Speaker
Description
There is an ongoing debate of whether bright or faint galaxies reionized the Universe. In this context, the galaxy COLA1 - a luminous rare double peaked Lyman-$\alpha$ (Ly$\alpha$) emitter in the COSMOS field at a redshift $z=6.6$ - provides an exceptional opportunity to unveil the anatomy of an ionized bubble in detail. The visibility of the blue peak of the Ly$\alpha$ line suggests that COLA1 resides inside a large highly ionized bubble, and allows us to infer the Lyman Continuum escape fraction. However, such ionized bubbles have not been found in recent reionization simulations. I will present the first view of COLA1 and its large-scale environment obtained from Wide Field Slitless Spectroscopic (WFSS) data from JWST/NIRCam. First, we prove that COLA1 is a single extremely compact bright galaxy ($M_{\rm UV}=-21.3$). We confirm the systemic redshift suggested by the Ly$\alpha$ double-peaked profile using detections of H$\gamma$, H$\beta$ and [OIII]. I will discuss how we measure an exceptionally high Ly$\alpha$ escape fraction of $f_{\rm esc}({\rm Ly}\alpha)=81\pm 5\%$ and how COLA1 has all the properties expected for strong ionizing galaxies with an inferred ionizing photon escape fraction of $f_{\rm esc}\approx 30\%$. Using the complete spectroscopic coverage of the WFSS mode, we identify a total of 141 spectroscopically selected [OIII] emitters in the $\sim 22$ arcmin$^2$ field at $z\sim 5$-$7$. While we find an overdensity ($\delta+1 = 1.96$; 4 galaxies) around COLA1, we use a semi-empirical forward modeling approach to show that this is a totally normal environment given COLA1s UV luminosity. Our results suggest that COLA1 itself makes a substantial contribution to the ionizing photon budget needed to power its bubble.
