The Lilly-Madau diagram from Madau & Dickinson (2014), showing the star formatio
ID: 106705 • Letter: T
Question
The Lilly-Madau diagram from Madau & Dickinson (2014), showing the star formation per unit volume as a function of redshift (and thus cosmological time). Measurements from a variety of wavelengths (including UV, infrared and radio) are combined in the left panel, while UV and infrared are shown separately in the right panel. (a) Describe why the ultraviolet, far-infrared and radio continuum all trace star formation within galaxies. (b) How are measurements of the UV and far-infrared impacted by the dust content (or lack thereof) within a galaxy?Explanation / Answer
Why the ultraviolet, far- infrared and radio continuum all trace star formation within galaxies?
Ultraviolet intensity The mass-lifetime relation for stars tells us that stars that are bright in the UV (beyond the level of the hot evolved stars in ellipticals) must be fairly young, less than 109 years for early A stars. Thus, there has been interesting in seeing massive and young stars directly in the UV, where one traces stars not massive or hot enough to produce H II regions. The measurement is analogous to the scaling used for H interpretation, except one deal now with luminosity in energy units rather than photon units, and we see the photospheric light directly. However, reddening (extinction) is a strong effect and correspondingly serious uncertainty. At these wavelengths, dust gives a "picket-fence" effect, with very little column density range between no significant extinction and no significant transmission. Comparison between H and UV imagery has proven very interesting, with the detailed correspondence being less than exact.
Far-Infrared Emission: The far-IR emission is ubiquitous among spiral and irregular galaxies, even some for which optical data suggested a very modest dust content. Some fraction of this emission is powered by young stars, making far-IR emission a very tempting indicator of star formation if we can understand it properly. For thermal emission, dust grains are heated by absorption of starlight, which operates most effectively in the blue and UV as the wavelength comes closer to the characteristic grain size. The grains cool by black-body emission, modified by a wavelength-dependent emissivity caused largely by the fact that the grains are smaller than the blackbody peak wavelength and thus cannot radiate such wavelengths as efficiently as a perfect radiator. The peak emission typically corresponds to a blackbody temperature 20-40 K; a significant range must be present to account for the far-IR spectral shapes. Sometimes several discrete components are fit, including very cool dust measured in the sub-mm window. The components are sometimes attributed to different regimes of heating: environments of OB stars, regions near cooler stars, and so-called infrared cirrus emission from a widely distributed grain population heated only by the average ambient stellar radiation field.
Radio Continuum Emission: Star-forming galaxies and galactic disks are copious emitters of centimeter-wavelength radio emission, much of which must be connected with star formation. The radio spectra of these systems are not the flat, thermal free-free spectra of individual H II regions, but nonthermal such as in synchrotron radiation. Apparently, there is a dominant role for particles accelerated perhaps in supernova remnants, radiating while spiraling through large-scale magnetic fields. The radio emission traces star formation through the supernova rate.
How are measurements of the UV and far-infrared impacted by the dust content within the galaxies?
The UV and far-infrared impacted by the dust content within the galaxies as some fraction of this emission is powered by young stars, making far-IR emission a very tempting indicator of star formation. For thermal emission, dust grains are heated by absorption of starlight, which operates most effectively in the blue and UV as the wavelength comes closer to the characteristic grain size. The grains cool by black-body emission, modified by a wavelength-dependent emissivity caused largely by the fact that the grains are smaller than the blackbody peak wavelength and thus cannot radiate such wavelengths as efficiently as a perfect radiator. The peak emission typically corresponds to a blackbody temperature 20-40 K, a significant range must be present to account for the far-IR spectral shapes. Sometimes several discrete components are fit, including very cool dust measured in the sub-mm window. The components are sometimes attributed to different regimes of heating: environments of OB stars, regions near cooler stars, and so-called infrared cirrus emission from a widely distributed grain population heated only by the average ambient stellar radiation field. The total energy removed by dust absorption in the visible and UV must emerge in the far-IR. For some galaxies, most of the UV light is absorbed by dust content.
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