the contribution of merger-induced starbursts to the smg
TRANSCRIPT
Chris Hayward, CfA ([email protected])Massive Galaxies Across Cosmic Time 3
November 8, 2010
The contribution of merger-induced starbursts to the SMG population
Collaborators
• Desika Narayanan (Arizona)
• Patrik Jonsson, Lars Hernquist (CfA)
• Dušan Kereš, Phil Hopkins (Berkeley)
• T.J. Cox (Carnegie Observatories)
What drives SF in high-z ULIRGs?
Hopkins, Younger, CCH+10
Submm galaxy selection yields sample of high-z ULIRGs
↑ gas content and compactness at high z → “normal disks” can be ULIRGs
Submm flux not necessarily simply related to LIR b/c dust T variation
Nature of SMG population unclear
Submm counts from SAMs
2005MNRAS.356.1191B
Baugh+05(ν = 850 μm)
•With standard IMF quiescent SF dominates but counts too low•Flat IMF in burst makes bursts dominant
Gadget-2 simulations
• Large suite of major & minor mergers, isolated disks; non-cosmological
• Gadget-2 N-body/SPH (Springel 05)
• Schmidt-Kennicutt SF recipe
• Two-phase ISM of Springel & Hernquist (03)
• BH growth & feedback (Springel+05)
Sunrise radiative transfer• Use Sunrise MC RT code (Jonsson 06; Jonsson, Groves, & Cox 10) in post-
processing to calculate SEDs of simulated galaxies
• Stellar SEDs from Starburst99 (Leitherer+99); Kroupa IMF
• AGN template of Hopkins+07
• Calculate dust attenuation and re-emission
• Solves for dust T iteratively (Juvela 05) to properly treat dust self-absorption -- key for high optical depths encountered in SMGs
• Yields UV-mm SEDs/images from multiple viewing angles
Isolated disk evolution
S850 ∝ SFR0.4
Isolated disk w/ Mhalo = 9e12, Mb = 4e11; initially 60% gas
Submm flux traces SFR well
Hard to produce classical SMG with even with such a massive, gas-rich disk
Merger of two z ~ 2 disks, each w/ Mhalo = 9e12, Mb = 4e11; initially 60% gas
Merger evolution
Two SF regimes:
1. Quiescent disk (during infall)
2. Merger-driven burst
S850 ∝ SFR0.25
S850 ∝ SFR0.4
Bursts inefficient at boosting submm flux (~40x in SFR but <2 in S850) b/c SED significantly hotter at burst and burst does not dominate Lbol
Our model for number counts
1. Submm duty cycles from simulations
2. Merger rates from “semi-empirical” model of Hopkins+10
3. Combine to get number counts:
dN(> Sλ)
dΩ=
∫dN
dV dtd logMbdµdfg(Mb, µ, fg, z)τ(> Sλ,Mb, µ, fg, z)
dV
dΩdz(z)d logMbdµdfgdz
Submm counts from starbursts
1.1 mm
N(>
S)
Merger-driven starburst counts close to observed (Austermann+10) but still below
However, starbursts are not the only way mergers contribute
SMG bimodality
N(>
S)
Engel+10
• Submm bolometers used to identify SMGs have beams ~15” (~130 kpc at z = 2) ⇒ easy
to fit two disks in beam
• Adding two disks is efficient way to boost submm flux
• Will look like “normal” disks; not yet interacting
• Supported by large fraction of SMGs w/ multiple radio counterparts & resolved close pairs (e.g., Engel+10)
• Overlap w/ BzKs Emanuele discussed
Burst+infall counts
Austermann+101.1 mm
N(>
S)
SHADES (0.7 deg2)
Mergers can match counts with standard IMF when both infall and burst stages included
Summary
• We have used high-resolution SPH sims + 3-D RT w/ full dust T calculation to model SMGs; combined with semi-empirical merger rates to predict SMG number counts
• Mergers create SMGs via 2 effects:
1. Starburst induced at coalescence
2. Pre-coalescence: two progenitor disks in submm beam
• Starbursts relatively inefficient at boosting submm flux; infall phase of two disks in submm beam very important
• Match # counts w/ standard IMF
• Unlike local ULIRGs, SMGs are a mix of quiescent and bursting sources; use caution when interpreting observations of high-z ULIRGs, b/c SF mechanism is not uniform