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Register a new userAbsolute Proper Motion of IRAS 00259+5625 with VERA : Indication of Superbubble Expansion Motion
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We present the first measurement of the absolute proper motions of IRAS 00259+5625 (CB3, LBN594) associated with the HI loop called the NGC281 superbubble that extends from the Galactic plane over ~300 pc toward decreasing galactic latitude. The proper motion components measured with VERA are (mu_alpha cos(delta), mu_delta) = (-2.48 +/- 0.32, -2.85 +/- 0.65) mas yr^{-1}, converted into (mu_l cos(b), mu_b) = (-2.72 +/- 0.32, -2.62 +/- 0.65) mas yr^{-1} in the Galactic coordinates. The measured proper motion perpendicular to the Galactic plane (mu_b) shows vertical motion away from the Galactic plane with a significance of about ~4-sigma. As for the source distance, the distance measured with VERA is marginal, 2.4^{+1.0}_{-0.6} kpc. Using the distance, an absolute vertical motion (v_{b}) of -17.9 +/- 12.2 km s^{-1} is determined with ~1.5-sigma significance. The tendency of the large vertical motion is consistent with previous VLBI results for NGC 281 associated with the same superbubble. Thus, our VLBI results indicate the superbubble expansion motion whose origin is believed to be sequential supernova explosions.
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We derived the absolute proper motion (PM) of the old, solar-metallicity Galactic open cluster M67 using observations collected with CFHT (1997) and with LBT (2007). About 50 galaxies with relatively sharp nuclei allow us to determine the absolute PM of the cluster. We find (mu_alpha cos(delta),mu_delta)_J2000.0 = (-9.6+/-1.1,-3.7+/-0.8) mas/yr. By adopting a line-of-sight velocity of 33.8+/-0.2 km/s, and assuming a distance of 815+/-50 pc, we explore the influence of the Galactic potential, with and without the bar and/or spiral arms, on the galactic orbit of the cluster.
In this paper we report a new estimate of the absolute proper motion (PM) of the globular cluster NGC 5139 ($omega$ Cen) as part of the HST large program GO-14118+14662. We analyzed a field 17 arcmin South-West of the center of $omega$ Cen and computed PMs with an epoch span of $sim$15.1 years. We employed 45 background galaxies to link our relative PMs to an absolute reference-frame system. The absolute PM of the cluster in our field is: $(mu_alpha cosdelta , mu_delta) = (-3.341 pm 0.028 , -6.557 pm 0.043)$ mas yr$^{-1}$. Upon correction for the effects of viewing perspective and the known cluster rotation, this implies that for the cluster center of mass $(mu_alpha cosdelta , mu_delta) = (-3.238 pm 0.028, -6.716 pm 0.043)$ mas yr$^{-1}$. This measurement is direct and independent, has the highest random and systematic accuracy to date, and will provide an external verification for the upcoming Gaia Data Release 2. It also differs from most reported PMs for $omega$ Cen in the literature by more than 5$sigma$, but consistency checks compared to other recent catalogs yield excellent agreement. We computed the corresponding Galactocentric velocity, calculated the implied orbit of $omega$ Cen in two different Galactic potentials, and compared these orbits to the orbits implied by one of the PM measurements available in the literature. We find a larger (by about 500 pc) perigalactic distance for $omega$ Cen with our new PM measurement, suggesting a larger survival expectancy for the cluster in the Galaxy.
Our knowledge of the dynamics and masses of galaxies in the Local Group has long been limited by the fact that only line-of-sight velocities were observationally accessible. This introduces significant degeneracies in dynamical models, which can only be resolved by measuring also the velocity components perpendicular to the line of sight. However, beyond the solar neighborhood, the corresponding proper motions have generally been too small to measure. This has changed dramatically over the past decade, especially due to the angular resolution and stability available on the Hubble Space Telescope. Proper motions can now be reliably measured throughout the Local Group, as illustrated by, e.g., the work of the HSTPROMO collaboration. In this review, I summarize the importance of proper motions for Local Group science, and I describe the current and future observational approaches and facilities available to measure proper motions. I highlight recent results on various Milky Way populations (globular clusters, the bulge, the metal-poor halo, hypervelocity stars, and tidal streams), dwarf satellite galaxies, the Magellanic Clouds and the Andromeda System.
We report on absolute proper-motion measurements of an H2O maser source in the NGC 281 West molecular cloud, which is located ~320 pc above the Galactic plane and is associated with an HI loop extending from the Galactic plane. We have conducted multi-epoch phase-referencing observations of the maser source with VERA (VLBI Exploration of Radio Astrometry) over a monitoring period of 6 months since May 2006. We find that the H2O maser features in NGC 281 West are systematically moving toward the southwest and further away from the Galactic plane with a vertical velocity of ~20-30 km/s at its estimated distance of 2.2-3.5 kpc. Our new results provide the most direct evidence that the gas in the NGC 281 region on the HI loop was blown out from the Galactic plane, most likely in a superbubble driven by multiple or sequential supernova explosions in the Galactic plane.
We present the first absolute proper motion measurement of Leo I, based on two epochs of HST ACS/WFC images separated by ~5 years. The average shift of Leo I stars with respect to ~100 background galaxies implies a proper motion of (mu_W, mu_N) = (0.1140 +/- 0.0295, -0.1256 +/- 0.0293) mas/yr. The implied Galactocentric velocity vector, corrected for the reflex motion of the Sun, has radial and tangential components V_rad = 167.9 +/- 2.8 km/s and V_tan = 101.0 +/- 34.4 km/s, respectively. We study the detailed orbital history of Leo I by solving its equations of motion backward in time for a range of plausible mass models for the Milky Way and its surrounding galaxies. Leo I entered the Milky Way virial radius 2.33 +/- 0.21 Gyr ago, most likely on its first infall. It had a pericentric approach 1.05 +/- 0.09 Gyr ago at a Galactocentric distance of 91 +/- 36 kpc. We associate these time scales with characteristic time scales in Leo Is star formation history, which shows an enhanced star formation activity ~2 Gyr ago and quenching ~1 Gyr ago. There is no indication from our calculations that other galaxies have significantly influenced Leo Is orbit, although there is a small probability that it may have interacted with either Ursa Minor or Leo II within the last ~1 Gyr. For most plausible Milky Way masses, the observed velocity implies that Leo I is bound to the Milky Way. However, it may not be appropriate to include it in models of the Milky Way satellite population that assume dynamical equilibrium, given its recent infall. Solution of the complete (non-radial) timing equations for the Leo I orbit implies a Milky Way mass M_MW,vir = 3.15 (-1.36, +1.58) x 10^12 Msun, with the large uncertainty dominated by cosmic scatter. In a companion paper, we compare the new observations to the properties of Leo I subhalo analogs extracted from cosmological simulations.
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