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Exploring the Structure of the Milky Way: Nucleus, Bulge, Bar, and Disk Characteristics

This exploration delves into the intricate structure of the Milky Way galaxy, examining its nucleus, bulge, bar, and disk configurations, which include both thick and thin disks. Key measurements such as distances in kpc, star density variations, metallicity, and age distributions are discussed. The interplay of gas and stellar components, especially in relation to spiral arms, is analyzed through various observational techniques. This study provides insights into the galaxy's dynamic nature, including its rotation curve and gas distribution across different regions.

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Exploring the Structure of the Milky Way: Nucleus, Bulge, Bar, and Disk Characteristics

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  1. The Milky Way Nucleus Bulge Bar Disk Halo

  2. Thick Disk Thin Disk Thick = 2 – 3 kpc Thin = .6 - .8 kpc Where it drops by a factor of e (2.72)

  3. Latitude (b), Longitude (l) +b l=90 l=0 l=270 l=180 l=180 -b

  4. Ecliptic

  5. Radio – 74 cm

  6. Radio - 21 cm

  7. Infrared

  8. Near IR

  9. Visible

  10. UV

  11. X-ray

  12. Gamma-rays

  13. Disk • Variations with distance from mid-plane • Star density (decreases out) • [Fe/H] (decreases out) • Age (increases out) • z-velocity (increases outward) • Thin/Thick disk correlate with Populations

  14. Thin disk • Population I, solar+ [Fe/H] • Spiral Arms (500 pc thick) • Outside of spirals, stellar density lower • Sun is in between arms, above midplane • How do we know this? • How do we find spiral arms?

  15. 2 kpc

  16. Surveying the Galaxy • Finding Disk, Spirals • Look for gas • H I for the disk • CO, GMCs for the Spiral arms • Other objects associated with spiral structure • What do you actually see? • What is actually measured?

  17. Observe gas along a line of sight • Measure radial velocities • NOT DISTANCES! • Need to assume a velocity pattern - rotation curve • Need to know Sun’s velocity, distance

  18. Disk Sections • Based upon how they move, and material concentration • Within 3 kpc of center, Bulge (<3kpc) • From 3 kpc to solar distance (3-7.5 kpc) • Beyond solar distance (>7 kpc) • Best velocity/material maps based on • H I • CO – also indicate amount of H2

  19. H I map • Near l=0º, very little H I < 3 kpc (vel high, but not strong) • “Horns” at l=30º, vel=150 km/s • Curve not symmetric, galaxy is warped • Rotation curve of galaxy is relatively flat

  20. CO Map • Not as spread out in velocity as H I • “Horns” at l=25, vel = 130 km/s – due to “molecular ring” • Assymetric • Rotation curve of galaxy is relatively flat • H I 3x greater than CO in 3 kpc<R<solar distance • Very little CO beyond solar distance

  21. H I Map “Fingers of God”

  22. Overall gas distribution • Molecular gas (CO) peaks at ~4 kpc • Within 3 kpc much less gas • Almost all CO within solar circle • Less than half of H I in solar circle • H I remains high from 3 kpc<R<solar circle • At solar circle (½ density thickness) • CO thickness = 50 pc • H I thickness = 100 pc

  23. Beyond solar circle • Severe warping apparent (>15 kpc) • R=24 kpc, warp = 4 kpc • H I peaks at 12-15 kpc • H I density drops strongly after 16 kpc

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