(Proposal ID) S16B-116

(PI) Morokuma, Tomoki

(Proposal Title)
A Search for 10^4M_sun Active Black Holes

(Abstract)
We propose 2-night optical spectroscopic observations with FOCAS to 
(i) identify low-mass (10^{4-6}M_sun) active black holes (BHs) and 
(ii) measure the BH masses in a nearby galaxy cluster. The candidates were 
selected via rapid optical variability by utilizing our high-cadence imaging 
data taken with Hyper Suprime-Cam in 2014. This method was proven to be 
effective by our previous observations for selecting low-mass active BHs 
with a small number of contaminations (Morokuma et al., 2016, PASJ, in press). 
The target field is a nearby galaxy cluster, Perseus cluster (Abell 426), at 
a distance of ~75 Mpc, which enhances the expected number of such low-mass 
active BHs down to 10^{4}M_sun by a factor of 10^{5} compared to blank field 
survey. The expected number of low-mass active BHs showing rapid variability 
is about 10 with a similar number of objects being background low-mass active 
BHs. We take spectra for 10 candidates around expected Halpha wavelength 
regions to determine the redshift (distance), measure the Halpha widths and 
luminosity, measure the BH masses, and aim for identifying 5 cluster (i.e., 
nearby) members with low-mass active BHs among which BH masses of a few 
objects are less than 10^{5}M_sun.