THE DENSITY OF LYMAN ALPHA EMITTERS AT VERY HIGH REDSHIFT

Esther M. Hu, Lennox L. Cowie, & Richard G. McMahon
1998, Submitted to ApJ Letters

Spectra of High-z Emitters

FIGURE: Spectra of Lyman alpha emitting galaxies at redshifts 5.64, 4.19, 4.02, and 3.05 with the position of the redshifted 1216 Å Hydrogen Lyman alpha line marked, over a wavelength region from approximately 4800 Å to 9700 Å.

High redshift galaxies showing strong Hydrogen Lyman alpha emission may be extremely faint, or undetected, in deep optical or IR continuum images - even in deep exposures made with the 10 m Keck Telescope or with the Hubble Space Telescope. These objects may provide us with a view of galaxies in the earliest stages of the formation process, when there is massive star formation that can excite the Lyman alpha emission line, but which has not yet produced substantial amounts of dust that can suppress this line. Important consequences for our understanding of the early universe are:

Lyman alpha emitters may provide our best means for identifying galaxies at redshifts above 5 for optical and infrared studies. And this type of galaxy may be an increasing fraction of the galaxies at higher redshifts.
Our picture of the global star formation history of the Universe, particularly at redshifts above 3, will need to be revised. The Lyman alpha emitting galaxies are currently missing from our census, but are significant contributors to the net star formation at high redshift.

In order to pick out such objects we must narrow our observed wavelength interval to provide the best contrast between the light in the line and the background sky. This may be done in two ways, either with a narrow wavelength band filter or with a spectroscopic image such as those shown above. The objects detected here were found in a survey with the Keck II Telescope that uses both these techniques to find these line emitting galaxies, in order to learn about their number density, physical properties, star formation rates, and their relationship to brighter high-redshift galaxies. This is the first time narrowband filters have ever been used on a 10 m telescope.

The emission line searches can pick out objects which are nearly invisible in the continuum bands as can be seen when we look at the z=4.19 emitter above in a deep HST image

The 10 m Keck II telescope Narrow Band Searches for high z Lyman alpha emitting galaxies are described in more detail in this companion paper. Most of the objects found by this method are star forming galaxies. A few of the emission line objects are AGN.

See our selected bibliography for additional papers on emission-line searches and deep galaxy surveys. A separate set of catalogs for the Hubble Deep Field is also available.

Spectral Searches for these extremely faint emission lines rely on being able to accurately subtract the night sky background to extremely high precision as is described here.

GIF images of Plates 1 and 2 from the paper

Deep Keck narrowband image showing z = 4.52 emission-line galaxies (396 Kb gif; 950 × 690)
Composite spectra of emission-line galaxies at high- and low-redshift (365 Kb gif; 1279 × 750)

The following postscript files are available:

Plate 1 with caption (1.1 Mb) and in gzip compressed form (399 Kb)
Plate 2 with caption (5.7 Mb) and in gzip compressed form (567 Kb)
Complete gzipped paper (1.05 Mb) A version of the paper with lower resolution figures is available as preprint astro-ph/9803011 from the Los Alamos preprint server (458 Kb)

Esther M. Hu

(hu@ifa.hawaii.edu)
The URL for this document is http://www.ifa.hawaii.edu/faculty/hu/emitters.html
Last Revision: March 5th 1998