Discovering Dust-Obscured Active Galaxies as They Grow

How did galaxies form and evolve during the 13.8-billion-year history of the universe? This question has been the subject of intense observational and theoretical investigation. Recent studies have revealed that almost all massive galaxies harbor a supermassive black hole whose mass reaches up to a hundred thousand or even a billion times the mass of the sun, and their masses are tightly correlated with those of their host galaxies. This correlation suggests that supermassive black holes and their host galaxies have evolved together, closely interacting as they grow.

The number density of DOGs (Dust Obscured Galaxies) as a function of infrared luminosity. Data represented by the red star is the HSC result (Hyper Suprime-Cam, or HSC, is a new wide-field camera mounted at the prime focus of the Subaru Telescope). The research team found that (i) their infrared luminosity exceeds 10 trillion suns, and (ii) their number density is about 300 per cubic gigaparsecs (1 gigaparsec is about 3×10²⁵ meter)

Young main-sequence (MS) stars and red-giant branch (RGB) stars around M81, M82, and NGC 3077

Astronomers using the Subaru Telescope’s Hyper Suprime-Cam prime-focus camera recently observed the nearby large spiral galaxy M81, together with its two brightest neighbors, M82 and NGC3077. The results of their observations are deep, super wide-field images of the galaxies and their populations of young stars. As part of a Galactic Archaeology study, the team discovered that the spatial distribution of the young stars around these galaxies follows very closely that of their distribution of neutral hydrogen.

Young main-sequence (MS) stars and red-giant branch (RGB) stars around M81, M82, and NGC 3077. Left: yellow is brighter stars, and blue is fainter stars. Right: color-coded for the metallicity, namely yellow is metal rich, blue is metal poor. Solid line shows the R25 radius of the galaxy measured in the visible light. Source: NAOJ

Unexpectedly Little Black-hole Monsters Rapidly Suck up Surrounding Matter

Schematic view of ULXs (looking from upper side) and SS 433 (looking from left side). Strong X-rays are emitted from the inner region of the supercritical accretion disk. Powerful winds are launched from the disk, which eventually emit spectral lines of helium ions and hydrogen atoms. Source: Subaru Telescope

Shallow ionized calcium absorption lines suggest that superflare stars have large starspots

 

Source: Subaru Telescope

Based on solar observations, astronomers know that if there are large dark star spots on a stellar surface, the “core depth” (the depth and width of a spectral line) of the Ca II 854.2 [nm] (ionized Calcium) absorption line becomes shallow. Using this, they investigated the core depth of Ca II 854.2 [nm] line, and found that superflare stars have large starspots compared to sunspots.

In the above figure, the bottom two images on the left show the Sun in visible light (left) and the Ca II line (right). The upper two images are imaginary drawings of a superflare star in visible light (left) and the Ca II line (right) where the areas around the starspots are bright. 

Superflare stars (the upper spectrum, shown in red) have a shallow (bright) core depth of the absorption line of Ca II 854.2 [nm] (ionized calcium) compared to the Sun (the bottom spectrum, in black). This suggests that superflare stars have large starspots.

Spectroscopic observations allow observers to estimate the rotation velocity of superflare stars

 

Source: Subaru Telescope

Top: Four neutral iron (Fe I) absorption lines are shown. Spectral lines show some broadening because of the Doppler effect on the light from the rotating stellar surface. Slowly rotating stars like the Sun have a narrow line profile, while rapidly rotating stars have a wide line profile. Measuring these broadenings allows an estimate of the stellar rotation velocity. 

Bottom: The wavelength of light from the surface of a rotating star shifts because of the Doppler effect. For example, the wavelength of light from point A becomes a bit short (is blue-shifted) since this point is approaching us (the observer). By contrast, the wavelength of light from point C is a bit long (is red-shifted) since this point moves away from us. The wavelengths of light from point B have no shifts since this point moves perpendicular to the line of sight. Finally, this line-shift effect results in broadening of spectral lines.

Subaru Telescope observes superflare stars with large starspots

Source: Subaru Telescope

A team of astronomers has used the High Dispersion Spectrograph on the Subaru Telescope to conduct spectroscopic observations of Sun-like “superflare” stars first observed and cataloged by the Kepler Space Telescope. The investigations focused on the detailed properties of these stars and confirmed that Sun-like stars with large starspots can experience super flares.

The team targeted a set of solar-type stars emitting very large flares that release total energies 10-10000 times greater than the biggest solar flares, and carried out spectroscopic observations on 50 solar-type superflare stars selected from the Kepler Space Telescope’s data. From the investigation of the detailed properties of spectral lines, the team obtained the following results:

1. More than half the observed 50 stars show no evidence of binarity (that is, they are not binary stars). The team confirmed the characteristics of the target stars as similar to those of the Sun.
2. On the basis of the Kepler data, superflare stars show somewhat regular, periodic changes in their brightnesses. The typical periods range from one day to a few tens of days. Such variations are explained by the rotation of the star and its starspots. As shown in figure, the stars seem to become dimmer when their starspots are on their visible sides. Moreover, the timescales of the brightness variations should correspond to the stars’ rotation speeds.

The artificial image of a superflare star seen with visible light shows a large superflare (shown in white) occurring in the large starspot area.