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SED Fits to ULIRG Galaxies   In order to test these "multiple HII region" models for the SEDs of Starburst Galaxies, Reuland et al. [71] have collected data from the literature for 41 ultra-luminous infrared galaxies (ULIRGs) in the local universe. The data consist of UV/optical fluxes from the Third Reference Catalogue of Bright Galaxies v3.9, aperture-corrected JHK-band photometry from Spignolio [80] and 3-1500 µm data from Klaas et al. [56, 57] and from Spoon [81]. The cosmological model used to derive luminosity distance is the concordance cosmology with OmegaM = 0.27, OmegaLambda = 0.73 and Ho = 71 km s-1 Mpc-1 [79, 86].

The variables characterising the fit are:

The result of this fitting exercise is shown in Fig 5 for four well-known ULIRGS. The remaining 37 objects give fits of comparable quality. The largest deviations are seen in the far-UV, but this is most likely due to the presence of a small number of relatively unobscured stars. These make a negligible contribution to the Bolometric flux, and hence to the estimated total SFR.

Figure 5

Figure 5. Here we show the fits to the SEDs of a few well-known ULIRGS in the local universe, scaled to a star formation rate of 1.0 Modot yr-1. The shape of the far-IR bump is used to fit the pressure; P/k. The visible and UV part of the spectrum is fitted by a combination of foreground screen attenuation with an effective Av, and a molecular cloud dissipation timescale, tauMyr. Finally, the overall scaling factor provides the estimate of the total star formation rate.

Implications for hi-z Starbursts   All of the fits ULIRGS are characterized by high pressures, log P / k > 6 cm-3 K, and therefore have far-IR bumps which peak below 100 µm . This result is not unexpected, because the star formation region can only be as compact as it is in ULIRGS if the star formation is occuring in a very high pressure and high density environment. The pressure can be indirectly inferred from measurements of pressure in the HII regions though measurements of the density-sensitive [S II] lambda6717, 6731 Å doublet in warm IRAS galaxies. This has been done by Kewley [50, 51] and indeed, pressures log P / k > 6 cm-3 K are inferred for most objects.

In the local universe, the "dust temperature" inferred from the modified Black-Body fits to the long wavelength side of the far-IR peak in starburst galaxies is observed to correlate with the absolute luminosity, which for these galaxies can be interpreted as proportional to the rate of star formation [9]. However, the same exercise applied to high-redshift submillimeter selected galaxies (SMGs) provides a similar correlation, but shifted to higher luminosity. At a given luminosity, the dust in SMGs is about 20K cooler than in ULIRGs in the local universe, and at a given dust temperature, the SMGs are typically 30 times as luminous as their ULIRG counterparts.

What does this mean? As reported above, Takagi et al [84, 85] had found that most ULIRGS have a constant surface brightness of order 1012 Lodot kpc-2. Our results show that this corresponds to an ISM with a pressure of order log P / k ~ 7 cm-3 K . These parameters probably characterise "maximal" star formation, above which gas is blown out into the halo of the galaxy and star formation quenched. Only mergers, which provide an additional ram-pressure confinement of the star formation activity may exceed this surface brightness. Thus, in order to scale the star formation up to the rates inferred for SMGs (~ 1000 - 5000 Modot yr-1), we must involve a greater area of the galaxy in star formation, rather than trying to cram more star formation into the same volume. For a typical value of 1013 Lodot kpc-2, we require "maximal" star formation over an area of ~ 10 kpc2, and the most luminous SMGs require star formation to be extended over an area of at least ~ 100 kpc2. We can therefore conclude:

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