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Register a new userDust in the Small Magellanic Cloud: Interstellar Polarization and Extinction
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Added by
Claudia Vilega Rodrigues
Publication date
1997
fields
Physics
and research's language is
English
Authors
C. V. Rodrigues
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To elucidate the dust properties in the SMC we have for the first time measured linear polarization in five colors in the optical region of the spectrum for a sample of reddened stars. For two of these stars, for which there were no existing UV spectrophotometric measurements, but for which we measured a relatively large polarization, we have also obtained data from the International Ultraviolet Explorer (IUE) in order to study the extinction. The main results are: (1) the wavelength of maximum polarization, $lambda_{max}$, in the SMC is typically smaller than that in the Galaxy; (2) however, AZV 456, which shows the UV extinction bump, has a $lambda_{max}$ typical of that in the Galaxy, its polarization curve is narrower, its bump is shifted to shorter wavelengths as compared to the Galaxy and its UV extinction does not conform to the Galactic analytical interpolation curve based on the ratio of total to selective extinction; (3) the typical, monotonic SMC extinction curve can be best fit with amorphous carbon and silicate grains; (4) the extinction towards AZV456 may only be explained by assuming a larger gas-to-dust ratio than the observed N(HI)/A(V) value, with a small amount of the available carbon in graphite form; (5) from an analysis of both the extinction and polarization data and our model fits it appears that the SMC has typically smaller grains than those in the Galaxy.
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We examine the three-dimensional structure and dust extinction properties in a ~ 200 pc $times$ 100 pc region in the southwest bar of the Small Magellanic Cloud (SMC). We model a deep Hubble Space Telescope optical color-magnitude diagram (CMD) of red clump and red giant branch stars to infer the dust extinction and galactic structure. We model the distance distribution of the stellar component with a Gaussian and find a centroid distance of 65.2 kpc (distance modulus $mu$ = 19.07 mag) with a FWHM $approx$ 11.3 kpc. This large extent along the line of sight reproduces results from previous studies using variable stars and red clump stars. Additionally, we find an offset between the stellar and dust distributions, with the dust on the near side relative to the stars by 3.22 $^{+1.69}_{-1.44}$ kpc, resulting in a 73% reddened fraction of stars. Modeling the dust layer with a log-normal $A_V$ distribution indicates a mean extinction $langle A_V rangle$ = 0.41 $pm$ 0.09 mag. We also calculate $A_V/N_H$ = 3.2 - 4.2 $times10^{-23}$ mag cm$^2$ H$^{-1}$ which is significantly lower than the Milky Way value but is comparable to previous SMC dust-to-gas ratio measurements. Our results yield the first joint dust extinction and 3D geometry properties in a key region in the SMC. This study demonstrates that CMD modeling can be a powerful tool to simultaneously constrain dust extinction and geometry properties in nearby galaxies.
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 x 100 pc region in the southwest bar of the Small Magellanic Cloud (SMC). The rich information provided by our 8-band ultra-violet 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 $R_{475} = A_{475}/(A_{475}-A_{814})$ = 2.65 $pm$ 0.11. This measurement is significantly larger than the equivalent values of published Milky Way $R_{V}$ = 3.1 ($R_{475} = 1.83$) and SMC Bar $R_{V}$ = 2.74 ($R_{475} = 1.86$) 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 a full-width half-max depth of 10 $pm$ 2 kpc in the stellar distribution of the SMC (5 $pm$ 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 broad-band HST imaging for simultaneously constraining dust and galactic structure outside the Milky Way.
In order to understand the evolution of the interstellar medium (ISM) of a galaxy, we have analysed the gas and dust budget of the Small Magellanic Cloud (SMC). Using the Spitzer Space Telescope, we measured the integrated gas mass-loss rate across asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the SMC, and obtained a rate of 1.4x10^-3 Msun yr-1. This is much smaller than the estimated gas ejection rate from type II supernovae (SNe) (2-4x10^-2 Msun yr-1). The SMC underwent a an increase in starformation rate in the last 12 Myrs, and consequently the galaxy has a relatively high SN rate at present. Thus, SNe are more important gas sources than AGB stars in the SMC. The total gas input from stellar sources into the ISM is 2-4x10^-2 Msun yr-1. This is slightly smaller than the ISM gas consumed by starformation (~8x10^-2 Msun yr-1). Starformation in the SMC relies on a gas reservoir in the ISM, but eventually the starformation rate will decline in this galaxy, unless gas infalls into the ISM from an external source. The dust injection rate from AGB and RSG candidates is 1x10^-5 Msun yr-1. Dust injection from SNe is in the range of 0.2--11x10^-4 Msun yr-1, although the SN contribution is rather uncertain. Stellar sources could be important for ISM dust (3x10^5 Msun yr-1) in the SMC, if the dust lifetime is about 1.4 Gyrs. We found that the presence of poly-aromatic hydrocarbons (PAHs) in the ISM cannot be explained entirely by carbon-rich AGB stars. Carbon-rich AGB stars could inject only 7x10^-9 Msun yr-1 of PAHs at most, which could contribute up to 100 Msun of PAHs in the lifetime of a PAH. The estimated PAH mass of 1800 Msun in the SMC can not be explained. Additional PAH sources, or ISM reprocessing should be needed.
The Tail region of the Small Magellanic Cloud (SMC) was imaged using the MIPS instrument on the Spitzer Space Telescope as part of the SAGE-SMC Spitzer Legacy. Diffuse infrared emission from dust was detected in all the MIPS bands. The Tail gas-to-dust ratio was measured to be 1200 +/- 350 using the MIPS observations combined with existing IRAS and HI observations. This gas-to-dust ratio is higher than the expected 500-800 from the known Tail metallicity indicating possible destruction of dust grains. Two cluster regions in the Tail were resolved into multiple sources in the MIPS observations and local gas-to-dust ratios were measured to be ~440 and ~250 suggests dust formation and/or significant amounts of ionized gas in these regions. These results support the interpretation that the SMC Tail is a tidal tail recently stripped from the SMC that includes gas, dust, and young stars.
We present an analysis of dust grain emission in the diffuse interstellar medium of the Small Magellanic Cloud (SMC). This study is motivated by the availability of 170 microns ISOPHOT data covering a large part of the SMC, with a resolution enabling to disentangle the diffuse medium from the star forming regions. After data reduction and subtraction of Galactic foreground emission, we used the ISOPHOT data together with HiRes IRAS data and ATCA/Parkes combined HI column density maps to determine dust properties for the diffuse medium. We found a far infrared emissivity per hydrogen atom 30 times lower than the Solar Neighborhood value. The modeling of the spectral energy distribution of the dust, taking into account the enhanced interstellar radiation field, gives a similar conclusion for the smallest grains (PAHs and very small grains) emitting at shorter wavelength. Assuming Galactic dust composition in the SMC, this result implies a difference in the gas-to-dust ratio (GDR) 3 times larger than the difference in metallicity. This low depletion of heavy elements in dust could be specific of the diffuse ISM and not apply for the whole SMC dust if it results from efficient destruction of dust by supernovae explosions.
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