The ALMA z=4 Survey (AR4S) ALMA studies of massive z~4-5 galaxies Corentin Schreiber Leiden University October 21 2016 In collaboration with: Maurilio Pannella, David Elbaz, Roger Leiton, Tao Wang, and Ivo Labbé.
The cosmic history of star formation in galaxies Madau & Dickinson 2014 Star formation was most intense in the past, around z=2, then declined. Where are all these stars formed? In which galaxies? Merger-driven bursts, or secular evolution? z<3 well constrained by MIR/FIR/radio observations, but higher redshifts remain a challenge Is our view of the high-z Universe biased in UV surveys?
Reaching a complete and robust census of star formation in galaxies complete: need to work with mass-complete samples (not IR selected, no Lyman Break Galaxies) robust: need to measure SFR from dust emission (not by correcting the UV continuum for dust attenuation) observationally challenging, but doable now with Hubble WFC3 and Herschel Rodighiero+11
The Herschel view Hubble CANDELS NIR imaging z=0 stacking deepest Herschel images z=0.5 z=1 z=2 z=3 Find correlation between SFR and M* from z=0.3 to z=3.5 Tight dispersion of 0.3 dex SFR Schreiber+15 z=1.5 >60% of the stars in the Universe were formed in the Main Sequence --> SF is mostly secular But very limited census at z>3, Herschel not deep enough -> need ALMA! stellar mass
The ALMA z=4 survey Schreiber+16b: arxiv:1606.06252 All 3.5 < zphot < 5 galaxies with logm* > 10.7 (Salpeter) In CANDELS: GOODS South, UDS and COSMOS 96 galaxies, only 50% of which are LBGs ALMA band 7 870um: 170um rest-frame --> measure SFR close to peak of dust IR emission 5.5h: 1.5 min/galaxy in Cycle 2, 0.2 mjy RMS (SFR > 300 Msun/yr) 30 detections at 3 sigma, only 33% of which are LBGs (we aimed for more, but underestimated Tdust in the proposal)
Some SEDs (ALMA non-detected)
Some SEDs (ALMA detected)
From ALMA flux to SFR: Tdust Schreiber+16b Stacking Herschel images at the position of the AR4S galaxies (ALMA detections / all the sample) sc r He Sp Schreiber+in prep. Tdust = 40+/-2 K l e h ALMA r e itz Similar to Tdust measured by other studies for z>4 galaxies Béthermin+15, da Cunha+15
First results: the z=4 Main Sequence Forward modeling observed ssfr (including non-detections) -> constrain normalization and scatter for ~1/3 of the SFR @ z=4 -> consistent with previous studies at z<4, scatter 0.3 dex Schreiber+16b
Constraints on dusty SFR IR AR4S (all M* > 10.7) UV AR4S (detections)
Discovered two new galaxies in COSMOS ALMA bright, IRAC bright, not in CANDELS and UltraVISTA DR2 catalogs. Not MIPS detected --> probably high-z! Top: no optical or NIR, very red IRAC color. ch1-ch2 = 0.7 K-ch1 > 2.2 Bottom: can detect something in HST and VISTA after smoothing and combining multiple bands.
SED fitting (+ FIR constraints) Not clear what they are... Certainly massive and dusty SF, and likely z>3.5 (because not MIPS detected) Got more ALMA time (3h) to search for the [CII] line at z=5.3 to 6.8 No line was found, so they are probably at 3.5<z<5.3.
A population of HST-dark dropout galaxies T. Wang: H-dropouts in the CANDELS catalogs 62 galaxies (ch2 < 24) 17 of them were observed with ALMA (rest of the sample will come soon) 80% detection rate! Important fraction of the population of high-z MS massive and dusty galaxies (10% to 30%)? Trying to get spectroscopic redshifts (ALMA, VLA, VLT...) Wang+15, Wang+in prep.
Spectroscopic confirmation Method: ALMA CO scan for a few galaxies (will try larger sample if successful) Goal: better control the photo-z and the degeneracies with attenuation (AV) Ranked A will be observed this cycle! (PI: Schreiber) redshift observed wavelength [um]
Conclusions Need complete samples and robust observables to study star formation in galaxies. AR4S: 96 massive galaxies at z=4 observed in sub-mm continuum with ALMA. ssfr average and dispersion in agreement with expectations (previous observations and numerical simulations). star formation still secular at z=4!... but missing HST-dark galaxies: H-dropouts. Unknown population, need to get redshifts (tough!). Much more will be done with this data set! (IRX-beta,...) + catalog will be public (Leiton+prep)