Harvard Astronomy 201b

ARTICLE: HII Regions and the Abundance Properties of Spiral Galaxies

In Journal Club, Journal Club 2011 on March 1, 2011 at 1:17 am

Read the paper by D. Zaritsky, R.C. Kennicutt Jr, & J.P. Huchra (1994)

Summary by: Courtney Dressing


We investigate the relationships between the characteristic oxygen abundance, the radial abundance gradient, and the macroscopic properties of spiral galaxies by examining the properties of individual H II regions within those galaxies. Our observations of the line flux ratio (O II) lambda lambda 3726, 3729 + (O III) lambda lambda 4959, 5007)/H beta for 159 H II regions in 14 spiral galaxies are combined with published data to provide a sample of 39 disk galaxies for which (O II) + (O III)/H beta has been measured for at least five H II regions. We find that the characteristic gas-phase abundances and luminosities of spiral galaxies are strongly correlated. This relationship maps almost directly onto the luminosity-metallicity relationship of irregular galaxies and is also quite similar to that found for elliptical and dwarf spheroidal galaxies. Within our sample of spirals, a strong correlation between characteristic abundance and Hubble type also exists. The correlation between luminosity and Hubble type complicates the issue, but we discuss several interpretations of the correlations. The relationship between circular velocity and characteristic abundance is also discussed. We find that the slopes of the radial abundance gradients, when expressed in units of dex/isophotal radius, do not significantly correlate with either luminosity or Hubble type. However, the hypothesis that both early and very late type spirals have shallower gradients than intermediate spirals is consistent with the data. We find suggestive evidence that the presence of a bar induces a flatter gradient and also briefly discuss whether abundance gradients are exponential, as is usually assumed. We investigate the properties of individual H II regions in a subset of 42 regions for which we have spectra that cover almost the entire spectral range from 3500 to 9800 A. We use those data to estimate the densitites and ionizing spectra within the H II regions. We confirm that the ionizing spectrum hardens with increasing radius and decreasing abundance. We find no correlation between the ionization parameter and either radius or abundance, but this may be due to significant scatter introduced by the simple conversion of line ratios to ionization parameter.

Goals of the Paper

  • How do the abundances of galaxies relative to their macroscopic properties?
  • Is the chemical abundance of a spiral galaxy predominately determined by Hubble type or luminosity or a combination of both?
  • What does that tell us about galaxy formation and evolution?
  • Is there a spiral galaxy equivalent of the fundamental plane?
  • Are the abundances of spiral galaxies determined by the intrinsic properties of the galaxies or by the local ISM?

Major Results

  • The characteristic abundance of spiral galaxies is strongly correlated with luminosity and Hubble type.
  • The slope of the radial abundance gradient (dex/isophotal radius) is not significantly correlated with luminosity or Hubble type.
  • Intermediate spirals have steeper radial abundance gradients than early and very late spirals.
  • Barred galaxies have flatter gradients than unbarred galaxies.
  • The ionizing spectrum in an H II region hardens with increasing radius and decreasing abundance.
  • The ionization parameter does not seem to be correlated with radius or abundance, but the correlation may be masked by the conversion from line ratios to ionization parameter.

Previous Work

  • First studies of chemical abundances in HII regions: Searle (1971) and Shields (1974)
  • Vila-Costas & Edmunds (1992) demonstrated that chemical abundances are related to global properties and not just local properties (e.g., surface density).

Why write this paper in 1994?

  • Larger sample of galaxies.
  • The tools to relate line ratios to temperatures and ionization parameters were already in place (Edmunds & Pagel 1984, Vilchez & Pagel 1988)

The Sample

  • 39 Sab – Sm galaxies
    • 32 barred + 7 unbarred
    • 14 new late-type spirals + 25 early-type spirals from OK
  • 577 measured HII regions (159 new + 57 from OK)
  • Absolute luminosities: -16.9 to -21.4 (most between -21 < MB < -19)
  • Larger than previous studies
  • Includes early-type spirals (fainter HII regions)
  • Require ≥ 5 measured HII regions per galaxy (some regions might be measured more than once)
  • Full list in Table 1

Abundance measurement

  • Metal line strength indicates electron temperature.
  • Oxygen cooling
    • Low temperature: fine-structure lines at 52 μm and 88 μm
    • High temperature: forbidden lines at 3727 Å, 4959 Å, and 5007 Å

    Schematic view of oxygen cooling

  • R23 = ([O II] λλ3726, 3729 + [O III] λλ4959,5007)/Hβ
    • Probes both singly and doubly ionized oxygen.
    • Best choice for determining temperature of high metallicity regions when temperature sensitive lines (e.g., [O III] λ4363) aren’t strong enough.
    • Stronger correlation with oxygen abundance than most bright-line ratios. (Pagel, Edmunds, & Smith 1979)
    • Precision: 0.2 dex (Pagel, Edmunds, & Smith 1980)

Observing Procedure

  • Select HII regions from continuum subtracted Hα CDD images acquired with
    • Steward 2.3 m
    • KPNO 0.9 m
    • KPNO Burrell Schmidt (0.9 m)
  • Target selection requirements
    • Bright enough for high S/N spectrum (most observations)
    • Good radial coverage of galactic disk (most observations)
    • ≥ 10 HII regions per galaxy (ended up with ≥ 5 for some)
  • Spectroscopy
    • Multiple Mirror Telescope (MMT)
      Example Long-Slit Spectrum

      Figure 1 from paper. Example Long-Slit Spectrum.

      • Red Channel Spectrograph (600 line/mm grating)
      • Long-slit mode
      • 2” x 180” slit
      • Covered 3500 – 5200 Å with ~9 Å resolution
      • Observing runs in 3/90, 1/91, 4/91
      • Most of fainter targets observed with MMT
      • Reduced using IRAF
    • Steward Observatory 2.3 m at Kitt Peak
      Example cross-dispersed spectrum

      Figure 2 from paper. Example Cross-Dispersed Spectrum.

      • B&C spectrograph with cross-disperser & echellette grating
      • Cross-dispersed mode (8 orders)
      • 4.5” x 16” slit
      • Covered 3600 – 9000 Å
      • Resolution of 2.5 Å (blue) to 4.8 Å (red)
      • Observing runs in 1/90, 3/90, 4/90
      • See Section 2.2 for reduction procedure
    Corrections to Spectra
    • Dust reddening
      • Determined from line ratios of Hα/Hβ or Hβ/Hγ
      • If Balmer decrement was not measureable (17% of HII regions), then AV was determined from a nearby region or set to zero.


    • Stellar absorption correction <10% for 75% of HII regions
    Measuring Line Fluxes
    • Two methods (difference of a few percent)
      1. Integrate under line between continuum crossings
      2. Fit a Gaussian & integrate the Gaussian between continuum crossings
    • Average two results to find measured flux
    • Require S/N ≥ 3 to keep measurement
    • Perform visual checks of all lines
    • Results in Table 2 (excerpt below)
    Excerpt of Table 2 from paper.

    Excerpt of Table 2 from paper. The offsets in columns 2 and 3 are the offsets from the nucleus in arcseconds. Column 4 is the deprojected radius in units of the isophotal radius. The Hβ flux in column 6 is in units of 10^-16 ergs cm^-2 s^-1.

    Properties of the HII Regions

    • Spectrum is influenced by
      • Elemental abundances
      • Gas density
      • Ionizing spectrum (characterized by temperature T)
      • Dust extinction
      • Ionization parameter U=Q/(4πR2nc)
        • Q=flux of hydrogen-ionizing photons
        • R=radius of ionized sphere of gas
        • n=number density of H atoms
        • c=speed of light
    • Determine these parameters using cross-dispersed data (need data redward of 5200 Å)
    • Combine data from different galaxies
      • Advantage: Increases statistical significance of trends
      • Disadvantage: Obscures trend if differences between galaxies are larger than the trends within galaxies.


      • Determined from [S II] λ6716/[S II] λ6731
      • Results from 42 regions in Figure 4
      • Low-density limit: 1.42 (Czyzak 1986)
      • Weighted mean 1.37 ± 0.02 → density ≤ 100 cm-3
        • Collisional effects are negligible
      Density versus Oxygen Abundance

      Figure 4 from paper. Plot of density versus oxygen abundance. The sulfur density proxy ratios are sufficiently close to the low-density limit that the density is constrained to be less than 100 particles/cm^3.

      Ionizing Spectrum

      • Previously known that Hβ EW correlates with oxygen abundance (Shields & Tinsley 1976; Shields & Serle 1978)
        • Due to change in mean temperature T of ionizing stars with metallicity?
      • Estimate T using η parameter (Vilchez & Pagel 1988)
        \log \eta =\log \eta' + \frac{0.14}{t}+0.16
        \eta'= \frac{[O II] \lambda\lambda 3726,3729}{[O III] \lambda\lambda 4959,5007} / \frac{[S II] \lambda\lambda 6716,6731}{[S III]\lambda\lambda9069,9532}
        where t = electron temperature in units of 10,000 K 
      • η parameter correlates with the hardness of the ionizing spectrum but does not (as of 1994) accurately measure effective temperature (Garnett 1989).
      Hardness Parameter versus Radius and Oxygen Abundance

      Figure 5 from paper. Left: Hardness versus Radius. Same correlation between hardness parameter and radius as in Vlichez & Pagel (1988). The vertical offset might not be significant. Right:Hardness versus oxygen abundance.

      Ionization Parameter U

      • Linked to density & effective temperature of ionization source.
      • Estimated using [S II] and [S III] as in Diaz et al. (1991)
        \log U = -1.69\log\left(\left[S II\right]/\left[S III\right]\right)-2.99 
      • Ionization parameter is not strongly correlated with radius or abundance.
      • The scatter might conceal a weak correlation.
      Ionization Parameter

      Figure 6 from paper. Left: Ionization parameter versus radius. No strong correlation is observed. Right: Ionization parameter versus oxygen abundance. No strong correlation is observed.

      Reddening and Extinction

      • If dust has a large influence on the spectra of HII regions, then the excitation of HII regions should be correlated with the observed reddening.
      • No correlation between reddening and abundance.
      • Slight hint of possible correlation between radius and reddening (Fig 7c).
      • Large variation in extinction from one HII region to another.
      • The strength of extinction seems dominated by local influences (e.g., proximity to dust lane).

      Figure 7 from paper. Left: Extinction versus radius. Very weak radial dependence is observed for regions with HBeta EW > 50 Angstroms. Right: Extinction versus oxygen abundance. No correlation is observed.

      Abundance Properties of Spiral Galaxies

      • Spirals have a wide range of mean abundances and abundance gradients.
      • Differences between galaxies > variations within a galaxy at the same radius → differences in abundances and abundance gradients occur on galactic scales
      • See Figure 8 (excerpt below)
      Abundances for spiral galaxies

      Excerpt of Figure 8 from paper. Abundances versus fractional isophotal radius for several of the spiral galaxies in the sample.

      Defining Terms: Normalization Radii

      • Absolute physical radius (kpc)
        • Useful for finding properties that depend on physical size
      • Dimensionless radii normalized to isophotal radius of disk (ρ/ρ0)
        • Isophotal radius ρ0 = radius where surface brightness = 25.0 mag/arcsec2
        • Easy to find in literature
        • Less sensitive to bulge/disk ratio selection effects.
      • Dimensionless radii normalized to disk exponential scale length (ρ/ρs)
        • Disk scale length ρs = radius found by fitting an exponential function to the surface brightness profile of the galaxy:
          (Binney & Tremaine, eqn. 1.7)

      Defining Terms: Abundances

      • Characteristic abundance z
        • Usual choice is projected central abundance, but that requires extrapolation to the center of the disk and might not agree with the actual abundance in the center.
        • Instead, the authors perform a weighted linear least-squares fit to the data to determine abundances at 3 kpc, 0.4ρ0, and 0.8ρS.
          • Characteristic abundances agree with actual abundances.
          • Very little extrapolation is required.
        • All three normalization radii result in roughly the same list of galaxies with increasing z.
      • Abundance gradient G
        • Three radius normalizations: dex/kpc, dex/ρ0, and dex/ρS
        • Choice of radius normalization affects the steepness of the gradient
        • Most results quoted for dex/ρ0
      Uncertainties in characteristic abundance and abundance gradient

      Figure 9 from paper. Top: Uncertainty in Characteristic Abundance versus Sample Size Uncertainty decreases most dramatically for sample sizes < 5. Bottom: Uncertainty in Abundance Gradient versus Sample Size The improvement with increased sample size decreases for sample sizes > 5, so 5 H II regions were required per galaxy to determine z and G.

      Discussion: Characteristic Abundance & Galaxy Properties

      • The wide range in z and G for different galaxies implies that abundances are global properties rather than local properties.
        1. Are the abundances an immediate consequence of macroscopic properties?
        2. Is the dependence of abundances on macroscopic properties mediated by local properties?
      • Question: Do z and G depend on Hubble type or mass?
      Numerical Hubble Diagram from Wikipedia

      Numerical Hubble type diagram from Wikipedia.

      Relationship between z and galaxy properties

      • All three scalings of z correlate strongly with
        1. Absolute blue magnitude (M¬B)
        2. Inclination-corrected circular velocity (VC)
        3. Hubble type (T)
      Abundance versus magnitude, circular velocity, and Hubble type

      Figure 10 from paper. Oxygen abundance versus absolute blue magnitude (left), circular velocity (middle), and Hubble type (right).

      Circular velocity versus Hubble Type

      Figure 11 from paper. Circular velocity versus Hubble Type. The point of this figure is that the observed correlation of characteristic abundance with circular velocity and Hubble Type in Figure 10 might actually be due to the correlation between Hubble type and mass found for spiral galaxies.

      Residuals to z-T relationship and z-Vc relationship

      The lack of a correlation in either panel of Figure 12 indicates that the abundances are determined by a single parameter that is correlated with both mass and Hubble type rather than by mass or Hubble type independently. 

      Abundance versus Luminosity

      • How does the abundance versus luminosity relation for spiral galaxies compare to that for ellipticals, irregulars, and dwarf spheroidals?
      Metallicity versus Luminosity

      Figure 13 from paper. Metallicity versus luminosity for dwarf ellipticals and ellipticals (top) and for spirals and irregulars (bottom).

      Spirals, ellipticals, dwarf ellipticals, and irregulars display similar correlations between abundance and luminosity. Since the mass-to-light ratios are more similar than the luminosities, the galaxies also have similar correlations between abundance and galaxy mass. 

      Abundance versus Gas Fraction

      • If the correlation between Hubble type and abundance is due to the gas fraction, then there should be a clear correlation between gas fraction μg and z. Is there?
      Abundance versus gas fraction and relative HI Content

      Figure 14 from paper. Abundance versus gas fraction (left) and relative HI content (right). There is no significant correlation observed in the left panel, and the apparent weak correlation in the right panel is only an artifact of the correlation between z and Vc.

      • This paper does not find a correlation between gas fraction and abundance, but Vila-Costas & Edmunds (1992) present evidence of a correlation between molecular gas fraction and abundance.

      Abundance Gradients and Galaxy Properties

      • Gradient in [O III]/Hβ cannot be fully explained by simply changing nebular elemental abundances.
      • Gradient is partially caused by changes in the mean temperature T of the ionizing stars.
      • Not all changes in the ionizing spectrum can be attributed to changes in abundance because other factors (e.g., variations in IMF) can explain differences in the ionizing spectrum in different regions.
      • The correlation between G and macroscopic quantities (if it is actually exists) is much weaker than correlation between z and macroscopic quantities.

      Relationship between G and Radial Normalization

      • Steep gradients are unstable in small galaxies because of the short diffusion timescale.
      • Large galaxies cannot destroy steep gradients as efficiently as small galaxies.
      • Small galaxies likely have smaller gradients than large galaxies when gradients are measured in units of dex/isophotal radius.
      • Small galaxies and large galaxies may have similar gradients when the gradients are reported in units of dex/kpc.
      Gradient versus Hubble Type

      Figure 15 from paper. Gradient in dex/kpc (left panel) or in dex/ρ0 (right panel) versus Hubble Type. Panel (a) reveals a 2 σ correlation between gradient and Hubble type in VE’s data, and panel (c) is only one galaxy away from a significant correlation. Neither panel b nor panel d shows a significant correlation between gradient in dex/ρ0 and Hubble type.

      Nature versus Nurture: Why does it matter whether z is governed by mass or Hubble type?

      • If Hubble type is dominant:
        • Chemical evolution is governed by environment and/or initial structure (likely via effects on star formation and IMF)
      • If mass is dominant:
        • Processes like galactic mass loss are more important
      • Either way, stochastic events (e.g., starbursts and accretion events) have little influence on global abundances.

      Paper Abbreviations

      • RC2: de Vaucouleurs, de Vaucouleurs, & Corwin 1976. Second Reference Catalog of Bright Galaxies
      • OK: Oey & Kennicutt 1993
      • VE: Vila-Costas & Edmunds 1992

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