Abundances in the blr
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Abundances in the BLR. Nathan Stock February 19, 2007. Motivation. Metallicity affects properties of the AGN (Ferland, 1996) Opacity Kinematics Structure Outflows enrich IGM (Friaca, 1998) Dust increases obscuration of high-z objects. Motivation.

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Abundances in the BLR

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Abundances in the BLR

Nathan Stock

February 19, 2007


Metallicity affects properties of the AGN (Ferland, 1996)




Outflows enrich IGM (Friaca, 1998)

Dust increases obscuration of high-z objects


Provides information on chemical history of the gas (Ferland 1996)

Representative of much larger region (~100’s pc) than BLR itself (>1 pc)

Constrain early nucleosynthesis / SFR in the host galaxy

Typical QSO Spectra

Hamann 1999

Collisionally excited lines

Line strengths of collisionally excited lines

High density limit

Low density limit

Hamann 1999

Recombination lines

Especially relevant for H, He

Line strength (oversimplification)

Hamann 1999

Calculating abundances

Take the ratio of two lines

Adjust from abundance of element in ionized state to total abundance

Express in standard form

Hamann 1999

Typical QSO Spectra

Hamann 1999

But wait…

  • CIV acts as a cooling

  • line: as C/H ↑,

  • temperature ↓

CIV/Lyα ratio does

not depend on


Hamann 1999

And there’s more…

We want the lines to be emitted from the same region of the BLR

i.e. elements have similar ionization energies

Differing ne, β in different regions

Different continua as well

Hamann 1999

And more…

We want similar values of ncrit

Removes possible dependence on ne

Both emitters either in high density or low density limit

Note: similar ncrit does not help if the emitters are in different regions of the BLR!

Hamann 1999

Problem 1: Appropriate element

To find metallicity, we cannot use C/H when

Z>.02 Zsun

Moreover, C and other elements don’t have ions

in the same range as HII

Ratios of elements to each other would also

seem counterproductive, but…

Hamann 1999

Secondary production of N

C, N, O produced in later stages of stars

N is also produced in

CNO cycle

Valid for Z>.2 Zsun

Result: N/O α O/H α Z

Hamann 1999

Caveat: secondary N is delayed

Delayed production will be important if Z enrichment is faster than stellar lifetimes

This is true in dense environments


q is a delay factor

q=0 in slow evolution limit

q~.5 in fastest evolutions

Hamann 2001

Problem 2: Appropriate ionization

We want lines that are emitted from the same region of the BLR (co-spatial)

Need to model ionization regions

Define U, nH, abundance, incident spectrum

Use CLOUDY to identify ionized regions

Hamann 2001

  • Results for a ‘typical’ BLR

    • nH = 10^10

    • U = .1

    • Solar abundances

Hamann 2001

Problem 3: Appropriate ncrit

We want critical densities to be similar

Ensures ions are in same

density limit in each part of the


Hamann 2001

Putting it together

We can model how emission line intensities (equivalent widths) vary…

…with the flux of incident ionizing photons (ΦH)

… with hydrogen density (nH)

Integrating the intensities over (ΦH, nH) space (with appropriate weighting) gives us the total line strength

Hamann 2001

Example: CIII] λ977

ΦHtoo low, C is neutral

ΦHtoo high, C is ionized

In both limits, the CIII]

equivalent width is weak

nH too high, exceed ncrit

Emission line is collisionally suppressed

nH too low, forbidden lines become efficient coolants

Gas temperature drops, weakening emission lines

Hamann 2001

Equivalent Widths of lines

Hamann 2001

Flux Ratios of lines

Hamann 2001

Total line strength

Integrate over the space

to find the line strength

714 for log nH

1724 for log ΦH

Assume equal weighting

Previously shown to reproduce AGN broad emission lines fairly accurately (Baldwin 1997)

Similarly, we can find the total line ratio

Hamann 2001

Finding line ratio – Z relations

Previously considered solar abundance

Now, vary the abundances

Recalculate emission line ratios

Hamann 2001

  • Dependence affected by shape of incident spectrum

    • Solid curve: MF87 spectrum

    • Dotted curve: α=-1 power law

    • Dashed curve: segmented power law

Hamann 2001

  • NIII]/OIII] and

  • NV/HeII:

  • N found in narrower region  line ratios will underestimate abundances

Things to note:

Hamann 2001

Possible NV contamination?


NV line at λ1240, Lyα at λ1216

If NV is moving away from Lyα at the right velocity, it can absorb and rescatter Lyα emission

v ~ 5900 km/s  achievable in BAL winds


Hamann 1999

BAL contributions likely small

Only ~30% of Lyα photons interact with NV in BAL

Most Lyα passes through

Given 12% BAL covering factor & typical BAL velocity profile


Moreover, BAL peak would be much wider

>10,000 km/s BAL vs. 2500 km/s BLR half-widths

Do not see this in spectra

Hamann 1996

Line ratios taken from the literature on quasars imply they have BLR regions which have greater than solar metallicities.


Hamann 2001

Abundances in Quasars

All the most robust line ratios show

Z~2-3Zsolar is typical all quasars

Abundances constitute a lower limit on actual Z because we assumed q=0

In quasars, q is almost certainly not 0!

Hamann 2001

More quasar data

70 quasars,

each Z found

by averaging

several N ratios

Average metallicty of the sample: Z~4 Zsolar

Dietrich, 2003

What does this high Z tell us?

High metallicity  significant chemical evolution occurred before our observations

Even at the highest redshift quasars!

Chemical evolution models imply most of the original gas has been processed by stars

Vigorous star formation and evolution likely precedes the epoch of quasar activity

Have ~1-2 Gyr of stellar formation time at z=5

Hamann 2001

Metallicity-Luminosity Relation?

Metallicity-tracking line ratios in the BLR do not appear to correlate with redshift

However, it DOES appear to correlate with quasar luminosity

Nagao 2006

Metallicity-Luminosity Relation?

Is this relation fundamental or apparent?

Evidence that it’s MBH that really matters

Warner 2007

Metallicity-MBH Relation

What brings about this relation?

MBHα Mhost (at least in local universe)

We expect a more massive host galaxy to have a higher metallicity

More stars producing elements

Evidence that MBHα Mhost relation applies at high redshift as well

A possible way to find host masses even at high redshift?

Hamann 2007

So what’s next?

Direct measurements of host galaxies

Mass, age, metallicity

More samples, over a variety of properties

Higher redshifts, lower luminosities

The usual pushing the limits of what we can observe

Other abundances

Not just C, N, O

Fe, Si?

Hamann 2007


Dietrich, M., Hamann, F., et al. 2003, 589, 722

Ferland, G.J., Baldwin, J.A., Korista, K.T., et al. 1996, ApJ, 461, 683

Hamann, F., Dietrich, F. & Ferland, G.J. astro-ph 0701503

Hamann, F., & Ferland, G.J. 1999, ARA&A, 37, 487

Hamann, F., & Korista, K.T. 1996, ApJ, 464, 158

Hamann, F., Korista, K.T., Ferland, G.J., et al. Astro-ph 0109006

Nagao, T., Marconi, A., & Maiolino, R. 2006, A&A, 447, 863

Sadat, R., Guiderdoni, B. Silk, J. 2001,°a, 369, 26

Warner, C., Hamann, F., & Dietrich, M. 2007, ApJ, submitted

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