Lunch talk on Sep. 19, 2022

Unveiling the nature of optically-dark galaxies: A substantial population of compact starbursts with high infrared optical depths

Speaker: Shuowen Jin (Cosmic Dawn Center & DTU Space)

Venue: Video Conference

Time: 12:30 PM, Monday, Sep. 19, 2022

Abstract: 

Optically-dark galaxies belong to a population that is invisible or extremely faint in deep optical imaging even with HST, but well-detected in IRAC, (sub)mm or radio. This population has been found to be dominant in the early Universe, and contributed significantly to the Cosmic Star Formation Density. However, their extreme faintness makes it difficult to measure the redshifts and ISM properties, preventing us from unveiling its nature. Recently, using NOEMA and ALMA 3mm line scans, we measured spectroscopic redshifts of ten optically-dark galaxies at 3.5<z<6 by solidly detecting CO and [CI](1-0) transitions. We provided first direct evidence that their cold dust SEDs are impacted by the cosmic microwave background (CMB). Remarkably, multiple pieces of evidence support that these galaxies have optically thick dust in FIR, which makes them deceivingly cold at the surface but are actually warm in their starbursting cores. The optically thick dust directly answered us why they are dark in optical, shedding lights on the nature of optically-dark galaxies.

Report PPT:  SWIFAR_Shuowen Jin.pdf