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Low-J CO Line Emission at High Redshift with ALMA Band 2 Leslie Hunt INAF-Osservatorio Astrofisico di Arcetri Firenze, Italy.

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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:

1 Low-J CO Line Emission at High Redshift with ALMA Band 2 Leslie Hunt INAF-Osservatorio Astrofisico di Arcetri Firenze, Italy

2 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

3 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

4 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 )

5 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 )

6 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.

7 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!

8 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

9 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

10 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).

11 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.

12 Low-J CO lines key to accurate molecular gas mass: ALMA Band 2 to the rescue

13 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.

14 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.

15 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

16 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.


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