Using the data from the Spitzer Space Telescope in the Deep HSC fields, we have found a mysterious population of objects that are detected at SNR>5 in the 3-5 micron (IRAC ch1 and ch2) but are either very red with Y-ch1>3.5 or totally invisible in the much deeper optical. Their median magnitudes are 23.95 and 23.68 AB in ch1 and ch2, respectively; 6% of the objects are also detected in IRAC ch3 (6 micron) and 50 objects have detection in all 4 IRAC channels. Several dozens of dropouts falls into the JWST PRIMER survey footprint and have clear detection.
Their extremely red Y-ch1 colours are difficult to explain. They cannot be artificially created by transient events, and cannot be objects at z>7. They are very abundant (surface density ~950/sq.deg) and only 2% of them are Galactic brown dwarfs based on their ch1-ch2 colours. Our tentative model suggests that this is a population of the z>3 massive (>10^10.5 solar mass) galaxies with their stellar population being as old as the age of the Universe.
We present our pilot study of those objects and put them into the context of the current galaxy evolution paradigm in the JWST era.
BIO
I got my PhD in the University of Missouri and now I work as a postdoc at the CASSACA (Chinese Academy of Sciences South America Center for Astronomy) at the NAOC. My main fields of interests are evolution of galaxies, stellar mass functions and contribution of the optically invisible galaxies to the total stellar mass budget of the Universe.
Host: Junkai Zhang
