Distances from time delays

Water-maser proper-motions by VLBI

Appears like the DawnFair as the MoonBright as the Sun

Majestic as the Stars

by Nari Jeong 2007. 7. 19

Distances from time delays

< Fundamental thinking >Astrophysical source’s light output changes

with time-> we can estimate its distance!!

The ring around Supernova 1987A

< Fact >IUE -> narrow emission lines from highly ionized

atoms in the spectrum of the supernova.

< Reason >Lines arise from radiation that has been

reprocessed bymaterial at some distance from the supernova.

< How to estimate distance >

Supernova : 52±3kpc

Gravitational lens time delays

< Fact >Large scales!! -> Gravitationally-lensed quasars.< Reason >Light passes a massive body -> path is bent by gravity.< How to estimate distance >The first term :

The second term : General-relativistic time dilation <- distortion of space-tim

e in the presence of a gravitational field

< Fact >Quasar has -> two images

< Reason >Different mass models would produce very different arrangements on the sky the multiple image.

< Kind >1. Multiple-image gravitational lens2. Luminous arc3. Radio ring4. Microlensing

< Q2237+0305, Einstein Cross >

Water-maser proper-motions by VLBI

Large distances!! - Bright enough to be readily detectable- Measure very small change = > H2O maser spots ( occur : active star formation )

VLBI ( very long baseline interferometry )

< Greenhill et at (1993) >H2O maser in M33 at two epochs separated by t = 479 days.Point out : large number of uncertainties

< Greenhill et al (1981) >Application of such a model to a Milky Way H2O maser Source.

< Miyoshi et al (1995) >VLBI observation : core of the nearby spiral galaxy NGC 4258.Distribution of H2O maser spots with a remarkably simple geometry.

< Figure 7.3 >Center – rotation as r^-1/2 – Keplerian mationMiddle – linearly – a single circular orbitEnd – similar center – disk’s inner edge=> Centripetally accelerated by the central mass

< Greenhill et al (1995) >Projected close to the center of the disk drift