71 research outputs found
Theory of pixel lensing towards M31 I: the density contribution and mass of MACHOs
POINT-AGAPE is an Anglo-French collaboration which is employing the Isaac
Newton Telescope (INT) to conduct a pixel-lensing survey towards M31. In this
paper we investigate what we can learn from pixel-lensing observables about the
MACHO mass and fractional contribution in M31 and the Galaxy for the case of
spherically-symmetric near-isothermal haloes. We employ detailed pixel-lensing
simulations which include many of the factors which affect the observables. For
a maximum MACHO halo we predict an event rate in V of up to 100 per season for
M31 and 40 per season for the Galaxy. However, the Einstein radius crossing
time is generally not measurable and the observed full-width half-maximum
duration provides only a weak tracer of lens mass. Nonetheless, we find that
the near-far asymmetry in the spatial distribution of M31 MACHOs provides
significant information on their mass and density contribution. We present a
likelihood estimator for measuring the fractional contribution and mass of both
M31 and Galaxy MACHOs which permits an unbiased determination to be made of
MACHO parameters, even from data-sets strongly contaminated by variable stars.
If M31 does not have a significant population of MACHOs in the mass range
0.001-1 Solar masses strong limits will result from the first season of INT
observations. Simulations based on currently favoured density and mass values
indicate that, after three seasons, the M31 MACHO parameters should be
constrained to within a factor four uncertainty in halo fraction and an order
of magnitude uncertainty in mass (90% confidence). Interesting constraints on
Galaxy MACHOs may also be possible. For a campaign lasting ten years,
comparable to the lifetime of current LMC surveys, reliable estimates of MACHO
parameters in both galaxies should be possible. (Abridged)Comment: 21 pages, 14 figures. Submitted to MNRA
A companion to a quasar at redshift 4.7
There is a growing consensus that the emergence of quasars at high redshifts
is related to the onset of galaxy formation, suggesting that the detection of
concentrations of gas accompanying such quasars should provide clues about the
early history of galaxies. Quasar companions have been recently identified at
redshifts up to . Here we report observations of Lyman-
emission (a tracer of ionised hydrogen) from the companion to a quasar at
=4.702, corresponding to a time when the Universe was less than ten per cent
of its present age. We argue that most of the emission arises in a gaseous
nebula that has been photoionised by the quasar, but an additional component of
continuum light -perhaps quasar light scattered from dust in the companion
body, or emission from young stars within the nebula- appears necessary to
explain the observations. These observations may be indicative of the first
stages in the assembly of galaxy-sized structures.Comment: 8 pages, 4 figures, plain LaTeX. Accepted for publication in Natur
On emission-line spectra obtained from evolutionary synthesis models I. Dispersion in the ionising flux and Lowest Luminosity Limits
(abriged) Stellar clusters with the same general physical properties (e.g.,
total mass, age, and star-formation mode) may have very different stellar mass
spectra due to the incomplete sampling of the underlying mass function; such
differences are especially relevant in the high-mass tail due to the smaller
absolute number of massive stars. The dispersion in the number of massive stars
also produces a dispersion in the properties of the corresponding ionising
spectra. In this paper, we lay the bases for the future analysis of this effect
by evaluating the dispersion in the ionising fluxes of synthetic spectra. As an
important consequence, we found that the intensities of synthetic fluxes at
different ionisation edges are strongly correlated, a fact suggesting that no
additional dispersion will result from the inclusion of sampling effects in the
analysis of diagnostic diagrams; this is true for HII regions on all scales.
Additionally, we find convincing suggestions that the He II lines are strongly
affected by sampling, and so cannot be used to constrain the evolutionary
status of stellar clusters. We also establish the range of applicability of
synthesis models set by the Lowest Luminosity Limit for the ionising flux, that
is the lowest limit in cluster mass for which synthesis models can be applied
to predict ionising spectra. This limit marks the boundary between the
situations in which the ionising flux is better modeled with a single star as
opposed to a star cluster; this boundary depends on the metallicity and age,
ranging from 10^3 to more than 10^6 Mo. As a consequence, synthesis models
should not be used to try to account for the properties of clusters with
smaller masses.Comment: Replaced with accepted versio
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