GALAXY SELECTIONa) Redshift range : 0.025 < z < 0.05Motivation:1) We want to detect HI and H2 down to levels of a few percent inM(gas)/M* in about an hour of integration time.2) We want to get an accurate estimate of the total cold gas mass with a singlepointing of both the Arecibo and IRAM telescopes.b) Stellar mass range: log M* > 10Motivation:1) Span a range of stellar masses encompassing both “active” star-forminggalaxies and “passive” systems => quantify how the transition between thesetwo populations is reflected in their cold gas content. Avoid any selection on morphology, environment etc.2) In the mass range, all galaxies have roughly solar metallicity: conversionfactor from CO luminosity to H2 mass is simplified.OBSERVING STRATEGY: Integrate until the galaxy is detected or a limitingHI/H2 mass fraction ~1.5-3% is reached, i.e. Our survey will quantify the fullcondensed baryon budget in these galaxies.
MILLENNIUM-II Simulations (Boylan-Kolchin et al 2009)
Models are based on the L-Galaxies Code developed by Springel, Croton, De Lucia, Guo et al.
Radially Resolved Models for Galactic Disk FormationScale radius of the infalling gas is proportional to λ Rvir (following Fall &Efstathioiu 1980; Mo, Mao, White 1998)The gas that falls is at one snapshot is simply superposed on the gas thathas already fallen in at earlier times.
Krumholz, McKee and Tumlinson 2009 Blitz & Rosolowsky 2004,2006
Comparison of ModelProfiles with THINGS/HERACLES data from Bigielet al 2008 GAS PROFILES ARE TOO FLAT AND THE MOLECULAR GAS ISRAPIDLY CONSUMED IN THEINNER DISK.PROBLEM CANNOT BE SOLVEDBY ADJUSTING FREEPARAMETERS.
Comparison of ModelProfiles with THINGS/HERACLES data from Bigielet al 2008 IF ENERGY FROM SUPERNOVAIS MORE EFFICIENTLYDISSIPATED IN DENSE INNERREGIONS OF THE DISK (i.e. LESSCOLD GAS IS REHEATED ), THENMODELS CAN FIT DATA.Caveat: Radialinflow of gas has notbeen considered
The Models Provide a Reasonable Match to the ObservedMass Functions of HI, H2 and Stars in the Local Universe
It is interesting to consider the global scaling relations between stellar mass,stomic gas mass and molecular gas mass for an ensemble of disk galaxies
What Are the Physical Drivers of these Scaling Relations? 1) The MASS OF THE DARK MATTER HALO determines the mass of baryons that is able to cool. Because supernovae feedback is less efficient in massive halos, a larger fraction of the baryons that cool are converted into stars.
What Are the Physical Drivers of these Scaling Relations? 1) The SPIN PARAMETER sets the contraction factor of the infalling gas. A higher spin parameter results in larger galaxies with lower gas surface densities.
What Are the Physical Drivers of these Scaling Relations? 1) The FRACTION OF GAS in the galaxy that was ACCRETED recently (i.e. Gas that has has less time to be turned into stars).
COMPARE WITH DR2 FROM THECOLD GASS SURVEY(300 galaxoes with both HIobservations from Arecibo and CO 1-0observations from the IRAM 30mtelescope)
PROCEDURE:1) Analyze detections and non-detections separately.2) Analysis of population of galaxies with detected HI/CO allows us to test the disk formation models .3) Analysis of galaxies without detected gas allows us totest whether “quenching” processes (in particularradio-mode AGN feedback) operates the same way in themodels and in the data.
Comparison of Mean HI and H2 scaling relationsAtomic Gas Molecular Gas
Saintonge et al 2011Molecular gas depletion times are shorter in more actively star-forming galaxies
Distribution Functions of the HI detections in bins of mass, stellar mass density, concentration
Trends in non-detected fraction do NOT agree with the models Fraction of non- detections is independent of stellar mass in the data, but depends strongly on stellar surface density and concentration (i.e. Bulge-to-disk ratio)
In the models, “radio-mode” AGN feedback prevents gasfrom cooling in massive galaxies (with black holes) located inhalos above ~10^11 M_sun. Observationally, this ismanifested as a strong threshold in STELLAR MASS
H2 detections and non-detections in 2-dimensionalplanes of stellar mass and galaxy structural parameters
Natural interpretation of the results: processes associated with the formation of galactic bulges andor black holes are most directly responsible for shutting down star formation in galaxies.
UGC 8802: A Case Study of a Star-Forming Outlierfrom the HI “plane”, with unusually high HI fractionM(HI) = 2 x 10 10 M_sol Moran et al 2010
As the HI gas fraction increases, galaxies become bluer on the outside relativeto the inside. This effect is weaker in control samples matched in stellar mass,structural parameters and specific star formation rate. Wang et al 2011
Preliminary work with Cheng Li:Find best photometric “predictors” of the HI content of a galaxy Update of HI gas fraction “plane” to include a colour gradient term -- gives much more accurate predictions for high Hi fraction galaxies.
GASS-SDSS cross-correlation functions: “real” versus “predicted”What is this useful for? Creating mock catalogues – tuning up future surveys with real data!