Observations of the Milky Way by the Hubble Space Telescope
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Hubble Space Telescope Observations of Milky Way Gas and Structure
The Hubble Space Telescope (HST) has provided key insights into the gas content and structure of the Milky Way. Early HST observations using the Faint Object Spectrograph revealed that about 44% of absorption lines in quasar spectra are due to gas in the Milky Way's disk and halo. These studies also measured the abundance of magnesium in high-velocity gas, finding values between 0.059 and 0.32 times the solar abundance, and showed that high-velocity hydrogen emission is always accompanied by metal-line absorption at similar velocities .
HST Photometry and the Milky Way’s Distance Scale
HST has played a crucial role in calibrating the cosmic distance ladder using Milky Way Cepheid variable stars. By combining precise HST photometry with Gaia parallaxes, researchers have achieved a 1% precision in calibrating Cepheid luminosities. This has led to a highly accurate measurement of the Hubble constant, with values around 73 km/s/Mpc, and has highlighted a significant tension with predictions from the standard cosmological model (ΛCDM) 2810. The HST’s spatial scanning mode has been essential in reducing photometric errors and improving calibration accuracy 28.
HST Studies of Milky Way Globular Clusters and Mass
HST has measured the absolute proper motions of distant Milky Way globular clusters, allowing astronomers to determine their 3D velocities and origins. These measurements have helped distinguish clusters formed within the Milky Way from those accreted from other galaxies, such as those associated with the Sagittarius stream. Using these data, the mass of the Milky Way within 39.5 kpc has been estimated at about 0.61 × 10¹² solar masses, with a total virial mass of about 2.05 × 10¹² solar masses 45. HST has also enabled the first precise age estimates for previously unstudied globular clusters, revealing both old and unusually young clusters in the Milky Way .
HST Observations of Ultra-Faint Dwarf Galaxies
Deep HST imaging has been used to study ultra-faint dwarf (UFD) galaxies around the Milky Way. These observations show that UFDs associated with the Large Magellanic Cloud (LMC) are structurally similar to those around the Milky Way, with no systematic differences in their magnitude–size relation. HST data also indicate that the stellar initial mass function in UFDs is more "bottom-light" (fewer low-mass stars) than in the Milky Way disk, and that the IMF slope correlates with galaxy metallicity 367. Simulations using different dark matter models do not show significant differences in the observed properties of UFDs, but more data are needed for stronger conclusions 67.
Conclusion
Hubble Space Telescope observations have transformed our understanding of the Milky Way by revealing the properties of its gas, refining the cosmic distance scale, mapping the motions and origins of globular clusters, and characterizing ultra-faint dwarf galaxies. These results have not only deepened our knowledge of our own galaxy but have also had a major impact on broader questions in cosmology and galaxy formation.
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