Dust Attenuation Curves in the Local Universe: Demographics and New Laws for Star-forming Galaxies and High-redshift Analogs (1804.05850v1)

Abstract: We study dust attenuation curves of 230,000 individual galaxies in the local universe, ranging from quiescent to intensely star-forming systems, using GALEX, SDSS, and WISE photometry calibrated on Herschel-ATLAS. We use a new method of constraining SED fits with infrared luminosity (SED+LIR fitting), and parameterized attenuation curves determined with the CIGALE SED fitting code. Attenuation curve slopes and UV bump strengths are reasonably well constrained independently from one another. We find that $A_{\lambda}/A_V$ attenuation curves exhibit a very wide range of slopes that are on average as steep as the SMC curve slope. The slope is a strong function of optical opacity. Opaque galaxies have shallower curves - in agreement with recent radiate transfer models. The dependence of slopes on the opacity produces an apparent dependence on stellar mass: more massive galaxies having shallower slopes. Attenuation curves exhibit a wide range of UV bump amplitudes, from none to MW-like; with an average strength 1/3 of the MW bump. Notably, local analogs of high-redshift galaxies have an average curve that is somewhat steeper than the SMC curve, with a modest UV bump that can be to first order ignored, as its effect on the near-UV magnitude is 0.1 mag. Neither the slopes nor the strengths of the UV bump depend on gas-phase metallicity. Functional forms for attenuation laws are presented for normal star-forming galaxies, high-z analogs and quiescent galaxies. We release the catalog of associated SFRs and stellar masses (GSWLC-2).

• The paper introduces a novel SED+LIR fitting method to derive attenuation curves for 230,000 galaxies, establishing new empirical laws for star-forming systems.
• It finds that dust attenuation curves display a wide range of slopes—from SMC-like to shallower curves—correlating with galaxy optical opacity.
• The analysis reveals that UV bump strengths vary significantly and do not depend on gas-phase metallicity, challenging traditional dust models.

Dust Attenuation Curves in the Local Universe: An Expert Review

The research paper titled "Dust Attenuation Curves in the Local Universe: Demographics and New Laws for Star-forming Galaxies and High-redshift Analogs" by Salim et al. provides a comprehensive analysis of dust attenuation curves across a vast sample of galaxies in the local universe. This paper employs data from the Sloan Digital Sky Survey (SDSS) and includes 230,000 galaxies, providing significant insights into the variations in dust attenuation laws among different galaxy populations. The paper introduces a novel methodology for deriving these curves, with implications for our understanding of galaxy evolution and the accurate determination of galactic parameters such as star formation rates (SFRs) and stellar masses.

Methodology and Findings

The authors leverage a sophisticated Spectral Energy Distribution (SED) fitting approach, constrained by infrared (IR) luminosity (SED+LIR fitting), to determine the attenuation curve parameters. This method integrates data from the Herschel-ATLAS survey, allowing for precise measurements of attenuation curve slopes and UV bump strengths. The paper confirms that the dust attenuation curves ($A_{\lambda}/A_V$) exhibit a diverse range of slopes, typically as steep as the Small Magellanic Cloud (SMC) extinction curve. This diversity is strongly correlated with the optical opacity of galaxies, with more opaque systems exhibiting shallower curves, consistent with recent radiative transfer model predictions.

Perhaps more surprising is the finding that the attenuation curves have a wide range of UV bump amplitudes, spanning from negligible to Milky Way (MW)-like strengths, although the average bump strength is about one-third that of the MW's. This paper also finds that the slope and the bump strength do not depend significantly on gas-phase metallicity, a result that challenges conventional theories linking dust properties with metallic composition.

Implications

From a practical standpoint, the paper's release of functional forms for attenuation laws applicable to different types of galaxies—normal star-forming, high-redshift analogs, and quiescent galaxies—provides valuable tools for researchers modeling galaxy evolution across cosmic time. These laws account for the average properties but also accommodate the observed diversity in attenuation behaviors.

Theoretically, the paper calls into question established assumptions, particularly those related to the calibration of SFRs and stellar mass estimates in galaxies where direct IR emission data might be absent, such as in high-redshift systems. The findings imply that previous estimates may have been biased by assuming universal attenuation laws not supported by the detailed demographic paper presented.

Future Directions

This research lays the groundwork for future galaxy evolution studies using the upcoming James Webb Space Telescope (JWST), which will observe early universe galaxies with limited direct IR emission data. The attenuation curves derived here will be crucial for interpreting such observations. Additionally, the paper's results provide benchmarks for refining radiative transfer models and enhancing the physical understanding of dust grain properties and their interaction with starlight in various interstellar environments.

Overall, Salim et al.'s work is a significant contribution to the precision tools needed for astrophysical research, offering robust, empirically derived principles that will guide the interpretation of galaxy observations in both the local and distant universe. The continued exploration of dust behavior in galaxies is essential for unraveling the complex interplay of starlight, star formation, and interstellar medium characteristics that shape the observable universe.