The Small Magellanic Cloud Investigation of Dust and Gas Evolution (SMIDGE): The Dust Extinction Curve from Red Clump Stars

Abstract

We use Hubble Space Telescope (HST) observations of red clump stars taken as part of the Small Magellanic Cloud Investigation of Dust and Gas Evolution (SMIDGE) program to measure the average dust extinction curve in a ∼200 pc×100 pc region in the southwest bar of the Small Magellanic Cloud (SMC). The rich information provided by our eight-band ultraviolet through near-infrared photometry allows us to model the color-magnitude diagram of the red clump accounting for the extinction curve shape, a log-normal distribution of A V , and the depth of the stellar distribution along the line of sight. We measure an extinction curve with = R 475 =  (-) A A A 2.65 0.11 475 475 814. This measurement is significantly larger than the equivalent values of published Milky Way (MW) R V =3.1 (= R 1.83 475) and SMC Bar R V =2.74 (= R 1.86 475) extinction curves. Similar extinction curve offsets in the Large Magellanic Cloud (LMC) have been interpreted as the effect of large dust grains. We demonstrate that the line-of-sight depth of the SMC (and LMC) introduces an apparent "gray" contribution to the extinction curve inferred from the morphology of the red clump. We show that no gray dust component is needed to explain extinction curve measurements when FWHM depth of 10±2 kpc in the stellar distribution of the SMC (5 ± 1 kpc for the LMC) is considered, which agrees with recent studies of Magellanic Cloud stellar structure. The results of our work demonstrate the power of broadband HST imaging for simultaneously constraining dust and galactic structure outside the MW.

SMASHing the LMC: A Tidally Induced Warp in the Outer LMC and a Large-scale Reddening Map

“…Since RC brightness responds to stellar age more sensitively than color for a given metallicity, a lack of skewness in the color distribution of individual stars can be understood. Nevertheless, the width (∼0.1 mag) of the individual stars' magnitude distribution is consistent with the theoretical dispersion in magnitude due to local age and metallicity spread (e.g., Girardi & Salaris 2001;Yanchulova Merica-Jones et al 2017). Furthermore, we find an excellent agreement between the width (0.031 mag) of the cells' distribution, which reflects a global variation in stellar population effect across the LMC disk, and the maxi-mum I-band magnitude difference of 0.036 mag that is measured among different regions in the LMC (Girardi & Salaris 2001) .…”

“…The HB is the metal-poor equivalent of the RC, thus appearing on the bluer side of the main RC. The LMC RGB appears as a single broad band unlike in M31 and the SMC where the RGB is well split into visibly distinct sequences (Dalcanton et al 2015;Yanchulova Merica-Jones et al 2017).…”

“…The line-of-sight depth of the LMC disk has been measured in the literature (e.g., Jacyszyn-Dobrzeniecka et al 2017;Yanchulova Merica-Jones et al 2017). We also measure a line-of-sight depth by looking at the distribution of individual stars along the Z-axis.…”

“…The LMC has an inclined, slightly elongated, rotating, and star-forming disk (Kim et al 1998;van der Marel 2001;van der Marel & Cioni 2001;van der Marel et al 2002;Subramanian & Subramaniam 2010) with a shallow stellar metallicity gradient (e.g., Carrera et al 2008;Feast et al 2010;Carrera et al 2011;Piatti & Geisler 2013;Choudhury et al 2016;Pieres et al 2016), and one prominent spiral arm in the central region with an off-centered bar (de Vaucouleurs & Freeman 1972;Zhao & Evans 2000;Zaritsky 2004;. Its depth along the line of sight is ∼5 kpc Yanchulova Merica-Jones et al 2017), and the northern outer disk shows signs of disturbance in its stellar structure (Mackey et al 2016).…”

METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble Program. I. Overview and Initial Results

“…Visual inspection of Figure 9 suggests that sources detected in three or more filters are affected by extinction up to A B 3 and A V 2. In the future, we will perform a study similar to Yanchulova Merica- Jones et al (2017) to model the CMD of the red clump and the red giant branch, and verify the NUV-NIR extinction curve in the LMC derived from spectroscopy. Furthermore, we can use the NUV-NIR photometry to individual stars to determine their stellar and dust parameters, and model the distribution of those stars in the dust disk to derive extinction maps, similar to Dalcanton et al (2015).…”