 GeV gamma-ray line emission from a region 1 :5 ( 200 parsec projected) away from Sgr A* in the Galactic plane.

PSEUDOBULGE FORMATION AS A DYNAMICAL RATHER THAN A SECULAR PROCESS

We investigate the formation and evolution of the pseudobulge in Eris, a high-resolution N-body + smoothed particle hydrodynamic cosmological simulation that successfully reproduces a Milky-Way-like massive late-type spiral in an cold dark matter universe. At the present epoch, Eris has a virial mass M vir 8 × 1011 M ☉, a photometric stellar mass M * = 3.2 × 1010 M ☉, a bulge-to-total ratio B/T = 0.26, and a weak nuclear bar. We find that the bulk of the pseudobulge forms quickly at high redshift via a combination of non-axisymmetric disk instabilities and tidal interactions or mergers, both occurring on dynamical timescales, not through slow secular processes at lower redshift. Its subsequent evolution is not strictly secular either, and is closely intertwined with the evolution of the stellar bar. In fact, the structure that we recognize as a pseudobulge today evolved from a stellar bar that formed at high redshift due to tidal interactions with satellites, was destroyed by minor mergers at z ~ 3, re-formed shortly after, and weakened again following a steady gas inflow at z 1. The gradual dissolution of the bar ensued at z ~ 1 and continues until the present without increasing the stellar velocity dispersion in the inner regions. In this scenario, the pseudobulge is not a separate component from the inner disk in terms of formation path; rather, it is the first step in the inside-out formation of the baryonic disk, in agreement with the fact that pseudobulges of massive spiral galaxies typically have a dominant old stellar population. If our simulations do indeed reproduce the formation mechanisms of massive spirals, then the progenitors of late-type galaxies should have strong bars and small photometric pseudobulges at high redshift.

STELLAR AND GASEOUS NUCLEAR DISKS OBSERVED IN NEARBY (U)LIRGs

We present near-infrared integral field spectroscopy of the central kiloparsec of 17 nearby luminous and ultra-luminous infrared galaxies undergoing major mergers. These observations were taken with OSIRIS assisted by the Keck I and II Adaptive Optics systems, providing spatial resolutions of a few tens of parsecs. The resulting kinematic maps reveal gas disks in at least 16 out of 19 nuclei and stellar disks in 11 out of 11 nuclei observed in these galaxy merger systems. In our late-stages mergers, these disks are young (stellar ages <30 Myr) and likely formed as gas disks that became unstable to star formation during the merger. On average, these disks have effective radii of a few hundred parsecs, masses between 10^8 and 10^(10) M_☉, and v/σ between 1 and 5. These disks are similar to those created in high-resolution hydrodynamical simulations of gas-rich galaxy mergers, and favor short coalescence times for binary black holes. The few galaxies in our sample in earlier stages of mergers have disks that are larger (r_(eff) ~ 200-1800 pc) and are likely remnants of the galactic disks that have not yet been completely disrupted by the merger.

The Host Galaxies of X-Ray Selected Active Galactic Nuclei to z - 2.5: Structure, Star-Formation and Their Relationships from CANDELS and Herschel/Pacs

We study the relationship between the structure and star formation rate (SFR) of X-ray selected low and moderate luminosity active galactic nuclei (AGNs) in the two Chandra Deep Fields, using Hubble Space Telescope imaging from the Cosmic Assembly Near Infrared Extragalactic Legacy Survey (CANDELS) and deep far-infrared maps from the PEP+GOODS-Herschel survey. We derive detailed distributions of structural parameters and FIR luminosities from carefully constructed control samples of galaxies, which we then compare to those of the AGNs. At z ~ 1, AGNs show slightly diskier light profiles than massive inactive (non-AGN) galaxies, as well as modestly higher levels of gross galaxy disturbance (as measured by visual signatures of interactions and clumpy structure). In contrast, at z ~ 2, AGNs show similar levels of galaxy disturbance as inactive galaxies, but display a red central light enhancement, which may arise from a more pronounced bulge in AGN hosts or extinguished nuclear light. We undertake a number of tests of both these alternatives, but our results do not strongly favor one interpretation over the other. The mean SFR and its distribution among AGNs and inactive galaxies are similar at z> 1.5. At z 1.5.

SIMULATIONS OF RECOILING MASSIVE BLACK HOLES IN THE VIA LACTEA HALO

The coalescence of a massive black hole (MBH) binary leads to the gravitational-wave recoil of the system and its ejection from the galaxy core. We have carried out N-body simulations of the motion of a MBH = 3.7x10^6 Msun MBH remnant in the Via Lactea I simulation, a Milky Way sized dark matter halo. The black hole receives a recoil velocity of Vkick = 80, 120, 200, 300, and 400 km/s at redshift 1.5, and its orbit is followed for over 1 Gyr within a live host halo, subject only to gravity and dynamical friction against the dark matter background. We show that, owing to asphericities in the dark matter potential, the orbit of the MBH is hightly non-radial, resulting in a significantly increased decay timescale compared to a spherical halo. The simulations are used to construct a semi-analytic model of the motion of the MBH in a time-varying triaxial Navarro-Frenk-White dark matter halo plus a spherical stellar bulge, where the dynamical friction force is calculated directly from the velocity dispersion tensor. Such a model should offer a realistic picture of the dynamics of kicked MBHs in situations where gas drag, friction by disk stars, and the flattening of the central cusp by the returning black hole are all negligible effects. We find that MBHs ejected with initial recoil velocities Vkick > 500 km/s do not return to the host center within Hubble time. In a Milky Way-sized galaxy, a recoiling hole carrying a gaseous disk of initial mass ~MBH may shine as a quasar for a substantial fraction of its wandering phase. The long decay timescales of kicked MBHs predicted by this study may thus be favorable to the detection of off-nuclear quasar activity.

Massive Black Hole recoil in high resolution hosts

The final inspiral and coalescence of a black hole binary can produce highly beamed gravitational wave radiation. To conserve linear momentum, the black hole remnant can recoil with “kick” velocity vkick ≤ 4000 km/s. We present two sets of full N-body simulations of recoiling massive black holes (MBH) in high-resolution, non-axisymmetric potentials. The host to the first set of simulations is the main halo of the Via Lactea I simulation (Diemand et al. 2007). The nature of the resulting orbits is investigated through a numerical model where orbits are integrated assuming an evolving, triaxial NFWpotential, and dynamical friction is calculated directly from the velocity dispersion along the major axes of the main halo of Via Lactea I. By comparing the triaxial case to a spherical model, we find that the wandering time spent by the MBH is significantly increased due to the asphericity of the halo. For kicks larger than 200 km/s, the remnant MBH does not return to the inner 200 pc within 1 Gyr, a timescale an order of magnitude larger than the upper limit of the estimated QSO lifetime. The second set of simulations is run using the outcome of a high-resolution gas-rich merger (Mayer et al. 2007) as host potential. In this case, a recoil velocity of 500 km/s cannot remove the MBH from the nuclear region. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)