Abstract
We study the projected spatial offset between the ultraviolet continuum and
Ly$\alpha$ emission for a sample of 65 lensed and unlensed galaxies in the
Epoch of Reionization (5<z<7), the first such study at these redshifts, in
order to understand the potential for these offsets to confuse estimates of the
Ly$\alpha$ properties of galaxies observed in slit spectroscopy. While we find
that ~40% of galaxies in our sample show significant projected spatial offsets
($|\Delta_Ly\alpha-UV|$), we find a modest average offset of 0.61$\pm$0.08
kpc. A small fraction of our sample, ~10%, exhibits offsets of 2-4 kpc, sizes
that are larger than the effective radii of typical galaxies at these
redshifts. An internal comparison and a comparison to studies at lower redshift
yielded no significant evidence of evolution of $|\Delta_Ly\alpha-UV|$ with
redshift. In our own sample, UV-bright galaxies showed offsets a factor of
three greater than their fainter counterparts, 0.89$\pm$0.18 vs. 0.27$\pm$0.05
kpc, respectively. We argue that offsets are likely not the result of merging
processes, but are rather due to anisotropic internal processes internal to the
galaxies as a result of stellar feedback, which facilitates Ly$\alpha$
fluorescence and/or backscattering from nearby or outflowing gas. The reduction
in the Ly$\alpha$ flux due to offset effects for various observational setups
was quantified through mock observations of simple simulations. It was found
that the loss of Ly$\alpha$ photons for galaxies with average offsets is not,
if corrected for, a limiting factor for all but the narrowest slit widths
(<0.4''). However, for the largest offsets, if such offsets are mostly
perpendicular to the slit major axis, slit losses were found to be extremely
severe in cases where slit widths of $łe$1'' were employed, such as those
planned for James Webb Space Telescope/NIRSpec observations. (abridged)
Description
The Size and Pervasiveness of Ly$\alpha$-UV Spatial Offsets in Star-Forming Galaxies at $z\sim6$
Links and resources
Tags