Abstract
We produce a set of 72 NIR-through-UV extinction curves by combining new
Hubble Space Telescope
/STIS optical spectrophotometry with existing
International Ultraviolet Explorer
...spectrophotometry (yielding gapless coverage from 1150 to 10000 Å) and NIR photometry. These curves are used to determine a new, internally consistent NIR-through-UV Milky Way mean curve and to characterize how the shapes of the extinction curves depend on
R
(
V
). We emphasize that while this dependence captures much of the curve variability, considerable variation remains that is independent of
R
(
V
). We use the optical spectrophotometry to verify the presence of structure at intermediate wavelength scales in the curves. The fact that the optical-through-UV portions of the curves are sampled at relatively high resolution makes them very useful for determining how extinction affects different broadband systems, and we provide several examples. Finally, we compare our results to previous investigations.
Abstract
We measured the mid-infrared (MIR) extinction using Spitzer photometry and spectroscopy (3.6–37
μ
m) for a sample of Milky Way sight lines (mostly) having measured ultraviolet extinction ...curves. We used the pair method to determine the MIR extinction that we then fit with a power law for the continuum and modified Drude profiles for the silicate features. We derived 16 extinction curves having a range of
A
(
V
) (1.8–5.5) and
R
(
V
) values (2.4–4.3). Our sample includes two dense sight lines that have 3
μ
m ice feature detections and weak 2175 Å bumps. The average
A
(
λ
)/
A
(
V
) diffuse sight-line extinction curve we calculate is lower than most previous literature measurements. This agrees better with literature diffuse dust grain models, though it is somewhat higher. The 10
μ
m silicate feature does not correlate with the 2175 Å bump, for the first time providing direct observational confirmation that these two features arise from different grain populations. The strength of the 10
μ
m silicate feature varies by ∼2.5 and is not correlated with
A
(
V
) or
R
(
V
). It is well fit by a modified Drude profile with strong correlations seen between the central wavelength, width, and asymmetry. We do not detect other features with limits in
A
(
λ
)/
A
(
V
) units of 0.0026 (5–10
μ
m), 0.004 (10–20
μ
m), and 0.008 (20–40
μ
m). We find that the standard prescription of estimating
R
(
V
) from
C
×
E
(
K
s
−
V
)/
E
(
B
−
V
) has
C
= −1.14 and a scatter of ∼7%. Using the IRAC 5.6
μ
m band instead of
K
s
gives
C
= −1.03 and the least scatter of ∼3%.
Abstract
Dust extinction is one of the fundamental measurements of dust grain sizes, compositions, and shapes. Most of the wavelength-dependent variations seen in Milky Way extinction are strongly ...correlated with the single parameter
R
(
V
) =
A
(
V
)/
E
(
B
−
V
). Existing
R
(
V
)-dependent extinction relationships use a mixture of spectroscopic and photometry observations, and hence do not fully capture all the important dust features or continuum variations. Using four existing samples of spectroscopically measured dust extinction curves, we consistently measure the
R
(
V
)-dependent extinction relationship spectroscopically from the far-ultraviolet (FUV) to mid-infrared for the first time. Linear fits of
A
(
λ
)/
A
(
V
) dependent on
R
(
V
) are done using a method that fully accounts for their significant and correlated uncertainties. These linear parameters are fit with analytic wavelength-dependent functions to determine the smooth
R
(
V
) (2.3–5.6) and wavelength (912 Å–32
μ
m) dependent extinction relationship. This relationship shows that the FUV rise, 2175 Å bump, and the three broad optical features are dependent on
R
(
V
), but the 10 and 20
μ
m features are not. Existing literature relationships show significant deviations compared to this relationship especially in the FUV and infrared (IR). Extinction curves that clearly deviate from this relationship illustrate that this relationship only describes the average behavior versus
R
(
V
). We find tentative evidence that the relationship may not be linear with
R
(
V
)
−1
especially in the ultraviolet (UV). For the first time, this relationship provides measurements of dust extinction that spectroscopically resolve the continuum and features in the UV, optical, and IR as a function of
R
(
V
), enabling detailed studies of dust grain properties and full spectroscopic accounting for the effects of dust extinction on astrophysical objects.
Abstract
Interstellar dust extinction curves provide valuable information about dust properties, including the composition and size of the dust grains, and are essential to correct observations for ...the effects of interstellar dust. In this work, we measure a representative sample of near-infrared (NIR; 0.8–5.5
μ
m) spectroscopic extinction curves for the first time, enabling us to investigate the extinction at wavelengths where it is usually only measured in broad photometric bands. We use IRTF/SpeX spectra of a sample of reddened and comparison stars to measure 15 extinction curves with the pair method. Our sample spans
A
(
V
) values from 0.78 to 5.65 and
R
(
V
) values from 2.43 to 5.33. We confirm that the NIR extinction curves are well fit by a power law, with indices and amplitudes differing from sight line to sight line. Our average diffuse NIR extinction curve can be represented by a single power law with index
α
= 1.7, but because of the sight line-to-sight line variations, the shape of any average curve will depend on the parental sample. We find that most of the variation in our sample can be linked to the ratio of total-to-selective extinction
R
(
V
), a rough measurement of the average dust grain size. Two sight lines in our sample clearly show the ice extinction feature at 3
μ
m, which can be fitted by a modified Drude profile. We find tentative ice detections with slightly over 3
σ
significance in two other sight lines. In our average diffuse extinction curve, we measure a 3
σ
upper limit of
A
(ice)/
A
(
V
) = 0.0021 for this ice feature.
Abstract
We explore evolution in the dust-to-gas ratio with density within four well-resolved Local Group galaxies—the LMC, SMC, M31, and M33. We do this using new Herschel maps, which restore ...extended emission that was missed by previous Herschel reductions. Combining this sensitivity to diffuse dust emission with excellent physical resolution allows us to probe the dust-to-gas ratio across 2.5 orders of magnitude in interstellar medium (ISM) surface density. We find a significant increase in the dust-to-gas ratio with density, with the dust-to-gas ratio varying within each galaxy by up to a factor 22.4, as density changes. We explore several possible reasons for this, and our favored explanation is that it is being driven by dust grain growth in denser regions of the ISM. We find that the evolution of the dust-to-gas ratio with ISM surface density is very similar between M31 and M33, despite their large differences in mass, metallicity, and star formation rate; conversely, we find M33 and the LMC to have very different dust-to-gas evolution profiles, despite their close similarity in those properties. Our dust-to-gas ratios address previous disagreement between UV- and far-IR-based dust-to-gas estimates for the Magellanic Clouds, removing the disagreement for the LMC, and considerably reducing it for the SMC—with our new dust-to-gas measurements being factors of 2.4 and 2.0 greater than the previous far-IR estimates, respectively. We also observe that the dust-to-gas ratio appears to fall at the highest densities for the LMC, M31, and M33; this is unlikely to be an actual physical phenomenon, and we posit that it may be due to a combined effect of dark gas, and changing dust mass opacity.
The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and life-cycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc ...resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and H alpha observations. We investigate the range of CO-to-Hsub 2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on XCO to be 6 x 10sup 20 cmsup -2 Ksup -1 kmsup -1 s in the LMC (Z = 0.5Z) at 15 pc resolution, and 4 x 1021 cmsup -2 Ksup -1 kmsup -1 s in the SMC (Z = 0.2Z) at 45 pc resolution. Our analysis demonstrates that obtaining robust ISM masses remains a non-trivial endeavor even in the local Universe using state-of-the-art maps of thermal dust emission.
The JWST Early Release Observations Pontoppidan, Klaus M.; Barrientes, Jaclyn; Blome, Claire ...
Astrophysical journal. Letters,
09/2022, Letnik:
936, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
The James Webb Space Telescope (JWST) Early Release Observations (EROs) is a set of public outreach products created to mark the end of commissioning and the beginning of science operations ...for JWST. Colloquially known as the “Webb First Images and Spectra,” these products were intended to demonstrate to the worldwide public that JWST is ready for science, and is capable of producing spectacular results. The package was released on 2022 July 12 and included images and spectra of the galaxy cluster SMACS J0723.3-7327 and distant lensed galaxies, the interacting galaxy group Stephan’s Quintet, NGC 3324 in the Carina star-forming complex, the Southern Ring planetary nebula NGC 3132, and the transiting hot Jupiter WASP-96b. This paper describes the ERO technical design, observations, and scientific processing of data underlying the colorful outreach products.
The measurement of precise absolute fluxes for stellar sources has been pursued with increased vigor since the discovery of dark energy and the realization that its detailed understanding requires ...accurate spectral energy distributions (SEDs) of redshifted Ia supernovae in the rest frame. The flux distributions of spectrophotometric standard stars were initially derived from the comparison of stars to laboratory sources of known flux but are now mostly based on calculated model atmospheres. For example, pure hydrogen white dwarf (WD) models provide the basis for the HST CALSPEC archive of flux standards. The basic equations for quantitative spectrophotometry and photometry are explained in detail. Several historical lab-based flux calibrations are reviewed; and the SEDs of stars in the major online astronomical databases are compared to the CALSPEC reference standard spectrophotometry. There is good evidence that relative fluxes from the visible to the near-IR wavelength of ∼2.5 μm are currently accurate to 1% for the primary reference standards, and new comparisons with lab flux standards show promise for improving that precision.
We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the Draine & Li dust model. We derive the abundance of ...the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction ( , the fraction of the dust mass in the form of PAHs) is smaller in the SMC ( %) than in the LMC ( %). We measure the PAH fraction in different gas phases (H ii regions, ionized gas outside of H ii regions, molecular gas, and diffuse neutral gas). H ii regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way (∼4.6%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas.
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