UV Spectral Slope and Nebular Dust Attenuation in Dwarf Galaxies at $1.4<z<2.6$

TL;DR

The article addresses how nebular dust attenuation relates to stellar mass and the UV spectral slope for extremely low-mass galaxies at $1.4<z<2.6$. Using 33 lensed dwarfs with Keck/MOSFIRE rest-frame optical spectra and deep HST photometry, it derives Balmer decrements, UV slopes, and stellar masses via SED fitting, then analyzes $ au_B$–$β$ and $E(B-V)_{neb}$–$M_*$ relations, including median-stacked spectra. The results show a β–τ_B relation consistent with an SMC-like, steep attenuation curve (and with low-metallicity galaxies at similar redshift), while the $E(B-V)_{neb}$–$M_*$ relation extends to $\log(M_*/M_\odot)\sim7$ with substantial scatter and no strong redshift evolution relative to local dwarfs. These findings imply metal-dependent dust attenuation in dwarfs at cosmic noon and provide crucial calibration for dust corrections in faint galaxies, motivating JWST studies of resolved dust in the low-mass regime.

Abstract

We analyze nebular dust attenuation and its correlation with stellar mass ($M_{*}$) and UV spectral slope ($β$) in 33 lensed, low-mass star-forming galaxies at $1.4\leq z \leq 2.6$, using Keck/MOSFIRE rest-frame optical spectroscopy. Located behind three massive lensing galaxy clusters Abell 1689, MACS J1149.5+2223, and MACS J0717.5+3745, galaxies in our sample have a median stellar mass of $\log(M_{*}/M_{\odot})=8.3$ and an intrinsic UV absolute magnitude range of $-20.9<M_{UV}<-13$. We measure nebular dust attenuation via Balmer optical depth ($τ_{B}$) defined as the H$α$/H$β$ ratio. We also derive physical properties from Hubble Space Telescope multi-wavelength photometry and construct composite spectra using median stacking in bins of $M_{*}$ and $β$. We find that the $τ_{B}-β$ relation for the dwarf galaxies in this study is best represented by SMC dust curve. This is consistent with previous studies of low-metallicity galaxies at similar redshifts, which show a steep attenuation curve similar to the SMC curve, in contrast to high-metallicity and more massive galaxies that exhibit a much shallower dust attenuation curve. We also investigate the relationship between nebular dust attenuation and stellar mass, $E(B-V)_{nebular}-M_{*}$, down to $\log(M_{*}/M_{\odot})\sim 7$. We demonstrate that this relation does not notably evolve with redshift and is consistent with what has been observed for local SDSS galaxies at similar low stellar masses.

Figures (8)

• Figure 1: Left: Distribution of the rest-frame UV absolute magnitudes at 1500 Å for our final sample. These $M_{UV}$ values are corrected for the lensing magnification. Right: Distribution of spectroscopic redshifts of our sample.
• Figure 2: Histogram of stellar mass $M^{*}$ for the 33 galaxies in our final sample. These mass values are from SED fitting and are corrected for magnification.
• Figure 3: Comparing the $\beta_{\mathrm{power}}$ and $\beta_{\mathrm{SED}}$ values. Upper row: The distribution of $\beta_{\mathrm{power}}$ and $\beta_{\mathrm{SED}}$ values are shown with the purple and blue histograms, respectively. Lower row: The difference between the $\beta_{\mathrm{power}}$ and $\beta_{\mathrm{SED}}$ measurements versus the $\beta_{\mathrm{power}}$ values on the x-axis. The dotted blue lines define the region where the difference between $\beta$ values is below 0.5.
• Figure 4: Ratio of H$\alpha$ to H$\beta$ emission lines versus lensing-corrected H$\alpha$ luminosity for our sample. The emission line fluxes have been adjusted for the underlying Balmer absorption. Additionally, the dashed purple line denotes the theoretical value of 2.86 for H$\alpha$/H$\beta$ under normal ISM conditions without dust. As seen here, a fraction of sources in our sample have the Balmer ratio lower than theoretical value. The arrows represent $3\sigma$ lower limit for the line ratio whenever H$\beta$ is undetected.
• Figure 5: Stacked spectra at rest frame. These composite spectra are generated using the median stacking technique (see Section \ref{['sec:stack']}). Prior to stacking, all individual spectra were normalized to their H$\alpha$ flux. The top (bottom) row exhibits the composite spectra at the H$\beta$ (H$\alpha$) wavelength for three bins of $\beta_{power}$, with the bluest on the left and the reddest on the right. Within each panel, the stack is represented in black, while the best fit is depicted with blue, green, and red lines, corresponding to the blueness of the UV spectral slope bin. The wavelength of the fitted line (H$\alpha$ or H$\beta$) is indicated with a dashed line.