What can emission lines tell us? lecture 2 Grazyna Stasinska

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What can emission lines tell us? lecture
2Grazyna Stasinska
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Diagnostics based on emission lines

• plasma diagnostics electron temperature, density
• ionic and elemental abundances - direct methods
• elemental abundances - statistical methods
• estimation of the effective temperature of the
  ionizing star- or of the effective hardness of
  the ionizing radiation field
• determining the star formation rate
• how to distinguish normal galaxies from AGN hosts?

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Diagnostics based on emission lines

• plasma diagnostics electron temperature, density
• ionic and elemental abundances - direct methods
• elemental abundances - statistical methods
• estimation of the effective temperature of the
  ionizing star- or of the effective hardness of
  the ionizing radiation field
• determining the star formation rate
• how to distinguish normal galaxies from AGN hosts?

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The most popular Te diagnostic
The most popular ne diagnostic

OIII4363/5007
SII6731/6716
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Some plasma diagnostics in X-rays

• Porquet Dubau (2000)
• He-like ions emit three main lines (n 2 shell),
  which are close in wavelengths
• resonance lines (called w),
• intercombination lines (x y),
• forbidden lines (z).
• the combination of the ratio of these lines can
  be used to derive
• the ionizing process (pure photoionized plasma or
  hybrid plasma)
• the electron density R(ne) z / x y
• the temperature G(Te) (x y) z /w

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Plasma diagnostic diagrams

Plasma diagnostic diagram for the planetary
nebula NGC 7027
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Diagnostics based on emission lines

• plasma diagnostics electron temperature, density
• ionic and elemental abundances - direct methods
• elemental abundances - statistical methods
• estimation of the effective temperature of the
  ionizing star- or of the effective hardness of
  the ionizing radiation field
• determining the star formation rate
• how to distinguish normal galaxies from AGN hosts?

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• The method for abundances from opticanl or IV
  lines
• Te and ne are obtained from plasma diagnostics
• Ionic abundance ratios are determined from line
  intensity ratios
• eg O/H (OIII5007/Hb) /
  (eOIII5007(Te)/eHb(Te))
• Elemental abundance ratios are obtained
• either by adding all the observed ions

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a note on ionization correction factors

• Ionization correction factors based on ionization
  potentials
• a first approximation promoted by
  Torres-Peimbert Peimbert 1977
• but risky eg (O ..)/O ? He/He (although
  O and He have the same ionization potential
  54.4 eV)
• there is nothing which empedes O ions to be
  present in the He zone
• Ionization correction factors based on model
  grids may be risky too
• observations often pertain only to a small
  fraction of the object while grids usually
  consider entire nebulae
• there is no robust formula to correct for He
• Cases when no icf is needed
• when all the expected ionization stages are
  observed
• however in this case beware of errors in
  determining ionic abundances
• from different spectral ranges
• from lines extremely sensitive to Te (lines with
  high excitation potential as UV lines or
  transauroral lines)

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a rough evaluation of Te-based methods

• the methods are easy to implement
• they depend on a very limited amount of
  assumptions
• error bars are relatively easy to estimate
• the abundances of the most important elements
  are expected to be correct (within error bars)
• they are very close to abundances obtained from
  successful tailored photoionization modelling
• from optical spectra abundances can be derived
  for He, N, O, Ne, S, Cl, Ar, Fe
• C is however a difficult subject

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a case of failure of Te-based abundances metal
rich HII r. Stasinska 2005

• with very large telescopes OIII4363/5007,
  NII5755/6584, SIII6312/9532 can be measured
  even at high metallicities (eg Bresolin et al
  2005)
• the problem
• at Z gt Z? strong Te gradients are predicted
• Te sensitive ratios strongly overestimate Te in
  the emitting zones

• O/H is strongly biased !
• the bias depends on
• what line is measured to derive Te
• what relation is adopted between T(O) and
  T(O)

T(O)TNII5755/6584 T(O)T(O)-3000/0.7
T(O) TNII5755/6584 T(O) TOIII4363/5007
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a further problem to derive Te at high
metallicity

• contamination of collisionally excited lines
  (CELs) by recombination
• at low Te, CELs with high excitation energy such
  as OII7330 or NII5755 may be dominated by
  recombination
• this effect, very strong in the case of
  TOII3727/7330 is usually not well corrected for
  in the literature (one should use the Te
  representative of the zone emitting the
  recombination line to correct for it)
• a similar effect is likely to occur for
  TSII4070/6720

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Diagnostics based on emission lines

• plasma diagnostics electron temperature, density
• ionic and elemental abundances - direct methods
• elemental abundances - statistical methods
• estimation of the effective temperature of the
  ionizing star- or of the effective hardness of
  the ionizing radiation field
• determining the star formation rate
• how to distinguish normal galaxies from AGN hosts?

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• In many cases, the weak OIII 4363 or NII5755
  lines are not available because
• the temperature is too low
• the spectra are of low signal-to-noise
• the data consist of narrow band images in the
  strongest lines only
• Strong line methods to derive abundances
• are statistical
• have to be calibrated
• Best known strong line methods the ones based on
  oxygen lines
• Pagel et al 1979 used (OIIOIII)/H??as an
  indicator of O/H
• this method, la??????????????????????????, has
  been calibrated many times
• Mc Gaugh 1994 refined the method to account for
  the ionization parameter U
• Pilyugin (2000, 2001 ..., 2005) proposed the most
  sophisticated approach

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Rationale of Mc Gaughs method

• there are 4 independent strong line ratios
• H??H?, OII/H?, OIII/H?, NII /H?
• there are 5 parameters determining them
• C(H?? , ltTgt, U, O/H, N/O
• underlying hypothesis of the method
• ltTgt is related to O/H
• (this is expected statistically for giant HII
  regions)
• the procedure
• both O/H and U are derived simultaneously from
• (OIIOIII)/Hb, and OIII/OII
• a problem
• (OIIOIII)/H??vs. O/H is double valued
• a way out

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McGaugh diagrams for the O23 method
???????????? versus ??????????????/???
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what lies behind the OIII5007/Hb vs O/H relation

• Intensity ratio OIII5007/Hb A
  n(O) / n(H) Te0.5 exp (-28800/Te)
• Thermal balance equation n(H) ne T B
  ni j ne Te-0.5 exp (- Eexc/Te)
• if 12 log O/H ltlt 8.2
• cooling is due to H Ly a,
• Te is independent of O/H
• OIII5007/Hb C T O/H
• if 12 log O/H gt 9
• cooling is due to OIII52,88m
• OIII5007/Hb C T f(Te)
• where f(Te) Te exp (- 28800/Te)
• which decreases
• with increasing O/H

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An evaluation of strong line methods

• Perez-Montero Diaz 2005

• uses a data base of 367 objects with measured Te
• including some giant HII regions in the inner
  parts of galaxies (expected to be metal rich)
• but ignores the strong bias due to low Te
  evidenced by Stasinska 05

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the strong line method recalibrated

• Pilyugin Thuan 2005

• upper branch calibration
• (ie high O/H)
• lower branch calibration
• (ie low O/H)
• uses a data base of over 700 objects with
  measured Te
• including some giant HII regions in the inner
  parts of galaxies (expected to be metal rich)
• uses only Te-derived abundances
• but ignores the strong bias due to low Te
  evidenced by Stasinska 05

the last word on abundances from strong line
methods is not said
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more on strong line methods for Giant HII
RegionsStasinska 2006

• Requirements for an ideal metallicity indicator
• should be single valued
• should have a behaviour dominated by a well
  understood physical reason
• should be unaffected by the presence of diffuse
  ionized gas
• should be independent of chemical evolution
• Looking for an ideal metallicity indicator
• data base of 670 objects in spirals, SDSS DR3
  and BCDs galaxies with Te measured
• using P calibration of Pilyugin 2001 when Te is
  not measured

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results two new well behaved metallicity
indicators

• ArIII/OIII
  
  SIII/OIII
• s0.23
  s0.25

but the lines are only moderately strong ...
nb all strong line methods will need
recalibration when we undertand better the
physics of metal-rich HII regions, (Stasinska
2005)
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comparison of O/H from various metallicity
indicators

• ArIII/OIII vs NII/Ha
• larger dispersion
• (effect of N/O and ionization variations)
• slight bias

• ArIII/OIII SIII/OIII
• very tight correlation (as expected)
• dispersion mostly from measurement errors

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Diagnostics based on emission lines

• plasma diagnostics electron temperature, density
• ionic and elemental abundances - direct methods
• elemental abundances - statistical methods
• estimation of the effective temperature of the
  ionizing star- or of the effective hardness of
  the ionizing radiation field
• determining the star formation rate
• how to distinguish normal galaxies from AGN hosts?

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Estimation of T by counting photons

• Zanstra 1931
• TZH is obtained assuming that all stellar Lyc
  photons are absorbed by the nebula,
• from the observed stellar visual magnitude and
  the total nebular H? flux

• for very hot stars (PN nuclei), one can also
  define TZHe using the He II 4686 flux as a
  measure of the number of photons with energies
  above 54.4 eV

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notes on Zanstra-type methods and on the
ionization of He

• results from model computations with PHOTO
• a . He I 5876 / H? measures T only in a small
  range (T lt 40 kK)
• due to competition between H and He to absorb
  photons with energies gt 54.4 eV
• c . HeII 4868 / H? saturates at T gt 150 kK
• c . HeII 4868 / H??depends on U at T gt 100 kK
• dependence on He/H
• c . HeII 4868 / H? does not depend on He/H
• e . HeII 4868 / He I 5876 depends on He/H
• not considered in empirical methods
• f . the H and He zones may have different Te

b
a
___ U10-2 He/H0.1 ___ U10-3 He/H0.1 ___
U10-2 He/H0.15
d
c
e
f
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T from observed ionization structure

• Kunze et al 1996
• The ionization structure depends on T
• -gt line ratios of two successive ions measure T
• but the ionization structure also depends on U
  !!!

T
(SIV/SIII) / (NeIII/NeII) vs NeIII/NeII
Morisset 2004 determination of T using a full
grid of atmospheres with WM-basic and taking
into account T, U and metallicity
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T from energy-balance methods

• Stoy 1931 Stasinska 1980
• L( ? CEL) / L(H?) f(T)
  Te is a function of O/H and T
• calibration by Preite-Martinez Pottasch 83

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Diagnostics based on emission lines

• plasma diagnostics electron temperature, density
• ionic and elemental abundances - direct methods
• elemental abundances - statistical methods
• estimation of the effective temperature of the
  ionizing star- or of the effective hardness of
  the ionizing radiation field
• determining the star formation rate
• how to distinguish normal galaxies from AGN hosts?

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star formation rate

• techiques
• UV continuum, FIR continuum, recombination
  lines, forbidden lines...
• each technique requires a calibration usually
  done with evolutionary stellar synthesis models
• basic parameters
• metallicity (Z)
• star formation history (SFH)
• description of the IMF
• stellar evolutionary tracks
• stellar model atmospheres
• see reviews by Kennicutt 1998 and Schaerer 1999

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star formation rate using L(H?)

• Kennicutt 1998
• SFR M? yr -1 7.910-42 L(H?)erg s-1 A(H?) /
  f
• where A(H?) is the extinction
• f is the fraction of Lyc photons absorbed by H

Scharer 1999 the SFR from L H? strongly depends
on assumed parameters for the stellar population
temporal evolution of models with cst SFR
IMF Mup Z/Z
? _____ Salpeter 100 1 .......... Salpeter
100 .05 _ _ _ _ Salpeter 100 2 _ . _
. Salpeter 30 1 ------ Scalo 100
1
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star formation rate using OII

• advantage of OII
• is seen in a broad redshift range, rather used at
  large redshifts ( 1)
• caution about OII
• calibrations by different authors differ strongly
  (see Kennicutt 1998)
• OII/H? is expected to vary with metallicity and
  U
• OII can be produced by ionization by an active
  galactic nucleus AGN and not by stars
• exemple of observed dispersion in OII/H?

• data from subsample of SDSS DR3
• normal star forming galaxies
• AGN host galaxies
• hybrid

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Diagnostics based on emission lines

• plasma diagnostics electron temperature, density
• ionic and elemental abundances - direct methods
• elemental abundances - statistical methods
• estimation of the effective temperature of the
  ionizing star- or of the effective hardness of
  the ionizing radiation field
• determining the star formation rate
• how to distinguish normal star forming galaxies
  from AGN hosts?

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Segregation of emission line objects in
emission-line ratio diagrams

• The BPT diagram
• Baldwin, Phillips, Terlevich 1981
• ??e????? ??????????????????? i??????????????????
  of the diagram
• Interpretation
• photons from PNe and AGNs are harder than those
  from massive stars that power GHRs
• ? they provide more heating
• ? collisionally excited lines will be brighter
  than in the case of ionization by massive stars
  only

OIII/Hb
PNe
AGNs
GHRs
NII/Ha
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The next step

• Veilleux Osterbrock 1987
• more diagrams, more points
• GHRs form a sequence in the OIII/Hb vs
  NII/Ha?and OIII/Hb vs SII/Ha
• comparison with sequences of photoionization
  models

OIII/Hb vs NII/Ha
OIII/Hb vs SII/Ha OIII/Hb
vs OI/Ha
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the Sloan Digital Sky Survey revolution

• Kauffmann et al 2003
• spectra of 100 000 galaxies
• subtraction of stellar continua obtained by
  population synthesis
• galaxies hosting AGNs also form a sequence!

galaxies in the BPT diagram now remind the wings
of a seagull
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modelling of the upper envelope of the left
wingStasinska Cid Fernandes Mateus Sodre Vale
Asari 2006

• motivation
• previous dividing lines were too generous for
  NSF galaxies
• the model
• (uses Starburst99 PHOTO)
• constant star formation
• abundance ratios taken from Izotov et al 2006
• result
• U decreases az Z increases
• OI and SII lines less well fitted (because
  of 1-zone model)

of the 4 diagrams, the OIII/H? vs NII/H? is
the best to distinguish NFSg and AGN hosts
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can one distinguish AGN hosts and NSF galaxies
with their NII/Ha?only ?

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distinguishing AGN hosts and NSF galaxies using
only NII/Ha

NSF
hybrid

• feasible
• allows one to consider more galaxies of the
  initial sample (intensities of OIII and Hb not
  needed)
• allows one to see relations with another
  parameter (here D4000)

AGN
all
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end of lecture 2