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

• 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)

Examples of SpectraVacca & Conti (1992)

POX 4

NGC 3125

Kunth & Schild (1986)

More Examples of Spectra

Schaerer, Contini, & Kunth (1999)

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

Model Predictions

• Arnault Kunth & Schild (1989)

• =2, Mupp= 120 M

• N(W-R) increases with Z• N(W-R)/N(O) for IB >> CSF

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

• 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)

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N(WNL) = Lobs(4686) /LWNL (4686)

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Q0obs =

α Bα Hβeff

Lobs(Hβ )

hν Hβ

Estimating N(WR) and N(O)

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N(WCE) = Lobs(5808) /LWCE (5808)

€

N(O) =Q0obs −N(WNL)Q0

WNL −N(WCE)Q0WCE

η 0(τ )Q0O7V

‘Standard’ Models (Schaerer & Vacca 1998)

• 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)

Example of Model Predictions

SV98

Comparisons with ModelsGuseva, Izotov, & Thuan (2000)

Guseva, Izotov, & Thuan (2000)

Comparisons with Models

Guseva, Izotov, & Thuan (2000)

Comparisons with Models

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”

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LSMCWN (4686) ≈ 0.25LLMC

WN ; LSMCWCE (5808) ≈ 0.3LLMC

WCE

I Zw 18 – A Challenge to the Models?

• 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

IB, =2.35 Mupp=150 M

De Mello et al. (1998)

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)

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

NGC 3125

NGC 3125 – A1Chandar, Leitherer, & Tremonti (2004)

Hadfield & Crowther (2006)

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 – A1Hadfield & Crowther (2006)

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

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

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)

Wolf-Rayet Galaxies in the SDSS

Brinchmann, Kunth, & Durret (2008)

Wolf-Rayet Galaxies at High Redshift!

811 Lyman Break Galaxies (Shapley et al. 2003) z ~ 3

Stellar He II 1640FWHM ~ 1500 km/sEW ~ 1.3 Å

Wolf-Rayet Galaxies at High Redshift!

• Bruzual & Charlot (2003) synth models

• SV98 + Crowther & Hadfield (2006) WR and Of line fluxes

• Chabrier (2003) IMF• SFR ~ exp(-t/); =15 Gyr

Brinchmann et al. (2008)

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)