Red quasars in the2XMMi/SDSS-DR7 cross-correlation

Angel Ruiz (INAF-OAB)&R. della Ceca, A. Caccianiga, P. Severgnini, F. X. Pineau, V. Braito

Introduction

Quasars typically selected as point-like objects bluer than stars (e.g. U-B< -0.44)

Blue colors related to the Big Blue Bump (BBB).

Red quasars Reddening by dust (Cutri et al. 2003)

New surveys discovered a population of intrinsic red quasars (no dust) Diminished BBB (Hall et al. 2006, Young et al. 2008)

Long term objectives:Build a large sample of intrinsic red quasars

Check the absence of the BBB (opt-UV SED)

Understand why the BBB has disappeared (e.g. low accretion rates)

Young et al. 2008: SDSS-dr3 / XMM Archive cross-correlation.Selection criteria: g-r>0.5

To avoid host galaxy (z>0.5) and Lya forest (z0.35

D(g-i)

g-r

D(g-i)

g-r

Outline

Separate dust reddening QSO and intrinsic red QSO.

Study of the X-ray spectra:XMM-Newton/EPIC spectra automatically extracted (V. Braito) bright sources (>100 counts): 25 sources

Goal: estimate X-ray absorption.

Spectral model:Power law + intrinsic absorption

Power law + NH upper limit (90% confidence)

Study of the optical spectra:Goal: estimate E(B-V)

Spectral model:Composite QSO template (Vanden Berk et al. 2001)

SMC extinction law (Gordon et al. 2003)

X-ray results

1-2 keV flux / erg s-1 cm-2

c2 / d.o.f.

All sources well fitted

X-ray results

1-2 keV flux (erg s-1 cm-2)

NH (x1022 cm-2)

Absorption limit

X-ray results

1-2 keV flux (erg s-1 cm-2)

G

Young et al. 2008

Mateos et al. 2005

= 1.7 0.4

X-ray results

1-2 keV flux (erg s-1 cm-2)

G

= 1.7 0.4

Absorption limit

X-ray results

Xray absorbed sources:NH > 1022 @ 90% confidence: 4 sources

NH UL > 1022 and G 1022 @ 90% confidence: 4 sources

NH UL > 1022 and G0

Normal blue PL cont.: D(u-r) ~ D(g-i) ~ D(r-z)

Dust reddening cont.: D(u-r) > D(g-i) > D(r-z)

Redder PL continuum: D(u-r) < D(g-i) > D(r-z)

D(u-r) D(g-i) > 0 Dust red. continuumD(u-r) D(g-i) < 0 Intrinsic red continuum

Continuum shape

E(B-V)

D(u-r) - D(g-i)

dust
continuum

red
continuum

Continuum shape

NH (x1022 cm-2)

D(u-r) - D(g-i)

X-ray absorption vs. optical extinction

NH (x1022 cm-2)

E(B-V)

High X-ray absorption Low X-ray absorption

GALEX UV colours

dust
continuum

red
continuum

15 / 19 sources with GALEX data

NUV - z

D(u-r) - D(g-i)

GALEX UV colours

dust
continuum

red
continuum

15 / 19 sources with GALEX data

FUV - z

D(u-r) - D(g-i)

Gas-to-dust ratio

NH / E(B-V)

Galactic ratio

high E(B-V) dust continuum int. red continuum

D(u-r) - D(g-i)

Conclusions

No clear link between X-ray absorption and optical obscuration (i.e. due to gas and dust) X-ray properties are not reliable to distinguish intrinsically red QSO.

3-8 intrinsically red QSO: 12-30% of the X-ray bright sample < 40% of Young et al. 2008

Conclusions

No clear link between X-ray absorption and optical obscuration (i.e. due to gas and dust) X-ray properties are not reliable to distinguish intrinsically red QSO.

3-8 intrinsically red QSO: 12-30% of the X-ray bright sample < 40% of Young et al. 2008

TODO:Correct optical photometry for strong emission lines.

Extend the analysis to the X-ray faint sources.

Build optical-to-X-ray SED of the continuum.