Nitya Kallivayalil

ARE THE MAGELLANIC CLOUDS ON THEIR FIRST PASSAGE ABOUT THE MILKY WAY

Recent proper-motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil and coworkers suggest that the 3D velocities of the Clouds are substantially higher (~100 km s-1) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we reexamine the orbital history of the Clouds using the new velocities and a ΛCDM-motivated MW model with virial mass Mvir = 1012 M☉ (e.g., Klypin and coworkers). We conclude that the LMC and SMC are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances 200 kpc (i.e., Mvir > 2 × 1012 M☉), that their orbital period and apogalacticon distance must be a factor of 2 larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of H I gas trailing the LMC and SMC that extends ~100° across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.

The proper motion of the large magellanic cloud using HST

The authors present a measurement of the systemic proper motion of the Large Magellanic Cloud (LMC) from astrometry with the High Resolution Camera (HRC) of the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). They observed LMC fields centered on 21 background QSOs that were discovered from their optical variability in the MACHO database. The QSOs are distributed homogeneously behind the central few degrees of the LMC. With 2 epochs of HRC data and a {approx} 2 year baseline they determine the proper motion of the LMC to better than 5% accuracy: {mu}{sub W} = -2.03 {+-} 0.08 mas yr{sup -1}, {mu}{sub N} = 0.44 {+-} 0.05 mas yr{sup -1}. This is the most accurate proper motion measurement for any Milky Way satellite thus far. When combined with HI data from the Magellanic Stream this should provide new constraints on both the mass distribution of the Galactic Halo and models of the Stream.

Is the SMC bound to the LMC? The Hubble space telescope proper motion of the SMC

We present a measurement of the systemic proper motion of the Small Magellanic Cloud (SMC) made using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). We tracked the SMCs motion relative to four background QSOs over a baseline of approximately 2 yr. The measured proper motion is μW = -1.16 ± 0.18 mas yr-1, μN = -1.17 ± 0.18 mas yr-1. This is the best measurement yet of the SMCs proper motion. We combine this new result with our prior estimate of the proper motion of the Large Magellanic Cloud (LMC) from the same observing program to investigate the orbital evolution of both Clouds over the past 9 Gyr. The current relative velocity between the Clouds is 105 ± 42 km s-1. Our investigations of the past orbital motions of the Clouds in a simple model for the dark halo of the Milky Way imply that the Clouds could be unbound from each other. However, our data are also consistent with orbits in which the Clouds have been bound to each other for approximately a Hubble time. Smaller proper-motion errors and better understanding of the LMC and SMC masses would be required to constrain their past orbital history and their bound versus unbound nature unambiguously. The new proper-motion measurements should be sufficient to allow the construction of improved models for the origin and properties of the Magellanic Stream. In turn, this will provide new constraints on the properties of the Milky Way dark halo.

A COLD MILKY WAY STELLAR STREAM IN THE DIRECTION OF TRIANGULUM

We present evidence for a new Milky Way stellar tidal stream in the direction of the Andromeda and Triangulum (M31 and M33) galaxies. Using a matched-filter technique, we search the Sloan Digital Sky Survey Data Release 8 by creating stellar density maps which probe the Milky Way halo at distances between 8 and 40 kpc. A visual search of these maps recovers all of the major known stellar streams, as well as a new stream in the direction of M31/M33 that we name the Triangulum stream. The stream spans 02 by 12° on the sky, or 75 pc by 5.5 kpc in physical units with a best-fitting distance of 26 ± 4 kpc. The width of the stream is consistent with being the tidal remnant of a globular cluster. A color-magnitude diagram of the stream region shows an overdensity which, if identified as a main-sequence turnoff, corresponds to an old (~12 Gyr) and metal-poor ([Fe/H] ~–1.0 dex) stellar population. Future kinematic studies of this and similar cold streams will provide tight constraints on the shape of the Galactic gravitational potential.

TiNy Titans: The Role of Dwarf–Dwarf Interactions in Low-mass Galaxy Evolution

We introduce TiNy Titans (TNT), the first systematic study of star formation and the subsequent processing of the interstellar medium in interacting dwarf galaxies. Here we present the first results from a multiwavelength observational program based on a sample of 104 dwarf galaxy pairs selected from a range of environments within the spectroscopic portion of the Sloan Digital Sky Survey and caught in various stages of interaction. The TNT dwarf pairs span mass ratios of M∗,1/M∗,2 < 10, projected separations < 50 kpc, and pair member masses of 7 < log(M∗/M⊙) < 9.7. The dwarf-dwarf merger sequence, as defined by TNT at z=0, demonstrates conclusively and for the first time that the star formation enhancement observed for massive galaxy pairs also extends to the dwarf mass range. Star formation is enhanced in paired dwarfs in otherwise isolated environments by factor of 2.3 (± 0.7) at pair separations < 50 kpc relative to unpaired analogs. The enhancement decreases with increasing pair separation and extends out to pair separations as large as 100 kpc. Starbursts, defined by Hα EQW > 100Å, occur in 20% of the TNT dwarf pairs, regardless of environment, compared to only 6-8% of the matched unpaired dwarfs. Starbursts can be triggered throughout the merger (i.e. out to large pair separations) and not just approaching coalescence. Despite their enhanced star formation and triggered starbursts, most TNT dwarf pairs have similar gas fractions relative to unpaired dwarfs of the same stellar mass. Thus, there may be significant reservoirs of diffuse, nonstarforming neutral gas surrounding the dwarf pairs or the gas consumption timescales may be long in the starburst phase. The only TNT dwarf pairs with low gas fractions (fgas < 0.4) and the only dwarfs, either paired or unpaired, with Hα EQW < 2Å are found near massive galaxy hosts. We conclude that dwarf-dwarf interactions are significant drivers of galaxy evolution at the low mass end, but ultimately environment is responsible for the quenching of star formation. This preliminary study is a precursor to an ongoing high resolution H i and optical imaging program to constrain the spatial distribution of star formation and gas throughout the course of the dwarf-dwarf merger sequence.We introduce TiNy Titans (TNT), the first systematic study of star formation and the subsequent processing of the interstellar medium in interacting dwarf galaxies. Here we present the first results from a multiwavelength observational program based on a sample of 104 dwarf galaxy pairs selected from a range of environments within the spectroscopic portion of the Sloan Digital Sky Survey and caught in various stages of interaction. The TNT dwarf pairs span mass ratios of M/M 100 Å, occur in 20% of the TNT dwarf pairs, regardless of environment, compared to only 6%–8% of the matched unpaired dwarfs. Starbursts can be triggered throughout the merger (i.e., out to large pair separations) and not just approaching coalescence. Despite their enhanced star formation and triggered starbursts, most TNT dwarf pairs have similar gas fractions relative to unpaired dwarfs of the same stellar mass. Thus, there may be significant reservoirs of diffuse, non-star-forming neutral gas surrounding the dwarf pairs, or the gas consumption timescales may be long in the starburst phase. The only TNT dwarf pairs with low gas fractions (f) and the only dwarfs, either paired or unpaired, with Hα EQW < 2 Å are found near massive galaxy hosts. We conclude that dwarf–dwarf interactions are significant drivers of galaxy evolution at the low-mass end, but ultimately environment is responsible for the quenching of star formation. This preliminary study is a precursor to an ongoing high-resolution H i and optical imaging program to constrain the spatial distribution of star formation and gas throughout the course of the dwarf–dwarf merger sequence.

The shape of the inner Milky Way halo from observations of the Pal 5 and GD-1 stellar streams

We constrain the shape of the Milky Ways halo by dynamical modeling of the observed phase-space tracks of the Pal 5 and GD-1 tidal streams. We find that the only information about the potential gleaned from the tracks of these streams are precise measurements of the shape of the gravitational potential---the ratio of vertical to radial acceleration---at the location of the streams, with weaker constraints on the radial and vertical accelerations separately. The latter will improve significantly with precise proper-motion measurements from Gaia. We measure that the overall potential flattening is 0.95 +/- 0.04 at the location of GD-1 ([R,z] ~ [12.5,6.7] kpc) and 0.94 +/- 0.05 at the position of Pal 5 ([R,z] ~ [8.4,16.8] kpc). Combined with constraints on the force field near the Galactic disk, we determine that the axis ratio of the dark-matter halos density distribution is 1.05 +/- 0.14 within the inner 20 kpc, with a hint that the halo becomes more flattened near the edge of this volume. The halo mass within 20 kpc is 1.1 +/- 0.1 x 10^{11} M_sun. A dark-matter halo this close to spherical is in tension with the predictions from numerical simulations of the formation of dark-matter halos.

REVISITING THE ROLE OF M31 IN THE DYNAMICAL HISTORY OF THE MAGELLANIC CLOUDS

We study the dynamics of the Magellanic Clouds in a model for the Local Group whose mass is constrained using the timing argument/two-body limit of the action principle. The goal is to evaluate the role of M31 in generating the high angular momentum orbit of the Clouds, a puzzle that has only been exacerbated by the latest Hubble Space Telescope proper motion measurements. We study the effects of varying the total Local Group mass, the relative mass of the Milky Way (MW) and M31, the proper motion of M31, and the proper motion of the Large Magellanic Cloud (LMC) on this problem. Over a large part of this parameter space, we find that tides from M31 are insignificant. For a range of LMC proper motions approximately 3σ higher than the mean and total Local Group mass >3.5 × 1012 M ☉, M31 can provide a significant torque to the LMC orbit. However, if the LMC is bound to the MW, then M31 is found to have negligible effect on its motion, and the origin of the high angular momentum of the system remains a puzzle. Finally, we use the timing argument to calculate the total mass of the MW-LMC system based on the assumption that they are encountering each other for the first time, their previous perigalacticon being a Hubble time ago, obtaining M MW + M LMC = (8.7 ± 0.8) × 1011 M ☉.

Identifying true satellites of the Magellanic Clouds

The hierarchical nature of ΛCDM suggests that the Magellanic Clouds must have been surrounded by a number of satellites before their infall into the Milky Way halo. Many of those satellites should still be in close proximity to the Clouds, but some could have dispersed ahead/behind the Clouds along their Galactic orbit. Either way, prior association with the Clouds constrains the present-day positions and velocities of candidate Magellanic satellites: they must lie close to the nearly polar orbital plane of the Magellanic Stream, and their distances and radial velocities must follow the latitude dependence expected for a tidal stream with the Clouds near pericentre. We use a cosmological numerical simulation of the disruption of a massive sub-halo in a Milky Way-sized ΛCDM halo to test whether any of the 20 dwarfs recently discovered in the Dark Energy Survey, the Survey of the MAgellanic Stellar History, Pan-STARRS, and ATLAS surveys are truly associated with the Clouds. Of the six systems with kinematic data, only Hor 1 has distance and radial velocities consistent with a Magellanic origin. Of the remaining dwarfs, six (Hor 2, Eri 3, Ret 3, Tuc 4, Tuc 5, and Phx 2) have positions and distances consistent with a Magellanic origin, but kinematic data are needed to substantiate that possibility. Conclusive evidence for association would require proper motions to constrain the orbital angular momentum direction, which, for true Magellanic satellites, must be similar to that of the Clouds. We use this result to predict radial velocities and proper motions for all new dwarfs, assuming that they were Magellanic satellites. Our results are relatively insensitive to the assumption of first or second pericentre for the Clouds

THE PROPER MOTION OF PALOMAR 5

Palomar 5 (Pal 5) is a faint halo globular cluster associated with narrow tidal tails. It is a useful system to understand the process of tidal dissolution, as well as to constrain the potential of the Milky Way. A well-determined orbit for Pal 5 would enable detailed study of these open questions. We present here the first CCD-based proper motion measurement of Pal 5 obtained using SDSS as a first epoch and new LBT/LBC images as a second, giving a baseline of 15 years. We perform relative astrometry, using SDSS as a distortion-free reference, and images of the cluster and also of the Pal 5 stream for the derivation of the distortion correction for LBC. The reference frame is made up of background galaxies. We correct for differential chromatic refraction using relations obtained from SDSS colors as well as from flux-calibrated spectra, finding that the correction relations for stars and for galaxies are different. We obtain mu_alpha=-2.296+/-0.186 mas/yr and mu_delta=-2.257+/-0.181 mas/yr for the proper motion of Pal 5. We use this motion, and the publicly available code galpy, to model the disruption of Pal 5 in different Milky Way models consisting of a bulge, a disk and a spherical dark matter halo. Our fits to the observed stream properties (streak and radial velocity gradient) result in a preference for a relatively large Pal 5 distance of around 24 kpc. A slightly larger absolute proper motion than what we measure also results in better matches but the best solutions need a change in distance. We find that a spherical Milky Way model, with V_0=220 km/s and V_(20 kpc), i.e., approximately at the apocenter of Pal 5, of 218 km/s, can match the data well, at least for our choice of disk and bulge parametrization.

On the effect of electron collisions in the excitation of cometary HCN

The electron-HCN collision rate for the excitation of rotational transitions of the HCN molecule is evaluated in comets C/1995 O1 (Hale-Bopp) and C/1996 B2 (Hyakutake). Based on theoretical models of the cometary atmosphere, we show that collisions with electrons can provide a significant excitation mechanism for rotational transitions in the HCN molecule. Computed values of the cross sectione-HCN can be as high as 1:3 ; 10 � 12 cm 2 , more than 2 orders of magnitude greater than the commonly assumed HCN-H2O cross section. For the ground rotational transitions of HCN, the electron-HCN collision rate is found to exceed the HCN-H2 Oc ollision rate at distances greater than 3000 km from the cometary nucleus of Hale-Bopp and 1000 km from that of Hyakutake. Collisional excitation processes dominate over radiative excitation processes up to a distance of 160,000 km from the cometary nucleus of Hale-Bopp and 50,000 km from that of Hyakutake. Excitation models that neglect electron collisions can underestimate the HCN gas production rates by as much as a factor of 2. Subject headingg comets: general — molecular processes