The nature of dark matter remains mysterious, with luminous material accounting for at most ∼25 per cent of the baryons in the Universe. We accordingly undertook a survey looking for the microlensing ...of stars in the Large Magellanic Cloud (LMC) to determine the fraction of Galactic dark matter contained in massive compact halo objects (MACHOs). The presence of the dark matter would be revealed by gravitational lensing of the light from an LMC star as the foreground dark matter moves across the line of sight. The duration of the lensing event is the key observable parameter, but gives non-unique solutions when attempting to estimate the mass, distance and transverse velocity of the lens. The survey results to date indicate that between 8 and 50 per cent of the baryonic mass of the Galactic halo is in the form of MACHOs (ref. 3), but removing the degeneracy by identifying a lensing object would tighten the constraints on the mass in MACHOs. Here we report a direct image of a microlens, revealing it to be a nearby low-mass star in the disk of the Milky Way. This is consistent with the expected frequency of nearby stars acting as lenses, and demonstrates a direct determination of a lens mass from a microlensing event. Complete solutions such as this for halo microlensing events will probe directly the nature of the MACHOs.
Overcoming the Barrier Geha, Rabih M; Dhaliwal, Gurpreet; Winston, Lisa G ...
The New England journal of medicine,
12/2018, Letnik:
379, Številka:
23
Journal Article
We present evidence for a counterrotating core in the low-luminosity (MB = -18.2) elliptical galaxy NGC 770 based on internal stellar kinematic data. This counterrotating core is unusual, as NGC 770 ...is not the primary galaxy in the region and lies in an environment with evidence of ongoing tidal interactions. We discovered the counterrotating core via single-slit Keck ESI echelle spectroscopy; subsequent integral field spectroscopy was obtained with the Gemini Multiobject Spectrograph. The counterrotating region has a peak rotation velocity of 21 km s-1 as compared to the main galaxy's rotation speed of greater than 45 km s-1 in the opposite direction. The counterrotating region extends to a radius of ~4'' (0.6 kpc), slightly smaller than the half-light radius of the galaxy, which is 53 (0.8 kpc). The photometry and two-dimensional kinematics suggest that the counterrotating component is confined to a disk whose scale height is less than 08 (0.1 kpc). The major axis of counterrotation is misaligned with that of the outer galaxy isophotes by 15°. We compute an age and metallicity of the inner counterrotating region of 3 ± 0.5 Gyr and Fe/H = 0.2 ± 0.2 dex based on Lick absorption-line indices. The lack of other large galaxies in this region limits possible scenarios for the formation of the counterrotating core. We discuss several scenarios and favor one in which NGC 770 accreted a small gas-rich dwarf galaxy during a very minor merging event. If this scenario is correct, it represents one of the few known examples of merging between two dwarf-sized galaxies.
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