Hi-GAL: The Herschel Infrared Galactic Plane Survey
S. M
OLINARI
,
1
B. S
WINYARD
,
2
J. B
ALLY
,
3
M. B
ARLOW
,
4
J.-P. B
ERNARD
,
5
P. M
ARTIN
,
6
T. M
OORE
,
7
A. N
ORIEGA
-C
RESPO
,
8
R. P
LUME
,
9
L. T
ESTI
,
10,11
A. Z
AVAGNO
,
12
A. A
BERGEL
,
13
B. A
LI
,
14
P. A
NDRÉ
,
15
J.-P. B
ALUTEAU
,
12
M. B
ENEDETTINI
,
1
O. B
ERNÉ
,
5
N. P. B
ILLOT
,
14
J. B
LOMMAERT
,
16
S. B
ONTEMPS
,
15,17
F. B
OULANGER
,
13
J. B
RAND
,
18
C. B
RUNT
,
19
M. B
URTON
,
20
L. C
AMPEGGIO
,
21
S. C
AREY
,
8
P. C
ASELLI
,
22
R. C
ESARONI
,
10
J. C
ERNICHARO
,
23
S. C
HAKRABARTI
,
24
A. C
HRYSOSTOMOU
,
25
C. C
ODELLA
,
10
M. C
OHEN
,
26
M. C
OMPIEGNE
,
27
C. J. D
AVIS
,
25
P.
DE
B
ERNARDIS
,
28
G.
DE
G
ASPERIS
,
29
J. D
I
F
RANCESCO
,
30
A. M.
DI
G
IORGIO
,
1
D. E
LIA
,
131
F. F
AUSTINI
,
1
J. F. F
ISCHERA
,
6
Y. F
UKUI
,
32
G. A. F
ULLER
,
33
K. G
ANGA
,
34
P. G
ARCIA
-L
ARIO
,
35
M. G
IARD
,
5
G. G
IARDINO
,
36
J: G
LENN
,
3
P. G
OLDSMITH
,
37
M. G
RIFFIN
,
38
M. H
OARE
,
22
M. H
UANG
,
39
B. J
IANG
,
40
C. J
OBLIN
,
5
G. J
ONCAS
,
41
M. J
UVELA
,
42
J. K
IRK
,
38
G. L
AGACHE
,
13
J. Z. L
I
,
39
T. L. L
IM
,
2
S. D. L
ORD
,
14
P. W. L
UCAS
,
43
B. M
AIOLO
,
21
M. M
ARENGO
,
24
D. M
ARSHALL
,
5
S. M
ASI
,
28
F. M
ASSI
,
10
M. M
ATSUURA
,
4
C. M
ENY
,
5
V. M
INIER
,
15
M.-A.
M
IVILLE
-D
ESCHÊNES
,
13
L. M
ONTIER
,
5
F. M
OTTE
,
15
T. G. M
ÜLLER
,
44
P. N
ATOLI
,
29
J. N
EVES
,
43
L. O
LMI
,
10
R. P
ALADINI
,
8
D. P
ARADIS
,
8
M. P
ESTALOZZI
,
2
S. P
EZZUTO
,
1
F. P
IACENTINI
,
28
M. P
OMARÈS
,
12
C. C. P
OPESCU
,
45
W. T. R
EACH
,
8
J. R
ICHER
,
46
I. R
ISTORCELLI
,
5
A. R
OY
,
6
P. R
OYER
,
16
D. R
USSEIL
,
12
P. S
ARACENO
,
1
M. S
AUV AGE
,
15
P. S
CHILKE
,
47
N. S
CHNEIDER
-
B
ONTEMPS
,
15
F. S
CHULLER
,
47
B. S
CHULTZ
,
14
D. S. S
HEPHERD
,
48
B. S
IBTHORPE
,
38
H. A. S
MITH
,
24
M. D. S
MITH
,
49
L. S
PINOGLIO
,
1
D. S
TAMATELLOS
,
38
F. S
TRAFELLA
,
21
G. S
TRINGFELLOW
,
3
E. S
TURM
,
44
R. T
AYLOR
,
50
M. A. T
HOMPSON
,
43
R. J. T
UFFS
,
51
G. U
MANA
,
52
L. V
ALENZIANO
,
53
R. V
AVREK
,
35
S. V
ITI
,
4
C. W
AELKENS
,
16
D. W
ARD
-T
HOMPSON
,
38
G. W
HITE
,
2
F. W
YROWSKI
,
47
H. W. Y
ORKE
,
37
AND
Q. Z
HANG
24
Received 2009 December 21; accepted 2010 January 14; published 2010 February 26
ABSTRACT. Hi-GAL, the Herschel infrared Galactic Plane Survey, is an Open Time Key Project of the Herschel
Space Observatory. It will make an unbiased photometric survey of the inner Galactic plane by mapping a 2° wide
strip in the longitude range ∣l∣ < 60° in five wavebands between 70 μm and 500 μm. The aim of Hi-GAL is to detect
the earliest phases of the formation of molecular clouds and high-mass stars and to use the optimum combination of
Herschel wavelength coverage, sensitivity, mapping strategy, and speed to deliver a homogeneous census of star-
forming regions and cold structures in the interstellar medium. The resulting representative samples will yield the
variation of source temperature, luminosity, mass and age in a wide range of Galactic environments at all scales from
massive YSOs in protoclusters to entire spiral arms, providing an evolutionary sequence for the formation of inter-
mediate and high-mass stars. This information is essential to the formulation of a predictive global model of the role
of environment and feedback in regulating the star-formation process. Such a model is vital to understanding star
formation on galactic scales and in the early universe. Hi-GAL will also provide a science legacy for decades to
come with incalculable potential for systematic and serendipitous science in a wide range of astronomical fields,
enabling the optimum use of future major facilities such as JWST and ALMA.
Online material: color figures
1. INTRODUCTION
Dust is the most robust tracer of the “Galactic ecology”—the
cycling of material from dying stars to the ionized, atomic, and
molecular phases of the ISM, into star-forming cloud cores, and
back into stars. While atoms, ions, and molecules are imperfect
tracers because they undergo complex phase changes, chemical
1
INAF-Istituto Fisica Spazio Interplanetario, Rome, Italy; sergio.molinari@
ifsi‑roma.inaf.it.
2
STFC, Rutherford Appleton Laboratory, Didcot, UK.
3
Center for Astrophysics and Space Astronomy (CASA), Department of
Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO.
4
Department of Physics and Astronomy, University College London, UK.
5
Centre d’Etude Spatiale du Rayonnement, CNRS, Toulouse, France.
6
University of Toronto, CITA, Canada.
7
Astrophysics Research Institute, John Moores University, Liverpool, UK.
8
Spitzer Science Center, California Institute of Technology, Pasadena, CA.
9
Department of Physics & Astronomy, University of Calgary, Calgary, Canada.
10
INAF—Osservatorio Astrofisico di Arcetri, Florence, Italy.
11
European Southern Observatory, Garching bei Muenchen, Germany.
12
Universitè de Provence, LAM, Marseille, France.
13
Institut d’Astrophysique Spatiale, Universit Paris-Sud, Orsay, France.
14
NASA Herschel Science Center, California Institute of Technology, Pasa-
dena, CA.
314
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UBLICATIONS OF THE
A
STRONOMICAL
S
OCIETY OF THE
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ACIFIC
, 122:314–325, 2010 March
© 2010. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A.
processing, and depletion onto grains, and are subject to complex
excitation conditions, dust is relatively stable in most phases of
the ISM. It is optically thin in the far-infrared (FIR) over most of
the Galaxy, so that its emission and absorption simply depend on
emissivity, column density, and temperature. Cold dust in parti-
cular (10 K ≤ T ≤ 40 K) traces the bulk of nonstellar baryonic
mass in all of these “habitats” of the Galactic ecosystem.
Temperature and luminosity and, as their byproduct, mass of
cold dust measured over the entire Galactic plane (GP), are, at
subparsec resolution, the critical observables needed to formulate
a global predictive model of the cycling process between the
Galactic ISM and star formation. This process drives the galactic
ecology in normal spirals, as well as the enhanced star-formation
rates of starburst galaxies and mergers, and a quantitative under-
standing of it is needed in order to follow the formation and
evolution of galaxies throughout the cosmos. The adequate
measurement of these key quantities has been beyond the capa-
bilities of the previous mid- to far-infrared surveys of the Galactic
plane (IRAS, Neugebauer et al. 1984; MSX, Price et al. 2001;
COBE/DIRBE and FIRAS, e.g., Sodroski et al. 1994; ISO,
Omont et al. 2003; Spitzer, Benjamin et al. 2003; Carey et al.
2009) either due to limited wavelength coverage and/or inade-
quate spatial resolution leading to confusion. The balloon-borne
BLAST experiment (Pascale et al. 2008) implements Herschel/
SPIRE detector arrays and is providing exciting anticipations of
what Herschel will do. The AKARI satellite (Murakami et al.
2007) improves over IRAS, and results from its FIR photometric
mapping of the GP are eagerly awaited.
Observing the distribution and temperature of dust across the
Galaxy will resolve many current debates such as the modes of
formation of molecular clouds and high-mass stars.
Molecular clouds are traditionally thought to follow a “slow
formation” scheme, where distributed material is accumulated by
large-scale perturbations such as the passage of a spiral arm.
Shielding by dust and surface reactions on grains promotes
the H
I → H
2
transition, which in turn allows the formation of
other molecules that cool the cloud. Gravity, mediated by mag-
netic fields, leads to star formation. In this scenario cloud life-
times are about ∼30 Myr (Leisawitz et al. 1989). This picture has
difficulty explaining the absence of quiescent, non–star-forming
GMCs (however, see Palla & Galli 1997) and the continuous
regeneration of turbulence needed to support GMCs for many
crossing times. Alternatively, a “fast formation” scenario has
been proposed (Hartmann et al. 2001) in which most MCs are
transient, short-lived structures (Stone et al. 1998; Padoan &
Nordlund 1999) created in the postshock regions of converging
large-scale flows. Stars form onvery short timescales (Elmegreen
2000). However, rapid MC formation requires rapid H
I → H
2
conversion (Goldsmith & Li 2005). Accelerated H
2
formation
requires either high-density preshock conditions (n ∼ 200 cm
3
,
T ≤ 100 K; Price et al. 2001), or strong turbulence (Glover &
Mac Low 2007), higher than observed.
On the other hand, the formation of high-mass stars and of
the star clusters hosti ng them is likely the most important
process that shapes the formation and evolution of galaxies.
Massive stars are responsible for the global ionization of the
ISM. Their energetic stellar winds and supernova blast waves
direct the dynamical evolution of the ISM, shaping its morphol-
ogy, energetics and chemistry, and influencing the formation of
subsequent generations of stars and planetary systems. Despite
their importance, remarkably little is known about how massive
stars form (McKee & Tan 2003). We lack a “fundamental
theory” or, rather, a galaxy-scale predictive model for star for-
mation. One of the main limitations to this goal is the lack of
statistically significant and well-characterized samples of young
massive stars in the various evolutionary stages and environ-
15
SAp CEA, Saclay, France.
16
Institute for Astronomy, Katholieke Universiteit Leuven, Leuven, Belgium.
17
Observatoire de Bordeaux, Bordeaux, France.
18
INAF—Istituto di Radioastronomia, Bologna, Italy.
19
School of Physics, University of Exeter, Exeter, UK.
20
School of Physics, University of New South Wales, Sydney, Australia.
21
Dipartimento di Fisica, Università del Salento, Lecce, Italy.
22
School of Physics & Astronomy, University of Leeds, Leeds, UK.
23
Centro de Astrobiología, CSIC-INTA, Madrid, Spain.
24
CfA, Harvard University, Cambridge, MA.
25
Joint Astronomy Center, Hilo, HI.
26
Radio Astronomy Lab, University of California, Berkeley, CA.
27
Canadian Institute for Theoretical Astrophysics, University of Toronto,
Toronto, Canada.
28
Dipartimento di Fisica, Università di Roma 1 “La Sapienza,” Rome, Italy.
29
Dipartimento di Fisica, Università di Roma 2 “Tor Vergata,” Rome, Italy.
30
Herzberg Institute of Astrophysics, NRCC, Victoria, Canada.
31
Observatòrio Astronomico de Lisboa, Lisboa, Portugal.
32
Department of Astrophysics, Nagoya University, Nagoya, Japan.
33
Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy,
University of Manchester, Manchester, M13 9PL, UK.
34
APC, Université Paris 7, Paris, France.
35
Herschel Science Center, ESAC/ESA, Madrid, Spain.
36
RSSD, ESTEC/ESA, Noordwijk, The Netherlands.
37
Jet Propulsion Laboratory, Pasadena, CA.
38
School of Physics and Astronomy, Cardiff University, Cardiff, UK.
39
National Astronomical Observatories, Chinese Academy of Sciences, Bei-
jing, China.
40
Department of Astronomy, Beijing Normal University, Beijing, China.
41
Département de physique, Université Laval, Québec, Canada.
42
Helsinki University Observatory, University of Helsinki, Helsinki, Finland.
43
Centre for Astrophysics Research, Science and Technology Research Insti-
tute, University of Hertfordshire, Hatfield, UK.
44
MPE-MPG, Garching bei Muenchen, Germany.
45
University of Central Lancashire, PR1 2HE, Preston, UK.
46
Cavendish Labs, Cambridge, UK.
47
MPIfR-MPG, Bonn, Germany.
48
National Radio Astronomy Observatory, Socorro, NM.
49
Centre for Astrophysics & Planetary Science, University of Kent,Canterbury,
UK.
50
Center for Radio Astronomy, University of Calgary, Calgary, Canada.
51
Max-Planck-Institut für Kernphysik, Heidelberg, Germany.
52
INAF-Osservatorio Astrofisico di Catania, Catania, Italy.
53
INAF-IASF, Bologna, Italy.
HI-GAL: HERSCHEL INFRARED GALACTIC PLANE SURVEY 315
2010 PASP, 122:314–325
ments on which a theory can be based. In turn, this results from
the difficulty of gathering observational data on a large number
of forming high-mass stars: they make up only a very small frac-
tion of the total number of stars in the Galaxy, their early evolu-
tionary phases of massive stars are more rapid than those of
low-mass stars, they lie at large distance and form in crowded
environments. There is thus a long list of questions that the
community has been addressing for some time, not finding
satisfactory answers. Here is an abridged list:
1. What is the temperature and density structure of the ISM?
How do molecular clouds form and evolve, and how are they
disrupted?
2. What is the origin of the stellar initial mass function
(IMF)? What is its relationship to the mass function (MF) of
ISM structures and cloud cores on all scales?
3. How do massive stars and clusters form and how do they
evolve? What are the earliest stages of massive star formation
and what are the timescales of these early phases?
4. How do the star-formation rate (SFR) and efficiency (SFE)
vary as a function of galactocentric distance and environmental
conditions such as the intensity of the interstellar radiation field
(ISRF), ISM metallicity, proximity to spiral arms or the molec-
ular ring, external triggers, and total pressure?
5. Does a threshold column density for star formation exist in
our Galaxy? What determines the value of this possible
threshold?
6. What are the physical processes involved in triggered star
formation on all scales and how does triggered star formation
differ from spontaneous star formation?
7. How do the local properties of the ISM and the rates of
spontaneous or triggered star formation relate to the global scal-
ing laws observed in external galaxies?
Using the Herschel telescope, the largest ever in space, Hi-
GAL will provide unique new data with which to address these
questions. Hi-GAL will make thermal infrared maps of the Ga-
lactic plane at a spatial resolution 30 times better than IRAS and
100 times better than DIRBE, from which a complete census of
compact source luminosities, masses, and spectral energy dis-
tributions (SEDs) will be derived. Source distances are a crucial
parameter in this respect, and a dedicated effort will be needed
(see § 4). Extraction of statistically significant samples of star-
forming regions and cold ISM structures will be possible in all
the environments of the Milky Way at all scales from massive
young stellar objects (YSOs) in individual protoclusters to com-
plete spiral arms.
In the following we present the specific characteristics of the
survey as well as some of the science outcomes that we expect
to obtain with this unique project.
2. HI-GAL OBSERVING STRATEGY
The area covered by Hi-GAL (∣ l∣≤60°, ∣b∣≤1°) contains
most of the star formation in the Galaxy, and it is the one which
offers the best coverage in ancillary data which will be critical in
the scientific analysis (see § 4). The b distribution and extent of
the survey is shown in Figure 1 along with the l b plot of
λ-rising SED IRAS sources (F
100
>F
60
>F
25
>F
12
) which
are potential YSOs. The Hi-GAL area (thick dashed lines in that
figure) represents the j bj ≤ 1° strip centered on the midplane and
contains about 80% of all potential YSOs contained in jbj ≤ 5°
strip, thus encompassing most of the potential star-formation
sites in the inner Galaxy.
The Herschel photometric cameras (PACS; Poglitsch et al.
2008) and SPIRE (Griffin et al. 2009) will be used in parallel
mode (pMode
54
) to maximize survey speed and wavelength
F
IG
.1.—Top panel: l b plot of λ-rising SED IRAS sources; solid line is the
Galactic midplane. The asterisks mark the median latitude of the sources com-
puted in 10°l bins. The dashed lines delimit the regions where 50% and 75% of
jbj ≤ 5° IRAS sources are contained. Bottom panel: b-distribution of the same
IRAS sources in the jlj ≤ 60° region.
54
In pMode, the Herschel telescope is scanning the sky in a raster fashion at
constant speed, while both PACS and SPIRE acquire data simultaneously.
316 MOLINARI ET AL.
2010 PASP, 122:314–325
coverage. Due to the instruments’ wavelength-multiplexing
capabilities, each pMode observation delivers maps at five
different wavelengths: 70 and 170 μm with PACS and 250,
350 and 500 μm
55
with SPIRE. Both cameras use bolometric
detector arrays to map the sky by scanning the spacecraft along
approximate great circles. Both instruments require their on-
board subkelvin coolers to be recycled to provide the detectors
with an operating temperature required of about 0.3 K in each
case. In pMode, both instruments are placed into their photo-
metric observing mode with the detectors at their correct oper-
ating
temperature, i.e., both instrument coolers are recycled, and data
are taken from the five arrays simultaneously as the spacecraft is
scanned across the sky.
The size and separation of the fields of view of PACS and
SPIRE are shown in Figure 2 as viewed in the spacecraft co-
ordinate system. Although the PACS array fully samples the
point-spread function (PSF) spatially from the telescope, it still
has gaps between the subarra ys, and the SPIRE arrays only
sparsely sample the sky. In order to make fully spatially-
sampled maps it is necessary to scan the SPIRE array at an angle
of 42.5° with respect to its short symmetry axis. Scanning at an
angle is also used for the PACS arrays to fill in for the gaps
between subarrays. To achieve redundancy in the data and re-
move instrumental effects such as high-frequency detector re-
sponse; slow drifts in gain; or stray ligh t, saturation, and
environmental (cirrus confusion) effects, it is also necessary
to make at least a second pass over the same region of the
sky using the other scan angle at 42:5°, which, quite conve-
niently, is nearly orthogonal to the first one (see Fig. 3).
The distance between each scan in parallel mode is set by the
size of the PACS array (being the smaller of the two), and
the effective length of each leg of the raster takes into account
the separation between the two fields of view. HSPOT, the
Herschel-SPOT observing tool,
56
automatically calculates these
parameters to ensure that the area required is covered. The dis-
tance between scans is approximately 155″ and the excess
length of the scan beyond the required length to cover the area
is typically 20′. An example of how the sky is covered in a Par-
allel Mode observation used in Hi-GAL is shown in Fig. 3.
The strategy employed to cover the 60° ≤ l ≤ 60°, jbj ≤ 1°
survey area is to conduct observations with series of 552:2°×
2:2° square tiles with two passes over each tile at the two above-
mentioned scan angles (see Fig. 2). These tiles will be spaced
every 2°, so that the overlap between tiles ensures that no cover-
age gaps are introduced by different tile orientation due to vari-
able satellite roll angles with time; Figure 3 shows a section of
the galactic plane with consecutive observing blocks overlaid
providing overlapping coverage.
Given the spatial separation required for PACS in the pMode
observations, the SPIRE data is heavily oversampled and we
F
IG
.2.—Field of view of the PACS and SPIRE instruments shown in the
context of the Herschel field of view as viewed in the coordinate system of
the spacecraft. þZ refers to the axis toward the Sun. The þX axis is the telescope
boresight and it would be oriented perpendicular to the plane of the page. The
scan directions used to map the sky are as indicated. The different photometric
channels of each instrument map the same region of the sky.
F
IG
.3.—Sample AORs (Astronomical Observation Request) overlaid on the
IRAS 100 μm image of a portion of the Galactic plane. From left to right, the
nominal and the orthogonal 2:2°×2:2° pMode AORs overlaid on one another.
See the electronic edition of the PASP for a color version of this figure.
55
The bandwidth of the filters are 60–85 μm and 125–210 μm for PACS
(Poglitsch et al. 2008), while for SPIRE they are such that λ=Δλ ∼ 3 (Griffin
et al. 2009).
56
At ftp://ftp.sciops.esa.int/pub/hspot/HSpot_download.html
HI-GAL: HERSCHEL INFRARED GALACTIC PLANE SURVEY 317
2010 PASP, 122:314–325
cover a greater area than required with each individual instru-
ment than would be required using them sequentially. Although
it might seem that sequential PACS and SPIRE scan mode
observations would be more efficient, in fact, the satellite
overheads, set up, calibration and pointing acquisition, etc., re-
quire 30% more time to cover the same area sequentially com-
pared to using the pMode.
In order to cover the maximum area in the shortest time,
Hi-GAL data will be taken at the maximum possible scan speed
for the satellite of 60
″
s
1
. This implies a beam-crossing time for
the short wavelength, 250 μm band of SPIRE of 3 Hz that is
well within the bandwidth available in the detectors of 5 Hz.
However, although the PACS detectors have a similar response
time, the much smaller PSF will be smeared out compared to
that achievable with a slower scan. Additionally, because of
the finite data transmission bandwidth between the Herschel
satellite and the ground, it is necessary to perform onboard data
compression for the PACS data which are the most demanding
in terms of number of pixels (2048 for the 70 μm array and 512
for the 170 μm array) at the frame acquisition rate of 40 Hz. The
baseline configuration for the pMode is then to average on
board groups of 8 frames at 70 μm and 4 frames at 170 μm.
Since the telescope is continuously scanning while acquiring,
this co-addition will result in a further degradation of the
PSF in the direction of the scan from its original diffraction-
limited shape; the effect will be more severe at 70 μm where
the degradation should be of a factor of 2 based on simulations.
This loss in imaging fidelity at the shortest wavelength is
considered acceptable for a survey like Hi-GAL because,
as discussed in § 1, our main focus is toward a large-scale
picture of the galaxy. Taking advantage of the orthogonal
cross scan observing strategy, we may be able to recover some
of the spatial resolution by careful deconvolution during
postprocessing.
2.1. Detection of Compact Sources
The SPIRE digital readout electronics impose a limitation on
the brightest sources that can be observed for a given offset set-
ting (DC voltage removal) before digitization (SPIRE Instru-
ment Users Manual, 2007). This problem can be alleviated
to some extent by choosing a bias setting that gives the largest
dynamic range per offset range. Simulations of the effect of bias
variation show that setting a bias hi gher (∼3x) than the pre-
dicted nominal value will approximately double the dynamic
range for most detectors under the conditions likely to be found
in orbit (telescope temperature and emissivity and sky back-
ground). The same simulations show that a significant
(>10%) fraction of the SPIRE 250 μm array detectors will sat-
urate on sources greater than 500 Jy. The situation is slightly
more relaxed for the 350 and 500 μm arrays. We take the
upper limit of detectable sources in the SPIRE bands as
500 Jy beam
1
, assuming that a strong source instrument setting
is used. This setting is required for all observations of bright
regions/sources with SPIRE and is not a special Hi-GAL con-
figuration. The saturation limits for PACS should be around
2000 Jy at nominal bias, which will be used for the Hi-GAL
survey.
The 1σ sensitivities provided by HSPOT, for a single Astro-
nomical Observation Request (AOR), are 17.6 and 26.8 mJy in
the two PACS 70 and 170 μm bands, and 12.8, 17.6, and
14.9 mJy for the SPIRE bands; co-addition of the orthogonal
scanning patterns will provide
ffiffiffi
2
p
better figures. These sensi-
tivities result from the adopted scanning strategy designed to
maximize redundancy and map fidelity especially for large-
scale diffuse structures. However, the limiting factor for the de-
tectability of sources and clouds will likely be cirrus confusion.
Estimates based on recent BLAST measurements (Roy et al.
2010) suggest values on the order of 75, 140, and 160 mJy
in the 170, 250, and 500 μm Herschel bands for a representative
region of the Galactic plane at l ¼ 45°; these values are greater
than the detector sensitivities. Figure 4 shows the expected flux
from a 20 M
⊙
envelope as a function of distance (in kpc) in
each of the three above-mentioned bands, and for three different
dust temperatures; we adopted β ¼ 2 and the dust opacity from
Preibisch et al. (1993). The horizontal dashed lines (color coded
with wavelength; see online article for color figures) represent
the predicted confusion noises based on BLAST images. Fig-
ure 4 shows that we will detect the representative 20 M
⊙
core
everywhere in the Galaxy except for very cold dust (T ≤ 10 K),
for which detectability is predicted to be limited within a dis-
tance of about 5 kpc. We may then conclud e that cirrus confu-
sion is not going to be a problem for the investigations of the
intermediate and high-mass star-formation studies that are the
“core science” of this project (see § 3.3 and 3.4).
F
IG
.4.—Flux expected in the 170, 250, and 500 μm Herschel bands from a
20 M
⊙
core (dust þ gas), with key in upper right showing symbols representing
different temperaturesand gray levels representing different wavelengths, as a
function of distance in kpc. The dashed lines represent the confusion noise ex-
pected in the three bands. See the electronic edition of the PASP for a color
version of this figure.
318 MOLINARI ET AL.
2010 PASP, 122:314–325
