Presentation on theme: "Low-J CO Line Emission at High Redshift with ALMA Band 2 Leslie Hunt INAF-Osservatorio Astrofisico di Arcetri Firenze, Italy."— Presentation transcript:
Low-J CO Line Emission at High Redshift with ALMA Band 2 Leslie Hunt INAF-Osservatorio Astrofisico di Arcetri Firenze, Italy
4mm spectral region rich in lines at z=0 These starbursts, LIRGs (top), ULIRGs (bottom), all show different line ratios (e.g., HCN, HCO+): can we assume the same physical conditions for all these, and at z>0? From Snell+ (2011), the FCRAO RSR 3mm survey
ALMA fundamental for dusty high-z galaxies Peak of star-formation activity, Star-Formation Rate Density SFRD, at z ~ 2-3. LIRGs, Luminous Infra Red Galaxies, – L are responsible for >50% of the SFRD from z=0.6 to z=2. ULIRGs account for 20% at z 50% at z>2.3 (See Reddy+ 2008, Rodighiero+ 2010, Murphy+ 2011). Dust-corrected UV-derived SFRD from Cucciati+ 2012, VVDS
f gas zz Galaxies more gas rich at higher redshifts Gas fraction f gas = M gas /(M gas +M*) depends on X CO (α CO = equivalent mass conversion), the conversion factor from integrated line flux (T b Δv) to column density N H2. Traditional X CO assumes two values, one for rotationally dominated systems (disks), and another for starbursts. (See Narayanan for continuously varying X CO )
Massive disks vs. starbursts (mergers) Kennicutt- Schmidt (K-S) relation between SF and gas mass surface densities shows bi-modal behavior or uni- modal behavior depending on CO conversion to total H 2 mass. (Plots taken from Genzel+ 2010; see also Daddi+ 2010, Tacconi )
High-J CO measurements are biased toward warm, dense gas Spectral-line energy distribution (SLED) of prototypical LIRG starburst, M82 (Weiss+ 2005). At z> 0.5, typical sub-mm observations trace H 2 mass with high-J CO [(3-2), (4- 3), …]. These transitions would detect only the inner 400 pc circumnuclear starburst region in M82, rather than the more extended, lower excitation gas.
Uncertainties when CO transitions trace gas mass Need to assume brightness temperature (T b ) ratios if transitions other than CO(1-0) are used to trace molecular mass. Need to assume a conversion factor X CO (α CO ) to convert CO luminosity to molecular gas mass. Both parameters depend on physical conditions, including gas volume density n(H 2 ), excitation temperature T ex, dynamical state (e.g., turbulence in clumps vs. rotation dominated), … Constraint: Using the same X CO for both starbursts and massive disks can sometimes give gas masses which exceed the dynamical mass!
X factor: Relating CO luminosity to H 2 mass CO emission is optically thick (e.g., Wilson+ 1974), hence traces surface area, not volume need proportionality constant X to relate CO intensity to mass or column density, N H2 Assumptions (e.g., Dickman+ 1986): (1)Extragalactic molecular emission distributed as an ensemble of independent discrete clouds (no overlap along LOS) (2)Individual clouds virialized (line width ~ dynamical mass) I(CO) = T b dv Σ T b Δv ~ Σ T b (M/r) ½ ~ T b Σ (n H2 ) ½ r ~ [T b (n H2 ) -½ ] N H2 Hence, N H2 = X * I(CO), where X ~ (n H2 ) ½ / T b virialization mass in homogeneous sphere N H2 = (n H2 ) r
Distinct gas phases in z=2 ISM SLED of lensed Sub-millimeter Galaxy (SMG) at z=2.3. With CO(1-0) from GBT, and CO(3-2), CO(4-3), CO(5-4), CO(6-5), CO(7-6), CO(8-7), CO(9-8) from IRAM 30-m, Danielson+ (2011) find 2 phases necessary: cool, less dense (disk) phase + warm, denser (clumps in starburst) phase. Factor of 4 temperature variation within various kinematic components! Jupper
High-J lines underestimate cool gas mass (Taken from Dannerbauer+ 2009; see also Aravena ) Near-IR selected (BzK) galaxies at z=1.5 mapped in CO(1-0) (VLA) and CO(2-1), CO(3-2) (PdBI) show a SLED in which the cool gas traced by CO(1-0) is underestimated by J upper > 2. Such SLEDs arise from spatially extended cool, less dense, gas, typical of the low-excitation conditions in quiescent disks (e.g., the local spirals).
Evidence for different K-S relations between different galaxy populations is weak (Taken from Ivison ) Observational K-S relation with different local and high-z galaxy populations (LIRGs, ULIRGs, BzKs, SMGs) with CO(1-0) (or CO(2-1) measurements. Offset shown as red arrow for translation when LCO(1-0) inferred from 3-2 transition. X CO values inferred from high-J observations could lead to the massive disk vs. starburst dichotomy proposed by several groups. Also lead to steeper slope.
Low-J CO lines key to accurate molecular gas mass: ALMA Band 2 to the rescue
5 transitions with Band 2 from z=0.29 to z=0.72, including CO(1- 0). 5-7 transitions with Band 2 from z=1.57 to z=2.44, including CO(2- 1). Band 2 enables low-J SLEDs around the peak of the cosmic SFRD.
ALMA Band 2 will enable accurate gas mass estimates at z>0.3 Band 2 will explore the low-J CO transitions, necessary to accurately infer molecular gas mass and the nature of star-formation activity at high redshift. This is fundamental for z>0.5 because from z=0, the number of LIRGs increases by almost two orders or magnitude (see Murphy+ 2011). However, massive star-forming disks with longer gas depletion times, contribute at least 50% of the SFRD at z~ (see, e.g., next slide). Band 2 will allow comparison of the total molecular gas mass in different galaxy populations, without relying on uncertain T b ratios or X CO conversion factors.
Selection of z= galaxies from MASSIV (Contini, Epinat, Vergani, … et al. 2011) used to study how disk and spheroidal systems grew through cosmic time (colors show the motions of the Hα gas in the galaxies) Need of low-J CO for mass measurements (cold dust and reliable CO/H 2 conversion factor) to study the fundamental relations (mass-size-velocity-SFR-metallicity) at low- and intermediate-z Need of low-J CO to trace the distribution and kinematics of the (cold, not dense) gas reservoir Credit: ESO/CFHT; ESO1212 Science Release Contini, Epinat, Vergani et al. The Messenger 2012
Thank you! Last, but not least, 13 CO transitions and other potentially optically thin transitions with Band 2 will help resolve degeneracies of temperature/density.