299 research outputs found

    Constraining the Galactic Bar Parameters with Red Clump Giants

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    We show that the small intrinsic spread in luminosities of red clump giants can be used to constrain the differences in the streaming motions of Galatic bulge stars on the near side and those on the far side. We propose two methods to select two samples with one preferentially on the near side and the other on the far side. In the first method, we divide red clump giants into a bright sample and a faint one; stars in the bright sample will be on average more on the near side and vice versa. The second method relies on the fact that lensed bulge stars lie preferentially on the far side due to the enhanced lensing probability by the stars on the near side and in the disk. If the radial streaming motion is ~50km/s, we find the difference in the average radial velocity between the bright and faint samples can reach ~33km/s while the corresponding difference is about ~10km/s between the lensed stars and all observed stars. The difference in the average proper motion between the bright and faint samples is about ~1.6mas/yr if there is a tangential streaming motion of 100km/s; the corresponding shift between the lensed stars and all observed stars is ~1mas/yr. To observe the shifts in the radial velocity and proper motion, roughly one hundred microlensing events, and/or bright/faint red clump giants, need to be observed either spectroscopically or astrometrically. The spectroscopic observations can be performed efficiently using multi-object spectrographs already available. The proper motion signature of microlensed objects can be studied using ground-based telescopes and HST. These observations will provide strong constraints on the Galactic bar parameters (abridged).Comment: MNRAS, 13 pages, very minor revisio

    Gravitational lensing effects on sub-millimetre galaxy counts

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    We study the effects on the number counts of sub-millimetre galaxies due to gravitational lensing. We explore the effects on the magnification cross section due to halo density profiles, ellipticity and cosmological parameter (the power-spectrum normalisation σ8\sigma_8). We show that the ellipticity does not strongly affect the magnification cross section in gravitational lensing while the halo radial profiles do. Since the baryonic cooling effect is stronger in galaxies than clusters, galactic haloes are more concentrated. In light of this, a new scenario of two halo population model is explored where galaxies are modeled as a singular isothermal sphere profile and clusters as a Navarro, Frenk and White (NFW) profile. We find the transition mass between the two has modest effects on the lensing probability. The cosmological parameter σ8\sigma_8 alters the abundance of haloes and therefore affects our results. Compared with other methods, our model is simpler and more realistic. The conclusions of previous works is confirm that gravitational lensing is a natural explanation for the number count excess at the bright end.Comment: 10 pages, 10 figures, accepted by MNRA