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A molecular line scan in the Hubble Deep Field North: constraints on the CO luminosity function and the cosmic H2 density

TL;DR: In this paper, a blind molecular line scan in the Hubble Deep Field North (HDF-N) using the IRAM Plateau de Bure Interferometer is presented, where the authors present direct constraints on the CO luminosity function at high redshift and the resulting cosmic evolution of the molecular gas density.
Abstract: We present direct constraints on the CO luminosity function at high redshift and the resulting cosmic evolution of the molecular gas density, ρH_2(z), based on a blind molecular line scan in the Hubble Deep Field North (HDF-N) using the IRAM Plateau de Bure Interferometer. Our line scan of the entire 3 mm window (79-115 GHz) covers a cosmic volume of ~7000 Mpc^3, and redshift ranges z 2. We use the rich multiwavelength and spectroscopic database of the HDF-N to derive some of the best constraints on CO luminosities in high redshift galaxies to date. We combine the blind CO detections in our molecular line scan (presented in a companion paper) with stacked CO limits from galaxies with available spectroscopic redshifts (slit or mask spectroscopy from Keck and grism spectroscopy from the Hubble Space Telescope) to give first blind constraints on high-z CO luminosity functions and the cosmic evolution of the H_2 mass density ρH_2(z) out to redshifts z ~ 3. A comparison to empirical predictions of ρH_2(z) shows that the securely detected sources in our molecular line scan already provide significant contributions to the predicted ρH_2(z) in the redshift bins 〈z〉 ~ 1.5 and 〈z〉 ~ 2.7. Accounting for galaxies with CO luminosities that are not probed by our observations results in cosmic molecular gas densities ρH_2(z) that are higher than current predictions. We note, however, that the current uncertainties (in particular the luminosity limits, number of detections, as well as cosmic volume probed) are significant, a situation that is about to change with the emerging ALMA observatory.

Summary (2 min read)

1. INTRODUCTION

  • The last decade has seen impressive advances in their understanding of galaxy formation and evolution based on deep field studies at various wavelengths.
  • It has been shown that the comoving cosmic star formation rate (SFR) density rose gradually from early epochs (at least z ∼ 6–8) to a peak level between z ∼ 3 and 1, after which it dropped by an order of magnitude toward the present (Hopkins & Beacom 2006; Bouwens et al. 2010).
  • Likewise, magnitude-selected samples (e.g., Le Fèvre et al. 2005; Lilly et al. 2007) provide a census of the star-forming population based on UV/optical flux rather than color.
  • In order to obtain an unbiased census of the molecular gas content in high-z galaxies, there is a clear need for a blind search of molecular gas down to mass limits characteristic of the normal star-forming galaxy population, i.e., a molecular deep field.
  • Such a molecular deep field has been out of reach using past instrumentation, both in terms of sensitivity and instantaneous bandwidth.

2.1. Complete Frequency Scan of the 3 mm Band

  • The observational details are discussed in D14.
  • Table 1 also gives the cosmic volume probed by their observations.
  • From this the authors conclude that they reach high spectroscopic completeness (i.e., >90%) down to H-band magnitudes of HAB < 24 mag for all redshift bins.

3.1. Stacked CO Limits Based on Known Spectroscopic Redshifts

  • In Figure 3 the authors show the spectra extracted at the pixel of the nominal positions of the galaxies with spectroscopic redshifts (Table 2), shifted to their respective redshifts; here they exclude sources that only have low-quality (quality 2) grism redshifts.
  • Or rest frame UV features showing systematic offsets (e.g., Steidel et al. 2010).
  • These higher uncertainties are related to poorer spectral resolution, confusion between spatial and spectral structure in slitless observations, and the intrinsic weakness of the lines.
  • For their stack the authors weight-average the primary-beam corrected spectra after realignment and rebinning .

3.2. Blind CO Detections from Molecular Line Scan

  • Seventeen potential candidate lines are discussed in D14 and here the authors concentrate on the ones with quality flag “high-quality” and “secure,” which leaves 13 sources.
  • (ID.Z22 in Table 2; coincident CO and grism redshift); in both cases the spectral energy distributions based on the available multiwavelength photometry are in excellent agreement with the derived redshifts (D14).
  • The authors record the number of blind line detections and limiting magnitudes in each redshift bin in Table 4.

4. IMPLICATIONS

  • The authors molecular line scan in the HDF-N constitutes the first systematic blind search for CO emission down to a mass limit 16 In D14 the authors have derived the number of likely spurious detections using simulated data cubes.
  • The authors find two sources classified as “high-quality/secure,” i.e., with spectroscopic S/N > 3.5, that are false detections.
  • If the authors assign one to each redshift bin this leads to a spurious fraction of 1/3 and 1/8 for the two highest redshift bins.
  • That is characteristic of galaxies that lie on the relatively tight “main sequence” SFR–M∗ relation (Daddi et al. 2007).
  • Its cosmic volume is well defined and characterized through the ancillary multiwavelength observations.

4.1. Constraints on CO Luminosity Function

  • The lower limits in the plots are thus from their secure detections.
  • 2. Constraints on ρH2 (z) We now proceed to convert their constraints on the CO luminosity function to constraints on the cosmic volume density of the molecular gas mass ρH2 (z).the authors.the authors.
  • In Figure 5 the authors also show the contribution of the galaxies for which they obtained a stacked upper limit (Section 3.1).
  • Like in the case of the blind detections, the authors do not attempt to correct this measurement for undetected sources at lower (or higher) CO luminosities.

5. SUMMARY AND OUTLOOK

  • The authors molecular line scan of the HDF-N (D14) allows us to place first direct “blind” limits on the molecular gas density in “normal” galaxies at high redshift.
  • F.W., D.R., and E.d.C. acknowledge the Aspen Center for Physics where parts of this manuscript were written.
  • This paper is based on observations with the IRAM Plateau de Bure Interferometer (PdBI).

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The Astrophysical Journal, 782:79 (7pp), 2014 February 20 doi:10.1088/0004-637X/782/2/79
C
2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
A MOLECULAR LINE SCAN IN THE HUBBLE DEEP FIELD NORTH: CONSTRAINTS
ON THE CO LUMINOSITY FUNCTION AND THE COSMIC H
2
DENSITY
F. Walter
1
, R. Decarli
1
, M. Sargent
2
, C. Carilli
3
, M. Dickinson
4
, D. Riechers
5
, R. Ellis
6
, D. Stark
7
,
B. Weiner
7
, M. Aravena
8,9
, E. Bell
10
, F. Bertoldi
11
,P.Cox
12
, E. Da Cunha
1
, E. Daddi
4
,D.Downes
12
,
L. Lentati
13
, R. Maiolino
13
, K. M. Menten
14
, R. Neri
12
, H.-W. Rix
1
, and A. Weiss
14
1
Max-Planck Institut f
¨
ur Astronomie, K
¨
onigstuhl 17, D-69117 Heidelberg, Germany; walter@mpia.de
2
Laboratoire AIM, CEA/DSM-CNRS-Universite Paris Diderot, Irfu/Service d’Astrophysique,
CEA Saclay, Orme des Merisiers, F-91191 Gif-sur-Yvette cedex, France
3
NRAO, Pete V. Domenici Array Science Center, P.O. Box O, Socorro, NM 87801, USA
4
National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
5
Cornell University, 220 Space Sciences Building, Ithaca, NY 14853, USA
6
Astronomy Department, California Institute of Technology, MC105-24, Pasadena, CA 91125, USA
7
Steward Observatory, University of Arizona, 933 North Cherry Street, Tucson, AZ 85721, USA
8
European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago, Chile
9
N
´
ucleo de Astronom
´
ıa, Facultad de Ingenier
´
ıa, Universidad Diego Portales, Av. Ej
´
ercito 441, Santiago, Chile
10
Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
11
Argelander Institute for Astronomy, University of Bonn, Auf dem H
¨
ugel 71, D-53121 Bonn, Germany
12
IRAM, 300 rue de la piscine, F-38406 Saint-Martin d’H
`
eres, France
13
Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
14
Max-Planck-Institut f
¨
ur Radioastronomie, Auf dem H
¨
ugel 69, D-53121 Bonn, Germany
Received 2013 August 16; accepted 2013 December 5; published 2014 January 30
ABSTRACT
We present direct constraints on the CO luminosity function at high redshift and the resulting cosmic evolution of the
molecular gas density, ρ
H
2
(z), based on a blind molecular line scan in the Hubble Deep Field North (HDF-N) using
the IRAM Plateau de Bure Interferometer. Our line scan of the entire 3 mm window (79–115 GHz) covers a cosmic
volume of 7000 Mpc
3
, and redshift ranges z<0.45, 1.01 <z<1.89 and z>2. We use the rich multiwavelength
and spectroscopic database of the HDF-N to derive some of the best constraints on CO luminosities in high redshift
galaxies to date. We combine the blind CO detections in our molecular line scan (presented in a companion paper)
with stacked CO limits from galaxies with available spectroscopic redshifts (slit or mask spectroscopy from Keck
and grism spectroscopy from the Hubble Space Telescope) to give first blind constraints on high-z CO luminosity
functions and the cosmic evolution of the H
2
mass density ρ
H
2
(z) out to redshifts z 3. A comparison to empirical
predictions of ρ
H
2
(z) shows that the securely detected sources in our molecular line scan already provide significant
contributions to the predicted ρ
H
2
(z) in the redshift bins z∼1.5 and z∼2.7. Accounting for galaxies with
CO luminosities that are not probed by our observations results in cosmic molecular gas densities ρ
H
2
(z) that are
higher than current predictions. We note, however, that the current uncertainties (in particular the luminosity limits,
number of detections, as well as cosmic volume probed) are significant, a situation that is about to change with the
emerging ALMA observatory.
Key words: cosmology: observations galaxies: evolution galaxies: formation infrared: galaxies
Online-only material: color figures
1. INTRODUCTION
The last decade has seen impressive advances in our under-
standing of galaxy formation and evolution based on deep field
studies at various wavelengths. In particular, the cosmic history
of star formation, and the build-up of stellar mass as a function of
galaxy type and mass, have been well quantified, starting within
1 Gyr of the big bang. It has been shown that the comoving
cosmic star formation rate (SFR) density rose gradually from
early epochs (at least z 6–8) to a peak level between z 3
and 1, after which it dropped by an order of magnitude toward
the present (Hopkins & Beacom 2006; Bouwens et al. 2010).
The build-up of stellar mass (i.e., the temporal integral) follows
this evolution (Ilbert et al. 2009;Belletal.2007). The redshift
range z 1–3 constitutes the “epoch of galaxy assembly, when
roughly half the stars in the universe formed.
While progress in deep field studies has been impressive,
current knowledge of the formation of the general galaxy
population is based almost exclusively on optical, near-IR, and
centimeter-radio deep field surveys of stars, star formation, and
ionized gas. For example, Lyman break selected samples have
revealed a major population of star-forming galaxies at z 3
(e.g., Steidel et al. 2004). Likewise, magnitude-selected samples
(e.g., Le F
`
evre et al. 2005; Lilly et al. 2007) provide a census
of the star-forming population based on UV/optical flux rather
than color. Radio-selected sources provide estimates of dust-
unbiased SFRs (e.g., Cowie et al. 2004; Dunne et al. 2009;
Karim et al. 2011).
The molecular gas content is the cause of the cosmic star
formation history. However, observations of the gas content have
to date been limited to follow-up studies of galaxies that are pre-
selected from optical/NIR deep surveys (or, in the extreme cases
of quasar host galaxies and submillimeter galaxies, through
selection in the submillimeter continuum; Carilli & Walter
2013). In all cases the selection is based on the star formation
properties of a given galaxy.
In order to obtain an unbiased census of the molecular gas
content in high-z galaxies, there is a clear need for a blind
1

The Astrophysical Journal, 782:79 (7pp), 2014 February 20 Walter et al.
Tab le 1
Redshift Range and Cosmic Volume Covered by Molecular Line Scan
Line z
min
z
max
z
a
Vo l u m e
b
(Mpc
3
)
CO(1–0) 0.0041 0.446 0.338 91.66
CO(2–1) 1.01 1.89 1.52 1442
CO(3–2) 2.01 3.34 2.75 2437
CO(4–3) 3.02 4.78 3.98 2966
CO(5–4) 4.02 6.23 5.21 3249
Notes.
a
Volume-averaged redshift of CO transition.
b
Cosmic comoving volume probed by redshift range. As sky area we
use the frequency-dependent size of the PdBI primary beam (FWHM =
55

× (86 (GHz))).
search of molecular gas down to mass limits characteristic of the
normal star-forming galaxy population, i.e., a molecular deep
field. Such a molecular deep field has been out of reach using
past instrumentation, both in terms of sensitivity and instanta-
neous bandwidth. However they are now becoming feasible, in
particular given the unparalleled sensitivity of Atacama Large
(Sub)Millimeter Array (ALMA). We here present results based
on a precursor program, using the IRAM Plateau de Bure In-
terferometer (PdBI), of the Hubble Deep Field North (HDF-N;
Williams et al. 1996), that is discussed in detail in Decarli et al.
(2014, hereafter D14). After a brief summary of the observa-
tions (Section 2) we discuss stacked molecular gas limits (based
on galaxies with known spectroscopic redshifts, Section 3.1).
Together with the “blind” CO line detections from D14
(Section 3.2) these give first constraints on the redshift depen-
dence of the CO luminosity function in the HDF-N and their
implications for the cosmic evolution of the molecular gas con-
tent in galaxies (Section 4). A short summary and outlook is
presented in Section 5. Throughout the paper we adopt a stan-
dard cosmology with H
0
= 70 km s
1
Mpc
1
, Ω
m
= 0.3 and
Ω
Λ
= 0.7.
2. DATA
2.1. Complete Frequency Scan of the 3 mm Band
We have observed the full 3 mm band of the PdBI
(79.7–114.8 GHz) to approximately uniform sensitivity,
reaching an average noise of 0.3 mJy beam
1
in a 90 km s
1
channel (pointing center: 12:36:50.300 +62:12:25.00). Observa-
tions were done in C-array configuration, resulting in an average
beam size of 3

,or25 kpc at redshifts 1. At this resolution
we do not expect to spatially resolve high-redshift galaxies. The
observational details are discussed in D14. Table 1 summarizes
the redshift ranges probed by the different CO transitions cov-
ered by our scan and Figure 1 shows the field covered by our
observations. Table 1 also gives the cosmic volume probed by
our observations. Here we take into account that the covered sky
area, as defined by the primary beam, changes as a function of
frequency.
2.2. Optical /NIR Spectroscopy in the HDF-N
In our analysis we use available multiwavelength information
of the galaxies in the HDF-N, in particular (spectroscopic)
redshift estimates, to improve our sensitivity to search for CO
emission.
The H-band selected catalog by Dickinson et al. (2003), based
on deep Hubble Space Telescope (HST)/NICMOS F160W
Tab le 2
Galaxies with Ground-based or HST Grism-based
Redshifts Covered by Molecular Line Scan
ID R.A. Decl. z
spec
z
grism
Grism Quality
(J2000.0) (J2000.0)
(1) (2) (3) (4) (5) (6)
ID.Z01 12:36:49.81 +62:12:48.8 3.233
ID.Z02 12:36:50.26 +62:12:49.6 [1.625] 2
ID.Z03 12:36:47.04 +62:12:36.9 0.3209 [0.321] 2
ID.Z04 12:36:47.61 +62:12:37.2 [0.423] 2
ID.Z05 12:36:46.24 +62:12:29.1 1.585 3
ID.Z06 12:36:46.22 +62:12:28.5 1.591 3
ID.Z07 12:36:47.28 +62:12:30.7 0.4233
ID.Z08 12:36:49.56 +62:12:36.1 2.014 3
ID.Z09 12:36:46.94 +62:12:26.1 2.970 3 3
ID.Z10 12:36:51.28 +62:12:33.8 1.862 3
ID.Z11 12:36:49.99 +62:12:26.3 1.284 3
ID.Z12 12:36:49.95 +62:12:25.5 1.204 1.205 5
ID.Z13 12:36:50.35 +62:12:23.0 [1.185] 2
ID.Z14 12:36:52.09 +62:12:26.3 1.224 1.166 3
ID.Z15 12:36:53.49 +62:12:31.7 1.125 3
ID.Z16 12:36:47.49 +62:12:11.2 1.58 3
ID.Z17 12:36:51.74 +62:12:21.4 [2.713] 2
ID.Z18 12:36:51.71 +62:12:20.2 0.300
ID.Z19 12:36:49.60 +62:12:12.7 2.012 3
ID.Z20 12:36:53.42 +62:12:21.7 1.715 4
ID.Z21 12:36:53.66 +62:12:23.7 1.731 1.739 3
ID.Z22 12:36:51.61 +62:12:17.3 2.044 5
ID.Z23 12:36:53.91 +62:12:24.5 [1.797] 2
ID.Z24 12:36:52.67 +62:12:19.8 0.401
ID.Z25 12:36:48.80 +62:12:02.1 [1.038] 2
ID.Z26 12:36:51.89 +62:12:08.1 [1.144] 2
ID.Z27 12:36:50.48 +62:12:50.4 4.345
Notes. Catalog of the galaxies with ground-based or HST grism-based redshift
(from optical/NIR observations) consistent with the CO redshift coverage of
our line scan (Table 1). (1) Line ID. (2–3) Right Ascension and declination
(J2000). (4) Spectroscopic (ground-based) redshift from Cowie et al. (2004),
Reddy et al. (2006), Barger et al. (2008), and Stark et al. (2010). (5) Grism-
based redshift from AGHAST (B. Weiner et al., in preparation). (6) Quality of
the grism redshift (5: highest, 2: lowest; we consider only quality 3–5 in our
analysis and have but the quality 2 redshifts in brackets).
photometry, lists 220 galaxies within 30

(i.e., roughly the
size of the primary beam from the pointing center of our
observations. Cowie et al. (2004) and Barger et al. (2008)
provide spectroscopic redshifts for 15 of these. We add to
this so far unpublished spectroscopic redshifts (based on Keck
spectroscopy; M. Dickinson et al., in preparation) for an
additional eight galaxies up to z = 4. One additional faint galaxy
is included at z = 4.355—this redshift is based on one line
(presumably Lyα) and no continuum is seen in the spectrum
(Stark et al. 2010). All spectroscopic redshifts based on ground
observations are from the Keck telescope. In addition to these,
we add secure grism-based redshifts (based on the detection
of emission lines) and lower-quality redshifts (e.g., based on
absorption features or on the shape of the continuum emission)
from the HST survey A Grism H-Alpha SpecTroscopic survey”
(AGHAST; B. Weiner et al., in preparation). A quality flag q is
assigned to all grism-based redshifts in Table 2. Higher values
(q = 3–5) are associated with grism redshifts based on emission
lines. q = 2 values are associated with more uncertain redshifts,
e.g., based on the shape of the continuum emission. Out of our
complete spectroscopic set of 47 galaxies, 27 have a redshift
that is covered by our scan (Table 1 and Figure 1).
2

The Astrophysical Journal, 782:79 (7pp), 2014 February 20 Walter et al.
Figure 1. HST/WFC3 F160W (1.6 μm) image from the CANDELS survey (Grogin et al. 2011; Koekemoer et al. 2011) of the region of the HDF-N covered by our
line scan from the CANDELS survey. The red circle shows the primary beam FWHM of our observations at the intermediate frequency of our scan (97.25 GHz). The
black ellipse in the bottom-right corner shows the synthesized beam of our observations. Galaxies are labeled with their redshift. Blue colors indicate redshifts that
are not covered by the frequency coverage of our 3 mm scan (Table 1). Circles indicate ground-based redshifts and squares indicate slitless (grism) redshifts (when
both are available, only ground-based redshifts are shown). Green color indicates stars in the field. We show the spectroscopic completeness as a function of H-band
magnitude in Figure 2 and CO spectra toward all galaxies with redshift information in Figure 3.
(A color version of this figure is available in the online journal.)
This final sample of 27 galaxies with redshifts covered by
our frequency scan is listed in Table 2. These galaxies and their
respective ID’s are marked by circles (ground-based redshift)
and squares (slitless HST grism redshifts) in Figure 1. In Figure 2
we show the spectroscopic completeness in the field as a function
of H-band magnitude (to first order a measure of the stellar
mass) for the redshift intervals covered by our molecular line
scan. From this we conclude that we reach high spectroscopic
completeness (i.e., >90%) down to H-band magnitudes of
H
AB
< 24 mag for all redshift bins. This corresponds to the
following stellar masses in each redshift bin: z=0.338:
5.0 × 10
7
M
, z=1.52: 3.3 × 10
9
M
, z=2.75:
7.0 × 10
9
M
(da Cunha et al. 2013).
3. ANALYSIS
We base our analysis on two measurements: (1) deep stacked
CO limits based on the available optical/NIR spectroscopy
(Section 3.1) and (2) the blind CO detections discussed in D14
(Section 3.2).
3.1. Stacked CO Limits Based on Known
Spectroscopic Redshifts
We here use the spectroscopic redshift information presented
in Section 2.2 to aid in our search for CO emission, and to obtain
a stacked CO limit in the galaxy samples. In Figure 3 we show
the spectra extracted at the pixel of the nominal positions of the
galaxies with spectroscopic redshifts (Table 2), shifted to their
respective redshifts; here we exclude sources that only have
low-quality (quality 2) grism redshifts. All spectra have been
corrected for primary beam attenuation, leading to different
noise properties in the spectra. None of the spectra show
convincing CO emission at the expected redshift. In our blind
search for CO (D14) we report a candidate CO line emission
for one of the sources shown in Figure 3, ID.Z22, which is
spatially consistent with the CO line candidate ID.19 and where
the grism redshift matches the CO redshift perfectly (see detailed
discussion in D14).
15
We also note that one galaxy, ID.Z27 at
z = 4.345 (Stark et al. 2010), shows a tentative CO(4–3) line
but we treat this as an upper limit in our analysis.
To stack the spectra, we first need to consider the accuracy
of the available optical/NIR redshifts: the typical uncertain-
ties of Keck spectroscopic redshifts z 1.6 are of order few
tens of km s
1
(Newman et al. 2013), and we consider these
uncertainties negligible for our stacking, given the expected
line widths of 300 km s
1
(e.g., Carilli & Walter 2013).
At higher redshifts, the uncertainties are higher (a few hun-
dred km s
1
) due to various observational and astrophysical
biases, e.g., lack of bright nebular lines, such as [O iii]or[Oii],
15
For this source, we also show the spectrum that corresponds to the CO
candidate ID.19 that is offset by 1.

5 from the optical/NIR counterpart for the
optical galaxy ID.Z22 in Figure 3. See detailed discussion of CO candidate
ID.19 in D14.
3

The Astrophysical Journal, 782:79 (7pp), 2014 February 20 Walter et al.
Figure 2. Histogram of number of galaxies covered in our line scan as a function
of H-band magnitude (x-axis) and redshift bin (three panels). The blue line
shows the distribution of galaxies with photometric redshifts in each redshift
bin (available for most of the galaxies in the field), whereas the colored regions
indicate the availability of ground-based spectroscopic redshifts (red) and high-
quality HST grism spectroscopy (green). Grism spectra with quality q = 2
(yellow) are the least reliable and we do not use them for our analysis. Ground-
based redshifts are preferred to HST grism ones, when both are available.
(A color version of this figure is available in the online journal.)
or rest frame UV features showing systematic offsets (e.g., Stei-
del et al. 2010). The average uncertainty in the grism redshifts
is δz/(1 + z) 0.0016 (B. Weiner et al., in preparation). These
higher uncertainties are related to poorer spectral resolution,
confusion between spatial and spectral structure in slitless ob-
servations, and the intrinsic weakness of the lines.
For our stack we weight-average the primary-beam corrected
spectra after realignment and rebinning (bottom panels of
Figure 3). Given the uncertainties in grism redshifts, we compute
the stacked flux as the integral (and its uncertainties) of the
stacked spectra over 1000 km s
1
(i.e., sufficient to encompass
any CO emission within the typical redshift uncertainties). A
tighter velocity range of 300 km s
1
wasassumedforthelowest
redshift bin, where all galaxies have a more accurate ground-
based redshift. The final stacked upper limits for the CO fluxes
and resulting luminosities are given in Table 3.
3.2. Blind CO Detections from Molecular Line Scan
In D14 we present a blind search for CO emission in
the molecular line scan above a luminosity limit of 6 ×
10
9
Kkms
1
pc
2
(to first order irrespective of CO transition;
see D14). Seventeen potential candidate lines are discussed in
D14 and here we concentrate on the ones with quality flag
“high-quality” and “secure, which leaves 13 sources. Two line
candidates (ID.08 and ID.17) belong to the highly obscured
galaxy HDF 850.1 at z = 5.183 (Walter et al. 2012). This source
has previously been selected as a submillimeter galaxy (and
our field center was chosen to include it) so we do not discuss it
further here. There are two additional galaxies in the “secure” list
for which we are certain of their redshifts: ID.03 at z = 1.7844
(redshift derived from three CO lines) and ID.19 at z = 2.0474
Figure 3. Spectra of the galaxies with high-quality spectroscopic redshifts
falling within the range of redshifts our scan covered for various CO transitions
(see Tables 1 and 2). Spectra are corrected for primary beam attenuation. No
galaxy is individually detected at high significance. Vertical dashed lines indicate
band edges in our scan (D14). ID.Z22 is spatially coincident with our blind CO
detection ID.19 (see discussion in Section 3.1) and we show the spectrum of
ID.19, extracted 1.

5 away from the optical positions, as a blue-dashed line for
reference. The bottom panels show the stacked spectra for each transition and a
stack of all CO emission. We note that ID.Z1 enters the latter stack twice as it
has two lines in our scan.
(A color version of this figure is available in the online journal.)
(ID.Z22 in Table 2; coincident CO and grism redshift); in both
cases the spectral energy distributions based on the available
multiwavelength photometry are in excellent agreement with
the derived redshifts (D14).
Based on the available multiwavelength information, D14
assigned each of the remaining line candidates a tentative
redshift. We stress that we expect some of the line candidates to
be spurious,
16
and consequently treat the number of candidate
detections in each redshift bin as an upper limit. Dedicated
follow-up observations in other CO transitions are needed
to confirm the reality and redshifts of our candidate lines.
We record the number of blind line detections and limiting
magnitudes in each redshift bin in Table 4.
4. IMPLICATIONS
Our molecular line scan in the HDF-N constitutes the first
systematic blind search for CO emission down to a mass limit
16
In D14 we have derived the number of likely spurious detections using
simulated data cubes. We find two sources classified as “high-quality/secure,
i.e., with spectroscopic S/N > 3.5, that are false detections. If we assign one to
each redshift bin this leads to a spurious fraction of 1/3and1/8forthetwo
highest redshift bins.
4

The Astrophysical Journal, 782:79 (7pp), 2014 February 20 Walter et al.
Tab le 3
Stacked CO Limits Based on the Available Spectroscopic Redshift Information
Line z No.
a
S
CO
L
CO
L
CO
L
CO(1
0)
b
Density
c
(Jy km s
1
)(10
6
L
)(10
9
Kkms
1
pc
2
)(10
9
Kkms
1
pc
2
)(10
3
Mpc
3
)
CO(1–0) 0.338 4 <0.177 <0.034 <0.68 <0.68 43
CO(2–1) 1.52 10 <0.174 <2.1 <5.2 <5.91 6.9
CO(3–2) 2.75 5 <0.443 <22.0 <17 <34 2.1
Notes. All upper limits are 5σ . The CO(2–1) and CO(3–2) limits account for the higher uncertainties in the grism redshifts (see
Section 3.1).
a
Number of galaxies in stack (from Table 2 and Figure 2).
b
We have converted our high-J CO L
CO
luminosity limits to L
CO(1
0)
assuming L
CO(2
1)
/L
CO(1
0)
= 0.84 and L
CO(3
2)
/L
CO(1
0)
=
0.5 (Dannerbauer et al. 2009).
c
Volume density of sources in redshift bin using Column 5 in Table 1.
Tab le 4
Number of Blind CO Detections and Limiting Luminosities in Scan
Line z No.
a
L
3σ
CO
L
3σ
CO(10)
Density
c
(10
9
Kkms
1
pc
2
)(10
9
Kkms
1
pc
2
)
b
(10
3
Mpc
3
)
CO(1–0) 0.338 0 1.0 1.0 <10.9
CO(2–1) 1.52 1–3 5.2 6.2 0.69–2.10
CO(3–2) 2.75 1–8 6.6 13.2 0.41–3.28
Notes.
a
Number of blind detections in the molecular line scan above our sensitivity limit (next column) as derived in D14 (see
Section 3.2).
b
See Table 3 caption for details on conversion from L
CO
to L
CO(1
0)
.
c
Volume density of sources in redshift bin using Column 2 in Table 1. In case of no detection (first redshift bin) we
assume an upper limit of one source.
that is characteristic of galaxies that lie on the relatively tight
“main sequence” SFR–M
relation (Daddi et al. 2007). Its
cosmic volume is well defined and characterized through the
ancillary multiwavelength observations. In the following we
discuss our constraints on the CO luminosity functions and the
cosmic evolution of the cosmic molecular gas density ρ
H
2
(z)in
the HDF-N.
4.1. Constraints on CO Luminosity Function
We now constrain the CO luminosity function at different red-
shifts based on our blind CO detections (Table 4, Section 3.2).
We compare our measurements to empirical predictions of the
CO luminosity function. In Figure 4 we show the CO(1–0)
luminosity function in the three redshift bins covered by our
line scan, as predicted by M. T. Sargent et al. (in prepara-
tion) based on (1) the evolution of the stellar mass function
of star-forming galaxies, (2) the redshift evolution of the spe-
cific SFR of main-sequence galaxies, (3) the distribution of
main-sequence and star-bursting galaxies in the SFR–M
–plane
(Sargent et al. 2012), (4) distinct prescriptions of the star for-
mation efficiency of main-sequence and star-bursting galaxies,
and (5) a metallicity-dependent conversion factor α
CO
.
In the same plot (Figure 4) we also show the predictions based
on semi-analytical cosmological models by Lagos et al. (2011)
and Obreschkow et al. (2009a, 2009b), both interpolated to our
relevant median z based on their predictions at lower/higher
redshift. The luminosities plotted here are computed for the
CO(1–0) line (i.e., L
CO(1
0
) and we have converted our higher-
J CO luminosities and upper limits to L
CO(1
0)
(see Table 3
caption). The CO luminosity function has to date only been
directly measured at z = 0 (see data points by Kere
ˇ
setal.2003
in the left panel of Figure 4).
In order to compare our measurements to these empirically
predicted luminosity functions, we need to normalize our
number densities in a consistent way. In the literature, the
volume densities are typically given in units of sources per Mpc
3
and dex in luminosity (L
CO(1
0)
). For our blind detections we
define luminosity bins to range from our 3σ limiting luminosity
(Table 4) to a luminosity that is a factor of 10 higher (i.e., 1 dex).
We then count the number of blind detections in this luminosity
bin. We have at least two secure CO detections (one in the z
1.52 redshift bin, one in the bin with z=2.75), and potentially
up to 11 (3 at z=1.52, 8 at z=2.75), if we include all the
“high-quality” line candidates in D14 (see Section 3.2). The
lower limits in the plots are thus from our secure detections.
The upper limits represent the case where all line candidates
are in fact real. In the limit of low number statistics we adopt
the Poisson errors following Gehrels (1986, their Tables 1 and
2). We plot the allowed parameter space of our observations as
blue-shaded regions in Figure 4. Our blind detections thus probe
the “knee” of the predicted CO luminosity functions.
4.2. Constraints on ρ
H
2
(z)
We now proceed to convert our constraints on the CO
luminosity function to constraints on the cosmic volume density
of the molecular gas mass ρ
H
2
(z). This is shown in Figure 5
where we compare our observations to the same models and
empirical predictions as discussed in Section 4.1 and Figure 4.
For a comparison we also show the evolution of ρ(M
H i
) based on
Bauermeister et al. (2010) and ρ(M
) based on the compilations
in Marchesini et al. (2009) and Fontana et al. (2006).
The blue-shaded area in Figure 5 (with appropriate error bars)
indicates the contribution of our blind CO detections to ρ
H
2
(z)
from Figure 4. To translate the observed CO luminosities to H
2
5

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    [...]

  • ...Walter et al. (2014) argue that the tension seen between several predictions of H2 in the literature and their observational estimates are, in reality, worse than it seems from comparisons such as the one presented here....

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  • ...Related to (ii), Walter et al. (2014) adopted a value of X to convert their CO measurements to H2 similar to the MW value....

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  • ...…factor were on average lower in galaxies with large abundances of H2, as suggested by the theoretical studies of Lagos et al. (2012), Narayanan et al. (2012) and Popping et al. (2014), the H2 masses would be overestimated by Walter et al. (2014), improving the consistency with the EAGLE results....

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2,088 citations

Journal ArticleDOI
Anton M. Koekemoer1, Sandra M. Faber2, Henry C. Ferguson1, Norman A. Grogin1, Dale D. Kocevski2, David C. Koo2, Kamson Lai2, Jennifer M. Lotz1, Ray A. Lucas1, Elizabeth J. McGrath2, Sara Ogaz1, Abhijith Rajan1, Adam G. Riess3, S. Rodney3, L. G. Strolger4, Stefano Casertano1, Marco Castellano, Tomas Dahlen1, Mark Dickinson, Timothy Dolch3, Adriano Fontana, Mauro Giavalisco5, Andrea Grazian, Yicheng Guo5, Nimish P. Hathi6, Kuang-Han Huang3, Kuang-Han Huang1, Arjen van der Wel7, Hao Jing Yan8, Viviana Acquaviva9, David M. Alexander10, Omar Almaini11, Matthew L. N. Ashby12, Marco Barden13, Eric F. Bell14, Frédéric Bournaud15, Thomas M. Brown1, Karina Caputi16, Paolo Cassata5, Peter Challis17, Ranga-Ram Chary18, Edmond Cheung2, Michele Cirasuolo16, Christopher J. Conselice11, Asantha Cooray19, Darren J. Croton20, Emanuele Daddi15, Romeel Davé21, Duilia F. de Mello22, Loic de Ravel16, Avishai Dekel23, Jennifer L. Donley1, James Dunlop16, Aaron A. Dutton24, David Elbaz25, Giovanni Fazio12, Alexei V. Filippenko26, Steven L. Finkelstein27, Chris Frazer19, Jonathan P. Gardner22, Peter M. Garnavich28, Eric Gawiser9, Ruth Gruetzbauch11, Will G. Hartley11, B. Haussler11, Jessica Herrington14, Philip F. Hopkins26, J.-S. Huang29, Saurabh Jha9, Andrew Johnson2, Jeyhan S. Kartaltepe3, Ali Ahmad Khostovan19, Robert P. Kirshner12, Caterina Lani11, Kyoung-Soo Lee30, Weidong Li26, Piero Madau2, Patrick J. McCarthy6, Daniel H. McIntosh31, Ross J. McLure, Conor McPartland2, Bahram Mobasher32, Heidi Moreira9, Alice Mortlock11, Leonidas A. Moustakas18, Mark Mozena2, Kirpal Nandra33, Jeffrey A. Newman34, Jennifer L. Nielsen31, Sami Niemi1, Kai G. Noeske1, Casey Papovich27, Laura Pentericci, Alexandra Pope, Joel R. Primack2, Swara Ravindranath35, Naveen A. Reddy, Alvio Renzini, Hans Walter Rix7, Aday R. Robaina, David J. Rosario2, Piero Rosati7, S. Salimbeni5, Claudia Scarlata18, Brian Siana18, Luc Simard36, Joseph Smidt19, D. Snyder2, Rachel S. Somerville1, Hyron Spinrad26, Amber N. Straughn22, Olivia Telford34, Harry I. Teplitz18, Jonathan R. Trump2, Carlos J. Vargas9, Carolin Villforth1, C. Wagner31, P. Wandro2, Risa H. Wechsler37, Benjamin J. Weiner21, Tommy Wiklind1, Vivienne Wild, Grant W. Wilson5, Stijn Wuyts12, Min S. Yun5 
TL;DR: In this paper, the authors describe the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS).
Abstract: This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at z 1.5-8, and to study Type Ia supernovae at z > 1.5. Five premier multi-wavelength sky regions are selected, each with extensive multi-wavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 infrared channel (WFC3/IR) and the WFC3 ultraviolet/optical channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers ~125 arcmin2 within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of ~800 arcmin2 across GOODS and three additional fields (Extended Groth Strip, COSMOS, and Ultra-Deep Survey). We summarize the observational aspects of the survey as motivated by the scientific goals and present a detailed description of the data reduction procedures and products from the survey. Our data reduction methods utilize the most up-to-date calibration files and image combination procedures. We have paid special attention to correcting a range of instrumental effects, including charge transfer efficiency degradation for ACS, removal of electronic bias-striping present in ACS data after Servicing Mission 4, and persistence effects and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible photometry. A more detailed overview of the science goals and observational design of the survey are presented in a companion paper.

2,011 citations

Journal ArticleDOI
TL;DR: In this article, the authors review the theoretical underpinning, techniques, and results of efforts to estimate the CO-to-H2 conversion factor in different environments, and recommend a conversion factor XCO = 2×10 20 cm −2 (K km s −1 ) −1 with ±30% uncertainty.
Abstract: CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H2 “conversion factor,” XCO, in different environments. In the Milky Way disk, we recommend a conversion factor XCO = 2×10 20 cm −2 (K km s −1 ) −1 with ±30% uncertainty. Studies of other “normal galaxies” return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that XCO increases with decreasing metallicity, turning up sharply below metallicity ≈ 1/3–1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations, XCO appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower XCO is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments. Subject headings: ISM: general — ISM: molecules — galaxies: ISM — radio lines: ISM

2,004 citations

Journal ArticleDOI
Anton M. Koekemoer, Sandra M. Faber, Henry C. Ferguson, Norman A. Grogin, Dale D. Kocevski, David C. Koo, Kamson Lai, Jennifer M. Lotz, Ray A. Lucas, Elizabeth J. McGrath, Sara Ogaz, Abhijith Rajan, Adam G. Riess, S. Rodney, Louis Gregory Strolger, Stefano Casertano, Marco Castellano, Tomas Dahlen, Mark Dickinson, Timothy Dolch, Adriano Fontana, Mauro Giavalisco, Andrea Grazian, Yicheng Guo, Nimish P. Hathi, Kuang-Han Huang, Arjen van der Wel, Haojing Yan, Viviana Acquaviva, David M. Alexander Omar Almaini, Matthew L. N. Ashby, Marco Barden, Eric F. Bell, Frédéric Bournaud, Thomas M. Brown, Karina Caputi, Paolo Cassata, Peter Challis, Ranga-Ram Chary, Edmond Cheung, Michele Cirasuolo, Christopher J. Conselice, Asantha Cooray, Darren J. Croton, Emanuele Daddi, Romeel Davé, Duilia F. de Mello, Loic de Ravel, Avishai Dekel, Jennifer L. Donley, James Dunlop, Aaron A. Dutton, David Elbaz, Giovanni G. Fazio, Alex V. Filippenko, Steven L. Finkelstein, Chris Frazer, Jonathan P. Gardner, Peter M. Garnavich, Eric Gawiser, Ruth Gruetzbauch, Will G. Hartley, Boris Häussler, Jessica Herrington, Philip F. Hopkins, Jiasheng Huang, Saurabh Jha, Andrew Johnson, Jeyhan S. Kartaltepe, Ali Ahmad Khostovan, Robert P. Kirshner, Caterina Lani, Kyoung-Soo Lee, Weidong Li, Piero Madau, Patrick J. McCarthy, Daniel H. McIntosh, Ross J. McLure, Conor McPartland, Bahram Mobasher, Heidi Moreira, Alice Mortlock, Leonidas A. Moustakas, Mark Mozena, Kirpal Nandra, Jeffrey A. Newman, Jennifer L. Nielsen, Sami Niemi, Kai G. Noeske, Casey Papovich, Laura Pentericci, Alexandra Pope, Joel R. Primack, Swara Ravindranath, Naveen A. Reddy, Alvio Renzini, Hans-Walter Rix, Aday R. Robaina, David J. Rosario, Piero Rosati, S. Salimbeni, Claudia Scarlata, Brian Siana, Luc Simard, Joseph Smidt, D. Snyder, Rachel S. Somerville, Hyron Spinrad, Amber Straughn, Olivia Telford, Harry I. Teplitz, Jonathan R. Trump, Carlos J. Vargas, Carolin Villforth, C. Wagner, P. Wandro, Risa H. Wechsler, Benjamin J. Weiner, Tommy Wiklind, Vivienne Wild, Grant W. Wilson, Stijn Wuyts, Min S. Yun 
TL;DR: The Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) as mentioned in this paper was designed to document the evolution of galaxies and black holes at $z\sim 1.5-8$, and to study Type Ia SNe beyond $z>1.5.
Abstract: This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at $z\sim1.5-8$, and to study Type Ia SNe beyond $z>1.5$. Five premier multi-wavelength sky regions are selected, each with extensive multiwavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 / infrared channel (WFC3/IR) and UVIS channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers \sim125 square arcminutes within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of \sim800 square arcminutes across GOODS and three additional fields (EGS, COSMOS, and UDS). We summarize the observational aspects of the survey as motivated by the scientific goals and present a detailed description of the data reduction procedures and products from the survey. Our data reduction methods utilize the most up to date calibration files and image combination procedures. We have paid special attention to correcting a range of instrumental effects, including CTE degradation for ACS, removal of electronic bias-striping present in ACS data after SM4, and persistence effects and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible photometry. A more detailed overview of the science goals and observational design of the survey are presented in a companion paper.

1,917 citations

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Q1. What are the contributions in "C: " ?

The authors present direct constraints on the CO luminosity function at high redshift and the resulting cosmic evolution of the molecular gas density, ρH2 ( z ), based on a blind molecular line scan in the Hubble Deep Field North ( HDF-N ) using the IRAM Plateau de Bure Interferometer. The authors combine the blind CO detections in their molecular line scan ( presented in a companion paper ) with stacked CO limits from galaxies with available spectroscopic redshifts ( slit or mask spectroscopy from Keck and grism spectroscopy from the Hubble Space Telescope ) to give first blind constraints on high-z CO luminosity functions and the cosmic evolution of the H2 mass density ρH2 ( z ) out to redshifts z ∼ 3. A comparison to empirical predictions of ρH2 ( z ) shows that the securely detected sources in their molecular line scan already provide significant contributions to the predicted ρH2 ( z ) in the redshift bins 〈z〉 ∼ 1. 5 and 〈z〉 ∼ 2. 7. Accounting for galaxies with CO luminosities that are not probed by their observations results in cosmic molecular gas densities ρH2 ( z ) that are higher than current predictions. The authors note, however, that the current uncertainties ( in particular the luminosity limits, number of detections, as well as cosmic volume probed ) are significant, a situation that is about to change with the emerging ALMA observatory.