An atlas of ultraviolet spectra of star-forming galaxies

Abstract

We stack the rest-frame ultraviolet spectra of N = 14 highly magnified gravitationally lensed galaxies at redshifts 1.6 < z < 3.6. The resulting new composite spans 900 < λ rest < 3000Å3000Å, with a peak signal-to-noise ratio of 103 per spectral resolution element (∼100 km s −1). It is the highest signal-to-noise ratio, highest spectral resolution composite spectrum of z ∼ 2-3 galaxies yet published. The composite reveals numerous weak nebular emission lines and stellar photospheric absorption lines that can serve as new physical diagnostics, particularly at high redshift with the James Webb Space Telescope (JWST). We report equivalent widths to aid in proposing for and interpreting JWST spectra. We examine the velocity profiles of strong absorption features in the composite, and in a matched composite of z ∼ 0 COS/HST galaxy spectra. We find remarkable similarity in the velocity profiles at z ∼ 0 and z ∼ 2, suggesting that similar physical processes control the outflows across cosmic time. While the maximum outflow velocity depends strongly on ionization potential, the absorption-weighted mean velocity does not. As such, the bulk of the high-ionization absorption traces the low-ionization gas, with an additional blueshifted absorption tail extending to at least −2000 km s −1. We interpret this tail as arising from the stellar wind and photospheres of massive stars. Starburst99 models are able to replicate this high-velocity absorption tail. However, these theoretical models poorly reproduce several of the photospheric absorption features, indicating that improvements are needed to match observational constraints on the massive stellar content of star-forming galaxies at z ∼ 2. We publicly release our composite spectra.

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“…A literature search of local ultraviolet observations reveals very few references to Si ii* fine-structure emission lines. The International Ultraviolet Explorer (IUE) atlas of star-forming galaxies compiled by Kinney et al (1993) contains a census of ultraviolet emission lines associated with nebular objects such as H ii regions, planetary nebulae, and supernova remnants, but does not include the Si ii* features. The spectral resolution of IUE is $1000 km s À1 , coarser than the resolution of the LBG composite spectra.…”

“…When the LBG composite spectra are smoothed to the resolution of the IUE data, the Si ii* lines are still visible at 5%-10% above the continuum level. Presumably, the individual IUE spectra contained in the Kinney et al (1993) atlas are of insufficient S/N to see these features. However, the Si ii* features are also not detected in composite starburst spectra containing 20 individual galaxy spectra drawn from the IUE atlas, each of which has continuum S/N of at least 10 .…”

Star Formation and Dust Extinction in Nearby Star Forming and Starburst Galaxies

“…The Balmer lines emission were measured by the authors within the same aperture as the IUE observations. The UV fluxes are taken in the bin 1863-1963Å (Kinney et al 1993) for consistency with the UV data for the SFG sample. 32 out of the 39 galaxies have been observed by IRAS at 60 and 100 µm.…”

“…The IUE sample: the fluxes are corrected for the Galactic extinction following Calzetti et al (1994). The UV fluxes are taken in the bin 1863-1963Å from Kinney et al (1993. Other data come from Calzetti et al (1994Calzetti et al ( , 1995. The galaxies are sorted according to their diameter.…”

“…The extinction derived by Calzetti et al (2000) is at 1600Å. Therefore, for the need of this calculation we have taken the flux at 1600Å F 1600 from the IUE atlas of star forming galaxies (Kinney et al 1993) by interpolating the fluxes in the bins 1431-1532Å and 1863-1963Å. The extinction at 1600Å is then translated to 2000Å by using the attenuation curve also re-calibrated by Calzetti et al (2000).…”