MIRI Science

 

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  MIRI brings tremendous advances in science capability through the combination of a cold, large aperture telescope in space (nominally providing about 50 times the sensitivity and 7 times the angular resolution of Spitzer), plus state-of-the-art detector arrays in a power, flexible instrument. It will make important contributions to all four of the mission design themes for JWST: 1.) discovery of the "first light"; 2.) assembly of galaxies: history of star formation, growth of black holes, preduction of heavy elements; 3.) how stars and planetary systems form; and 4.) evolution of planetary systems and conditions for life.  

 

 

 

First Light

Finding the first episode of star formation - the 'first light' in the Universe - is often described as the defining observation for the JWST. Initial surveys will be made by the near infrared imager, NIRCam, to find candidates through the very strong absorption by hydrogen gas, galaxies called 'Lyman dropouts' because they become undetectably black at wavelengths shorter than the Lyman limit (the wavelength needed to strip electrons in the lowest orbits from hydrogen atoms). (emergence of galaxies, simulation from Cosmology on a Computer, University of Illinois)

The role of the MIRI will be to demonstrate that these objects are really undergoing their first episode of star formation. A galaxy that had previously formed stars and was undergoing a second such event would look very similar to a true 'first light' object to NIRCam. The older stars would be glaringly obvious to the MIRI.

The importance of the MIRI observations is illustrated above. The figure shows the stellar continuum spectra of a galaxy truly undergoing its first episode of star formation, with no stars older than 5 million years, and of a galaxy that is undergoing a second 5-million-year-old episode, following one 80 million years previously, with the two of equal mass. The spectra have been normalized to the same flux in the NIRCam bands. The galaxies are assumed to be at a redshift of z = 15. We assume at these large redshifts that there is a high degree of absorption starting just to the blue of Lyman alpha. The differences between the spectra are subtle in the NIRCam bands, but the older stars produce about three times greater relative signal in the MIRI bands for the older galaxy. The figure also shows a composite quasar spectrum. Although its continuum slope is different from that for the galaxies, with the large emission line equivalent widths, its photometric spectral energy distribution will be similar to those of the star forming galaxies in the NIRCam bands. However, the quasar is very easily identified by MIRI, due to the rapid rise of its continuum toward longer wavelengths.

The image below is another version: it represents real data on a galaxy at a redshift of z = 6.6 - 6.8. The short wavelength measurements, obtained with HST and from the ground, are analogous to the NIRCam role above, while the long wavelength ones obtained with Spitzer are analogous to the MIRI points. The Spitzer data (Egami et al. 2005) show that the galaxy is past the peak of its star formation episode, with an age of 50 million years or so.

 

 

Assembly of Galaxies

As galaxies assemble through collisions and mergers, they grow bulges and activate supermassive black holes in their nuclei, creating active galaxy nuclei. MIRI can probe obscured active galactic nuclei to determine the evolution of supermassive black holes in the heyday of quasars at redshifts of z ~ 2 - 2.5. (galaxy merger simulation by C. Mihos)

Most of the high redshift AGNs are heavily obscured (e.g. Gilli et al. 2000) Understanding AGN evolution requires understanding this dominant population, which can be identified from deep X-ray images with Chandra and XMM-Newton:

The Ne VI line at 7.65 microns allows measure of power of obscured AGNs. This line requires ultraviolet photons of very high energy to be excited, too high an energy to be produced by stars. The extinction at this line is less than 2% of the extinction in the visible. The line is in the MIRI spectral range to z ~ 2.5. 

The following spectrum obtained with the short wavelength spectrometer on ISO shows this strong line because the Circinus galaxy is the site of an obscured active nucleus.

Even at a redshift of 2.5, the line is readily measurable to a luminosity of only twice that of the nearby obscured moderately luminous active nucleus in NGC 1068.

In addition, MIRI Ha measurements can measure star formation rates in very young galaxies. A rate of 10 solar masses per year can be readily detected at a redshift of z ~ 10.

 

How stars and planetary systems form

MIRI can probe how cold cloud cores collapse into stars. This process has been hidden from us because of the immense optical depth of these cores, which blocks all the visible and near infrared emission from escaping.(animation from M. Ressler)

Very young protostars Class 0 have been observed in the sub-mm but are relatively faint in the mid-infrared They appear to be T~20K blackbodies from sub-mm observations. The spectral "windows" are indicated by peaks in the mid-infrared.
 

JWST can probe through these windows and see what is going on inside. By measuring the shadows cast by the protostellar cores, it can determine their density profiles. These observations are key to understanding the first stages of collapse into protostars.

 

Evolution of planetary systems and conditions for life

MIRI can trace the evolution of planetary systems from their formation to mature systems identified through their leftover debris. (animation from C. J. Hamilton)

  

 

 

    For example, the image to the far left shows the Spitzer/MIPS image of the debris disk around Fomalhaut at 24 microns, as released in the initial Spitzer press conference, compared with how the system might look to MIRI (near left). HST has found a massive planet that shepherds the inner edge of this disk (Kalas et al. 2008). We expect to find many more such complex systems with MIRI.
MIRI spectra will let us probe the products of violent collisions between planetesimals, such as the SiO gas and amorphous silica in this Spitzer spectrum of the young star HD 172555 (Lisse et al. 2008)

Measurements of secondary eclipses of transiting planets will let us make maps of their distribution of surface temperatures and determine their energy balance. (right image from H. Knutson)