0 Assigned ID: 648 ----- Attempting Submission 1 (Fri Apr 08 18:11:32 GMT 2016) ----- HST Phase I Proposal 648 (Phase 1 Proposal.apt.2016_04_01_00_48.aptbackup.2016_04_03_18_51.aptbackup.2016_04_04_17_23.aptbackup) successfully submitted. Receipt: # 648-1 ----- Attempting Submission 2 (Fri Apr 08 20:07:44 GMT 2016) ----- It was once generally assumed that galaxies start as disks, and that galactic spheroids form from disk mergers. However, the axis ratio distribution of high redshift HST images shows that most galaxies start not as disks but rather as elongated (prolate) stellar systems. Consistent with these observations, our high-resolution cosmological zoom-in galaxy simulations show that forming galaxies start as prolate stellar systems aligned with the cosmic dark matter filaments on which they form, and only become round stellar systems -- disks or spheroids -- when their centers become gravitationally dominated by ordinary matter. More massive simulated galaxies make this transition at redshifts z > 2.5, which should be accessible to JWST. In order to clarify the observational implications of the transition from elongated galaxies to galactic disks or spheroids, we propose to make realistic mock images and spectra from our simulations using our Sunrise code and compare them with HST images and grism spectra of star-forming galaxies. The distribution of axis ratio b/a versus semi-major axis a will make clearer the distinction between elongated and rounder galaxies. This will be further clarified by taking into account the amount of dust attenuation, which is predicted to be greater for edge-on disks than for edge-on elongated galaxies. Our mock images will allow us to quantify how the effects of dust and resolution make simulated images rounder, increasing the apparent axis ratio. We are also making mock images and spectra for large ground-based telescopes, which will allow further comparisons of predictions and observations. All outputs and codes will be publicly released. Galaxies 0 Regular false false true SUPPORTED