Mid-Infrared Properties of Low Metallicity Blue Compact Dwarf Galaxies From Spitzer Yanling Wu, Vassilis Charmandaris, Jim Houck, Jeronimo Bernard-Salas, Jessica Rosenberg, Leslie Hunt and the IRS Team Cornell University

OUTLINE Introduction to Blue Compact Dwarf Galaxies (BCDs) Introduction to Blue Compact Dwarf Galaxies (BCDs) Why should we be interested in low-Z galaxies? Why should we be interested in low-Z galaxies? Studying the BCDs with Spitzer Studying the BCDs with Spitzer  Presence of Polycyclic Aromatic Hydrocarbons (PAHs)  Estimating Elemental Abundances using infrared lines  Infrared to radio correlation Case Studies: IZw18 and SBS Case Studies: IZw18 and SBS Conclusions Conclusions

Introduction Blue Compact Dwarf galaxies : Blue Compact Dwarf galaxies : –blue optical color –small size (R<~1kpc) –low luminosity (M B >-18) Dominated by one or more recent bursts of star formation Dominated by one or more recent bursts of star formation Low chemical abundances Low chemical abundances Nearby, a few tens of Mpc Nearby, a few tens of Mpc

What is a Blue Compact Dwarf galaxy? Definition Definition Thuan & Martin 1981 : Thuan & Martin 1981 : –blue optical color –small size (R<~1kpc) –low luminosity (M B >-18) Gil de Paz et al : – µ B,peak - µ R,peak ≤1 – µ B,peak < 22mag/arcsec 2 –Dwarf: M K >-21 IZw 18: credit Izotov & Thuan

Gil de Paz et al. 2003

The chemical abundances

The Morphology of BCDs BCDs and dIrr BCDs and dIrr – The central morphology is often irregular – Central+surrounding host galaxy: a mixed bag ---- irregular; symmetric outer envelopes (more common); isolated HII regions (e.g. POX186) isolated HII regions (e.g. POX186)

Why do we study BCDs? The hierarchical galaxy formation scenario proposes that bigger structures are built from smaller galaxies, however, building-block galaxies are too faint and small to be studied at high-redshift. The hierarchical galaxy formation scenario proposes that bigger structures are built from smaller galaxies, however, building-block galaxies are too faint and small to be studied at high-redshift. Despite the discovery of large numbers of galaxies at high-redshift, it is hard to find truly young galaxies in the distant universe. Despite the discovery of large numbers of galaxies at high-redshift, it is hard to find truly young galaxies in the distant universe. Galaxies in the local universe can be used as laboratories to study SF and chemical evolution with better sensitivity, precision and spatial resolution. Galaxies in the local universe can be used as laboratories to study SF and chemical evolution with better sensitivity, precision and spatial resolution.

IRS spectroscopy IRS spectroscopy Composed of Composed of four modules: four modules: SL,LL — low-res SL,LL — low-res SH,LH — high- res SH,LH — high- res “ Peak up ” “ Peak up ” Options Options

Observations (I) --- our IRS/GTO sample A total of ~64 BCDs have been observed. A total of ~64 BCDs have been observed. For ~30 of them, we used all four Infrared Spectrograph (IRS, 5-38  m) modules to obtain spectra. For ~30 of them, we used all four Infrared Spectrograph (IRS, 5-38  m) modules to obtain spectra. The remaining were observed using the peak- up camera at 16  m and 22  m for broadband imaging. The remaining were observed using the peak- up camera at 16  m and 22  m for broadband imaging. IRAC and MIPS imaging were obtained also for part of the sample. IRAC and MIPS imaging were obtained also for part of the sample.

Observations (II) -- other Low-Z Dwarf Galaxies NDWFS/KISS Sample （ Rosenberg et al. 2006) NDWFS/KISS Sample （ Rosenberg et al. 2006) –A total of 19 star forming galaxies with M B > -18mag within the NDWFS area –Spectroscopy available via KISS (AGN excluded) –Abundances: 12+log(O/H) = > 9.1 (median metallicity 8.05) –IRAC, MIPS and IRS16  m data available MIPS/GTO Sample (Engelbracht et al. 2005,2008) MIPS/GTO Sample (Engelbracht et al. 2005,2008) GO Sample (Thuan, Hunt et al) GO Sample (Thuan, Hunt et al)

SMART The IRS Spectroscopy Modeling Analysis and Reduction Tool The IRS Spectroscopy Modeling Analysis and Reduction Tool written in IDL to reduce and analyze IRS data from all four modules including peak-up arrays written in IDL to reduce and analyze IRS data from all four modules including peak-up arrays

Studying the PAHs using IRS spectroscopy

Polycyclic Aromatic Hydrocarbon (PAH) Large molecules with ~50 to ~100s of carbon atoms Large molecules with ~50 to ~100s of carbon atoms Formed in the outflows of evolved and dying stars Formed in the outflows of evolved and dying stars Dominate the mid-IR (5-20  m) flux in normal galaxies and quiescent star forming regions via the so-called Unidentified IR Bands (UIBs) or IR Emission Features (IEFs): 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7  m Dominate the mid-IR (5-20  m) flux in normal galaxies and quiescent star forming regions via the so-called Unidentified IR Bands (UIBs) or IR Emission Features (IEFs): 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7  m More PAH bands: 14.3  m complexes, 17  m complexes. More PAH bands: 14.3  m complexes, 17  m complexes.

PAH Bands 10-20% of the total IR luminosity of a galaxy Bending, stretching modes  3.3,6.2,7.7,8.6,11.2,12.7  m PAH ratios  ionized or neutral, sizes, radiation field, etc. Leger & Puget (1984) Sellgren (1984) Desert, et al. (1990) Draine & Li, (2001) Peeters, et al. (2004)

Measuring the PAH feature Read in a spectrum and plot it on the screen Read in a spectrum and plot it on the screen Define the continuum around 6.2 micron by clicking the cursor on the screen three times and fitting the spectrum using “spline” routine Define the continuum around 6.2 micron by clicking the cursor on the screen three times and fitting the spectrum using “spline” routine Choose the beginning and ending points for integration of the emission feature by clicking on the screen around 6.2 micron twice Choose the beginning and ending points for integration of the emission feature by clicking on the screen around 6.2 micron twice Define the flux of the continuum as the flux at the point on the continuum that corresponds to the peak of PAH feature Define the flux of the continuum as the flux at the point on the continuum that corresponds to the peak of PAH feature PAH=area of emission/flux of continuum(EW) PAH=area of emission/flux of continuum(EW)

Example of PAH Measurement

SBS E and NGC7714 SBS E and NGC7714 Houck et al / 9.53

IRS spectroscopy of low metallicity galaxies Wu et al. 2006

MIR slope vs metallicity MIR slope vs metallicity Wu et al Engelbracht et al. 2005

PAH EW vs metallicity PAH EW vs metallicity Wu et al. 2006

PAH EW vs [NeIII]/[NeII] Starbursts see Brandl et al Wu et al [NeII] = 21.56eV [NeIII] = 40.96eV

PAH EW vs [NeIII]/[NeII]*L 22  m /V PAH EW vs [NeIII]/[NeII]*L 22  m /V Wu et al. 2006

PAH EW vs [NeIII]/[NeII]*(L 22  m /V)*(1/Z) Wu et al. 2006

Metallicity and presence of PAH Rosenberg et al. 2008

Elemental Abundances

MIR high-resolution spectra from Spitzer IRS MIR high-resolution spectra from Spitzer IRS Wu et al. 2006

Elemental Abundances: Infrared vs Optical Pros: Pros: –Metallicity measurements from the optical suffer from the dust extinction. –The infrared lines are much less sensitive to the uncertainties in the electron temperature. –Reduced the need of Ionization Correction Factors. Caveats: Caveats: –Hu  (12.37  m) line is usually very faint in BCDs. H  converted from H  suffers extinction effects. H  converted from H  suffers extinction effects. Need to make aperture correction so that H  flux corresponds to the same area as covered by the IRS SH slit. Need to make aperture correction so that H  flux corresponds to the same area as covered by the IRS SH slit.

Elemental Abundances: II Parameters: Parameters: –Extinction: adopted E(B-V) from optical studies (exception: SBS E — extinction estimated from silicate feature) –T e from optical if available, or assume T e =10,000 K –N e from optical if available, or assume N e = 100 cm -3 Lines Lines –We use the lines in the SH module to remove the uncertainty on the scaling factors between the SH and LH spectra. Ne: [NeII]  m, [NeIII]  m Ne: [NeII]  m, [NeIII]  m S: [SIV]  m, [SIII]  m S: [SIV]  m, [SIII]  m –Other Ionization stages: Ionization Correction Factors (ICFs) For Neon: [NeIV] <~1% For Neon: [NeIV] <~1% For S: [SII] ~10% For S: [SII] ~10%

Elemental Abundances: III Method: Method: –Solve the statistical equilibrium equation for a five level system –Effective collisional strengths from IRON project If there are embedded dust enshrouded regions they will likely have different metallicities than the regions exposed in the optical.

Ne/H (IR) vs S/H (IR) Wu et al. (2008a) Verma et al. (2003)

Ne/H (IR) vs O/H (Optical), S/H (IR) vs O/H (Optical) Wu et al. (2008a)

Neon and Sulfur abundances - IR vs Optical Wu et al. (2008a)  Overall consistent results between optical and IR studies  Slightly elevated Ne values as estimated in the IR

Ne/S (IR) vs Ne/H (IR) Wu et al. (2008a)

Ne/S (IR) vs O/H (Optical) Wu et al. (2008a) Infrared Optical  Discrepancy in Ne/S estimates in IR and optical, but consistent with Ne/S ratios from HII regions in other infrared studies  Possibly due to optical Ionization Correction Factors

IR/Radio correlation in BCDs

Radio continuum: Radio continuum: Three types of radio continuum emission: Three types of radio continuum emission: –Thermal free-free emission from HII regions (proportional to Lyman continuum flux, nearly flat spectral index, f ~ -0.1, accurate SFR estimator) –Non-thermal: synchrotron radiation from relativistic electrons accelerated in supernovae remnants, f ~ , -1.2<  < -0.4 f ~ , -1.2<  < -0.4 –Free-free absorption (young, dense, heavily embedded clusters,  > 0.0) IR continuum: IR continuum: –Thermal re-radiation of starlight from dust surrounding HII regions

FIR Luminosity vs 1.4GHz luminosity The low luminosity dwarf galaxies appear to have a similar slope as compared to that of normal galaxies. The low luminosity dwarf galaxies appear to have a similar slope as compared to that of normal galaxies. ~2.3 ± 0.2 (Condon 1992) ~2.3 ± 0.2 (Condon 1992) ~2.37 ± 0.23 (this sample) ~2.37 ± 0.23 (this sample) Wu et al. (2008b) q FIR =log[1.26× (2.58S 60  m + S 100  m )/(3.75×10 12 S 1.4GHz )]

Mid-IR to Radio Correlation Mid-IR emission also traces star formation activity, and shows more variation than FIR. Mid-IR emission also traces star formation activity, and shows more variation than FIR. Mid-IR luminosities are becoming available for low luminosity systems from deep Spitzer surveys. Mid-IR luminosities are becoming available for low luminosity systems from deep Spitzer surveys. ~1.25 ± 0.41 ~1.25 ± 0.41 (Wu et al. 2008b) (Wu et al. 2008b) ~0.84 ± 0.28 ~0.84 ± 0.28 or k-corrected or k-corrected ~0.94 ± 0.23 ~0.94 ± 0.23 (Appleton et al. 2004) (Appleton et al. 2004) Wu et al. (2008b) q 24 =log(S 24  m / S 1.4GHz )

q 24 as a function of metallicity No clear correlation is seen between the q 24 and the galaxy metallicities. No clear correlation is seen between the q 24 and the galaxy metallicities. q 24 ratios appear to be lower in metal- poor sources q 24 ratios appear to be lower in metal- poor sources SBS E is an outlier. SBS E is an outlier. Wu et al. (2008b)

Star Formation Rate Estimates General agreement between the radio and infrared (Wu et al. 2005) derived SFRs General agreement between the radio and infrared (Wu et al. 2005) derived SFRs A difference of ~ a factor of 4 in the radio or mid-IR (Calzetti et al. 2007) estimated SFRs. A difference of ~ a factor of 4 in the radio or mid-IR (Calzetti et al. 2007) estimated SFRs. though none might reflect the “ true ” SFRs. though none might reflect the “ true ” SFRs. Both SFRs from IR and radio are underestimated, but with a similar factor (Bell 2003). Both SFRs from IR and radio are underestimated, but with a similar factor (Bell 2003). Wu et al. (2008b)

Case Studies IZw18 and SBS E IZw18 and SBS E

Credit: Y. Izotov & T. Thuan

SBS E

The archytype BCD: IZw18 First member in the BCD family discovered by Zwicky in 1966 First member in the BCD family discovered by Zwicky in 1966 Extremely low metallicity: 0.03 Z solar Extremely low metallicity: 0.03 Z solar -main body (NW and SE region) -main body (NW and SE region) IZw18--| IZw18--| - “ C ” component - “ C ” component On-going star-formation in the main body; On-going star-formation in the main body; “ C ” component embedded in the same HI envelope with the main body (vanZee et al. 1998) “ C ” component embedded in the same HI envelope with the main body (vanZee et al. 1998)

Age controversy: Is IZw18 old or young? Main body: massive young star clusters, with ages ranging from 3 to 10 Myr (Aloisi et al. 1999; Recchi et al. 2002; Hunt et al. 2003) Main body: massive young star clusters, with ages ranging from 3 to 10 Myr (Aloisi et al. 1999; Recchi et al. 2002; Hunt et al. 2003) Age of the dominant underlying stellar population: Age of the dominant underlying stellar population: –Bona fide young galaxy with no evolved stars older than 500 Myr (Izotov et al. 2004) –A population of 0.5~5 Gyr old AGB stars is present (Aloisi et al. 1999; Ostlin et al. 2000)

Wu et al. 2007

IZw 18

Wu et al. 2007

Comparison with SBS E and NGC7714 IZw18 and SBS E have similar metallicity: 0.03 solar, 0.04 solar IZw18 and SBS E have similar metallicity: 0.03 solar, 0.04 solar No PAH emission in both galaxies No PAH emission in both galaxies Extremely hard radiation field : [NeIII]/[NeII]>5 Extremely hard radiation field : [NeIII]/[NeII]>5 NGC7714 : a typical starburst galaxy with over half solar metallicity NGC7714 : a typical starburst galaxy with over half solar metallicity

Wu et al The resemblance keeps even in the far-IR, indicating similar grain size and temperature distribution. The resemblance keeps even in the far-IR, indicating similar grain size and temperature distribution. Very different spectral slope : SBS E peaks at ~ 28  m in f space. Very different spectral slope : SBS E peaks at ~ 28  m in f space. Striking similarity in IRS spectra of IZw18 and NGC7714 (mid-IR spectral slope) Striking similarity in IRS spectra of IZw18 and NGC7714 (mid-IR spectral slope)

Conclusions  We detected PAH emission, at 6.2, 7.7, 8.6, 11.2 and 12.8  m, though their strength varies substantially in our sample.  The elemental abundances derived from the infrared fine-structure lines agree well with the optical results.  The FIR/radio correlation in BCDs is similar to that in normal galaxies. The mid-IR is also correlated with the radio luminosity, though the scatter is larger.