Nanyao Lu

Global Relation between Aromatic Features in Emission (AFEs) and Star Formation in Galaxies

( Does the Former Trace the Latter?)

Nanyao Lu (IPAC/SSC/Caltech)

Nanyao Lu

AFEs (or PAH Emission features) Nearly Ubiquitous in Disk Galaxies

λ(um)

Flux

den

sity

(J

y)(Lu et al. 2003)

Two elliptical galaxies

Average spectrum for spiral galaxies

Nanyao Lu

Exceptions: Some Low-Metallicity Starburst Dwarf Galaxies

(Houck et al. 2004)SBS0335-052 (12+log[O/H] = 7.30)

Why? PAHs are more servely destroyed by far-UV photons, or PAHs have not been produced enough in such a young galaxy (i.e., an intrinsic low PAH abundance w.r.t. large dust grains).

Nanyao Lu

Controversy on AFEs as a Tracer of Star Formation

Pro Evidences Anti Evidences

• Global surface brightness of AFEs correlates with that of Hα emission (e.g. Roussel et al. 2001).

• AFEs peak spatially near HII regions (i.e., PDRs)

• Surface brightness of AFEs correlates with that of cold dust emission at 850um (e.g., Haas et al. 2002).

• AFEs detected in diffuse ISM in our own Galaxy by COBE (Dwek et al. 1997), ISO (e.g., Lemke et al 1998), and Spitzer (e.g., Lu 2004).

• A possible way out of this controversy is that AFEs arise from both warm star-forming regions and the colder, diffuse ISM; and along the line-of-sight through high surface density regions, there is always a mix of these two components.

(for an example, see Dale & Helou (2002) who assumed a power law distribution of dust mass over heating intensity and predicted a correlation between AFEs and the 850um fluxes)

Nanyao Lu

IRAS/ISO Data Suggest a Two Component Scenario Lu et al 2003

Green line – a Galactic reflection nebula; red line – two Galactic HII regions (from Werner, Gautier & Cawlfeild 1994)

Nanyao Lu

A Simplified, Two-Component Model for AFEs

FPAH = a Fw + b Fc

Flux of AFE

Flux of large grain emssion from star-forming regions(i.e., the warm component).

Flux of large grain emissionfrom diffuse ISM (i.e., the coldcomponent).

where b is a universal constant, while a could have a dependence on some characteristics (e.g., the hardness) of the mean radiation field in star-forming regions.

FPAH = νfν at some wavelength where AFEs are dominant (e.g, 8um or 12um).

Nanyao Lu

Modelling the Far-Infrared Dust Emission

fν = Nw νβB(ν, Tw) + Nc νβB(ν, Tc)

We assume:

β = 2 Tc = 20K.

Solve Tw, Nw, and Nc using IRAS flux densities at 60 and 100μm and SCUBA flux density at 850μm. Then the integrated fluxes are:

Fw = Nw∫νβB(ν, Tw) dν, and

Fc = Nw∫νβB(ν, Tc) dν.

Searched the literature for IRAS galaxies with available 850um fluxes: 106 galaxies, all in IRAS Bright Galaxy Catalog (Soifer et al. 1987) with the 850um fluxes mostly from the SCUBA Local Universe Galaxy Survey (Dunne et al. 2000).

Nanyao Lu

Modelling the Far-Infrared Dust Emission

Nanyao Lu

AFEs and Cold Dust Component

Least-squares fit on 97 galaxies: slope b = 0.295 (+/–0.028);

Y-intercept a = 0.127(+/–0.016); C. Coef. = 0.74

where FPAH = νfν(12um).

Nanyao Lu

AFEs vs. FIR Flux for the Warm Dust Component

Least-squares fit on 97 galaxies by forcing the fit through (0, 0):

Slope a = 0.116 (+/– 0.010).

Nanyao Lu

AFEs-to-FIR Flux Ratio for the Warm Dust Component

Nanyao Lu

Fractional AFEs from the Warm Dust Component

Nanyao Lu

Cold-Component AFEs in Dwarf Irregular Galaxies

N1569

Recall FPAH / Fw = a + b Fc / Fw, and The slope “b” depends on R, the abundance ratio of PAHs to large dust grains.

If lower-metallicity dwarf irregulars have a lower R, one should expect to see a shallower slope for these galaxies. This is not obviously the case here though a definitive confirmation needs more dwarf galaxies.

Filled squares: Dwarfs.Open squares: Spirals.

Nanyao Lu

Predictions for Spitzer

For the Tc=20K cold dust component: νfν(12μm) ≈ 0.29 Fc.

For the warm, star-forming component at Tw: νfν(12μm) ≈ 0.12 Fw.

Assuming fν(8μm)/fν(12μm) ~ 1.1 (i.e, cirrus-like color), we havethe following ratios for Spitzer fluxes:

Component fν(8um)/fν(70um) fν(8um)/fν(160um) fν(8um)/fν(850um)

Diffuse ISM (20K) 0.17 0.032 2.0

SF Regions (40K) 0.024 0.075 20

SF Regions (50K) 0.033 0.173 58

Nanyao Lu

A Spitzer Test Case: NGC6946

70um 8um (stars subtracted) Gray: 24um; Contours: ratio 8um/70um

Contour step = 0.2; red contours: ratios > 1; blue contours: ratios < 1

Credit: Data taken from SINGS observations released in the Spitzer Public Archive

Nanyao Lu

A Spitzer Test Case: NGC6946

70um 8um (stars subtracted) Gray: 24um; Contours: ratio 8um/70um

Contour step = 0.2; red contours: ratios > 1; blue contours: ratios < 1

Credit: Data taken from SINGS observations released in the Spitzer Public Archive

Nanyao Lu

A Spitzer Test Case: NGC6946

Model prediction for diffuse ISM

Nanyao Lu

Conclusions AFEs or PAH emissions arise from both star-forming regions as well as diffuse ISM. Only in more actively star-forming galaxies, is the global PAH emission dominated by star-forming regions.

The PAH/FIR luminosity ratio in star-forming regions is lower than that in diffuse ISM, and may vary significantly as functions of factors such as the hardness of the radiation field.

We showed some preliminary indication that lower PAH feature strengths in lower-metallicity dwarf irregulars is not a result of an intrinsic deficiency of its carriers.