The physical origin of long gas depletion times in galaxies

Vadim SemenovAndrey Kravtsov

Nick Gnedin(University of Chicago)

why?

Semenov, Kravtsov & Gnedin 2017, ApJ 845,133 (arXiv:1704.04239)

2. longer than local depletion time in actively star-forming regions:

Star formation is surprisingly inefficient

→ orbital period of galaxy

→ gravitational collapse time

→ turbulent crossing time

1. much longer than any relevant dynamical timescale:

few %

Semenov, Kravtsov & Gnedin 2016, ApJ 826, 200Semenov, Kravtsov & Gnedin 2017, ApJ 845,133

Hydrodynamical simulations● ~L*-sized isolated galaxy:

Mdisk ~ 4.3x1010 Msun, Rdisk ~ 3.5 kpc, fgas = 0.2; Δ = 40 pc

● N-body+hydrodynamics with Adaptive Mesh Refinement ART code

● Z-dependent heating + cooling and self-shielding calibrated against RT simulations(Safranek-Shrader et al 2017)

● Star formation feedback calibrated against supernova remnant simulations (Martizzi et al 2015)

● Subgrid turbulence model (Schmidt et al 2014)

Temp

era ture (K)

Subg

rid t urb

ulent velo

city (k m/s)

Density ( cm

-3)

● Star formation recipe motivated by molecular cloud simulations (Padoan et al 2012; see also I-Ting Ho's talk yesterday)

Semenov, Kravtsov & Gnedin 2017, ApJ 845, 133 (arXiv:1704.04239)

Simulation reproduces observed depletion times

Surface density of HI+H2 (Msun/pc

2)Surface density of H2 only

(Msun/pc2)

Surf

ace

den

sity

of s

tar

form

atio

n ra

te

(Msu

n/yr

/kp

c2)

Gal

acto

cent

ric

rad

ius

(kp

c)

Milky Way

Bigiel et al 2008

Bigiel et al 2010Leroy et al 2013

Total gas Molecular gas

Star formation threshold:

Density (cm-3)

ther

mal

+ tu

rbul

ent v

elo

city

dis

per

sio

n

Slow star formation as a result of gas evolutionSt

rong

er s

upp

ort

Stronger gravity

Temp

era ture (K)

Star-forming gas

Non-star-forming gas

slow star formation small net gas inflow into star-forming state

Steady state:

Fsf

Star formation threshold:

Density (cm-3)

ther

mal

+ tu

rbul

ent v

elo

city

dis

per

sio

n

Slow star formation as a result of gas evolutionSt

rong

er s

upp

ort

Stronger gravity

Temp

era ture (K)

Star-forming gas

Non-star-forming gas

slow star formation small net gas inflow into star-forming state

Steady state:

Gas evolution is rapidth

erm

al +

turb

ulen

t ve

loci

ty d

isp

ersi

on

(km

/s)

Density (cm-3)

Temp

era ture (K)

Removal (feedback, expansion)

Supply (gravity, compression)

Time o

n which a

vir changes b

y a fac tor o

f 10 (Myr)

Net flux

Net flux is small due to cancellation of

strong opposite fluxes:

Density (cm-3)

Star

-form

ing

gas

Star

-form

ing

gas

ther

mal

+ tu

rbul

ent

velo

city

dis

per

sio

n (k

m/s

)

Vir

ial p

aram

eter

Long depletion time is a result of rapid gas cycling

Density (cm-3)

Required # of cycles:

star

-form

ing

stat

e

non-star-forming state

total time in star-forming state before gas is converted into stars

Semenov, Kravtsov & Gnedin 2017, ApJ 845, 133 (arXiv:1704.04239)

long because of large Nc (~10 - 100)

Example of application: self-regulation of star formation

Orr et al (2017), Hopkins et al (2017)

Stronger feedback shortens tsf:

Semenov, Kravtsov & Gnedin 2017 ApJ 845,133Semenov, Kravtsov & Gnedin 2017b, in prep

feedback strength

Efficient feedback:

At sufficiently high efficiency:

Independent of efficiency!

Global SFR is independent

of local SF efficiency

but scales with the feedback strength

Summary

Large # of cycles is required Significant fraction of time is spent in the non-star-forming state

Resolved puzzle of inefficient star formation:

This framework can be used to predict dependence of depletion time on properties of galaxies and physics of local star formation and feedback

For example, it explains how efficient feedback self-regulates global star formation rate in simulations

Semenov, Kravtsov & Gnedin 2017, ApJ 845,133 (arXiv:1704.04239)

feedbackISM dynamicsproperties of GMCs

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