Article

A first measurement of the Proper Motion of the Leo II dwarf spheroidal galaxy

The Astrophysical Journal (Impact Factor: 6.28). 08/2011; 741(2). DOI: 10.1088/0004-637X/741/2/100;
Source: arXiv

ABSTRACT We use 14-year baseline images obtained with the Wide Field Planetary Camera
2 on board the Hubble Space telescope to derive a proper motion for one of the
Milky Way's most distant dwarf spheroidal companions, Leo II, relative to an
extragalactic background reference frame. Astrometric measurements are
performed in the effective point spread function (ePSF) formalism using our own
developed code. An astrometric reference grid is defined using 3,224 stars that
are members of Leo II that are brighter than magnitude 25 in the F814W band. We
identify 17 compact extra-galactic sources, for which we measure a systemic
proper motion relative to this stellar reference grid. We derive a proper
motion [\mu_{\alpha},\mu_{\delta}]=[+104+/-113,-33+/-151] microarcseconds/yr
for Leo II in the heliocentric reference frame. Though marginally detected, the
proper motion yields constraints on the orbit of Leo II. Given a distance of
230 Kpc and a heliocentric radial velocity +79 km/s, and after subtraction of
the solar motion, our measurement indicates a total orbital motion
266.1+/-128.7 km/s in the Galactocentric reference frame, with a radial
component +21.5+/-4.3 km/s and tangential component 265.2+/-129.4 km/s. The
small radial component indicates that Leo II either has a low-eccentricity
orbit, or is currently close to perigalacticon or apogalacticon distance. We
see evidence for systematic errors in the astrometry of the extragalactic
sources which, while close to being point sources, are slightly resolved in the
HST images. We argue that more extensive observations at later epochs will be
necessary to better constrain the proper motion of Leo II. We provide a
detailed catalog of the stellar and extragalactic sources identified in the HST
data which should provide a solid early-epoch reference for future astrometric
measurements.

0 Followers
 · 
115 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Dwarf galaxies in the Local Group appear to be stripped of their gas within 270 kpc of the host galaxy. Color-magnitude diagrams of these dwarfs, however, show clear evidence of episodic star formation (\Delta{}t ~ a few Gyr) over cosmic time. We present a simple model to account for this behaviour. Residual gas within the weak gravity field of the dwarf experiences dramatic variations in the gas cooling time around the eccentric orbit. This variation is due to two main effects. The azimuthal compression along the orbit leads to an increase in the gas cooling rate of ~([1+\epsilon]/[1-\epsilon])^2. The Galaxy's ionizing field declines as 1/R^2 for R>R_disk although this reaches a floor at R~150 kpc due to the extragalactic UV field ionizing intensity. We predict that episodic SF is mostly characteristic of dwarfs on moderately eccentric orbits (\epsilon>0.2) that do not come too close to the centre (R>R_disk) and do not spend their entire orbit far away from the centre (R>200 kpc). Up to 40% of early infall dwarf spheroidals can be expected to have already had at least one burst since the initial epoch of star formation, and 10% of these dwarf spheriodals experiencing a second burst. Such a model can explain the timing of bursts in the Carina dwarf spheroidal and restrict the orbit of the Fornax dwarf spheroidal. However, this model fails to explain why some dwarfs, such as Ursa Minor, experience no burst post-infall.
    The Astrophysical Journal 09/2011; 748(2). DOI:10.1088/0004-637X/748/2/149 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We analyze subhalos in the Via Lactea II (VL2) cosmological simulation to look for correlations among their infall times and z = 0 dynamical properties. We find that the present day orbital energy is tightly correlated with the time at which subhalos last crossed into the virial radius. This energy-infall correlation provides a means to infer infall times for Milky Way satellite galaxies. Assuming that the Milky Way's assembly can be modeled by VL2, we show that the infall times of some satellites are well constrained given only their Galactocentric positions and line-of-sight velocities. The constraints sharpen for satellites with proper motion measurements. We find that Carina, Ursa Minor, and Sculptor were all accreted early, more than 8 Gyr ago. Five other dwarfs, including Sextans and Segue 1, are also probable early accreters, though with larger uncertainties. On the other extreme, Leo T is just falling into the Milky Way for the first time while Leo I fell in \sim 2 Gyr ago and is now climbing out of the Milky Way's potential after its first perigalacticon. The energies of several other dwarfs, including Fornax and Hercules, point to intermediate infall times, 2 - 8 Gyr ago. We compare our infall time estimates to published star formation histories and find hints of a dichotomy between ultrafaint and classical dwarfs. The classical dwarfs appear to have quenched star formation after infall but the ultrafaint dwarfs tend to be quenched long before infall, at least for the cases in which our uncertainties allow us to discern differences. Our analysis suggests that the Large Magellanic Cloud crossed inside the Milky Way virial radius recently, within the last \sim 4 billion years.
    Monthly Notices of the Royal Astronomical Society 10/2011; 425(1). DOI:10.1111/j.1365-2966.2012.21432.x · 5.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Forthcoming astrometric missions such as the Gaia satellite will bring to the fore the problem of estimating the enclosed mass from a set of positions, radial velocities and proper motions of tracer stars. Here, we show how to construct the tracer mass estimator when the proper motion data are available in addition to the usual line-of-sight velocity data. Notably, the mass estimators do not require any assumption on the anisotropy, as it is naturally incorporated through the different components of proper motions. In addition, the separate treatment of the proper motions and the line-of-sight velocities is desirable because they are observationally independent and thus the propagation of the combined uncertainties is rather straightforward. The extension to projected data is also sketched, together with a possible application of measuring the masses of Galactic globular clusters.
    Monthly Notices of the Royal Astronomical Society 11/2011; 420(3). DOI:10.1111/j.1365-2966.2011.20226.x · 5.23 Impact Factor

Preview

Download
0 Downloads
Available from