# Extinction curve variations

1. Oct 7, 2015

### wolram

All though i do not understand all this i wonder what others think, thees extinction seem significant

arXiv:1510.01321 [pdf, ps, other]
Interstellar Extinction Curve Variations Toward the Inner Milky Way: A Challenge to Observational Cosmology
David M. Nataf, Oscar A. Gonzalez, Luca Casagrande, Gail Zasowski, Christopher Wegg, Christian Wolf, Andrea Kunder, Javier Alonso-Garcia, Dante Minniti, Marina Rejkuba, Roberto K. Saito, Elena Valenti, Manuela Zoccali, Radoslaw Poleski, Grzegorz Pietrzynski, Jan Skowron, Igor Soszynski, Michal K. Szymanski, Andrzej Udalski, Krzystof Ulaczyk, Lukasz Wyrzykowski
Comments: 16 pages, 13 figures, 3 tables
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA)

We investigate interstellar extinction curve variations toward $\sim$4 deg$^{2}$ of the inner Milky Way in $VIJK_{s}$ photometry from the OGLE-III and $VVV$ surveys, with supporting evidence from diffuse interstellar bands and $F435W,F625W$ photometry. We obtain independent measurements toward $\sim$2,000 sightlines of $A_{I}$, $E(V-I)$, $E(I-J)$, and $E(J-K_{s})$, with median precision and accuracy of 2%. We find that the variations in the extinction ratios $A_{I}/E(V-I)$, $E(I-J)/E(V-I)$ and $E(J-K_{s})/E(V-I)$ are large (exceeding 20%), significant, and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how $R_{V}$ is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve has at least two degrees of freedom, and not one (e.g. $R_{V}$), which we conform with a principal component analysis. We derive a median value of $<A_{V}/A_{Ks}>=13.44$, which is $\sim$60% higher than the "standard" value. We show that the Wesenheit magnitude $W_{I}=I-1.61(I-J)$ is relatively impervious to extinction curve variations.
Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that $R_{V}$ variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by $\sim$100%, and lack sensitivity to extinction curve variations.
arXiv:1510.01321 [pdf, ps, other]
Interstellar Extinction Curve Variations Toward the Inner Milky Way: A Challenge to Observational Cosmology
David M. Nataf, Oscar A. Gonzalez, Luca Casagrande, Gail Zasowski, Christopher Wegg, Christian Wolf, Andrea Kunder, Javier Alonso-Garcia, Dante Minniti, Marina Rejkuba, Roberto K. Saito, Elena Valenti, Manuela Zoccali, Radoslaw Poleski, Grzegorz Pietrzynski, Jan Skowron, Igor Soszynski, Michal K. Szymanski, Andrzej Udalski, Krzystof Ulaczyk, Lukasz Wyrzykowski
Comments: 16 pages, 13 figures, 3 tables
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA)

We investigate interstellar extinction curve variations toward $\sim$4 deg$^{2}$ of the inner Milky Way in $VIJK_{s}$ photometry from the OGLE-III and $VVV$ surveys, with supporting evidence from diffuse interstellar bands and $F435W,F625W$ photometry. We obtain independent measurements toward $\sim$2,000 sightlines of $A_{I}$, $E(V-I)$, $E(I-J)$, and $E(J-K_{s})$, with median precision and accuracy of 2%. We find that the variations in the extinction ratios $A_{I}/E(V-I)$, $E(I-J)/E(V-I)$ and $E(J-K_{s})/E(V-I)$ are large (exceeding 20%), significant, and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how $R_{V}$ is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve has at least two degrees of freedom, and not one (e.g. $R_{V}$), which we conform with a principal component analysis. We derive a median value of $<A_{V}/A_{Ks}>=13.44$, which is $\sim$60% higher than the "standard" value. We show that the Wesenheit magnitude $W_{I}=I-1.61(I-J)$ is relatively impervious to extinction curve variations.
Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that $R_{V}$ variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by $\sim$100%, and lack sensitivity to extinction curve variations.

2. Oct 7, 2015

### Chronos

I suspect the variance noted by the authors relates more to our ability, or lack thereof, to accurately quantify extinction effects than systematics in cosmic distance ladder measurements.