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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