BLAST
Balloon-borne Large-Aperture Submillimeter Telescope

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Introduction High Redshift Star Formation Redshift Determination

Star Formation History of Submillimeter Galaxies:

There are good reasons to believe that the submillimeter and millimeter galaxies detected in the current SCUBA and MAMBO surveys are forming stars at very high rates (>> 100 M(sun)/yr), comparable to those in the local population of Ultra-Luminous IR Galaxies (ULIRGS).

The preliminary data for SCUBA and MAMBO galaxies suggests that the majority of this (sub)millimeter population have redshifts in the range 2<z<4. The upper-limit to this redshift distribution is still poorly constrained. Furthermore, the general shape of the spectral energy distribution (SED) of starburst galaxies produces a strong negative K-correction that enables some submillimeter and millimeter wavelength observations to probe the high-redshift Universe. A starburst galaxy with a fixed luminosity will have almost constant submillimeter or millimeter fluxes as the galaxy increases in redshift from z ~ 1 to z ~ 8. (These K-corrections are demonstrated in the diagram at the bottom left of this page.) Therefore, even without knowing the precise redshifts for individual objects, we can still be confident that the submillimeter fluxes provide a good measurement of the submillimeter luminosities. By adopting a typical starburst galaxy SED, we can then estimate the rest-frame FIR luminosities of submillimeter galaxies. Finally, we use the proportionality between the FIR luminosity and the current star formation rate (SFR) to demonstrate that this high-redshift submillimeter population appear to be forming stars with rates of ~100 - 1000 M(sun)/yr.

Although the number-density of submillimeter galaxies is ~10 times less than the high-redshift optical population of starburst galaxies, the star formation rate in individual submillimeter starbursts are ~10 to 100 times greater than in their optical counterparts. There are still uncertainties in these numbers, yet the fact remains that the population of submillimeter galaxies must contribute significantly to the luminosity density (due to star formation) over a wide range of redshifts.

BLAST will provide powerful constraints on the evolutionary history of star formation in dusty, optically-obscured galaxies by conducting simultaneous surveys at 250, 350 and 500 μm over a wide range of areas and depths. These BLAST surveys will provide photometric redshifts, constraints on the shape of the rest-frame FIR SEDs, and measurements of the individual SFRs (M(sun)/yr) for thousands of submillimeter galaxies.

General Starburst Galaxy SED and K-correction
K-corrections at submillimeter and millimeter wavelengths. This figure shows the dependence of flux density with redshift for a starburst galaxy of fixed luminosity (e.g. Arp 220, LFIR = 3x1012 Lsun). The strength of the negative K-correction at wavelengths greater than 850 μm is sufficient to completely compensate for the increasing cosmological distance of the source (assuming a lambda CDM model). Although the effect of the K-correction is weaker at shorter wavelengths, observations with BLAST at 250, 350 and 500 μm can still detect luminous starforming galaxies (300 to 1000 M(sun)/yr) at high redshift, 0<z<5.

The history of star formation
The history of star formation, as summarized by Smail et al. 2002. This figure illustrates the wide variation in the estimates of the luminosity density due to star formation at any given redshift. These variations are due to differences in the observational methods used to obtain estimates of the SFRs for the different galaxy populations. We draw attention to the large discrepancy at z~2 - 4 between the submillimeter estimate (open red circle) and the optical estimates (filled blue circles). There is reasonable agreement between these different methods, however, when the optical estimates are corrected for the effects of heavy obscuration by dust (open red squares). Therefore, the true picture of star formation at high-redshift remains incomplete until robust estimates of the redshift distribution and luminosities of submillimeter galaxies can be made.

 

Send questions or comments to Mark Devlin