Recent sensitive ALMA observations have revealed that a substantial fraction of galaxies which are bright in submm emission are often dark in optical images, making it exceedingly difficult to detect optical emission lines and obtain an unambiguous redshift. This can have broad consequences since the uncertainties in redshift can severely hamper our ability to efficiently conduct surveys which will ultimately connect various scientific fields, for example, blind galaxy/cluster searches, starburst-AGN-QSO connection, deep line surveys towards the lensed QSO, spatial variation of the CGM, reproducibility in cosmological simulations. The scope of this workshop is to review the recent progress in understanding the nature of optically-dark submm galaxy population and discuss possible science cases and strategies for the next call for proposals, including ALMA Cy-8. Other highly related and complementary topics will be covered in the workshop as well.

Goals of the workshop:

(i) summarise ancillary data of Cloverleaf, including astrochemical conditions, chemical evolution and gas excitation;

(ii) review of the redshift identification of DV;

(iii) understand the evolution of AGN at their transit phase in the early Universe, and explore the science cases;

(iv) design future observations (another target and DV) including follow-up observation of the blind detection.

This workshop is motivated by a blind detection of a strong millimetre line emitter, 5” from Cloverleaf (a well-known quadruply-lensed QSO at z = 2.55), which has been dubbed “Darth Vader” (DV). Deep HST images in the JHK filters show no detections (>27.2 mag.), whereas Chandra detects intense hard X-rays, possibly indicating an AGN in a transit stage from a starburst to a QSO. The possibility of associations with the 36 known HI/metal absorption systems and ~20 galaxies around the Cloverleaf has been already excluded. Recent ALMA Cy-7 observations finally determined its redshift (Hayatsu et al. in prep.). We have also found another blind detection close to Cloverleaf, but at an unknown redshift.

We look forward to seeing you at the workshop!

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