Color Anomaly in Multiple Quasars Dust Inhomogeneity or Quasar Microlensing

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Color Anomaly in Multiple Quasars-
Dust Inhomogeneity or Quasar
Microlensing -
in progress

Atsunori Yonehara (Univ. Tsukuba) with
Hiroyuki Hirashita (Nagoya Univ.)
Philip Richter (Arcetri Obs. ?)
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Topics

• Multiple Quasars
• Observed Color Anomaly
• Inhomogeneity in Lens Galaxy
• Quasar Microlensing
• Discussion

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1. Multiple Quasars

• What is multiple quasars ?
• Gravitationally lensed quasars with multiple
  (generally, 2 or 4) images.
• Lens object is a foreground galaxy (some system
  has no apparent lens object or nearby cluster
  contribution).
• How many ?
• A several tenth of such objects have been
  detected.
• The number is still increasing thanks to many
  surveys.
• They are rear, but useful astrophysical tools.

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Samples of multiple quasars
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Properties

• Image separation 1 (arcsec) 1 (kpc) at zl
• ? typical lens size for singular isothermal
  sphere with s200km/s
• Lensed images are nicely fitted by a point
  source.
• Corresponding images show similar spectral
  features.

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2. Observed Color Anomaly

• In principle, gravitational lens phenomenon
  should have no wavelength dependence.
• ? Images created from the same quasar should be
  observed with an identical color.
• However, not all but large number of multiple
  quasars show color anomaly.
• ? 16/23 lens galaxies show median differential
  extinction with ?E(B-V)0.04. (Falco et al.
  1999) non-zero differential extinction (?)

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Results in Falco et al.s paper

• Falco et al. (1999) have summarized color anomaly
  in lens galaxy (CASTLEs survey).

? Reference bluest image error 0.01mag.
(min.) observed B- V- mag.
? non-negligible color anomaly exists in
many systems. patchy nature of gas/dust ?
They only consider 2 colors.
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?EA - ?EB diagram

• Differential extinction - differential extinction
  diagram from CASTLEs Web page.
• Sample selection
• zl and zs are measured
• 3 photometric data are available (F160W, F555W,
  and F814W filter of HST)
• total 15 objects
• ? different from Falco et al. (1999)s sample.

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Possible explanations

• This may due to the intervening lens galaxy.
• Some inhomogeneity in lens galaxy
• Gas-to-dust ratio
• Ingredients of dust
• Column density of ISM
• Quasar microlensing
• Optical depth for quasar microlensing is order of
  unity for all multiple quasars.
• SADM microlensing will show color change.

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3. Inhomogeneity in Lens Galaxy

• Even if all galaxy has the same extinction
  properties as Milky Way, inhomogeneity of the
  (gas) density (e.g., spiral arms) may produce
  observed, differential extinctions.

By using Hirashita et al. (2003)s results, we
randomly select locations in a galaxy and obtain
gas density at the positions. ? calculate
extinctions and compare their value
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?EA - ?EB for inhomogeneity

• Two differential extinction show positive
  correlation.
• No negative ?EB .

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4. Quasar Microlensing

• When a stellar object in lens galaxy passes in
  front of an image, microlensing will occur.
• If matter in the lens galaxy consist only from
  stellar objects, optical depth for quasar
  microlensing can be order of unity.
• Einstein ring radius is comparable to the size of
  accretion disk in quasars, and finite size source
  effect is important for the quasar microlensing.
  ? color change

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Explanation for color change
Extended source
Compact source
Lens Object
Lens Object
Flux
Flux
time
time

• If one of multiple image suffers microlensing,
  color of the image will change.
• In general, all image can always be suffer quasar
  microlensing, independently.

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Magnification pattern
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An example of color change
? Light curve for quasar microlensing.
zs2.0, zl1.0 MBH108M? mass acc. rate
critical value typical caustic size
Event time scale a several yr

• Randomly pick up epochs from this light curve and
  compare colors at different epochs. ?
  differential extinction-like

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?EA - ?EB for microlensing

• No apparent correlation between two differential
  extinctions.
• Both of positive and negative ?EB exist.
• (Average magnification, µave, for both image
  10 .
• µtotµaveµqml(t) )

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5. Discussion

• Except some special case, negative correlations
  between two differential extinction cannot be
  produced in the case of inhomogeneity in lens
  galaxies.
• For positive correlation part, differential
  extinction can be explained by patchy extinction
  properties (more things to do).
• However, quasar microlensing can easily
  reproduce observed color anomaly.

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