The Galactic dust-to-metals ratio and metallicity using gamma-ray bursts

Abstract

The metallicity and dust-to-metals ratio of the Galaxy are fundamental parameters in understanding the interstellar medium (ISM). Currently, there is still some uncertainty surrounding these parameters. In this paper, the dust-to-metals ratio in the Galaxy is determined using the photoelectric absorption of the soft X-ray afterglows of a large sample of several hundred gamma-ray bursts (GRBs) to determine the metal column density in combination with Galactic dust maps to determine the line-of-sight dust extinction through the Galaxy in the direction of the GRB. GRB afterglows often have large extragalactic soft X-ray absorptions and therefore the GRB sample's upper-bound will define the Galactic dust-to-metals relation. Using a two-dimensional two-sample Kolmogorov-Smirnoff test, we determine this upper-bound and so derive the dust-to-metals ratio of the Galaxy. We find N H = 2.2 +0.3 −0.4 × 10 21 cm −2 A V asassuming solar, Anders & Grevesse (1989, Geochim. Cosmochim. Acta, 53, 197), metallicity. This result is consistent with previous findings using bright X-ray sources in the Galaxy. Using the same technique but substituting the H i maps from the Leiden-ArgentineBonn survey for the dust maps, allows us to place a limit on the metallicity in the Galaxy. We find a metallicity consistent with the Anders & Grevesse (1989) solar values often used in X-ray fitting. Based on this and previous studies, we suggest that the metallicity of a typical ISM sightline through the Galaxy is ∼0.25 dex higher than the current best estimate of the solar metallicity. We further show that the dust-to-gas ratio seems to be correlated with the total gas column density, and that this may be due to the metallicity gradient observed toward the Galactic centre. Based on the non-constant nature of the dust-to-gas ratio, we propose that the dust column density, at N H = 2.2 × 10 21 cm −2 A V , represents a better proxy for the soft X-ray absorption column density than H i maps.

Evidence for Thermal X-Ray Line Emission From the Synchrotron-Dominated Supernova Remnant Rx J1713.7-3946

“…For the BG components, we fixed all the parameters at the values in Table 2, except for the absorbing column density, which we reduced from N H ∼ 1 × 10 22 cm −2 (Table 3) to N H = 0.7 × 10 22 cm −2 . This is because our spectral extraction region is located within the absorption hole, where the visual extinction A V is ∼1.5 mag less than that in the surrounding region (Sano et al 2015), which is equivalent to ∆N H = 3×10 21 cm −2 , according to the N H -A V relation, N H = 2.2 × 10 21 cm −2 A V mag (Gorenstein 1975;Watson 2011). The N H value we employed is consistent with that measured for the CCO, ∼ 0.7 × 10 22 cm −2 (Cassam-Chenaï et al 2004).…”

The Metallicity and Dust Content of a Redshift 5 Gamma-Ray Burst Host Galaxy

“…where DTM LG ≡ 10 −21.3 mag cm 2 is the DTM in the Local Group (Watson 2011). Recently, Zafar & Watson (2013), using different classes of objects, found that DTM is independent of galaxy type or age, redshift, or metallicity and is very close to the value in the Local Group.…”

The Earliest Phases of Star formation (EPoS) observed withHerschel: the dust temperature and density distributions of B68

“…The pixel scale is 5 × 5 , resulting in a pixel area of A pix = 1.26 × 10 32 cm 2 at an assumed distance of 150 pc. The conversion factor of N H /A V = 2.2 × 10 21 cm −2 mag −1 (Ryter 1996;Güver & Özel 2009;Watson 2011) was applied. Then we extracted the source from this map via a 2D Gaussian fit after an unsharp masking procedure by convolving the original extinction map to three resolutions, 30 , 40 and 50 .…”

“…For a more recent characterisation of that ratio (Ryter 1996;Güver & Özel 2009;Watson 2011), the numerical value in this conversion is modified by about 18%. To be able to convert the quoted ratio N H /A V to N H /E B−V , we have assumed R V = 3.1 as the mean value for the diffuse ISM, and replaced the corresponding ratio in the equation above:…”