Color Anomaly in Multiple Quasars

- Dust Inhomogeneity

or Quasar Microlensing -

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

in progress

Topics

1.1. Multiple QuasarsMultiple Quasars

2.2. Observed Color AnomalyObserved Color Anomaly

3.3. Inhomogeneity in Lens GalaxyInhomogeneity in Lens Galaxy

4.4. Quasar MicrolensingQuasar Microlensing5.5. DiscussionDiscussion

1. Multiple Quasars

What is multiple quasars ?What is multiple quasars ? Gravitationally lensed quasars with multiple (generally, Gravitationally lensed quasars with multiple (generally,

2 or 4) images.2 or 4) images. Lens object is a foreground galaxy (some system has Lens object is a foreground galaxy (some system has

no apparent lens object or nearby cluster contribution).no apparent lens object or nearby cluster contribution).

How many ?How many ?A several tenth of such objects have been detected. A several tenth of such objects have been detected.

The number is still increasing thanks to many surveys.The number is still increasing thanks to many surveys.

They are rear, but useful astrophysical tools.They are rear, but useful astrophysical tools.

Samples of multiple quasarsQ0957 Q2237 RXJ0911

B1938 H1413 B1359

Properties

Image separation :Image separation : ～ ～ 1 (arcsec) 1 (arcsec) ～ ～ 1 (kpc) 1 (kpc) at zat zll

≒ ≒ typical lens size for singular isothermal sphere with σtypical lens size for singular isothermal sphere with σ ～～ 200km/s200km/s

Lensed images are nicely fitted by a point source.Lensed images are nicely fitted by a point source.

Corresponding Corresponding images show similar spectral featuresimages show similar spectral features..

Source redshift

Len

s re

dsh

iftsource

observerlens

image A

image B

2. Observed Color Anomaly

In principle, gravitational lens phenomenon In principle, gravitational lens phenomenon should have no wavelength dependence. should have no wavelength dependence. ⇒ ⇒ Images created from the same quasar Images created from the same quasar

should be observed with an should be observed with an identicalidentical color color..

However, not all but large number of However, not all but large number of multiple quasars show color anomalymultiple quasars show color anomaly..

⇒ ⇒ 16/23 lens galaxies show median differential 16/23 lens galaxies show median differential extinction with extinction with ΔE(B-V)ΔE(B-V) ～～ 0.040.04. . (Falco et al. 1999) (Falco et al. 1999)

… non-zero differential extinction (?) … non-zero differential extinction (?)

Results in Falco et al.’s paper

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

← 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 ?

Nu

mb

er o

f im

ages

ΔE(B-V) [mag.]

σ＝ 0.01 Gaussian

σ＝ 0.1 Gaussian

They only consider 2 colors.

ΔEA - ΔEB diagram

Differential extinction - diDifferential extinction - differential extinction differential extinction diagram from CASTLEs agram from CASTLEs Web page. Web page.

Sample selection:Sample selection: zzll and z and zss are measured are measured

3 photometric data are a3 photometric data are available (F160W, F555vailable (F160W, F555W, and F814W filter of W, and F814W filter of HST) HST)

total: 15 objectstotal: 15 objects

↑ ↑ different from Falco edifferent from Falco et al. (1999)’s sample.t al. (1999)’s sample.

Possible explanations

This may due to the intervening lens galaxy.This may due to the intervening lens galaxy.

1.1. Some inhomogeneity in lens galaxySome inhomogeneity in lens galaxyGas-to-dust ratio Gas-to-dust ratio

Ingredients of dustIngredients of dust

Column density of ISMColumn density of ISM

2.2. Quasar microlensingQuasar microlensingOptical depth for quasar microlensing is order Optical depth for quasar microlensing is order of unity for all multiple quasars. of unity for all multiple quasars.

SADM microlensing will show color change. SADM microlensing will show color change.

3. Inhomogeneity in Lens Galaxy

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

Two differential Two differential extinction show extinction show positive correlation.positive correlation.

No negative ΔENo negative ΔEBB . .

Extinction curve for various n(H) (RV=3.1) ← Cardelli et al. (1989)

ΔEA-ΔEB diagram

4. Quasar Microlensing

When a stellar object in lens galaxy passes in When a stellar object in lens galaxy passes in front of an image, microlensing will occur. front of an image, microlensing will occur.

If matter in the lens galaxy consist only from If matter in the lens galaxy consist only from stellar objects, stellar objects, optical depth for quasar optical depth for quasar microlensing can be order of unitymicrolensing can be order of unity..

Einstein ring radius is comparable to the size Einstein ring radius is comparable to the size of accretion disk in quasars, and of accretion disk in quasars, and finite size finite size source effectsource effect is important for the quasar is important for the quasar microlensing. ⇒microlensing. ⇒ “color change”“color change”

Explanation for color change

If one of multiple image suffers microlensing, If one of multiple image suffers microlensing, color of the image will changecolor of the image will change..

In general, all image can always be suffer In general, all image can always be suffer quasar microlensing, independently.quasar microlensing, independently.

Extended source Compact source

Lens Object

time

Flux

Lens Object

time

Flux

Different color !

Magnification pattern

An example of color change

Randomly pick up epochs from this light Randomly pick up epochs from this light curve and compare colors at different curve and compare colors at different epochs. ⇒epochs. ⇒ “differential extinction”-like“differential extinction”-like

← 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]

ΔEA - ΔEB for microlensing

No apparent correlatiNo apparent correlation between two diffon between two differential extinctions.erential extinctions.

Both of positive and nBoth of positive and negative ΔEegative ΔEBB exist. exist.

(Average magnification, μ(Average magnification, μaveave, ,

for both image = 10 .for both image = 10 .

μμtottot=μ=μaveave+μ+μqmlqml(t) )(t) )

ΔEA-ΔEB diagram

5. Discussion

Except some special case, negative correlExcept some special case, negative correlations between two differential extinction cations between two differential extinction cannot be produced in the case of inhomogannot be produced in the case of inhomogeneity in lens galaxies. eneity in lens galaxies.

For positive correlation part, differential extFor positive correlation part, differential extinction can be explained by patchy extinctiinction can be explained by patchy extinction properties (on properties (more things to domore things to do). ).

However, quasar microlensing can easily However, quasar microlensing can easily reproduce observed color anomalyreproduce observed color anomaly..

Das Ende