Hydrogen Emission and Absorption in the Halos of the Milky Way and Nearby Galaxies

Using a sample of nearly half million galaxies, intersected by over 7 million lines of sight from the Sloan Digital Sky Survey Data Release 12, we trace H-alpha + [NII] emission from a galactocentric projected radius, r_p, of 5 kpc to more than 100 kpc. We obtain consistent results using only the H-alpha or [NII] flux. We measure a stronger signal for the bluer half of the target sample than for the redder half on small scales, r_p < 20 kpc. We obtain a 3 sigma detection of H-alpha+[NII] emission in the 50 to 100 kpc r_p bin, and the detection is 34 times fainter than a previous strict limit obtained using deep narrow-band imaging. The faintness of the signal demonstrates why it has been so difficult to trace recombination radiation out to large radii around galaxies. This signal, combined with published estimates of number density, lead us to estimate the temperature of the gas to be 12,000 K.   Using nearly one million lines of sight from the SDSS DR12, we present the first evidence for a widely distributed, neutral, excited hydrogen component of the Milky Way Galaxy's halo. It is observed as the slight, (0.779 +- 0.006)%, absorption of flux near the rest wavelength of H-alpha in the combined spectra of millions of galaxy spectra. The velocities of this gas reach the Galactic escape speed, but the gas is on average neither rotating about the Galaxy nor rapidly falling inward or expanding outward from the Galaxy.

Huanian Zhang
1st meeting room, KIAA
Fri, 2017-06-30 12:00 to 13:00