Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Pierluigi Rinaldi, Pierluigi Rinaldi, , ,
| Summary:
Little Red Dots (LRDs) are compact, red sources discovered by JWST at high
redshift ($z gtrsim 4$), marked by distinctive "V-shaped" spectral energy
distributions (SEDs) and often interpreted as rapidly accreting AGNs. Their
evolution remains unclear, as identifying counterparts at lower redshifts is
challenging. We present WISEA J123635.56+621424.2 (here dubbed it the
Saguaro), a $z=2.0145$ galaxy in GOODS-North, as a possible analog of
high-redshift LRDs and a potential missing link in their evolutionary path
toward lower-redshift systems. It features a compact LRD-like nucleus
surrounded by a face-on spiral host. Its connection to LRDs includes that: (1)
its nuclear spectrum shows a clear "V-shaped" SED; and (2) when redshifted to
$z=7$, surface brightness dimming makes the host undetectable, thus mimicking
an LRD. This suggests that high-redshift LRDs may be embedded in extended
hosts. To test this, we stack rest-frame UV images of 99 photometrically
selected LRDs, revealing faint, diffuse emission. Stacking in redshift bins
reveals mild radial growth, consistent with the expected galaxy size evolution.
A simple analytic model confirms that surface brightness dimming alone can
explain their compact appearance. Lastly, we show that it the Saguaro is not
unique by describing similar objects from the literature at $zlesssim3.5$.
Taken together, our results support a scenario in which LRDs may not be a
distinct population, but could be the visible nuclei of galaxies undergoing a
short-lived, AGN-dominated evolutionary phase, with their compact, red
appearance driven largely by observational biases.
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