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Register a new userThe HST large programme on $omega$ Centauri -- III. Absolute proper motion
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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.
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In this second installment of the series, we look at the internal kinematics of the multiple stellar populations of the globular cluster $omega$ Centauri in one of the parallel Hubble Space Telescope (HST) fields, located at about 3.5 half-light radii from the center of the cluster. Thanks to the over 15-year-long baseline and the exquisite astrometric precision of the HST cameras, well-measured stars in our proper-motion catalog have errors as low as $sim 10 mu$as yr$^{-1}$, and the catalog itself extends to near the hydrogen-burning limit of the cluster. We show that second-generation (2G) stars are significantly more radially anisotropic than first-generation (1G) stars. The latter are instead consistent with an isotropic velocity distribution. In addition, 1G have excess systemic rotation in the plane of the sky with respect to 2G stars. We show that the six populations below the main-sequence (MS) knee identified in our first paper are associated to the five main population groups recently isolated on the upper MS in the core of cluster. Furthermore, we find both 1G and 2G stars in the field to be far from being in energy equipartition, with $eta_{rm 1G}=-0.007pm0.026$ for the former, and $eta_{rm 2G}=0.074pm0.029$ for the latter, where $eta$ is defined so that the velocity dispersion $sigma_mu$ scales with stellar mass as $sigma_mu propto m^{-eta}$. The kinematical differences reported here can help constrain the formation mechanisms for the multiple stellar populations in $omega$ Centauri and other globular clusters. We make our astro-photometric catalog publicly available.
In the fourth paper of this series, we present -- and publicly release -- the state-of-the-art catalogue and atlases for the two remaining parallel fields observed with the Hubble Space Telescope for the large programme on omega Centauri. These two fields are located at ~12 from the centre of the globular cluster (in the West and South-West directions) and were imaged in filters from the ultraviolet to the infrared. Both fields were observed at two epochs separated by about 2 years that were used to derive proper motions and to compute membership probabilities.
The galactic globular cluster Omega Centauri is the most massive of its kind, with a complex mix of multiple stellar populations and several kinematic and dynamical peculiarities. Different mean proper motions have been detected among the three main sub-populations, implying that the most metal-rich one is of accreted origin. This particular piece of evidence has been a matter of debate because the available data have either not been sufficiently precise or limited to a small region of the cluster to ultimately confirm or refute the result. Using astrometry from the second Gaia data release and recent high-quality, multi-band photometry, we are now in a position to resolve the controversy. We reproduced the original analysis using the Gaia data and found that the three populations have the same mean proper motion. Thus, there is no need to invoke an accreted origin for the most metal-rich sub-population.
The radio galaxy 3C 273 hosts one of the nearest and best-studied powerful quasar jets. Having been imaged repeatedly by the Hubble Space Telescope (HST) over the past twenty years, it was chosen for an HST program to measure proper motions in the kiloparsec-scale resolved jets of nearby radio-loud active galaxies. The jet in 3C 273 is highly relativistic on sub-parsec scales, with apparent proper motions up to 15$c$ observed by VLBI (Lister et al., 2013). In contrast, we find that the kpc-scale knots are compatible with being stationary, with a mean speed of $-$0.2$pm$0.5$c$ over the whole jet. Assuming the knots are packets of moving plasma, an upper limit of 1c implies a bulk Lorentz factor $Gamma<$2.9. This suggests that the jet has either decelerated significantly by the time it reaches the kpc scale, or that the knots in the jet are standing shock features. The second scenario is incompatible with the inverse Compton off the Cosmic Microwave Background (IC/CMB) model for the X-ray emission of these knots, which requires the knots to be in motion, but IC/CMB is also disfavored in the first scenario due to energetic considerations, in agreement with the recent finding of Meyer & Georganopoulos (2014) which ruled out the IC/CMB model for the X-ray emission of 3C 273 via gamma-ray upper limits.
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.
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