596 research outputs found
Estimating the Correlation Length r_0 from the Number of Galaxy Pairs with Similar Redshifts
We discuss methods that can be used to estimate the spatial correlation
length r_0 of galaxy samples from the observed number of pairs with similar
redshifts. The standard method is unnecessarily noisy and can be compromised by
errors in the assumed selection function. We present three alternatives, one
less noisy, one that responds differently to systematic errors, the third
insensitive to the selection function, and quantify their performance by
applying them to a cosmological N-body simulation and to the Lyman-break survey
of galaxies at redshift z~3. Researchers adopting the standard method could
easily conclude that the Lyman-break galaxy comoving correlation length was r_0
\~ 11 Mpc/h, several times larger than the correct value. The use of our
proposed methods would make this error impossible, except in the small sample
limit. When N_gal<~20, major errors in estimates of r_0 occur alarmingly often.Comment: 19 pages, accepted for publication in the Ap
Photon Mass Bound Destroyed by Vortices
The Particle Data Group gives an upper bound on the photon mass eV from a laboratory experiment and lists, but does not adopt, an
astronomical bound eV, both of which are based on the
plausible assumption of large galactic vector-potential. We argue that the
interpretations of these experiments should be changed, which alters
significantly the bounds on . If arises from a Higgs effect, both limits
are invalid because the Proca vector-potential of the galactic magnetic field
may be neutralized by vortices giving a large-scale magnetic field that is
effectively Maxwellian. In this regime, experiments sensitive to the Proca
potential do not yield a useful bound on . As a by-product, the non-zero
photon mass from Higgs effect predicts generation of a primordial magnetic
field in the early universe. If, on the other hand, the galactic magnetic field
is in the Proca regime, the very existence of the observed large-scale magnetic
field gives kpc, or eV.Comment: 9 pages, discussion of primordial magnetic field adde
X-ray properties of UV-selected star forming galaxies at z~1 in the Hubble Deep Field North
We present an analysis of the X-ray emission from a large sample of
ultraviolet (UV) selected, star forming galaxies with 0.74<z<1.32 in the Hubble
Deep Field North (HDF-N) region. By excluding all sources with significant
detected X-ray emission in the 2 Ms Chandra observation we are able to examine
the properties of galaxies for which the emission in both UV and X-ray is
expected to be predominantly due to star formation. Stacking the X-ray flux
from 216 galaxies in the soft and hard bands produces significant detections.
The derived mean 2-10 keV rest-frame luminosity is 2.97+/-0.26x10^(40) erg/s,
corresponding to an X-ray derived star formation rate (SFR) of 6.0+/-0.6
Msolar/yr. Comparing the X-ray value with the mean UV derived SFR, uncorrected
for attenuation, we find that the average UV attenuation correction factor is
\~3. By binning the galaxy sample according to UV magnitude and colour,
correlations between UV and X-ray emission are also examined. We find a strong
positive correlation between X-ray emission and rest-frame UV emission. A
correlation between the ratio of X-ray-to-UV emission and UV colour is also
seen, such that L(X)/L(UV) increases for redder galaxies. Given that X-ray
emission offers a view of star formation regions that is relatively unaffected
by extinction, results such as these can be used to evaluate the effects of
dust on the UV emission from high-z galaxies. For instance we derive a
relationship for estimating UV attenuation corrections as a function of colour
excess. The observed relation is inconsistent with the Calzetti et al. (2000)
reddening law which over predicts the range in UV attenuation corrections by a
factor of ~100 for the UV selected z~1 galaxies in this sample (abridged).Comment: 10 pages, 7 figures, accepted for publication in MNRA
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