The
Interstellar Medium In Galaxies
Seen
A Billion Years
After T
he Big Bang
P.L. Capak
1,2*
, C. Carilli
3,4
, G. Jones
5
, C.
M.
Casey
6
, D. Riechers
7
, K. Sheth
8
, C.
M. Carollo
9
, O. Ilbert
10
, A. Karim
11
, O. LeFevre
10
, S. Lilly
8
, N. Scoville
2
, V.
Smolcic
12
, L. Y
an
1,2
1
Infrared Processing and Analysis Center (IPAC), 1200 E. California Blvd., Pasadena, CA, 91125, USA E
-mail:
capak@ipac.caltech.edu
2
California Institute of Technology, 1200 E. California Blvd., Pasadena,
CA, 91125, USA
3
National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, USA
4
Astrophysics Group, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, UK
5
New Mexico Institute of Mining and Technology, 801 Leroy Pl, Socorro, NM 87801
, USA
6
Department of A
stronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, Texas 78712, USA
7
Department of Astronomy, Cornell University, 220 Space Sciences Building, Ithaca, NY 14853, USA
8
National Radio Astronomy Observatory,
520 Edgemont Road, Charlottesville, VA 22903, USA
9
Institute for Astronomy, ETH Zurich, CH
-8093 Zurich, Switzerland
10
Aix Marseille Université, CNRS, LAM (Laboratoire d'Astrophysique de Marseille), UMR 7326, 13388, Marseille, France
11
Argelander
-Institut f
ür Astronomie, Auf dem Hügel 71, D
-53121 Bonn, Germany
12
Physics Department, University of Zagreb, Bijeni
č
ka cesta 32, 10002 Zagreb, Croatia
Evolution
in the measured rest frame u
ltraviolet spectral slope and
ultraviolet to optical flux ratios indicate a rapid evolution in the dust
obscuration of galaxies durin
g the first 3 billion years of cosmic time (z>4)
1-
3
. This evolution implies a change in the average
interstellar medium
properties
, but the measurements are systematically uncertain due to
untested assumptions
4,5
,
and
the inability to measure
heavily obscured
regions of the galaxies. Previous attempts to directly measure the
interstellar medium in normal galaxies at these redshifts have failed for a
number of reasons
6-9
with one notable exception
10
. Here we report
measurements of the [CII] gas and dust emiss
ion in 9 typical (~1-
4L
*
) star-
forming galaxies ~1 billon years after the big bang (z~5-
6).
We find these
galaxies have >12x less thermal emission compared with similar sys
tems
~2 billion years later, and enhanced [CII] emission relative to the far-
infrared continuum, confirming a strong evolution in the interstellar
medium
properties in the early universe. The gas is distributed over scales
of 1-
8 kpc, and shows diverse dyn
amics within the sample. These results
are
consistent with early galaxies having
significantly less dust than
typical galaxies seen at z<3 and being comparable to local low-
metallicity
systems
11
.
We have obtained
rest
-frame far-infrared (FIR) (3-
1000
μ
m) measurements
of 9 “normal” (~1-
4 L
*
) galaxies and one low luminosity quasar redshifts of z=5-
6,
~1 billion years after the big bang
. These data were taken with an early 20-
antenna version of Atacama Large Millimeter Array (ALMA) in band 7 (1090-
800
μ
m) continuum mode with 7.5GHz of bandwidth, ~20 minutes of integration,
and
with the compact array yielding a resolution of ~0.6 arc seconds. One of the
four side bands was always centered on the 158
μ
m [CII] line
, which is the
dominant
FIR
cooling line for neutral gas in normal star forming galaxies
12,13
and
hence is a good indicator of galaxy dynamics and the spatial extent of the
ISM
13,14
. The other three spectral bands yield a measurement of the dust
continuum emission at
λ
o
~150
μ
m, which is a good indicator of the total infrared
luminosity
for normal FIR spectral energy distributions (SEDs) (Method 4).
The sample of objects was selected as Lyman Break Galaxies (LBGs)
from the 2 square degree Cosmic Evolution Survey (COSMOS) field
15
and have
spectroscopically determined absorption line redshifts from the Deep
Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) on the W. M. Keck
-II
Observatory in Hawaii. The objects were selected to be “characteristic” with
luminosities between 1 and 4
L
*
and with UV spectral s
lopes (
β
) between -
1.4
and -0.7. Three objects (1, 2, and 10) also meet the selection criteria for Ly
-
α
emitters at z~5.7. When possible, UV morphologies are measured from the
Hubble
-Space
-Telescope (HST) Advanced Camera For Surveys (ACS) F814W
(0.8
μ
m) d
ata
16
. S
tellar masses were determined by fitting SED models to
existing COSMOS data and new 3-
5
μ
m photometry from the Spitzer-
SPLASH
survey
17
(Method 3). Rest frame infrared luminosities were determined by
assuming the range of far infrared properties measured at z~2-
5
18
and have a
systematic uncertainty of 0.3 dex due to the uncertainty in the shape of the FIR
SED (M
ethod
4).
We detect 4 of 9 galaxies in the dust continuum (Figure 1, Extended Data
Table 1
), and a mean combination (stacking) the data on the re
maining six
objects yields a continuum detection of 35±13
μ
Jy. T
his is
more than 12x lower
than
expected for systems with similar UV properties at z<3 assuming the worst
-
case systematics
in our FIR determination (Method 5). This can be quantified
with the
Infrared
-Excess to UV
-slope (IRX
-
β
) relation
4
(Figure 2), which relates
the amount of dust absorption measured in the UV to the amount of infrared re
-
emission and is sensitive to both dust properties and column density. We find
our continuum detected objects are consis
tent with the
Small Magella
nic cloud
(SMC)
19
, which has lower metallicity gas, and less thermal FIR emission
20
than
typical galaxies at z<3. Our undetected systems have even less thermal
emission
than the SMC
. Hence, we conclude
z~6 galaxies are significantly
less
dust obscured than more evolved systems at z<3, confirming previous results
1-3,6
.
The very low IRX values at a
β
~
-1.2 implied by our non-
detections are
difficult to explain in the context of the very young sy
stems expected at z=5-
6
21
.
The simplest explanation is going to the extremes of the 1
σ
systematics in our
estimates of
β
and LIR, which would imply relatively un-
obscured systems with
hot dust (Method 4). But, we cannot exclude changes in the dust geometry or
rapid dis
ruption of the molecular clouds that could lead to lower IRX vales at a
given
β
with more normal dust temperatures
22
. Higher signal-
to-noise near
-
infrared photometry and shorter wavelength FIR data will ultimately be needed to
understand these sources
.
A corollary of this result is that U
V derived star
-formation rates at high
redshift (z>5) should use the SMC like IRX
-
β
relation or assume no dust rather
than the currently assumed Meurer IRX
-
β
relation. This will decrease the UV
derived star
-formation rates by a typical factor of ~2-
4 for individual galaxies
from
those implied by the Meurer relation for similar value
s of
β
. The effect on the
global star
-formation history is much smaller, <40
%, because the majority of star
-
formation is in low luminosity (<L*) galaxies that were already assumed to have
little or no dust extinction (Method
8, Extended Data Figure
4).
In contrast to the dust emission, we find >3
σ
detections of the [CII] line in
all 9 normal galaxies (Figure 3). The line emission is spectrally resolved in all
cases, with [CII] velocity dispersions of
σ
=63-
163 km/s and marginally resolved
at our spatial resolution of ~0.5-
0.9” indicating galaxies with M
dyn
~10
9-11
M
¤
(Method
2, 4). We also detect two optically faint [CII] emitters at redshifts
consistent with the targeted objects. HZ5a is detected near HZ5 at a redshift
consistent with the in-falling gas
seen in the optical spectra of HZ5
23
. HZ8W
corresponds to an optically faint companion to HZ8 and has a similar redshift.
Taken together the direct and serendipitous detections suggest ubiquitous and
enhanced [CII] emission in early galaxies similar to that seen in local low-
metallicity systems
11
(Figure 4).
The
[CII] enhancement in local systems is caused by
a lower dust to gas
ratio which allows the UV radiation field to penetrate a larger volume
of molecular
cloud
11
. Our significantly lower IRX values and enhanced [CII]/FIR ratios would
suggest a similar effect is happening in high redshift galaxies.
But, e
volution in
metal abundances which change the intrinsic UV slope, changes in the dust
properties, and differences in the
dust geometry
, have also been suggested as
possible causes of the transition in obscuration properties with redshift
24
. The
systems in th
is study were selected to have broad UV absorption features in their
spectra that indicate a relatively homogeneous metal abundance
of
≈
0.25 solar.
At this metallicity the UV spectral slopes are expected to be similar
to
those of
solar metallicity systems
25
. Furthermore, the population these objects were
selected from
have
dust attenuation
properties similar to lower redshift objects
26
.
So changes in the dust properties
that would be measureable in the attenuation
curve
are likely not the primary factor. At fixed stellar mass, our sample has
physical properties (gas velocity d
ispersions, sizes, and star-
formation rates
) that
imply geometries consistent with z~1
-3 galaxies that exhibit dust
properties like
those found in the Meurer model
27
. So, changes in the average cloud geometry
are no
t obvious
, but we cannot fully exclude this possibility since it would explain
the observed IRX properties. So we conclude
, a significant decrease
in the dust
to gas ratio is
the most likely explanation for the evolution in extinction properties.
Thes
e results are seemingly at odds with previous measurements
6-9
that
failed to detect [CII] emission at high redshift. Our sample contains objects with
both
Ly-
α
emission and absorption, and the [CII] emission strength is un-
correlated with the Ly
-
α
properties (Extended Data Table 1), which means
the
use of Ly
-
α
redshifts is not the cause. The most likely explanation for the lack of
detections is observational uncertainty in the expected line flux, line width, and
frequency. Even obscuration uncorrected UV star formation rates will over-
estimate the expected FIR continuum flux because less than 20% of the star
formation is leading to FIR emission in our sampl
e
6
(Figure 2). This means the
FIR emission of galaxies will be
more difficult to detect than previously expected
based on z<3
scaling relations (Method 5,6) so longer integration times are
required
. That said, th
ese data show
[CII] is enhanced and readily detectable
with ALMA
at z~
6 if
moderate
inte
gration times are used. Finally, the
[CII] lines
are resolved in velocity space so single channel sensitivity should not be used for
limits (Method 6).
References
1
Madau, P. & Dickinson, M. Cosmic Star-
Formation History.
Annual Rev.
Astron. & Astrophys.
52
, 415
-486, (2014).
2
Bouwens, R. J.
et al.
UV
-continuum Slopes at z ~ 4-
7 from the
HUDF09+ERS+CANDELS Observations: Discovery of a Well-
define
d UV
Color
-Magnitude Relationship for z >= 4 Star-
forming Galaxies.
Astrophys.
J.
754
, 83, (2012).
3
Finkelstein, S. L.
et al.
Candels: The Evolution of Galaxy Rest
-frame
Ultraviolet Colors from z = 8 to 4.
Astrophys. J.
756
, 164, (2012).
4
Reddy, N. A., E
rb, D. K., Pettini, M., Steidel, C. C. & Shapley, A. E. Dust
Obscuration and Metallicity at High Redshift: New Inferences from UV, H
α
,
and 8
μ
m Observations of z ~ 2 Star-forming Galaxies.
Astrophys. J.
712
,
1070-
1091, (2010).
5
Boquien, M.
et al.
The IRX
-beta relation on subgalactic scales in star-
forming galaxies of the Herschel Reference Survey.
Astron. Astrophys.
539
, A145, (2012).
6
Ota, K.
et al.
ALMA Observation of 158
μ
m [C II] Line and Dust Continuum
of a z = 7 Normally Star-
forming Galaxy in the E
poch of Reionization.
Astrophys. J.
792
, 34, (2014).
7
Ouchi, M.
et al.
An Intensely Star-
forming Galaxy at z ~ 7 with Low Dust
and Metal Content Revealed by Deep ALMA and HST Observations.
Astrophys. J.
778
, 102, (2013).
8
Maiolino, R.
et al.
The assembly
of ''normal'' galaxies at z=7 probed by
ALMA.
ArXiv e-prints
, (2015).
9
Schaerer, D.
et al.
New constraints on dust emission and UV attenuation
of z = 6.5-
7.5 galaxies from millimeter observations.
Astron. Astrophys.
574
, A19, (2015).
10
Watson, D.
et al.
A dusty, normal galaxy in the epoch of reionization.
Nat.
519
, 327
-330, (2015).
11
Israel, F. P. & Maloney, P. R. C
+
emission from the Magellanic Clouds. II.
[C II] maps of star
-forming regions LMC
-N 11, SMC
-N 66, and several
others.
Astron. Astrophys.
531
, A19, (2011).
12
Malhotra, S.
et al.
Infrared Space Observatory Measurements of [C II] Line
Variations in Galaxies.
Astrophys. J. Lett.
491
, L27
-L30, (1997).
13
Carilli, C. L. & Walter, F. Cool Gas in High-
Redshift Galaxies.
Annual Rev.
Astron. & Astrophys.
51
, 105
-161, (2013).
14
Diaz
-Santos, T.
et al.
Extended [CII] Emission in Local Luminous Infrared
Galaxies.
ArXiv e-prints
, (2014).
15
Leauthaud, A.
et al.
Weak Gravitational Lensing with COSMOS: Galaxy
Selection and Shape Measurements.
Astrophys. J. Supp.
172
, 219
-238,
(2007).
16
Koekemoer, A. M.
et al.
The COSMOS Survey: Hubble Space Telescope
Advanced Camera for Surveys Observations and Data Processing.
Astrophys. J. Supp.
172
, 196
-202, (2007).
17
Speagle, J. S., Steinhardt, C. L., Capak, P. L. & Sil
verman, J. D. A Highly
Consistent Framework for the Evolution of the Star-
Forming ''Main
Sequence'' from z~0-
6.
Astrophys. J. Supp.
, (2014).
18
Casey, C. M. Far
-infrared spectral energy distribution fitting for galaxies
near and far.
Mon. Not. R. Astron. Soc.
425
, 3094
-3103, (2012).
19
Pettini, M.
et al.
Infrared Observations of Nebular Emission Lines from
Galaxies at z~= 3.
Astrophys. J.
508
, 539
-550, (1998).
20
Gordon, K. D., Clayton, G. C., Misselt, K. A., Landolt, A. U. & Wolff, M. J.
A Quantitative Com
parison of the Small Magellanic Cloud, Large
Magellanic Cloud, and Milky Way Ultraviolet to Near-Infrared Extinction
Curves.
Astrophys. J.
594
, 279
-293, (2003).
21
Kong, X., Charlot, S., Brinchmann, J. & Fall, S. M. Star formation history
and dust content of galaxies drawn from ultraviolet surveys.
Mon. Not. R.
Astron. Soc.
349
, 769
-778, (2004).
22
Charlot, S. & Fall, S. M. A Simple Model for the Absorption of Starlight by
Dust in Galaxies.
Astrophys. J.
539
, 718
-731, (2000).
23
Capak, P. L.
et al.
A massive protocluster of galaxies at a redshift of
z~5.3.
Nat.
470
, 233
-235, (2011).
24
Reddy, N.
et al.
GOODS
-Herschel Measurements of the Dust Attenuation
of Typical Star-
forming Galaxies at High Redshift: Observations of
Ultraviolet
-selected Galaxies at z ~ 2.
Astrophys. J.
744
, 154, (2012).
25
Leitherer, C., Tremonti, C. A., Heckman, T. M. & Calzetti, D. An Ultraviolet
Spectroscopic Atlas of Local Starbursts and Star-
forming Galaxies: The
Legacy of FOS and GHRS.
Astron. J.
141
, 37, (2011).
26
Scoville, N.
et a
l.
Dust Attenuation in High Redshift Galaxies -- 'Diamonds
in the Sky'.
Astrophys. J.
800
, 108, (2015).
27
Förster Schreiber, N. M.
et al.
The SINS Survey: SINFONI Integral Field
Spectroscopy of z ~ 2 Star-
forming Galaxies.
Astrophys. J.
706
, 1364
-
1428, (2009).
28
Meurer, G. R., Heckman, T. M. & Calzetti, D. Dust Absorption and the
Ultraviolet Luminosity Density at z ~ 3 as Calibrated by Local Starburst
Galaxies.
Astrophys. J.
521
, 64
-80, (1999).
29
Diaz
-Santos, T.
et al.
Explaining the [C II]157.7 um Deficit in Luminous
Infrared Galaxies -- First Results from a Herschel/PACS Study of the
GOALS Sample.
Astrophys. J.
774
, 68, (2013).
Acknowledgements
:
Support for th
is work was provided by NASA through an
award issued by JPL/Caltech. We would like to thank the ALMA staff for
facilitating the observations and aiding in the calibration and reduction process.
ALMA
is a partnership of ESO (representing its member states), NSF (USA) and
NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in
cooperation with the Republic of Chile. The Joint ALMA Observatory is operated
by ESO, AUI/NR
AO and NAOJ.
This work is based in part on observations made
with the Spitze
r Space Telescope, and the W.M. Keck Observatory along with
Archival data from the NASA/ESA Hubble Space Telescope, the Subaru
Telescope
, the Canada
-France
-Hawaii
-Telescope, the ESO Vista telescope
obtained from the
NASA
/ IPAC Infrared Science Archive. VS acknowledges
funding by the European Union's Seventh Frame
-work program under grant
agreement 337595 (ERC Starting Grant, 'CoSMass').
Author Contributions
P.L. Capak proposed and carried out the observations,
conduced the analysis in this paper, and autho
red the majority of the text. C.
Carilli, G. Jones, and K. Sheth carried out the reduction and direct analysis of the
ALMA data.
C. Casey consulted on the spectral energy distribution fitting
,
interpretation of the data. She also conducted a blind test of the FIR luminosity,
[CII] line luminosity, and
β
measurements along with testing for sample selection
effects. D. Riechers conducted the spectral line analysis and carried out an
independent
blind check of
the ALMA data reduction. O. Ilbert carried ou
t the
spectral energy distribution fitting and consulted on their interpretation. C.
Carollo, A. Karim, O. LeFevre, S. Lilly, N. Scoville, V. Smolcic, and L. Yan
contributed to the overall interpretation of the results and various aspects of the
analysis.
Author Information:
Please send correspondence to Peter Capak,
capak@astro.caltech.edu
. This paper makes use of ALMA data:
ADS/JAO.ALMA#2012.1.00523.S. ALMA
The authors have no competing financial intere
sts