DIFFUSE Lyα EMITTING HALOS: A GENERIC PROPERTY OF HIGH-REDSHIFT STAR-FORMING GALAXIES

Abstract

Using a sample of 92 UV continuum-selected, spectroscopically identified galaxies with z = 2.65, all of which have been imaged in the Lyα line with extremely deep narrow-band imaging, we examine galaxy Lyα emission profiles to very faint surface brightness limits. The galaxy sample is representative of spectroscopic samples of Lyman break galaxies (LBGs) at similar redshifts in terms of apparent magnitude, UV luminosity, inferred extinction, and star formation rate and was assembled without regard to Lyα emission properties. Approximately 45% (55%) of the galaxy spectra have Lyα appearing in net absorption (emission), with 20% satisfying commonly used criteria for the identification of "Lyα emitters" (LAEs; W 0 (Lyα) 20 Å). We use extremely deep stacks of rest-UV continuum and continuum-subtracted Lyα images to show that all sub-samples exhibit diffuse Lyα emission to radii of at least 10 (∼80 physical kpc). The characteristic exponential scale lengths for Lyα line emission exceed that of the λ 0 = 1220 Å UV continuum light by factors of ∼5-10. The surface brightness profiles of Lyα emission are strongly suppressed relative to the UV continuum light in the inner few kpc, by amounts that are tightly correlated with the galaxies' observed spectral morphology; however, all galaxy sub-subsamples, including that of galaxies for which Lyα appears in net absorption in the spectra, exhibit qualitatively similar diffuse Lyα emission halos. Accounting for the extended Lyα emission halos, which generally would not be detected in the slit spectra of individual objects or with typical narrow-band Lyα imaging, increases the total Lyα flux (and rest equivalent width W 0 (Lyα)) by an average factor of ∼5, and by a much larger factor for the 80% of LBGs not classified as LAEs. We argue that most, if not all, of the observed Lyα emission in the diffuse halos originates in the galaxy H ii regions but is scattered in our direction by H i gas in the galaxy's circum-galactic medium. The overall intensity of Lyα halos, but not the surface brightness distribution, is strongly correlated with the emission observed in the central ∼1 -more luminous halos are observed for galaxies with stronger central Lyα emission. We show that whether or not a galaxy is classified as a giant "Lyα blob" (LAB) depends sensitively on the Lyα surface brightness threshold reached by an observation. Accounting for diffuse Lyα halos, all LBGs would be LABs if surveys were sensitive to 10 times lower Lyα surface brightness thresholds; similarly, essentially all LBGs would qualify as LAEs.

A young star-forming galaxy at z = 3.5 with an extended Lyman α halo seen with MUSE

“…With a magnification-corrected rest-frame UV magnitude of −21.14 AB, this galaxy has a typical L * luminosity at this redshift (Steidel et al 2011). It has a global equivalent width of 32 Å, classifying it as a Lyα emitter, and a Lyα luminosity of 5.6 × 10 42 erg s −1 , similar to the typical luminosity of narrow band surveys for redshifts z ∼ 3 (Garel, Guiderdoni & Blaizot 2015).…”

“…does the Lyα extension depend on the nature/mass/environment of galaxies? Using the Lyα scale length from the double exponential model, we obtain a ratio of 4.6, whereas Steidel et al (2011) obtain a ratio of 7.4 for the global sample of LBGs and 8.8 for a subsample of Lyα emitters, suggesting that this system Lyα emission is less extended than these bigger objects compared to the continuum emission. We note that some of the differences may arise from the surrounding Mpc-scale environment, particularly the density of the circum-galactic medium, as concluded by Matsuda et al (2012).…”

“…For example, the continuum scale length of this particular object is 10 times smaller than typical lengths of the Steidel et al (2011) sample. In a similar study, Momose et al (2014) also make a careful analysis of stacked images in redshift bins from 2 to 6.6, finding a Lyα scale length of 5 to 10 kpc, for a redshift close to 3.5, which is higher than the scale length we obtained from the double exponential fit.…”

“…Steidel et al 2011) because in these studies only the more extended part of the profile are fitted, discarding the central region. Additionally, our previous approach does not fully reproduce the Lyα profile in the outer part of the galaxy, which prompts us to try to disentangle the contribution of the central part of the galaxy from the halo.…”

“…Shapley et al 2003;Erb et al 2014; Hayes et al 2014; Momose et al 2014), allow the study of statistically significant properties of high-redshift galaxies. In particular, their rest-frame UV and optical nebular lines as well as their spatial extension both in continuum and in Lyα can be probed (Steidel et al 2011), providing an essential clue to understand when most galaxies gather their mass. However, these techniques erase the structure, kinematics and other resolved properties of individual sources, and with it the possibility of learning something new about the detailed physical processes that shape galaxy evolution.…”