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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14 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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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

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, 10
11



10
12

L


are
responsible for >50% of the
SFRD from z=0.6 to z=2.
ULIRGs account for 20% at
z<1.6, increasing to > 50% at
z>2.3


(
S
ee Reddy+ 2008,
Rodighiero
+ 2010,
Murphy+ 2011).

D
ust
-
corrected UV
-
derived SFRD from
Cucciati
+ 2012, VVDS

f
gas

z

z

Galaxies more gas rich at higher
redshifts

Gas fraction
f
gas

=
M
gas
/(
M
gas
+M
*) depends on X
CO


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+ 2012 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
+
2010.)

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


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
+ 2010.)

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
+ 2011.)

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 L’CO(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

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~ 1.5
-
2.5 (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=0.9
-
1.9 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.