1. Searching for Metal Absorbers up to Redshift 7
Sarah Bosman, IoA
George Becker
(University of California Riverside)
Martin Haehnelt

2. What are Metal Absorbers?
Detected as foreground in continuum quasar light
Lyman-α ↔ neutral H
Eg: C IV 1548 ↔ triply ionised carbon
Wavelength (A)
Ion. Potential (eV)
CIV
1548, 1550
(CIII 47.89)
MgII
2796, 2803
15.03
CII
1334
24.38
SiII
1526
16.34
FeII
2382, 2600,
16.18
Unlike objects detected through the light they emit, metal absorbers are detected through the light they absorb from a background source
In the same way as the Lyman-alpha forest originates from intergalactic gas containing neutral hydrogen, metal absorption results from gas in the correct ionisation state to absorb a particular frequency of light.
Sarah Bosman Wednesday lunch seminar

3. What are Metal Absorbers?
This is a classic illustration of the lyman-alpha forest by andrew pontzen
So, we've got a background source (quasar) emitting continuum light as well as specific broad emission lines; the light redshifts as it moves along the line of sight towards us, and when the intervening material contains neutral hydrogen one of these thin absorption features appears at the redshift of the absorber.
(play again)
This line over here is carbon 4, and analogously to the lyman-alpha forest, if an intervening system contains triply-ionised carbon it will leave an inprint in the continuum light at the redshift of the absorber.
In this illustration this happens when the light psses through a galaxy and a couple of absorption features are produced: DLA, LLS
But unlinke the lyman-alpha forest, these metal forests are discrete even at high redshift, that is, an absorption feature is only produced from regions containing the right metal and right ionisation state, which is typically around objects rather than the intergalactic medium itself
Credit: Andrew Pontzen
Sarah Bosman Wednesday lunch seminar

4. Why do we care?
Reason 1: Physical state of the intergalactic medium (IGM) and circumgalactic medium (CGM)
number density of absorbers,
metal enrichment
at z > 6.
Sarah Bosman Wednesday lunch seminar

5. Why do we care? Reason 1: Physical state of the IGM and CGM
eg: CIV at low-z traces gas in the CGM of galaxies photoinised by the ultraviolet background (UVB)
C IV region
C IV region
CIV at low-z traces gas in the CGM of galaxies LARGELY THOUGHT TO BE photoinised by the UVB
UV background
UV background
low-z: hard UV photons from
surrounding objects
C ejected in outflows
Sarah Bosman Wednesday lunch seminar

6. Why do we care? Reason 1: Physical state of the IGM and CGM
eg: CIV at high-z will be rarer due to softer UVB
C IV region
C IV region
CIV at low-z traces gas in the CGM of galaxies LARGELY THOUGHT TO BE photoinised by the UVB
UV background
UV background
High-z: CIV rarer
and weaker
C ejected in outflows
Sarah Bosman Wednesday lunch seminar

7. Why do we care? Reason 1: Physical state of the IGM and CGM
CIV appears
to drop.
- less C?
- less hard
Photons?
Strong and weaker systems
Strong systems only
D'Odorico+13 (black)
and others
Sarah Bosman Wednesday lunch seminar

8. Why do we care? Reason 1: Physical state of the IGM and CGM
eg: different gas phases around galaxies probed by absorption systems
SIMPLIFIED PICTURE
These circles represent average cross-sections of regions but we don't know they're spherical
DLA region
Sarah Bosman Wednesday lunch seminar

9. Why do we care? Reason 1: Physical state of the IGM and CGM
Strong absorption systems have smaller cross-section and probe preferentially the inner regions of galactic haloes
DLA region
Lyman-limit system
Sarah Bosman Wednesday lunch seminar

10. Why do we care? Reason 1: Physical state of the IGM and CGM - MgII
Cross-section ↔
Incidence rate
DLA region
Lyman-limit system
Sarah Bosman Wednesday lunch seminar

11. Why do we care? Reason 1: Physical state of the IGM and CGM - MgII
Cross-section ↔
Incidence rate
filaments
+ cold inflows
complicate the picture
In practise weak MgII systems can be associated with H density (much) lower than LLS
DLA region
MgII absorption
Lyman-limit system
Sarah Bosman Wednesday lunch seminar

12. Why do we care? Reason 1: Physical state of the IGM and CGM - MgII
Cross-section ↔
Incidence rate
filaments:
cold inflows
Especially at high redshift where galaxy formation is active
High z: MgII doesn't decline
dramatically but evidence
that DLA region shrinks.
Change in relative size of regions
due to softer UVB?
Link to filament collapse?
Mixing?
Galaxy growth?
Strong MgII systems go missing at a rate which follows global SFR density
DLAs have lower number density, which is classically interpreted as the regions shriking
DLA region
outflows
CIV region
probes warmer,
ionised gas in
haloes
MgII absorption
Lyman-limit system
Sarah Bosman Wednesday lunch seminar

13. Why do we care? Reason 2: Study early Galaxy formation and Evolution
Metal absorption is one of the only
observables for galaxies at z>6.
Emission: cold gas outflows
SFR, stellar mass
Metal absorption:
Different gas phases
Warmer, ionised flows
metal absorbers provide information on galaxy formation & evolution that complements other probes (studying galaxies in emission).
This is particularly useful at high redshifts, where galaxy evolution is in its early stages.
Metal abs. probes the baryonic cycle at high redshift.
→ enrichment, in/outflows, etc
+ reionisation: sources not visible in Lyman-a forest, emission weak, absorption lines could be useful
Maiolino+15 z~7 object
Sarah Bosman Wednesday lunch seminar

14. Why do we care?
Reason 3: Particular abundance ratios of metals could be first direct evidence of Pop III stars
Not polluted by later generations of stars
Oxygen/iron many time solar
Potentially carbon/iron many times solar
INTRO CCL:
Constraints on various metal absorbers which correlate with various gas phases give us OBSERVABLES which models and simulations try to reproduce,
at the interface of galaxy evolution models and large-scale properties and structures.
Cooke & Madau 14
Sarah Bosman Wednesday lunch seminar

15. How do we find them? Bright, distant quasar sight lines.
z=7 quasar J : unique opportunity to probe highest redshifts yet
Discovered by MORTLOCK ET AL 11
This is a redshift 7 quasar, the only one know so far
Various features are
(a) very strong absorption bluewards of Lyman-alpha, could indicate presence of neutral gas – I will not talk about this today
(b) this is a composite spectra made from lower-z quasars, it's remarkable how well it fits, which indicates the underlying quasar process is remarkably similar across redshift
(c) these absorption features which is what I'm gonna talk about
But for that we need a better spectrum
Mortlock+11
Sarah Bosman Wednesday lunch seminar

16. How do we find them? Higher quality spectrum obtained: 32 hours on
VLT's X-SHOOTER vis+nir spectrograph
Resolution 36 km/s with SNR ~ 25
And a better spectrum is exactly what we got
The best and only quasar spectrum at those redshifts
Proposal by
Becker, McMahon,
Hewett, Haehnelt +
10000
10500
12000
12500
13000
Sarah Bosman Wednesday lunch seminar

17. How do we find them?
Look for pairs of absorption lines, from same transition or species which correlate:
Left: C IV absorbs at
1548A and 1550A
always in ratio 2:1
+ use species with
similar ionisation
threshold eg Si II and O I
Additional single lines
by looking at the same
redshift as found pairs
Optical depths are in ratio 2:1
Similar ionisation threshold, Or any species you might expect to find in the same absorber
Flux
Wavelength (A)
12410
12420
12430
12440
Sarah Bosman Wednesday lunch seminar

18. How do we find them?
Automated code trained on half of the z>5 spectrum
Fake lines
introduced to
test
completeness:
LogN (CIV) cm-2
Sarah Bosman Wednesday lunch seminar
Percent fake lines recovered

19. Results: 6 systems at z>5.5
-200
-100
+100
+200
Velocity (km s-1)
Sarah Bosman Wednesday lunch seminar

20. Results: CIV
Previous work indicates decline in CIV systems at z>5, especially strong systems
Occurrence rate
per pathlength
explored
Black:
D'Odorico+13
Only strong systems
Log f
Log N(CIV)
Strength of systems
Sarah Bosman Wednesday lunch seminar

21. Results: CIV
Previous work indicates decline in CIV systems at z>5, especially strong systems
Occurrence rate
per pathlength
explored
Red, black:
D'Odorico+13
Only strong systems
Log f
Log N(CIV)
Strength of systems
Sarah Bosman Wednesday lunch seminar

22. Results: CIV
Previous work indicates decline in CIV systems at z>5, especially strong systems
Occurrence rate
per pathlength
explored
Red, black:
D'Odorico+13
Blue:
This work
Only strong systems
Log f
Log N(CIV)
Strength of systems
Sarah Bosman Wednesday lunch seminar

23. Results: CIV Cosmic mass density (x 10-8) Redshift
Intuitive explanation: from 4 to low redshift increase driven by build up of carbon in the CGM from SF
If CIV does stay flat 6-7, would suggest that while overall abundance is increasing (must be) the fraction visible as CIV is decreasing
Changes in density:
The winds are reaching more rarefied regions
Structure formation+expansion competing
Changes in UVB
Less metal-rich stars → softer UVB
Most likely power-law parameters computed by Poisson-statistics Monte-Carlo
Weak (~1 sigma) evidence for steepening ie abundance of strong systems declines while weak ones don't
Overall CIV abundance consistent across range
5.2 < z < 7.0, after declining by factor ~2 from z = 4.5 and factor ~10 from z = 1.5
1.5 – 4.5 – 5.2 – 7
4.3 – 1.3 – 1.1 – 0.7
Redshift
Cosmic mass density (x 10-8)
Sarah Bosman Wednesday lunch seminar

24. Results: MgII Contrast to CIV Also only strong MgII systems
Significant lack of
detections of
strong MgII
systems
Contrast to CIV
Also only strong MgII systems
Strong MgII systems with EW > 2 Å
Sarah Bosman Wednesday lunch seminar

25. Results: MgII
Overabundance of weak systems compared to previous work with same sensitivity:
Abundance of
systems per
pathlength
Matejek & Simcoe 12
Rest EW (MgII)
Strength of systems
Sarah Bosman Wednesday lunch seminar

26. Results: MgII
Overabundance of weak systems compared to previous work with same sensitivity:
Abundance of
systems per
pathlength
Matejek & Simcoe 12
Rest EW (MgII)
Strength of systems
Sarah Bosman Wednesday lunch seminar

27. Results: MgII
Overabundance of weak systems compared to previous work with same sensitivity:
Abundance of
systems per
pathlength
Matejek & Simcoe 12
Rest EW (MgII)
Strength of systems
Sarah Bosman Wednesday lunch seminar

28. Results: MgII
Overabundance of weak systems compared to previous work with same sensitivity:
Abundance of
systems per
pathlength
Matejek & Simcoe 12
Rest EW (MgII)
Strength of systems
Sarah Bosman Wednesday lunch seminar

29. Results: MgII
Overabundance of weak systems compared to previous work with same sensitivity:
Abundance of
systems per
pathlength
Apparent increase of weak MgII systems.
Fewer hard photons?
Matejek & Simcoe 12
Rest EW (MgII)
Strength of systems
Sarah Bosman Wednesday lunch seminar

30. Results: MgII
Overabundance of weak systems compared to previous work with same sensitivity:
Best fit PL
much steeper and
higher intercept:
underlying z~5 (red)
distribution is very
(>3 sigma) unlikely
Abundance of
systems per
pathlength
The non-detections are important too
Evidence of downsizing of MgII systems with redshift:
Strong systems getting progressively rarer starting at z>2
Weak systems becoming more abundant at z>5.5
Important caveat:
Statistics might not be Poisson: large-scale structure (eg filaments, voids) makes (non-)detections non-independent.
→ need more sigthlines to estimate cosmic variance in addition to statistical error bars
Strong evidence
of evolution from
lower z.
Rest EW (MgII)
Strength of systems
Sarah Bosman Wednesday lunch seminar

31. Why do we care? Reason 1: Physical state of the IGM and CGM - MgII
Cross-section ↔
Incidence rate
filaments:
cold inflows
Physical interpretation(s):
Small to big: with time, some of these weak systems become strong and some don't; early SFR bursts?
Fewer hard photons in UVB allows MgII regions to extend further from host: increases the chance of seeing weak systems
DLA region
outflows
CIV region
probes warmer,
ionised gas in
haloes
MgII absorption
Lyman-limit system
Sarah Bosman Wednesday lunch seminar

32. Results: CII - briefly
Models predict CII to CIV ratio, expect equal occurrence cross-over at high enough z
CIV
Number density
J1120 data folded into highest-z bin
Does CII/CIV increase?
UV background becomes softer?
Link to helium reionisation: if helium were completely neutral, hard UV photons generated by stars couldn't travel through IGM.
Remaining CIV system are close to objects? In any case there is at least a bubble around the objects where He is first-stage ionised.
CII
Redshift
Sarah Bosman Wednesday lunch seminar

33. Conclusions First quasar line of sight up to redshift 7
↔ metals probe different gas phases
CIV is still present at redshift 6.5
There are still some hard photons present.
MgII, tracing the slightly ionised gas around galaxies is appearing as weaker systems at z=6 compared to z=5 and lower
Fewer metals? UV background spectrum softer?
CII/CIV appears to decrease (maybe)
Sarah Bosman Wednesday lunch seminar

34. The future Need more data! More sightlines to quasars at z>6.5
Observing these high-z intervening systems with MUSE : what are they?
Robust comparison to simulations which resolve metal enrichment

35. Thanks!

36. Density-temperature distribution of metals
Laura Keating