Probing the Star Formation Main Sequence down to ${10}^8$ M${}_{\odot }$ at $1.0<z<3.0$

We investigate the star formation main sequence (MS) (SFR-M${}_{\star }$) down to 10${}^{8-9}{\mathrm{M}}_{\odot }$ using a sample of 34,061 newly-discovered ultra-faint ($27\lesssim i\lesssim 30$ mag) galaxies at $1<z<3$ detected in the GOODS-N field. Virtually these galaxies are not contained in previous public catalogs, effectively doubling the number of known sources in the field. The sample was constructed by stacking the optical broad-band observations taken by the HST/GOODS-CANDELS surveys as well as the 25 ultra-deep medium-band images gathered by the GTC/SHARDS project. Our sources are faint (average observed magnitudes $<i>\sim 28.2$ mag, $<H>\sim 27.9$ mag), blue (UV-slope $<\beta >\sim -1.9$), star-forming (rest-frame colors $<U-V>\sim 0.10$ mag, $<V-J>\sim 0.17$ mag) galaxies. These observational characteristics are identified with young (mass-weighted age $<{\mathrm{t}}_{\mathrm{M}-\mathrm{w}}>\sim 0.014$ Gyr) stellar populations subject to low attenuations ($<\mathrm{A}(\mathrm{V})>\sim 0.30$ mag). Our sample allows us to probe the MS down to ${10}^{8.0}{\mathrm{M}}_{\odot }$ at $z=1$ and ${10}^{8.5}{\mathrm{M}}_{\odot }$ at $z=3$, around 0.6 dex deeper than previous analysis. In the low-mass galaxy regime, we find an average value for the slope of 0.97 at $1<z<2$ and 1.12 at $2<z<3$. Nearly $\sim$60% of our sample presents stellar masses in the range ${10}^{6-8}$ M${}_{\odot }$ between $1<z<3$. If the slope of the MS remained constant in this regime, the sources populating the low-mass tail of our sample would qualify as starburst galaxies.