3,606 research outputs found
Dynamical virial masses of Lyman-break galaxy haloes at z=3
We improve on our earlier dynamical estimate of the virial masses of the
haloes of Lyman-break galaxies (LBGs) at redshift z=3 by accounting for the
effects of seeing, slit width, and observational uncertainties. From an
analysis of the small number of available rotation curves for LBGs we determine
a relation Vc7=(1.9+/-0.2)sigma between circular velocity at a radius of 7kpc,
and central line velocity width. We use this relation to transform the measured
velocity widths of 32 LBGs to the distribution of circular velocities, for the
population of LBGs brighter than R=25.5. We compare this distribution against
the predicted distribution for the 'massive-halo' model in which LBGs pinpoint
all of the highest mass dark matter haloes at that epoch. The observed LBG
circular velocities are smaller than the predicted circular velocities by a
factor >1.4+/-0.15. This is a lower limit as we have ignored any increase of
circular velocity caused by baryonic dissipation. The massive-halo model
predicts a median halo virial mass of 10^12.3 Msol, and a small spread of
circular velocities. Our median estimated dynamical mass is <10^(11.6+/-0.3)
Msol, which is significantly smaller; furthermore, the spread of our circular
velocities is much larger than the massive-halo prediction. These results are
consistent with a picture which leaves some of the most-massive haloes
available for occupation by other populations which do not meet the LBG
selection criteria. The median halo mass recently estimated by Adelberger et
al. from the measured clustering of LBGs is 10^(11.86+/-0.3) Msol. Our
dynamical analysis appears to favour lower masses and to be more in line with
the median mass predicted by the collisional starburst model of Somerville et
al., of 10^11.3 Msol. [abridged]Comment: 6 pages, 5 figures, MNRAS Letters, Accepte
HST Images of a Galaxy Group at z=2.81, and the Sizes of Damped Lyman Alpha Galaxies
We present HST WFPC2 observations in three bands (F450W, F467M and F814W) of
a group of three galaxies at z=2.8 discovered in a ground-based narrow-band
search for Lyman alpha emission near the z=2.8 quasar PKS0528-250. One of the
galaxies is a damped (DLA) absorber and these observations bear on the relation
between the DLA clouds and the Lyman-break galaxies and the stage in the
evolution of galaxies they represent. We describe a procedure for combining the
undersampled WFPC2 images pointed on a sub-pixel grid, which largely recovers
the full sampling of the WFPC2 point spread function (psf). These three
galaxies have similar properties to the Lyman-break galaxies except that they
have strong Lyman alpha emission. The three galaxies are detected in all three
bands, with average B~26, I~25. Two of the galaxies are compact with intrinsic
(i.e. after correcting for the effect of the psf) half-light radii of ~0.1
arcsec (~0.4/h kpc, q_o=0.5). The third galaxy comprises two similarly compact
components separated by 0.3 arcsec. The HST images and a new ground-based Lyman
alpha image of the field provide evidence that the three galaxies are more
extended in the light of Lyman alpha than in the continuum. The measured impact
parameters for this DLA galaxy (1.17 arcsec), for a second confirmed system,
and for several candidates, provide a preliminary estimate of the
cross-section-weighted mean radius of the DLA gas clouds at z~3 of less than
13/h kpc, for q_o=0.5. Given the observed sky covering factor of the absorbers
this implies that for q_o=0.5 the space density of DLA clouds at these
redshifts is more than five times the space density of spiral galaxies locally,
with the actual ratio probably considerably greater. For q_o=0.0 there is no
evidence as yet that DLA clouds are more common than spiral galaxies locally.Comment: 11 pages, LaTeX, 6 Figures total (4 colour GIF-format, 2 PostScript),
accepted for publication in MNRA
Cloud information for FIRE from surface weather reports
Surface weather observations of clouds were analyzed to obtain a global cloud climatology (Warren et al, 1986; 1988). The form of the synoptic weather code limits the types of cloud information which are available from these reports. Comparison of surface weather reports with instrumental observations during the FIRE field experiments can help to clarify the operational definitions which were made in the climatology because of the nature of the synoptic code. The long-term climatology from surface weather observations is also useful background for planning the location and timing of intensive field experiments
The lens and source of the optical Einstein ring gravitational lens ER 0047-2808
(Abridged) We perform a detailed analysis of the optical gravitational lens
ER 0047-2808 imaged with WFPC2 on the Hubble Space Telescope. Using software
specifically designed for the analysis of resolved gravitational lens systems,
we focus on how the image alone can constrain the mass distribution in the lens
galaxy. We find the data are of sufficient quality to strongly constrain the
lens model with no a priori assumptions about the source. Using a variety of
mass models, we find statistically acceptable results for elliptical
isothermal-like models with an Einstein radius of 1.17''. An elliptical
power-law model (Sigma \propto R^-beta) for the surface mass density favours a
slope slightly steeper than isothermal with beta = 1.08 +/- 0.03. Other models
including a constant M/L, pure NFW halo and (surprisingly) an isothermal sphere
with external shear are ruled out by the data. We find the galaxy light profile
can only be fit with a Sersic plus point source model. The resulting total
M/L_B contained within the images is 4.7 h_65 +/-0.3. In addition, we find the
luminous matter is aligned with the total mass distribution within a few
degrees. The source, reconstructed by the software, is revealed to have two
bright regions, with an unresolved component inside the caustic and a resolved
component straddling a fold caustic. The angular size of the entire source is
approx. 0.1'' and its (unlensed) Lyman-alpha flux is 3 x 10^-17 erg/s/cm^2.Comment: 13 pages, 5 figures. Revised version accepted for publication in
MNRA
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