14,024 research outputs found
A comparison of UV surface brightness and HI surface densities for spiral galaxies
Shaya and Federman (1987) suggested that the ambient ultraviolet flux at 1000 A permeating a spiral galaxy controls the neutral hydrogen (HI) surface density in the galaxy. They found that the atomic envelopes surrounding small molecular clouds, because of their great number, provide the major contribution to the HI surface density over the stellar disk. The increase in HI surface density with later Hubble types was ascribed to the stronger UV fields from more high-mass stars in later Hubble types. These hypotheses are based on the observations of nearby diffuse interstellar clouds, which show a sharp atomic-to-molecular transition (Savage et al. 1977), and on the theoretical framework introduced by Federman, Glassgold, and Kwan (1979). Atomic envelopes around interstellar clouds in the solar neighborhood arise when a steady state is reached between photodissociation of H2 and the formation of H2 on grains. The photodissociation process involves photons with wavelengths between 912 A and 1108 A. Shaya and Federman used H-alpha flux as an approximate measure for the far UV flux and made their comparisons based on averages over Hubble type. Here, researchers compare, on an individual basis, UV data obtained with space-borne and balloon-borne instruments for galaxies with measurements of HI surface density (Warmels 1988a, b). The comparisons substantiate the conclusion of Shaya and Federman that the far UV field controls the HI content of spiral galaxies
The Spectroscopic Properties of Ly{\alpha}-Emitters at z 2.7: Escaping Gas and Photons from Faint Galaxies
We present a spectroscopic survey of 318 faint , ,
Ly{\alpha}-emission-selected galaxies (LAEs) at 2.5<z<3. A sample of 32 LAEs
with rest-frame optical spectra from Keck/MOSFIRE are used to interpret the LAE
spectra in the context of their systemic redshifts. We find that the Ly{\alpha}
emission of LAEs is typically less spectrally extended than among samples of
more luminous continuum-selected galaxies (LBGs) at similar redshifts. Using
the MOSFIRE subsample, we find that the peak of the Ly{\alpha} line is shifted
by +200 km/s with respect to systemic across a diverse set of galaxies
including both LAEs and LBGs. We also find a small number of objects with
significantly blueshifted Ly{\alpha} emission, a potential indicator of
accreting gas. The Ly{\alpha}-to-H{\alpha} line ratios suggest that the LAEs
have Ly{\alpha} escape fractions %,
significantly higher than typical LBG samples. Using redshifts calibrated by
our MOSFIRE sample, we construct composite LAE spectra, finding the first
evidence for metal-enriched outflows in such intrinsically-faint high-redshift
galaxies. These outflows have smaller continuum covering fractions and velocities km/s, km/s than those associated with typical LBGs, suggesting that
gas covering fraction is a likely driver of the high Ly{\alpha} and
Ly-continuum escape fractions of LAEs. Our results suggest a similar scaling of
outflow velocity with star formation rate as is observed at lower redshifts
and indicate that a substantial
fraction of gas is ejected with
- …