Far-Infrared Photometric Redshifts: A New Approach to a Highly Uncertain Enterprise

Abstract

I present a new approach at deriving far-infrared photometric redshifts for galaxies based on their reprocessed emission from dust at rest-frame far-infrared through millimeter wavelengths. Far-infrared photometric redshifts ("FIR-$z$") have been used over the past decade to derive redshift constraints for highly obscured galaxies that lack photometry at other wavelengths like the optical/near-infrared. Most literature FIR-z fits are performed through ${\chi }^2$minimization to a single galaxy's far-infrared template spectral energy distribution (SED). The use of a single galaxy template, or modest set of templates, can lead to an artificially low uncertainty estimate on FIR-$z$'s because real galaxies display a wide range in intrinsic dust SEDs. I use the observed distribution of galaxy SEDs (for well-constrained samples across $0<z<5$) to motivate a new far-infrared through millimeter photometric redshift technique called MMpz. The MMpz algorithm asserts that galaxies are most likely drawn from the empirically observed relationship between rest-frame peak wavelength, ${\lambda }_{\mathrm{peak}}$, and total IR luminosity, L${}_{\mathrm{IR}}$; the derived photometric redshift accounts for the measurement uncertainties and intrinsic variation in SEDs at the inferred L${}_{\mathrm{IR}}$, as well as heating from the CMB at $z>5$. The MMpz algorithm has a precision of ${\sigma }_{\Delta z/(1+z)}\approx 0.3-0.4$, similar to single-template fits, while providing a more accurate estimate of the FIR-$z$ uncertainty with reduced chi-squared of order $\mathcal{O}({\chi }_{\nu }^2)=1$, compared to alternative far-infrared photometric redshift techniques (with $\mathcal{O}({\chi }_{\nu }^2)\approx 10-10^3$).

12 pages, 4 figures; accepted for publication in ApJ. For associated code, see http://www.as.utexas.edu/~cmcasey/mmpz.html

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• Characterization of Two 2mm-detected Optically-Obscured Dusty Star-Forming Galaxies
• Not-so-little Red Dots: Two massive and dusty starbursts at z~5-7 pushing the limits of star formation discovered by JWST in the COSMOS-Web survey
• A large population of strongly lensed faint submillimetre galaxies in future dark energy surveys inferred from JWST imaging
• The Physical Drivers of the Luminosity-Weighted Dust Temperatures in High-Redshift Galaxies
• High-z Sudoku: A diagnostic tool for identifying robust (sub)mm redshifts
• Early Science with the Large Millimeter Telescope: a 1.1 mm AzTEC Survey of Red-$Herschel$ dusty star-forming galaxies
• Improving Optical Photo-z Constraints for Dusty Star-forming Galaxies Using Submillimeter-based Priors
• The NIKA2 cosmological legacy survey at 2 mm: catalogs, colors, redshift distributions, and implications for deep surveys
• Searching Far and Long I: Pilot ALMA 2mm Follow-up of Bright Dusty Galaxies as a Redshift Filter