arXiv subjects

Nitya Kallivayalil

Publications and source records attributed to Nitya Kallivayalil.

104 records · Page 6Linked to original sources

arXiv2013

Third-Epoch Magellanic Cloud Proper Motions II: The Large Magellanic Cloud Rotation Field in Three Dimensions

We present the first detailed assessment of the large-scale rotation of any galaxy based on full three-dimensional velocity measurements. We do this for the LMC by combining our HST average proper motion (PM) measurements for stars in 22 fields, with existing line-of-sight (LOS) velocity measurements for 6790 individual stars. We interpret these data with a model of circular rotation in a flat disk. The PM and LOS data paint a consistent picture of the LMC rotation and their combination yields several new insights. The PM data imply a stellar dynamical center that coincides with the HI dynamical center, and a rotation curve amplitude consistent with that inferred from LOS velocity studies. The implied disk viewing angles agree with the range of values found in the literature, but continue to indicate variations with stellar population and/or radius. Young (RSG) stars rotate faster than old (RGB/AGB) stars due to asymmetric drift. Outside the central region, the circular velocity is approximately flat at Vcirc = 91.7 +/- 18.8 km/s. This is consistent with the baryonic Tully-Fisher relation, and implies an enclosed mass M(8.7 kpc) = (1.7 +/- 0.7) x 10^10 solar masses. The virial mass is larger and depends on the full extent of the dark halo. The tidal radius is 22.3 +/- 5.2 kpc (24.0 +/- 5.6 degrees). Combination of the PM and LOS data yields kinematic distance estimates for the LMC, but these are not yet competitive with other methods.

astro-ph.CO↗

arXiv2013

Third-Epoch Magellanic Cloud Proper Motions I: HST/WFC3 data and Orbit Implications

We present proper motions for the Large & Small Magellanic Clouds (LMC & SMC) based on three epochs of \textit{Hubble Space Telescope} data, spanning a $\sim 7$ yr baseline, and centered on fields with background QSOs. The first two epochs, the subject of past analyses, were obtained with ACS/HRC, and have been reanalyzed here. The new third epoch with WFC3/UVIS increases the time baseline and provides better control of systematics. The three-epoch data yield proper motion random errors of only 1-2% per field. For the LMC this is sufficient to constrain the internal proper motion dynamics, as will be discussed in a separate paper. Here we focus on the implied center-of-mass proper motions: mu_W(LMC) = -1.910 +/- 0.020 mas/yr, mu_N(LMC) = 0.229 +/- 0.047 mas/yr, and mu_W(SMC) = -0.772 +/- 0.063 mas/yr, mu_N(SMC) = -1.117 +/- 0.061 mas/yr. We combine the results with a revised understanding of the solar motion in the Milky Way to derive Galactocentric velocities: v_{tot,LMC} = 321 +/- 24 km/s and v_{tot,SMC} = 217 +/- 26 km/s. Our proper motion uncertainties are now dominated by limitations in our understanding of the internal kinematics and geometry of the Clouds, and our velocity uncertainties are dominated by distance errors. Orbit calculations for the Clouds around the Milky Way allow a range of orbital periods, depending on the uncertain masses of the Milky Way and LMC. Periods $\lesssim 4$ Gyr are ruled out, which poses a challenge for traditional Magellanic Stream models. First-infall orbits are preferred (as supported by other arguments as well) if one imposes the requirement that the LMC and SMC must have been a bound pair for at least several Gyr.

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arXiv2012

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 which we name the Triangulum stream. The stream spans 0.2 deg by 12 deg 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 turn-off, corresponds to an old (~12 Gyr) and metal-poor ([Fe/H] \sim -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.

astro-ph.GA↗

arXiv2012

The Role of Dwarf Galaxy Interactions in Shaping the Magellanic System and Implications for Magellanic Irregulars

We present a novel pair of numerical models of the interaction history between the Large and Small Magellanic Clouds (LMC and SMC, respectively) and our Milky Way (MW) in light of recent high precision proper motions (Kallivayalil et al. 2006a,b). Given the new velocities, cosmological simulations of structure formation favor a scenario where the Magellanic Clouds (MCs) are currently on their first infall towards our Galaxy (Boylan-Kolchin et al. 2011, Busha et al. 2011). We illustrate here that the observed irregular morphology and internal kinematics of the MCs (in gas and stars) are naturally explained by interactions between the LMC and SMC, rather than gravitational interactions with the MW. This picture further supports a first infall scenario (Besla et a. 2007). In particular, we demonstrate that the Magellanic Stream, a band of HI gas trailing behind the MCs 150 degrees across the sky, can be accounted for by the action of LMC tides on the SMC before the system was accreted by the MW. We further demonstrate that the off-center, warped stellar bar of the LMC and its one-armed spiral, can be naturally explained by a recent direct collision with the SMC. Such structures are key morphological characteristics of a class of galaxies referred to as Magellanic Irregulars (de Vaucouleurs & Freeman 1972), the majority of which are not associated with massive spiral galaxies. We infer that dwarf-dwarf galaxy interactions are important drivers for the morphological evolution of Magellanic Irregulars and can dramatically affect the efficiency of baryon removal from dwarf galaxies via the formation of extended tidal bridges and tails. Such interactions are important not only for the evolution of dwarf galaxies but also have direct consequences for the buildup of baryons in our own MW, as LMC-mass systems are believed to be the dominant building blocks of MW-type halos.

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arXiv2010

Simulations of the Magellanic Stream in a First Infall Scenario

Recent high precision proper motions from the Hubble Space Telescope (HST) suggest that the Large and Small Magellanic Clouds (LMC and SMC, respectively) are either on their first passage or on an eccentric long period (>6 Gyr) orbit about the Milky Way (MW). This differs markedly from the canonical picture in which the Clouds travel on a quasi-periodic orbit about the MW (period of ~2 Gyr). Without a short period orbit about the MW, the origin of the Magellanic Stream, a young (1-2 Gyr old) coherent stream of HI gas that trails the Clouds ~150 degrees across the sky, can no longer be attributed to stripping by MW tides and/or ram pressure stripping by MW halo gas. We propose an alternative formation mechanism in which material is removed by LMC tides acting on the SMC before the system is accreted by the MW. We demonstrate the feasibility and generality of this scenario using an N-body/SPH simulation with cosmologically motivated initial conditions constrained by the observations. Under these conditions we demonstrate that it is possible to explain the origin of the Magellanic Stream in a first infall scenario. This picture is generically applicable to any gas-rich dwarf galaxy pair infalling towards a massive host or interacting in isolation.

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arXiv2009

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 $HST$ proper motion measurements. We study the effects of varying the total Local Group mass, the relative mass of the Milky Way and M31, the proper motion of M31, and the proper motion of the 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 $> 3.5\times 10^{12} M_\odot$, 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 $M_{\rm MW} + M_{\rm LMC} = (8.7 \pm 0.8) \times 10^{11} M_\odot$.

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arXiv2008

Kinematical Structure of the Magellanic System

We review our understanding of the kinematics of the LMC and the SMC, and their orbit around the Milky Way. The line-of-sight velocity fields of both the LMC and SMC have been mapped with high accuracy using thousands of discrete traces, as well as HI gas. The LMC is a rotating disk for which the viewing angles have been well-established using various methods. The disk is elliptical in its disk plane. The disk thickness varies depending on the tracer population, with V/sigma ranging from 2-10 from the oldest to the youngest population. For the SMC, the old stellar population resides in a spheroidal distribution with considerable line-of-sight depth and low V/sigma. Young stars and HI gas reside in a more irregular rotating disk. Mass estimates based on the kinematics indicate that each Cloud is embedded in a dark halo. Proper motion measurements with HST show that both galaxies move significantly more rapidly around the Milky Way than previously believed. This indicates that for a canonical 10^12 solar mass Milky Way the Clouds are only passing by us for the first time. Although a higher Milky Way mass yields a bound orbit, this orbit is still very different from what has been previously assumed in models of the Magellanic Stream. Hence, much of our understanding of the history of the Magellanic System and the formation of the Magellanic Stream may need to be revised. The accuracy of the proper motion data is insufficient to say whether or not the LMC and SMC are bound to each other, but bound orbits do exist within the proper motion error ellipse.

astro-ph↗

arXiv2008

New Analysis of the Proper Motions of the Magellanic Clouds using HST/WFPC2

In HST Cycles 11 and 13 we obtained two epochs of ACS/HRC data for fields in the Magellanic Clouds centered on background quasars. We used these data to determine the proper motions of the LMC and SMC to better than 5% and 15% respectively. The results had a number of unexpected implications for the Milky Way-LMC-SMC system. The implied three-dimensional velocities were larger than previously believed and close to the escape velocity in a standard 10^12 solar mass Milky Way dark halo, implying that the Clouds may be on their first passage. Also, the relative velocity between the LMC and SMC was larger than expected, leaving open the possibility that the Clouds may not be bound to each other. To further verify and refine our results we requested an additional epoch of data in Cycle 16 which is being executed with WFPC2/PC due to the failure of ACS. We present the results of an ongoing analysis of these WFPC2 data which indicate good consistency with the two-epoch results.

astro-ph↗

arXiv2008

The Binarity of the Magellanic Clouds and the Formation of the Magellanic Stream

The HST proper motion (PM) measurements of the Clouds have severe implications for their interaction history with the Milky Way (MW) and with each other. The Clouds are likely on their first passage about the MW and the SMC's orbit about the LMC is better described as quasi-periodic rather than circular. Binary L/SMC orbits that satisfy observational constraints on their mutual interaction history (e.g. the formation of the Magellanic Bridge during a collision between the Clouds ~300 Myr ago) can be located within 1 sigma of the mean PMs. However, these binary orbits are not co-located with the Magellanic Stream (MS) when projected on the plane of the sky and the line-of-sight velocity gradient along the LMC's orbit is significantly steeper than that along the MS. These combined results ultimately rule out a purely tidal origin for the MS: tides are ineffective without multiple pericentric passages and can neither decrease the velocity gradient nor explain the offset stream in a polar orbit configuration. Alternatively, ram pressure stripping of an extended gaseous disk may naturally explain the deviation. The offset also suggests that observations of the little-explored region between RA 21h and 23h are crucial for characterizing the full extent of the MS.

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arXiv2007

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 et al (2006a,b) suggest that the 3D velocities of the Clouds are substantially higher (~100 km/s) 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 re-examine the orbital history of the Clouds using the new velocities and a LCDM-motivated MW model with virial mass Mvir = 1e12 Msun (e.g. Klypin et al 2002). We conclude that the Clouds 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 > 2e12 Msun), that their orbital period and apogalacticon distance must be a factor of two 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 HI gas trailing the LMC and SMC that extends roughly 100 degrees 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.

astro-ph↗

arXiv2006

Identification of the Microlens in Event MACHO-LMC-20

We report on the identification of the lens responsible for microlensing event MACHO-LMC-20. As part of a \textit{Spitzer}/IRAC program conducting mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields, we discovered a significant flux excess at the position of the source star for this event. These data, in combination with high resolution near-infrared \textit{Magellan}/PANIC data has allowed us to classify the lens as an early M dwarf in the thick disk of the Milky Way, at a distance of $\sim 2$ kpc. This is only the second microlens to have been identified, the first also being a M dwarf star in the disk. Together, these two events are still consistent with the expected frequency of nearby stars in the Milky Way thin and thick disks acting as lenses.

astro-ph↗

arXiv2006

Is the SMC Bound to the LMC? The HST 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 \textit{Hubble Space Telescope} (\textit{HST}). We tracked the SMC's motion relative to 4 background QSOs over a baseline of approximately 2 years. The measured proper motion is : $μ_W = -1.16 \pm 0.18 \masyr, μ_N = -1.17 \pm 0.18 \masyr$. This is the best measurement yet of the SMC's 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 $105 \pm 42 \kms$. 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 vs. 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.

astro-ph↗

arXiv2005

The Proper Motion of the Large Magellanic Cloud using HST

We 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). We 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 ~2 year baseline we determine the proper motion of the LMC to better than 5% accuracy: mu_W = -2.03 +/- 0.08 mas/yr; mu_N = 0.44 +/- 0.05 mas/yr. 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.

astro-ph↗

arXiv2004

Spitzer Space Telescope Observations of the Aftermath of Microlensing Event MACHO-LMC-5

We have carried out photometry of the microlensing event MACHO-LMC-5 with Spitzer's IRAC ten years after the magnification of the LMC source star was recorded. This event is unique in the annals of gravitational microlensing: the lensing star itself has been observed using HST (once with WFPC2 and twice with ACS/HRC). Since the separation between the source and lens at the epoch of the Spitzer observations was $\sim0.24''$, the two stars cannot be resolved in the Spitzer images. However, the IRAC photometry clearly establishes that the lens is a M5 dwarf star from its infrared excess, which in turn yields a mass of $\sim0.2 M_{\odot}$. This demonstrates the potential of Spitzer to detect the lenses in other gravitational microlensing events.

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