Absorbers at z 23 Photoionized by Quasars or Tracers of Hot Gas Andrew Fox ESOChile Jacqueline Bergeron amp Patrick Petitjean IAPParis H I H II Si III Si ID: 377845
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Slide1
O VI Absorbers at z=2-3Photoionized by Quasars or Tracers of Hot Gas?
Andrew Fox (ESO-Chile)
Jacqueline Bergeron & Patrick
Petitjean
(IAP-Paris)Slide2
H
I
–H
II
Si III-Si IV C III-C IV He II-He III N IV-N V O V-O VI
Metal lines as tracers of ionization level
13.6
eV 33.5 eV 47.9 eV 54 eV 77.9 eV 113.9 eV
O
VI advantages: O VI is most highly ionized line available in rest-frame UV Oxygen is most abundant metal in Universe O VI doublet at 1031, 1037 Å is intrinsically strong
O VI disadvantage: O VI falls in Ly-a forest blending/contamination. Only detectable at z2-3.
EnergySlide3
O VI
absorbers have power-law column density distribution (Bergeron & Herbert-Fort 2005)
“
Associated
” or “proximate” absorbers (at dv<5000 km s-1 from QSO) often removed from sample affected by ionization conditions close to QSO. This talk: Examine this practice (Fox, Bergeron, & Petitjean 2008, MNRAS)VLT/UVES, Keck/HIRES studies Schaye et al. 2000 Bergeron et al. 2002 Carswell
et al. 2002 Simcoe et al. 2002,2004,2006
Levshakov et al. 2003 Reimers et al. 2001, 2006
Bergeron & Herbert-Fort 2005 Lopez et al. 2007 Gonçalves et al. 2008O VI probes
IGM ionization and enrichmentIs there a proximity effect in O
VI?Slide4
VLT/UVES Large Program
20 QSOs, high resolution (FWHM 6.6 km s
-1
) and high S/N (~40–60)
Searched for O VI absorbers within 8000 km s-1 of zQSO.zQSO is determined from several QSO emission lines, allowing for systematic shifts (Tytler & Fan 1992)35 proximate O VI systems detected:26 weak systems9 strong systemsUVES Spectra
-200 0 km/s 200
-200 0 km/s 200Slide5
Two Populations of Proximate O VI
WEAK
log
N
(O VI)≤14.5
Weak
N V
and C IV
1 or 2 components
Velocities < zQSO
No evidence for partial coverageSTRONG log N(O VI) ≥ 15 Strong N V and C IV Multiple components
Velocities clustered around
z
QSO
Occasional evidence for partial coverage of continuum source.
Truly intrinsic: inflow/outflow near AGN central engine (several mini-BALs)Slide6
“Proximity Effect”: change in dN/dz at 2000 km s-1
Proximity zone extends over ~2000 km s
-1
, not 5000 km s
-1.Intervening systems(Bergeron & Herbert-Fort 2005)Slide7
Weak O VI absorbers: trends with proximity
At 2000 km s
-1
, see change in
N(H I) and in N(C IV) but not in N(O VI)Slide8
Furthermore, O VI/HI offsets are observed
Significant
velocity
centroid offsets
between O VI and H I are seen in ~50% of the weak O VI absorbers two ions are not co-spatial. (similar fraction of low-z O VI absorbers show offsets; Tripp et al. 2008)Slide9
Median b-values O
VI
<2000 km s
-1
from QSO: b=12.3 km s-1 Intervening O VI: b =12.7 km s-1 T <1.6x105 K Intervening N V: b =6.0 km s-1
(Fechner & Richter 2009) O VI
and N V trace different regions
O VI Component Line Width Distribution
b
=(2kT/m +
b2non-thermal)Slide10
O
VI
absorbers (even narrow) may not be
photoionized
; can be formed in non-equilibrium cooling gasResults of Gnat & Sternberg (2007)Frozen-in ionization can lead to O VI being present in gas down to ~104 K if the metallicity is close to solarSlide11
Are there any physical reasons why such hot gas should exist at z=2-3?
YES: Galactic Winds
YES: Hot-mode accretion
Simulations from
Kawata & Rauch (2007)Simulations from Dekel & Birnboim (2007)See also
Fangano, Ferrara, & Richter (2007)Slide12
Comparison of high-ion ratiosObservations
vs
theory (Gnat & Sternberg)
Cooling gas models can explain data
if elemental abundance ratios are non-solar:Need -1.8<[N/O]<0.4 -1.9 <[C/O]<0.6Slide13
Single-phase photoionization models for IGM O VI absorbers are too simplistic, becauseO VI-H
I
velocity offsets imply O
5+
and H0 occupy different regionsO5+ may be collisionally- rather than photo-ionizedDon’t know EGB shape above 100 eV that wellUse caution when combining O VI/H I ratio + CLOUDY IGM metallicity
Implications for O VI absorbers in general
H
0
, O
5+
, T~104 KH0, T~104 K
O
5+
,
T
~10
5
K
O
6
+
, O
7+
T
≥
10
6
K
EGB
What you see in H
I
What you see in O
VISlide14
2000 km/s Proximity
Warm plasma
photoionized
as you approach z(QSO), not hot plasma
QSO
N(H I)~10
15
N(O VI)~10
13.5
N(H I
)~10
14
N(O VI)~10
13.5Slide15
Almost 1
dex
uncertainty in
Jn at 113.9 eV!!!Simcoe et al. (2004)Slide16
Sawtooth modulation by He II Lyman series exacerbates the situation
Madau
&
Haardt
(2009)We don’t really know what’s happening out here!Slide17
In 20 high-quality QSO spectra from UVES, we search for O VI within 8000 kms-1 of zQSO
, finding
9
strong
absorbers (truly intrinsic, gas near AGN)26 weak absorbersAmong weak O VI absorbers:dN/dz increases by factor of 3 inside 2000 km s-1dN/dz in range 2000-8000 km s-1 matches intervening.N(H I) and N(C IV) show a proximity effect (dependence on
Dv), N(O VI
) does not.O VI-H I
velocity centroid offsets imply at least half the absorbers are multiphase.Cannot use O VI absorbers to probe high-energy tail of EGB: too many systematic uncertainties. Narrow O VI can form in radiatively-cooling hot gas, in interface regions that result from galactic winds/hot-mode accretion
Survey for Proximate O VI: SummarySlide18Slide19Slide20
Partial Coverage of Continuum SourceSlide21
O VI absorber size is <200 kpc, based on lack of Hubble broadening.
Simcoe et al. 2002Slide22
Strong O VI: Yes, we see strong O VI clustered around z
QSO
Weak O
VI
: Yes, we see dN/dz increase by a factor of three within 2000 km s-1 (but galaxies are clustered near quasars).No, the internal properties (b-values, log N) of the O VI absorbers do not depend on Dv, unlike H I and C IVIs there a line-of-sight proximity effect in O VI?
Is there
observational evidence (at
z
=2-3) for an extended (~10
Mpc) QSO proximity zone of E>100
eV photons?No: properties of weak O VI do not require photons at E>100 eV
(you can create the O
VI
with [cooled] hot gas)Slide23
C IV ionization fraction in hot gas Proximity zone extends over ~2000 km s
-1
, not 5000 km s
-1
.convert QSO B-magnitude and zQSO to L912 (Rollinde et al. 2005)Determine size of “Stromgren Sphere” where QSO radiation density exceeds estimated EGB radiation density at z=2.5 ( ~10 Mpc)Convert size to velocity assuming Hubble Flow and H(z=2.5)=250 km s-1 Mpc
-1 (1500-2500 km s-1)