Transcript Color Anomaly in Multiple Quasar

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. ?)
Topics
4.
Multiple Quasars
Observed Color Anomaly
Inhomogeneity in Lens Galaxy
Quasar Microlensing
5.
Discussion
1.
2.
3.
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.
Samples of multiple quasars
Q0957
B1938
Q2237
H1413
RXJ0911
B1359
Properties
source
lens
image B
observer
Lens redshift
image A
Source redshift
Image separation : ～ 1 (arcsec) ～ 1 (kpc) at zl
≒ typical lens size for singular isothermal sphere with σ～200km/s
Lensed images are nicely fitted by a point source.
Corresponding images show similar spectral features.
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 ΔE(B-V)～0.04. (Falco et al. 1999)
… non-zero differential extinction (?)
Results in Falco et al.’s paper
Number of images
Falco et al. (1999) have summarized color
anomaly in lens galaxy (CASTLEs survey).
σ＝0.01 Gaussian
σ＝0.1 Gaussian
← Reference: bluest image
error: 0.01[mag.] (min.)
observed B- & V- mag.
↓
non-negligible color anomaly
exists in many systems.
… patchy nature of gas/dust ?
ΔE(B-V) [mag.]
They only consider 2 colors.
ΔEA - ΔEB 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.
Possible explanations
This may due to the intervening lens galaxy.
1. Some inhomogeneity in lens galaxy
Gas-to-dust ratio
Ingredients of dust
Column density of ISM
2.
Quasar microlensing
Optical depth for quasar microlensing is
order of unity for all multiple quasars.
SADM microlensing will show color change.
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
ΔEA - ΔEB for inhomogeneity
Extinction curve for various n(H)
(RV=3.1) ← Cardelli et al. (1989)
ΔEA-ΔEB diagram
Two differential
extinction show
positive correlation.
No negative ΔEB .
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”
Explanation for color change
Extended source
Compact source
Lens Object
Lens Object
Flux
Flux
time
time
Different color !
If one of multiple image suffers microlensing,
color of the image will change.
In general, all image can always be suffer
quasar microlensing, independently.
Magnification pattern
An example of color change
← Light curve for
quasar microlensing.
zs=2.0, zl=1.0
MBH=108M◎
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
ΔEA - ΔEB for microlensing
ΔEA-ΔEB diagram
No apparent
correlation between
two differential
extinctions.
Both of positive and
negative ΔEB exist.
(Average magnification, μave,
for both image = 10 .
μtot=μave+μqml(t) )
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.
Das Ende