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Register a new userA Subaru/Suprime-Cam Survey of M31s spheroid along the South-East minor axis
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We have used Suprime-Cam on the Subaru Telescope to conduct a V- and I-band imaging survey of fields sampling the spheroid of the Andromeda galaxy along its south-east minor axis. Our photometric data are deep enough to resolve stars down to the red clump. Based on a large and reliable sample of red giant stars available from this deep wide-field imager, we have derived metallicity distributions vs. radius and a surface brightness profile over projected distances of R=23-66 kpc from the galaxys center. The metallicity distributions across this region shows a clear high mean metallicity and a broad distribution ([Fe/H] ~ -0.6 +/- 0.5), and indicates no metallicity gradient within our observed range. The surface brightness profile at R>40 kpc is found to be flatter than previously thought. It is conceivable that this part of the halo samples as yet unidentified, metal-rich substructure.
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We present rapidly rising transients discovered by a high-cadence transient survey with Subaru telescope and Hyper Suprime-Cam. We discovered five transients at z=0.384-0.821 showing the rising rate faster than 1 mag per 1 day in the restframe near-ultraviolet wavelengths. The fast rising rate and brightness are the most similar to SN 2010aq and PS1-13arp, for which the ultraviolet emission within a few days after the shock breakout was detected. The lower limit of the event rate of rapidly rising transients is ~9 % of core-collapse supernova rates, assuming a duration of rapid rise to be 1 day. We show that the light curves of the three faint objects agree with the cooling envelope emission from the explosion of red supergiants. The other two luminous objects are, however, brighter and faster than the cooling envelope emission. We interpret these two objects to be the shock breakout from dense wind with the mass loss rate of ~10^{-3} Msun yr^{-1}, as also proposed for PS1-13arp. This mass loss rate is higher than that typically observed for red supergiants. The event rate of these luminous objects is >~1 % of core-collapse supernova rate, and thus, our study implies that more than ~1 % of massive stars can experience an intensive mass loss at a few years before the explosion.
We perform a $z$-band survey for an optical counterpart of a binary neutron star coalescence GW170817 with Subaru/Hyper Suprime-Cam. Our untargeted transient search covers $23.6$ deg$^2$ corresponding to the $56.6%$ credible region of GW170817 and reaches the $50%$ completeness magnitude of $20.6$ mag on average. As a result, we find 60 candidates of extragalactic transients, including J-GEM17btc (a.k.a. SSS17a/DLT17ck). While J-GEM17btc is associated with NGC 4993 that is firmly located inside the 3D skymap of GW170817, the other 59 candidates do not have distance information in the GLADE v2 catalog or NASA/IPAC Extragalactic Database (NED). Among 59 candidates, 58 are located at the center of extended objects in the Pan-STARRS1 catalog, while one candidate has an offset. We present location, $z$-band apparent magnitude, and time variability of the candidates and evaluate the probabilities that they are located inside of the 3D skymap of GW170817. The probability for J-GEM17btc is $64%$ being much higher than those for the other 59 candidates ($9.3times10^{-3}-2.1times10^{-1}%$). Furthermore, the possibility, that at least one of the other 59 candidates is located within the 3D skymap, is only $3.2%$. Therefore, we conclude that J-GEM17btc is the most-likely and distinguished candidate as the optical counterpart of GW170817.
We report the discovery of a new ultra-faint dwarf satellite companion of the Milky Way based on the early survey data from the Hyper Suprime-Cam Subaru Strategic Program. This new satellite, Virgo I, which is located in the constellation of Virgo, has been identified as a statistically significant (5.5 sigma) spatial overdensity of star-like objects with a well-defined main sequence and red giant branch in their color-magnitude diagram. The significance of this overdensity increases to 10.8 sigma when the relevant isochrone filter is adopted for the search. Based on the distribution of the stars around the likely main sequence turn-off at r ~ 24 mag, the distance to Virgo I is estimated as 87 kpc, and its most likely absolute magnitude calculated from a Monte Carlo analysis is M_V = -0.8 +/- 0.9 mag. This stellar system has an extended spatial distribution with a half-light radius of 38 +12/-11 pc, which clearly distinguishes it from a globular cluster with comparable luminosity. Thus, Virgo I is one of the faintest dwarf satellites known and is located beyond the reach of the Sloan Digital Sky Survey. This demonstrates the power of this survey program to identify very faint dwarf satellites. This discovery of VirgoI is based only on about 100 square degrees of data, thus a large number of faint dwarf satellites are likely to exist in the outer halo of the Milky Way.
We present the first results of the Subaru/Hyper Suprime-Cam (HSC) survey of the interacting galaxy system, NGC4631 and NGC4656. From the maps of resolved stellar populations, we identify 11 dwarf galaxies (including already-known dwarfs) in the outer region of NGC4631 and the two tidal stellar streams around NGC4631, named Stream SE and Stream NW, respectively. This paper describes the fundamental properties of these tidal streams. Based on the tip of red giant branch method and the Bayesian statistics, we find that StreamSE (7.10 Mpc in Expected a posteriori, EAP, with the 90% credible intervals of [6.22, 7.29] Mpc) and StreamNW (7.91 Mpc in EAP with the 90% credible intervals of [6.44, 7.97] Mpc) are located in front of and behind NGC4631, respectively. We also calculate the metallicity distribution of stellar streams by comparing the member stars with theoretical isochrones on the color-magnitude diagram. We find that both streams have the same stellar population based on the Bayesian model selection method, suggesting that they originated from a tidal interaction between NGC4631 and a single dwarf satellite. The expected progenitor has a positively skewed metallicity distribution function with [M/H]_EAP=-0.92 with the 90% credible intervals of [-1.46, -0.51]. The stellar mass of the progenitor is estimated as 3.7 x 10e+8 Msun with the 90% credible intervals of [5.8 x 10e+6, 8.6 x 10e+9] Msun based on the mass-metallicity relation for Local group dwarf galaxies. This is in good agreement with an initial stellar mass of the progenitor presumed in the previous N-body simulation.
We present a photometric survey of the stellar halo of the Andromeda galaxy, using Suprime-Cam on the Subaru Telescope. A detailed analysis of VI color-magnitude diagrams of the resolved stellar population is used to measure properties such as line-of-sight distance, surface brightness, metallicity, and age, and these are used to isolate and characterize different components of the M31 halo: (1) several substructures, and (2) the smooth halo. First, we study M31s halo substructure along the north-west/south-east minor axis out to R ~ 100 kpc and the south-west major axis region at R ~ 60 kpc. We confirm two substructures in the south-east halo reported by Ibata et al. (2007) and discover two overdense substructures in the north-west halo. We investigate the properties of these four substructures as well as other structures including the western shelf and find that differences in stellar populations among these systems, thereby suggesting each has a different origin. Our statistical analysis implies that the M31 halo as a whole may contain at least 16 substructures, each with a different origin. Second, we investigate the properties of an underlying, smooth and extended halo component out to R > 100 kpc. We find that the surface density of this smooth halo can be fitted to a Hernquist model of scale radius ~ 17 kpc or a power-law profile with ~ R^{-2.17 +/- 0.15}. In contrast to the relative smoothness of the halo density profile, its metallicity distribution appears to be spatially non-uniform with non-monotonic variations with radius, suggesting that the halo population has not had sufficient time to dynamically homogenize the accreted populations. Further implications for the formation of the M31 halo are discussed.
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