Dust Complicates Determinations of the Distance to Galactic Center

Article written: 25 Apr , 2013
Updated: 26 Apr , 2016
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Obtaining an accurate distance between the Sun and the center of our Galaxy remains one of the principal challenges facing astronomers. The ongoing lively debate concerning this distance hinges partly on the nature of dust found along that sight-line. Specifically, are dust particles lying toward the Galactic center different from their counterparts near the Sun? A new study led by David Nataf asserts that, yes, dust located towards the Galactic center is anomalous. They also look at accurately defining both the distance to the Galactic center and the reputed bar structure that encompasses it.

The team argues that characterizing the nature of small dust particles is key to establishing the correct distance to the Galactic center, and such an analysis may mitigate the scatter among published estimates for that distance (shown in the figure below).  Nataf et al. 2013 conclude that dust along the sight-line to the Galactic center is anomalous, thus causing a non-standard ‘extinction law‘.  

The extinction law describes how dust causes objects to appear fainter as a function of the emitted wavelength of light, and hence relays important information pertaining to the dust properties.

The team notes that, “We estimate a distance to the Galactic center of [26745 light-years] … [adopting a] non-standard [extinction law] thus relieves a major bottleneck in Galactic bulge studies.”

Various estimates for the distance to the Galactic center tabulated by Malkin 2013. The x-axis describes the year, while the y-axis features the distance to the Galactic center in kiloparsecs (image credit: Fig 1 from Malkin 2013/arXiv/ARep).

Nataf et al. 2013 likewise notes that, “The variations in both the extinction and the extinction law made it difficult to reliably trace the spatial structure of the [Galactic] bulge.”  Thus variations in the extinction law (tied directly to the dust properties) also affect efforts to delineate the Galactic bar, in addition to certain determinations of the distance to the Galactic center.  Variations in the extinction law imply inhomogeneities among the dust particles.

“The viewing angle between the bulge’s major axis and the Sun-Galactic centerline of sight remains undetermined, with best values ranging from from  13  to …  44 [degrees],” said Nataf et al. 2013 (see also Table 1 in Vanhollebekke et al. 2009).  The team added that, “We measure an upper bound on the tilt of 40 [degrees] between the bulge’s major axis and the Sun-Galactic center line of sight.”

However, the properties of dust found towards the Galactic center are debated, and a spectrum of opinions exist.  While Nataf et al. 2013 find that the extinction law is anomalously low, there are studies arguing for a standard extinction law.  Incidentally, Nataf et al. 2013 highlight that the extinction law characterizing dust near the Galactic center is similar to that tied to extragalactic supernovae (SNe), “The … [extinction] law toward the inner Galaxy [is] approximately consistent with extra-galactic investigations of the hosts of type Ia SNe.”

The delineation of the bar at the center of our Milky Way galaxy by Nataf et al. 2013. The bar is closer toward the Sun in the 1st Galactic quadrant. The center line represents the direction toward the constellation of Sagitarrius (image credit: Fig 17 from Nataf et al. 2013/arXiv/ApJ).

Left, the delineation of the bar at the center of the Milky Way by Nataf et al. 2013. The centerline represents the direction towards Sagittarius (image credit: Fig 17 from Nataf et al. 2013/arXiv/ApJ).  Right, a macro view of the Galaxy highlighting the general orientation and location of the Galactic bar (image credit: NASA/Wikipedia).  The Galactic bar is not readily discernible in the distribution of RR Lyrae variables.

Deviations from the standard extinction law, and the importance of characterizing that offset, is also exemplified by studies of the Carina spiral arm.  Optical surveys reveal that a prominent spiral arm runs through Carina (although that topic is likewise debated), and recent studies argue that the extinction law for Carina is higher than the standard value (Carraro et al. 2013Vargas Alvarez et al. 2013).  Conversely, Nataf et al. 2013 advocate that dust towards the Galactic center is lower by comparison to the standard (average) extinction law value.

The impact of adopting an anomalously high extinction law for objects located in Carina is conveyed by the case of the famed star cluster Westerlund 2, which is reputed to host some of the Galaxy’s most massive stars.  Adopting an anomalous extinction law for Westerlund 2 (Carraro et al. 2013Vargas Alvarez et al. 2013) forces certain prior distance estimates to decrease by some 50% (however see Dame 2007).  That merely emphasizes the sheer importance of characterizing local dust properties when establishing the cosmic distance scale.

In sum, characterizing the properties of small dust particles is important when ascertaining such fundamental quantities like the distance to the Galactic center, delineating the Galactic bar, and employing distance indicators like Type Ia SNe.

The Nataf et al. 2013 findings have been accepted for publication in the Astrophysical Journal (ApJ), and a preprint is available on arXiv.  The coauthors on the study are Andrew Gould, Pascal Fouque, Oscar A. Gonzalez, Jennifer A. Johnson, Jan Skowron, Andrzej Udalski, Michal K. Szymanski, Marcin Kubiak, Grzegorz Pietrzynski, Igor Soszynski, Krzysztof Ulaczyk, Lukasz Wyrzykowski, Radoslaw Poleski.  The Nataf et al. 2013 results are based partly on data acquired via the Optical Graviational Lensing Experiment (OGLE).  The interested reader desiring additional information will find the following pertinent: Udalski 2003Pottasch and Bernard-Salas 2013Kunder et al. 2008Vargas Alvarez et al. 2013Carraro et al. 2013Malkin 2013Churchwell et al. 2009, Dame 2007Ghez et al. 2008Vanhollebekke et al. 2009.

The Nataf et al. 2013 results are based partly on observations acquired by the OGLE survey (image credit: OGLE team).

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2 Responses

  1. Snow Shovel says

    Dust is important. Extragalactic distances rely mostly on the red shift. If the red shift is caused predominately by dust (a real possibility) then the distances are wrong, the universe may not be expanding and there may not have been a Big Bang.

  2. The Latinist says

    Do you have any evidence that dust can cause a redshift in light?

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