Research column

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2017

Title: Mass Discrepancy-Acceleration Relation in Einstein Rings
McGaugh et al. (2016) found that the total acceleration (dynamical acceleration) tightly related to their baryonic acceleration in 153 spiral galaxies. It is consistent with the Mass Discrepancy-Acceleration Relation (MDAR) (McGaugh 2004). The mass ratio between observational mass (M_O) and baryonic mass (M_B) increases as the baryonic acceleration (g_B) decreases. It provides a constant a_0 (10^-10 m/s^2) in dark matter problem. Dark matter dominates the dynamics only if the acceleration of the system is smaller than a_0. Tian & Ko (2017) tested 57 Einstein rings from SLACS elliptical lenses database. The Einstein ring is the gravitational lensing image when the source is right behind the lensing object. Our results showed that the MDAR is also valid in elliptical galaxies with the relativistic effect.

A typical QSO spectrum usually shows blue color. Recently, a group of QSOs is discovered with red color. These QSOs are considered to be reddened by dust obscuration. Most previous studies on red QSOs rely on photometric selection, which implies that photometric data from the same filter representing different waveband properties at different redshifts. In our study, we developed a new method for classifying red QSOs with a statistical definition based on "relative spectral flux" at rest frame. The advantage is that we could find "slightly-red QSOs", which can not be found through photometric selection. We investigate the properties of our red QSO samples using SDSS, VLA, and WISE data at z=0.3-1.2. We found that our red QSO samples at lower z and at higher z show different properties. There might be at least two types of red QSOs in our red samples. One type might be associated with dust obscuration, but the other type might not be.

Distant luminous quasars provide important information on the growth of the first supermassive black holes, their host galaxies and the epoch of reionization. The identification of quasars is usually performed through detection of their Lyman-α line redshifted to 0.9 microns at z > 6.5. Here, we report the discovery of a very Lyman-α luminous quasar, PSO J006.1240 + 39.2219 at redshift z = 6.618, selected based on its red colour and multi-epoch detection of the Lyman-α emission in a single near-infrared band.

We studied the period-color and amplitude color relations for RR Lyrae at maximum and minimum light. Theory predicts that at minimum light, period-color relation for RR Lyrae will be flat, and this was shown by the Magellanic Clouds RR Lyrae in (V-R) and (V-I) colors. In this work we investigated the colors based on SDSS filters, and found that the period-color relations at maximum and minimum light are much more complicated for SDSS-based colors. This will help us understand more about the atmospheric structure of RR Lyrae and their applications in astrophysics.