THE MOSDEF SURVEY: MEASUREMENTS OF BALMER DECREMENTS AND THE DUST ATTENUATION CURVE AT REDSHIFTSz∼ 1.4–2.6

Abstract

We present results on the dust attenuation curve of z ∼ 2 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Our sample consists of 224 star-forming galaxies with nebular spectroscopic redshifts in the range z spec = 1.36 − 2.59 and high S/N measurements of, or upper limits on, the Hα and Hβ emission lines obtained with the MOSFIRE spectrograph on the Keck I telescope. Using deep multi-wavelength photometry, we construct composite spectral energy distributions (SEDs) of galaxies in bins of specific star-formation rate (SFR/M * ) and Balmer optical depth. These composites are used to directly constrain the shape and normalization of the dust attenuation curve over the full wavelength range from the UV through near-IR for typical star-forming galaxies at high redshift (z 1.4). Our results imply an attenuation curve that is very similar in shape and normalization to the SMC extinction curve at wavelengths λ 2500Å. At shorter wavelengths, the shape of the curve is identical to that of the Calzetti et al. (2000) starburst attenuation relation, but with a lower normalization (R V ), implying less attenuation at a fixed wavelength for a given SED shape. Hence, the new attenuation curve results in SFRs that are ≈ 20% lower, and stellar masses that are ∆ log(M * /M ) 0.16 dex lower, than those obtained with the starburst attenuation curve. We find marginal evidence for excess absorption at 2175Å. Moreover, we find that the difference in the reddening-and the total attenuation-of the ionized gas and stellar continuum correlates strongly with SFR, such that for dust-corrected SFRs 20 M yr −1 , assuming a Chabrier (2003) IMF, the nebular emission lines suffer an increasing degree of obscuration relative to the continuum. A simple model that can account for these trends is one in which the UV through optical stellar continuum is dominated by a population of less reddened stars, while the nebular line and bolometric luminosities become increasingly dominated by dustier stellar populations for galaxies with large SFRs, as a result of the increased dust enrichment that accompanies such galaxies. Consequently, UV-and SED-based SFRs may underestimate the total SFR at even modest levels of ≈ 20 M yr −1 .

An extreme [O III] emitter atz= 3.2: a low metallicity Lyman continuum source

“…A detailed comparison of a large sample of z ∼ 2 star-forming galaxies observed with MOSFIRE and with measured Balmer decrement for most of this sample shows that the relation between E(B − V) nebular and E(B − V) stellar is in fact SFR (and sSFR) dependent (see also de Barros et al 2015). To derive the emission line attenuation, we use the relation provided by Reddy et al (2015): Reddy et al (2015) emphasize that Eq. (A.2) is affected by a large scatter (σ ∼ 0.12).…”

“…Therefore, in this work, we assume a constant star formation history and the MOSDEF attenuation curve. While the β slope is consistent with no or low dust attenuation, we use the relations derived in Reddy et al (2015) to translate the β slope in color excess. This leads to E(B − V) stellar ≤ 0.04, while the Meurer relation (Meurer et al 1999) leads to no dust attenuation because of the higher intrinsic β slope in this latter relation.…”

Dust attenuation inz~ 1 galaxies fromHerscheland 3D-HST Hαmeasurements

“…Calzetti et al (2000) 0.93 Calzetti et al (2000) Fitzpatrick (1999) 0.66 Reddy et al (2015) Reddy et al (2015) 0.96 Reddy et al (2015) Fitzpatrick (1999) 1.19…”

Predicting dust extinction properties of star-forming galaxies from Hα/UV ratio

“…A growing number of extensive NIR spectroscopic observational campaign with new facilities are now under way (e.g. Steidel et al 2014;Reddy et al 2015), and also, future thirty-meter class telescopes will of course deliver high-quality spectra for high-redshift galaxies. We believe that those new observations will revolutionise the situation and make it easier to access Balmer decrement for high-z galaxies, allowing us to test if our new empirical recipes for predicting dust extinction properties established in the local Universe really work for high-redshift galaxies.…”

“…Indeed, it is becoming possible to obtain unprecedentedly high-quality NIR spectra for high-z galaxies (e.g. Steidel et al 2014;Reddy et al 2015), but even with such high-quality data, it is still challenging to detect Hβ lines from individual galaxies.…”