POINT—AGAPE is an Anglo-French collaboration which is employing the Isaac Newton Telescope (INT) to conduct a pixel-lensing survey towards M31. Pixel lensing is a technique which permits the ...detection of microlensing against unresolved stellar fields. The survey aims to constrain the stellar population in M31, and also the distribution and nature of massive compact halo objects (MACHOs) in both M31 and the Galaxy. 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 that affect the observables, such as non-uniform sampling and signal-to-noise ratio degradation owing to changing observing conditions. For a maximum MACHO halo we predict an event rate in V of up to 100 per observing season for M31 and 40 per season for the Galaxy. However, the Einstein radius crossing time is measurable for less than 10 per cent of the events, and the observed full-width at half-maximum duration provides only a weak tracer of lens mass. None the less, 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 M⊙, 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 of 4 uncertainty in halo fraction and an order of magnitude uncertainty in mass (90 per cent confidence). Interesting constraints on Galaxy MACHOs may also be possible. For a campaign lasting 10 years, comparable to the lifetime of current LMC surveys, reliable estimates of MACHO parameters in both galaxies should be possible.
New constraints on a triaxial model of the Galaxy Sevenster, Maartje; Saha, Prasenjit; Valls-Gabaud, David ...
Monthly notices of the Royal Astronomical Society,
08/1999, Volume:
307, Issue:
3
Journal Article
Peer reviewed
Open access
We determine the most likely values of the free parameters of an N-body model for the Galaxy developed by Fux via a discrete-discrete comparison with the positions on the sky and line-of-sight ...velocities of an unbiased, homogeneous sample of OH/IR stars. Via Monte Carlo simulation, we find the plausibility of the best-fitting models, as well as the errors on the determined values. The parameters that are constrained best by these projected data are the total mass of the model and the viewing angle of the central bar, although the distribution of the latter has multiple maxima. The other two free parameters, the size of the bar and the (azimuthal) velocity of the Sun, are less well-constrained. The best model has a viewing angle of ∼ 44°, a semimajor axis of 2.5 kpc (corotation radius 4.5 kpc, pattern speed 46 km s−1 kpc−1), a bar mass of 1.7×1010 M⊙ and a tangential velocity of the local standard of rest of 171 km s−1. We argue that the lower values that are commonly found from stellar data for the viewing angle (∼25°) arise when too few coordinates are available, when the longitude range is too narrow or when low latitudes are excluded from the fit. The new constraints on the viewing angle of the Galactic bar from stellar line-of-sight velocities decrease further the ability of the distribution of the bar to account for the observed microlensing optical depth toward Baade's window: our model reproduces only half the observed value. The signal of triaxiality diminishes quickly with increasing latitude, fading within approximately 1 scaleheight (≲3°). This suggests that Baade's window is not a very appropriate region in which to sample bar properties.
We have carried out a survey of the Andromeda galaxy for unresolved microlensing (pixel lensing). We present a subset of four short timescale, high signal-to-noise microlensing candidates found by ...imposing severe selection criteria: the source flux variation exceeds the flux of an $R=21$ magnitude star and the full width at half maximum timescale is less than 25 days. Remarkably, in three out of four cases, we have been able to measure or strongly constrain the Einstein crossing time of the event. One event, which lies projected on the M 31 bulge, is almost certainly due to a stellar lens in the bulge of M 31. The other three candidates can be explained either by stars in M 31 and M 32 or by MACHOs.
The nature of galaxy structures on large scales is a key observational prediction for current models of galaxy formation. The Sloan Digital Sky Survey (SDSS) and the 2dF galaxy survey have revealed a ...number of structures on 40-150 h -1 Mpc scales at low redshifts, and some even larger ones. To constrain galaxy number densities, luminosities, and stellar populations in large structures at higher redshift, we have investigated two sheet-like structures of galaxies at z = 0.8 and 1.3 spanning 150 h -1 comoving Mpc embedded in large quasar groups (LQGs) extending over at least 200 h -1 Mpc. We present first results of an analysis of these sheet-like structures using two contiguous 1 deg Galaxy Evolution Explorer (GALEX) fields (FUV and NUV) cross-correlated with optical data from the SDSS. We derive a sample of 462 Lyman break galaxy (LBG) candidates coincident with the sheets. Using the GALEX and SDSS data, we show that the overall average spectral energy distribution of a LBG galaxy at z ~ 1 is flat (in f Delta *l) in the rest-frame wavelength range from 1500 A to 4000 A, implying evolved populations of stars in the LBGs. From the luminosity functions we get indications for overdensities in the two LQGs compared to their foreground regions. Similar conclusions come from the calculation of the 2-point correlation function, showing a 2 Delta *s overdensity for the LBGs in the z ~ 0.8 LQG on scales of 1.6 to 4.8 Mpc, indicating similar correlation scales for our LBG sample as their z ~ 3 counterparts.
Discovery of a high-redshift Einstein ring Cabanac, R. A.; Valls-Gabaud, D.; Jaunsen, A. O. ...
Astronomy and astrophysics (Berlin),
06/2005, Volume:
436, Issue:
2
Journal Article
Peer reviewed
Open access
We report the discovery of a partial Einstein ring of radius 1$\farcs$48 produced by a massive (and seemingly isolated) elliptical galaxy. The spectroscopic follow-up at the VLT reveals a ...2$L_\star$ galaxy at $z=0.986$, which is lensing a post-starburst galaxy at $z=3.773$. This unique configuration yields a very precise measure of the mass of the lens within the Einstein radius, $(8.3 \pm 0.4) \times 10^{11}$$h_{70}^{-1}$$M_{\odot}$ . The fundamental plane relation indicates an evolution rate of ${\rm d}$$\log (M/L)_{B} / {\rm d}z = -0.57\pm0.04$, similar to other massive ellipticals at this redshift. The source galaxy shows strong interstellar absorption lines indicative of large gas-phase metallicities, with fading stellar populations after a burst. Higher resolution spectra and imaging will allow the detailed study of an unbiased representative of the galaxy population when the universe was just 12% of its current age.
The light curve of PA-99-N2, one of the recently announced microlensing candidates toward M31, shows small deviations from the standard Paczynski form. We explore a number of possible explanations, ...including correlations with the seeing, the parallax effect, and a binary lens. We find that the observations are consistent with an unresolved red giant branch or asymptotic giant branch star in M31 being microlensed by a binary lens. We find that the best-fit binary lens mass ratio is approx1.2 x 10 super(-2), which is one of the most extreme values found for a binary lens so far. If both the source and lens lie in the M31 disk, then the standard M31 model predicts the probable mass range of the system to be 0.02-3.6 M sub(o) (95% confidence limit). In this scenario, the mass of the secondary component is therefore likely to be below the hydrogen-burning limit. On the other hand, if a compact halo object in M31 is lensing a disk or spheroid source, then the total lens mass is likely to lie between 0.09 and 32 M sub(o), which is consistent with the primary being a stellar remnant and the secondary being a low-mass star or brown dwarf. The optical depth (or, alternatively, the differential rate) along the line of sight toward the event indicates that a halo lens is more likely than a stellar lens, provided that dark compact objects comprise no less than 15% (or 5%) of halos.
The NAROO digitization center Robert, V.; Desmars, J.; Lainey, V. ...
Astronomy and astrophysics (Berlin),
08/2021, Volume:
652
Journal Article
Peer reviewed
Open access
The New Astrometric Reduction of Old Observations (NAROO) center can be found at the Paris Observatory in Meudon, and it is dedicated to the measurement of astrophotographic plates and the analysis ...of old observations. The NAROO digitizer consists of a granite-based Newport-Microcontrol open-frame air-bearing
XY
positioning table, a scientific sCMOS camera, and a telecentric optical system. The plate holder assembly is suited for mounting glass plates up to 350 mm squared. The machine positioning stability is better than 15 nm, and its repeatability is better than 40 nm. With real photographic plate data, we were able to produce measurements with an accuracy better than 65 nm. The renewed interest about photographic plates concerns the expansion of the database of transient objects evolving in time, since digitization now makes it possible to measure images with a high level of accuracy and to identify all the available objects. The information extracted from such materials can be of an astrometric, photometric, and spectroscopic nature, when not purely imaging, with consequences in planetology, near-Earth asteroid risk assessment, astrophysical phenomena, and general relativity, to mention but a few. Through our scientific program in the
Gaia
era, we detail examples of current and upcoming uses for the community. We invite researchers to use our facilities and digitize their collection by answering our call for proposals.
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