Spectroscopy of Gaseous Nebulae in Galaxies

M. J. Barlow, Dept of Physics & Astronomy, UCL OUTLINE:

(i) Overview of ionized nebulae: HII regions, planetary nebulae and supernova remnants (ii) Determining nebular temperatures, densities and ionic abundances from collisionally excited lines (iii) Measuring element abundances in external galaxies (iv) Probing nebulae with deep spectra: s-process element forbidden lines and recombination lines from C, N, O and Ne. (v) Heavy element abundances from forbidden lines vs. those from optical recombination lines Photoionized Nebulae: two main types

HII regions: the brightest examples are the result of the formation of massive stars (> 20 Msun), which then ionize and disperse the natal cloud. The ionizing stars have temperatures of 25,000 K < Tstar < 50,000 K.

Planetary Nebulae: the final stage in the life of low and intermediate mass stars before they become white dwarfs. The outer envelope is ejected during the red giant AGB phase and then photoionized by the hot remnant star, with 25,000 K < Tstar < 200,000 K. The Orion Nebula (M42)

An HII region blister at the front of the Orion Molecular Cloud

Tstar ~ 37,000K Large quantities of dust can be present in HII regions 5
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 A starburst galaxy with many HII regions VLT image of NGC 1232.

HII regions show up as white in this image.

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Up to 500 Lsun in narrow (15 km/s) [OIII] 5007A line; use narrow-band filter . 
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Mosaic imagers + automated detection methods now allow hundreds of intergalactic PN candidates to be found in a single run (Feldmeier et al. 2003, 2004; Okamura et al. 2003; Arnaboldi et al. 2003). Mendez et al. (1997) detected 535 PNe in the elliptical NGC 4697 using filter images and slitless spectra. The former gave the [OIII] PN Luminosity Function. The latter gave radial velocities for 531 of the PNe, allowing an accurate Mass/Light ratio to be derived for the galaxy. Artist‘s impression of a dusty torus around an Active Galactic Nucleus

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Silicate dust absorption Supernova Remnants œ Collisionally Ionized Nebulae

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e.g. [OIII] optical forbidden line ratios and infrared fine-structure line ratios can provide measures of a) electron temperatures, by comparing the fluxes of lines from levels with different excitation energies (e.g. 4363A and 5007Å) b) electron density diagnostics (e.g. the 52um to 88um line ratio) using flux rations from lines with similar, or negligible, excitation energies b) O2+ abundances relative to H+, by comparing with hydrogen recombination line fluxes /

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Garcia-Rojas et al. (2015) Near-Infrared surveys for s-process element forbidden lines

Sterling et al. (2007), Sterling & Dinerstein (2008), Sterling et al. (2015)

œ First large-scale surveys for n-capture elements in PNe

œ [Kr III] 2.199, [Se IV] 2.287 um lines (Dinerstein 2001)

œ 120 Planetary Nebulae - 81 detections of Kr/Se

œ Increased by a factor of 10 the number of PNe with known n-capture element abundances

œ Kr and Se abundances are ~5x solar, confirming AGBs/PNe as s-process element sources

Sterling (2008) Temperature-dependence of different emission lines

Heavy element optical recombination lines (ORLs) have a similar inverse Te temperature dependence to hydrogen recombination lines.

So ionic abundances relative to hydrogen from ORLs should be insensitive to Te and red=recombination potentially more reliable than abundances from collisionally excited lines (= CELs = forbidden lines) Recombination lines (RLs): can also yield ionic abundances, given the availability of temperature-dependent recombination coefficients. ORLs: optical recombination lines from C,N,O and heavier elements. These lines are weak, scaling with the elemental abundance relative to hydrogen (typically less than a few x10-4 the strength of hydrogen recombination lines).

The OII Grotrian diagram • prominent optical recombination lines are shown in green œ no significant collisional excitation.

• until recently all calculations of the line intensities were carried out in LS-coupling.

• 4f-3d transitions might be expected to be well enough represented by hydrogenic approximations to use these lines to determine abundances of O2+ relative to H+.

• Should expect good reliability since hydrogen recombination lines have very similar dependence on temperature. The HIRES@Keck objects HII regions in M33

NGC 604 NGC 595 from Esteban (2009) The HIRES@Keck data NGC 604 (M 33) from Esteban (2009)

1 hour integration time, 135 lines

CII line at 4267Å OII M1 lines at ~4650Å Orion Nebula Balmer Jump => Te(BaC) = 7900+-600 K; close to Te(O III) and Te(N II) for this nebula

Esteban et al. (2004, MNRAS, 355, 247) HII region abundances from O II ORLs (Garcia-Rojas et al.)

Multiplet 1 

Multiplet 1  HII regions show a narrow range of ADF‘s ADF = abundance discrepancy factor; the ratio of ORL/CEL abundances for the same ion, e.g. O2+

Esteban et al. (2002); Peimbert (2003); Tsamis et al. (2003); Peimbert & Peimbert (2005); López-Sánchez et al. (2006)

García-Rojas & Esteban (2007)

HII region ADFs (~2) appear consistent with temperature fluctuations, in which case ORLs should give more accurate abundances than CELs Temperature fluctuations (Peimbert 1967) in nebulae will bias lines with strong Boltzmann factor dependences (e.g. collisionally excited optical and UV lines) towards higher temperature regions. Derived temperatures will be too high, with the result that subsequently derived ion abundances will be too low.

Only small t2 values are needed for M42 and most other HII regions HST and VLT spectra by Tsamis et al. (2011) of the embedded proplyd LV2 in M42 indicate that outflows from such objects are creating hot shocked regions that skew the CEL Te‘s to higher values, so that CELs will underestimate the true abundances of heavy elements.

The proplyd LV2 in M42 Collisionally-excited lines (CELs) and optical recombination lines (ORLs) in the spectrum of a planetary nebula

• Nebula formed from NGC 7009 stellar ejecta

• Ionization is maintained by UV radiation from the central star

• Classical temperature and density diagnostics give electron temperatures near 104 K and electron densities 102-105 cm-3

• But Balmer jump temperatures are much lower than those derived from CELs The HeI and OII ORL diagnostic ratios for the PNe listed below indicate that these lines originate from cooler plasma. One hypothesis is that they originate from super-metal-rich inclusions within the nebulae. These may be similar to hydrogen-deficient knots observed directly in the nebulae A30 and A78.

(ADF)

The measured ADF values are too large to be explained by Te fluctuations Hf 2-2 (PN G005.1-08.9)

[O III] H -

33".5þ33".5 33".5þ33".5

Hf 2-2 has a variable central star with a period of 0.398571 days N II 4042 N ADF = = ADF84 II + O II Hf Hf 2-2 O II 4089

C II 4267 C/ 0.133 = He/H Ne/ N/ O O O = 0.52 = = 0.51 = = 0.30 =

He I 4471 see Storey & Sochi (2014) Te(hot) = 8800K Te(cold) = 540K

see Storey & Sochi (2014) A two-component model for planetary nebulae

The very low Te(BJ)‘s compared to Te(O III])‘s observed in high-ADF planetary nebulae suggest that these nebulae contain at least two distinct emission regions:

̉ 4 A 'normal' component with a ''normal'' temperature (Te ~ 10 K) and ''normal'' abundances, from which the strong CELs originate.

̉ A 'Hydrogen-deficient' component, which is inferred to have a very low 3 temperature (Te ~ 10 K) and very high heavy elemental abundances. This emits most of the observed flux from the heavy element ORLs but emits essentially no optical CEL emission.

̉ In this scenario, ORLs and CELs yield different element abundances simply because they trace two completely different ionized regions. The ORLs reveal a separate nebular component, cold plasma enriched in heavy elements. Nature and origin of H-deficient inclusions?

̉ Probably a common but transitory phenomenon

̉ Born-again nuclear-processed PNe? (Liu et al. 2001)?

̉ Self-novae? (Pèquignot et al., 2004)

̉ AGB intershell material processed by CNO-cycle burning? (H:He:C:N:O:Ne=1:2.5:0.36:0.28:0.96:0.36 by mass in NGC 6153)

̉ Planetary system remnants? (planets, asteroids, comets, Kuiper-belt objects, Oort Cloud) around the progenitor star undergoing evaporation? ORL or CEL abundances for PNe ?

Planetary nebulae appear to behave differently from HII regions, with planetary nebulae showing significantly higher ORL/CEL abundance discrepancy factors (ADFs).

For HII regions, temperature fluctuations may account for the observed ORL/CEL abundance discrepancies. If so, ORL abundances should be used for HII regions

For planetary nebulae, temperature fluctuations (Peimbert 1967; 1971) or density inhomogeneities (Rubin 1989; Viegas & Clegg 1994) are inadequate to explain the factors of ten or more discrepancies between ORL and CEL abundances measured for some planetary nebulae.

However, for most PNe, forbidden CELs yield oxygen abundances for the hotter Te component that agree with the solar oxygen abundance: use CEL abundances for PNe Summary

The advent of 8-m telescopes equipped with very sensitive instrumentation has enabled the accurate measurement of large numbers of very faint emission lines, from nearby and from distant nebulae

The ions of many additional elements have become accessible for the first time, potentially enabling the abundance of many elements beyond the iron peak to be measured.

Deep observations have found significant differences between CEL and ORL abundances for CNO ions in both HII regions and planetary nebulae and have revealed the existence of `cold plasma‘ in some planetary nebulae.

The demand for new experimental and theoretical atomic data for a wide range of ions will continue to be strong.