SPHEREx: Star Formation in Abell 2029

SPHEREx provides low-resolution (R ≈ 40) spectrophotometry from 0.75 to 5 μm in 102 channels over the whole sky. We use it to test how well the star-formation rates of cluster galaxies can be measured once SPHEREx is added to standard broadband data. The testbed is Abell 2029 (z ≈ 0.078), where our spectroscopic membership catalog is nearly complete out to 3R₂₀₀ and contains about 1,100 member galaxies.

Pipeline

I perform forced photometry on the SPHEREx maps at the positions of known members, calibrate the SPHEREx flux scale against SDSS and WISE, merge the result with GALEX, SDSS, DECaLS, and WISE broadband photometry, and fit the SEDs with CIGALE. I run two independent CIGALE configurations (my own setup and the DESI value-added-catalog setup) to keep model-choice systematics visible.

What SPHEREx adds, and what it does not

The core of the work is a set of controlled experiments in which only the band set changes. We fit the same galaxies with the 15 broadband filters alone and with the same 15 filters plus the 102 SPHEREx channels.

• The 1–5 μm continuum constrains the old stellar population, and the M_* posterior narrows from σ(log M_*) ≈ 0.09 to 0.06 dex.
• Refitting with six different star-formation-history prescriptions, the scatter of M_* across SFH choices shrinks by 10–30%, and the envelope of best-fit SEDs narrows by a factor of two inside the SPHEREx window.
• SPHEREx does not set the SFR scale. Rest-UV and mid-IR coverage and the SFH prior govern the SFR; SPHEREx alone over-predicts SFR by about 1 dex because 0.75–5 μm cannot separate attenuation from young stars. Adding SPHEREx to the full broadband set leaves the median SFR unchanged (−0.02 dex) but reduces the typical per-galaxy SFR uncertainty by about 0.3 dex.