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Wolf-Rayet Galaxies: An Overview

Wolf-Rayet Galaxies: An Overview. William D. Vacca (USRA-SOFIA). Wolf-Rayet Galaxies. Subset of emission-line galaxies (or major portions thereof) in whose integrated (optical) spectra the signatures (emission features) of W-R stars are found

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Wolf-Rayet Galaxies: An Overview

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  1. Wolf-Rayet Galaxies:An Overview William D. Vacca (USRA-SOFIA)

  2. Wolf-Rayet Galaxies • Subset of emission-line galaxies (or major portions thereof) in whose integrated (optical) spectra the signatures (emission features) of W-R stars are found • Defined by detection of broad (stellar) He II 4686 or “blue bump” (= He II 4686 + N III 4640 + C III 4650) from W-R stars • Other broad lines: He II 1640, C III 5696, C IV 5808 • Most are “H II” galaxies – photoionization powered by hot stars – e.g., BCDs, although the class encompasses a wide range of galaxy types and morphologies (LINERs, Sy 2’s, ULIRGs) • Represent the more luminous extension of extragalactic GHIIRs (Conti 1991)

  3. Examples of Spectra Vacca & Conti (1992) Kunth & Schild (1986) NGC 3125 POX 4

  4. More Examples of Spectra Schaerer, Contini, & Kunth (1999)

  5. Ancient (pre-1998) History • First example (He 2-10) found in 1976 (Allen et al.) • First catalogue (Conti 1991) had 37 objects, found serendipitously • Large N(WR) (102-105) and large N(WR)/N(O) (> 0.1-1) derived from L(He II 4686) and L(He II 4686)/L(H) • Because W-R stars are short-lived descendants of the most massive O stars, this suggested W-R galaxies represented a brief (t < few Myr) burst of massive star formation observed at a “propitious” time ( < few Myr later) (Kunth & Sargent 1981; Durret et al. 1985; Armus et al. 1988; VC92) • Early Pop Syn Models (Arnault, Kunth & Schild 1989; Mas-Hesse & Kunth 1991; Krüger et al. 1992; Cervino & Mas-Hesse 1994) confirmed general picture • Short Burst, Salpeter IMF, Mupp>30 M, 3 <  < 6 Myr • Strong variation of N(WR) and N(WR)/N(O) with metallicity Z

  6. Model Predictions • Arnault Kunth & Schild (1989) • =2, Mupp= 120 M • N(W-R) increases with Z • N(W-R)/N(O) for IB >> CSF

  7. More recently… • Second catalogue (Schaerer, Contini, & Pindao 1999) listed 139 objects • ~40% have both WN and WC stars • Strong variation in N(WR), N(WC)/N(WN), and N(WR)/N(O) with Z • Larger samples and better optical data with higher S/N and R have enabled detailed studies of numerous objects • Schaerer et al. (1997) – WN, WC stars in SSCs in NGC5253 • Izotov et al. (1997); Legrand et al. (1997) – WN, WC stars in I Zw 18 • Schaerer, Contini, & Kunth (1999) – WC stars in W-R galaxies • Guseva, Izotov, & Thuan (2000) – W-R populations in 39 BCDs • Schaerer et al. (2000) – extended bursts in Z>Z W-R galaxies • Starburst regions in W-R gals composed of compact SSCs • Presence of W-R stars provides means of age-dating • UV and optical data for W-R galaxies but still no convincing detections of W-R features in the IR

  8. From FWHM of He II 4686 and NIII 4640≤He II 4686, dominant WN subtype is usually WNL From FWHM of CIV 5808 and absence of C III 5696, dominant WC subtype is usually WCE N(O) is estimated from L(H) (which yields Q0obs) and EW(H) (which yields , derived from models) Estimating N(WR) and N(O)

  9. Geneva (non-rotating) stellar evolution tracks with enhanced mass-loss rates as function of metallicity (0.05< Z/Z < 2.0) CoStar theoretical fluxes for O stars Spherical, expanding, unblanketed, non-LTE models of Schmutz et al. (1992) for W-Rs Empirical estimates of Of and W-R line fluxes from Gal and LMC stars No scaling of W-R models or line fluxes with Z Nebular continuum Instant. Burst (t = 0) with Salpeter IMF (=2.35), Mupp = 100 M Predict relative W-R numbers, luminosities of lines and W-R blue bump L/L(H), and EWs as a function of Z, age , EW(H) Extended to lower Z, finite duration bursts, non-Salpeter IMFs, inclusion of R136-type stars, newer line-blanketed O and W-R models (de Mello et al. 1998; Schaerer et al. 1999, 2000; Pindao et al 2002; Smith et al. 2002) ‘Standard’ Models (Schaerer & Vacca 1998)

  10. Example of Model Predictions SV98

  11. Comparisons with Models Guseva, Izotov, & Thuan (2000)

  12. Comparisons with Models Guseva, Izotov, & Thuan (2000)

  13. Comparisons with Models Guseva, Izotov, & Thuan (2000)

  14. Caveats and Problems • Calibration of LWN(4686) and LWCE(5808) based on Gal, LMC W-Rs • Huge range in line luminosities within any single WR subtype • For ZSMC Crowther & Hadfield (2006) find smaller line fluxes: • Contamination in low resolution spectra by nebular emission • Disentangling contributions to W-R broad features from WC and WN stars can be difficult • L(Hβ) and EW(Hβ) may not accurately reflect hot star population in either number or age • Narrow slit captures only fraction of L(Hβ) – “geometric dilution” • Stars and emitting gas may be spatially separated • Stars and gas may have different extinction values • Dust absorbs some of the ionizing photons • Nebula may not be ionization bounded (photon leakage) • Underlying older population contributes to L(4861) – “continuum dilution”

  15. I Zw 18 – A Challenge to the Models? IB, =2.35 Mupp=150 M • With Z ~Z/50, I Zw 18 should have few W-Rs and even fewer WC stars • Izotov et al. (1997) find N(WNL)=17, N(WCE)=5, N(WC)/N(WN) ~ 0.3, N(W-R)/N(O) ~ 0.02 • Re-analysis by De Mello et al. (1998) gives N(WNL) ~ 4, N(WCE) ~ 4, for N(WC)/N(WN) ~ 1 ! • Std IB models can reproduce observed EWs and N(W-R)/N(O) but not line fluxes • Crowther & Hadfield (2006) use SMC line luminosities to estimate N(WCE) ≥ 30 and N(WNL) ~ 10-200, so that N(W-R)/N(O) ~ 0.02-0.1 ! • May require models with rotation and/or binaries to produce more WRs at low Z De Mello et al. (1998)

  16. A Better Way… • Target ‘simple’, isolated objects representing SSPs formed in Instantaneous Bursts (t = 0, no continuum dilution e.g., SSCs) • Use model fits to UV spectral line profiles to determine the age  • Use observed slope of the UV continuum compared to models to estimate extinction • Match models to continuum levels to derive Mass, N(O) • Use synthetic or empirical ‘generic’ W-R spectra to match both UV and optical emission features and derive N(WN) and N(WC) • Not perfect (sensitive to extinction law, matched UV and optical apertures) but avoids problems of deriving N(O) from gas • Applied (in various forms) to: • 16 W-R galaxies – Mas-Hesse & Kunth (1999) • NGC 3049 – Gonzalez Delgado et al. (2002) • NGC 3125 – Chandar, et al. (2004); Hadfield & Crowther (2006) • He 2-10 – Chandar et al. (2003) • Tol 89 – Sidoli, Smith, & Crowther (2006)

  17. NGC 3125 - An example(Hadfield & Crowther 2006; Chandar et al. 2004)

  18. NGC 3125 NGC 3125 – A1 Chandar, Leitherer, & Tremonti (2004) Hadfield & Crowther (2006)

  19. NGC 3125(Hadfield & Crowther 2006) • Fitting SB99 models to wind line profiles gives •  = 4 Myr • Continuum fit gives • M = 2x105 M • N(O) = 550 • He II 1640 line gives • N(WN) ~ 110 • N(WR)/N(O) ~ 0.2 NGC 3125 – A1 Hadfield & Crowther (2006)

  20. NGC 3125(Hadfield & Crowther 2006) • Fit LMC template spectra (Z ~0.5Z) • For A1: • N(WN5-6) ~ 105 • N(WCE) ~ 20 • Agree with UV analysis • For B: • N(WN5-6) ~ 40 • N(WCE) ~ 20

  21. NGC 3125(Hadfield & Crowther 2006) • SB99 models with Kroupa IMF, Mupp = 100 M at  = 4 Myr yields optical cont. fits consistent with UV and pop analyses • A: • N(O) = 1150 • N(WR)/N(O) = 0.16 • M = 4.2 x 105 M • B: • N(O) = 450 • N(WR)/N(O) = 0.13 • M = 1.6 x105M

  22. Wolf-Rayet Galaxies in the SDSS • Zhang et al. (2007) constructed a sample of 174 W-R galaxies • Brinchmann, Kunth & Durret (2008) generated a sample of 570 W-R galaxies with z < 0.22 ! • Compared to SB99 and BC03 models with SV98/Crowther & Hadfield (2006) W-R and Of line fluxes • Considered finite burst durations t between 1 Myr and 0.5 Gyr • Serious discrepancy with models at lowest Z • Suggest models with rotation and binaries are needed Brinchmann et al. (2008)

  23. Wolf-Rayet Galaxies in the SDSS Brinchmann, Kunth, & Durret (2008)

  24. Wolf-Rayet Galaxies at High Redshift! 811 Lyman Break Galaxies (Shapley et al. 2003) z ~ 3 Stellar He II 1640 FWHM ~ 1500 km/s EW ~ 1.3 Å

  25. Wolf-Rayet Galaxies at High Redshift! Brinchmann et al. (2008) • Bruzual & Charlot (2003) synth models • SV98 + Crowther & Hadfield (2006) WR and Of line fluxes • Chabrier (2003) IMF • SFR ~ exp(-t/); =15 Gyr

  26. Summary • W-R galaxies are the result of short bursts of massive star formation observed during a brief and special time shortly after the onset of the burst • “WR phenomena in starburst galaxies are a normal part of evolution of young starbursts.” (Conti 1999) • Now have a sample of 570 plus some at high redshift! • ‘Integrated’, multi-wavelength analysis provides best way of comparing observations with models • Updated ‘standard’ models do a reasonably good job of matching the observed EWs and relative line fluxes at most metallicities, and overall trends with metallicity, with Salpeter IMF and large Mupp (> 30 M) • General picture is probably correct • But serious problems at the lowest metallicities • May require models with rotation and/or binaries • New models are under development • “So quick bright things come to confusion.” (Shakespeare, Midsummer Night’s Dream, Act I scene 1)

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