Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L43 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014664 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
Herschel
observations of water
vapour in Markarian 231
E. González-Alfonso1 -
J. Fischer2 -
K. Isaak3 -
A. Rykala3 -
G. Savini4 -
M. Spaans5 -
P. van der Werf6 -
R. Meijerink6 -
F. P. Israel6 -
A. F. Loenen6 -
C. Vlahakis6 -
H. A. Smith7 -
V. Charmandaris8,21 -
S. Aalto9 -
C. Henkel10 -
A. Weiß10 -
F. Walter11 -
T. R. Greve11,12 -
J. Martín-Pintado13 -
D. A. Naylor14 -
L. Spinoglio15 -
S. Veilleux16 -
A. I. Harris16 -
L. Armus17 -
S. Lord17 -
J. Mazzarella17 -
E. M. Xilouris18 -
D. B. Sanders19 -
K. M. Dasyra20 -
M. C. Wiedner21 -
C. Kramer22 -
P. P. Papadopoulos23 -
G. J. Stacey24 -
A. S. Evans25 -
Y. Gao26
1 - Universidad de Alcalá de Henares, Departamento de Física,
Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
2 - Naval Research Laboratory, Remote Sensing Division,
Washington, DC 20375, USA
3 -
ESA Astrophysics Missions Div/ Research and Scientific Support Dept
ESTEC/SRE-SA Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
4 - Department of Physics & Astronomy, University College London,
Gower Street, London WC1E 6BT, UK
5 -
Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
6 - Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
7 -
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
8 - University of Crete, Department of Physics, 71003, Heraklion, Greece
9 - Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala,
Sweden
10 - MPIfR, Auf dem Hügel 69, 53121 Bonn, Germany
11 - Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg,
Germany
12 - Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
13 - Centro de Astrobiología (CSIC-INTA), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
14 -
Space Astronomy Division, Institute for Space Imaging Science,
Department of Physics and Astronomy, University of Lethbridge,
Lethbridge, Alberta Canada, T1K 3M4, Canada
15 - Istituto di Fisica dello Spazio Interplanetario, CNR via Fosso del Cavaliere 100, 00133 Roma, Italy
16 - Department of Astronomy, University of Maryland, College Park, MD 20742 USA
17 - IPAC, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
18 - Institute of Astronomy and Astrophysics, National Observatory of Athens, P. Penteli, GR-15236 Athens, Greece
19 - University of Hawaii, Institute for Astronomy, 2680 Woodlawn Drive,
Honolulu, HI 96822, USA
20 -
Laboratoire AIM, CEA/DSM, CNRS, Université Paris Diderot,
Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, 91191
Gif sur Yvette Cedex, France
21 - Laboratoire d'Études du Rayonnement et de la Matière en
Astrophysique (LERMA), UMR 8112 du CNRS, OP, ENS, UPMC, UCP, 61 Av. de
l'Observatoire, 75014 Paris, France
22 - Instituto Radioastronomia Milimetrica (IRAM),
Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
23 - Argelander Institut fuer Astronomy, Auf dem Huegel 71, 53121, Germany
24 - Department of Astronomy, Cornell University, Ithaca, NY, USA
25 - Department of Astronomy, University of Virginia, 530
McCormick Road, Charlottesville, VA 22904, USA
26 -
Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road,
Nanjing 210008, PR China
Received 31 March 2010 / Accepted 28 April 2010
Abstract
The Ultra luminous infrared galaxy (ULIRG) Mrk 231 reveals up to seven
rotational lines of water (H2O)
in emission, including a very high-lying (
K) line detected at a 4
level, within the Herschel/SPIRE
wavelength range (
), whereas PACS
observations show one H2O line at 78
m in absorption, as found
for other H2O lines previously detected by ISO.
The absorption/emission dichotomy is caused by the
pumping of the rotational levels by far-infrared radiation emitted by
dust, and subsequent relaxation through lines at longer wavelengths,
which allows us to estimate both the column
density of H2O and the general characteristics of the underlying
far-infrared continuum source.
Radiative transfer models including excitation through both absorption of
far-infrared radiation emitted by dust and collisions are used to
calculate the equilibrium level populations of H2O and the
corresponding line fluxes.
The highest-lying H2O lines detected in emission, with levels at
300-640 K above the ground state, indicate that the source of
far-infrared radiation responsible for the pumping is compact
(radius
= 110-180 pc) and warm (
K),
accounting for at least 45% of the bolometric luminosity. The high
column density,
cm-2, found
in this nuclear component, is most probably the consequence of
shocks/cosmic rays, an XDR chemistry, and/or an ``undepleted chemistry''
where grain mantles are evaporated. A more extended region, presumably
the inner region of the 1-kpc disk observed in other molecular species,
could contribute to the flux observed in low-lying H2O lines through
dense hot cores, and/or shocks. The H2O 78
m line observed with
PACS shows hints of a blue-shifted wing seen in absorption, possibly
indicating the occurrence of H2O in the prominent outflow detected in
OH (Fischer et al. 2010, A&A, 518, L41). Additional PACS/HIFI observations of
H2O lines are required to constrain the kinematics of the nuclear
component, as well as the distribution of H2O relative to the
warm dust.
Key words: ISM: molecules - galaxies: ISM - galaxies: individual: Mrk 231 - line: formation - infrared: ISM - submillimeter: galaxies
1 Introduction
One key question in the study of composite infrared (IR) merging galaxies
and quasi-stellar objects (QSOs) is what fraction of their luminosity is
generated in the nuclear region (<200 pc) associated with the active
galactic nucleus (AGN) and a possible extreme nuclear starburst, and what
fraction arises from a more extended kpc-scale starburst
(e.g. Veilleux et al. 2009; Armus et al. 2007). The
ULIRG Markarian 231 (Mrk 231) is the most luminous (
)
galaxy in the local Universe (z<0.1),
and thus provides a unique template for such studies.
Since the bulk of
the luminosity in ULIRGs arises at far-IR wavelengths, where
sub-arc-second resolution observations are not available, an alternative
technique is required to constrain the compactness of the far-IR
emission and its physical origin.
In a previous work based on observations with the ISO, González-Alfonso et al. (2008, hereafter G-A08) have argued that the observation of molecular species such as OH and H2O at far-IR wavelengths is ideal for such a purpose, because their high-lying rotational levels are pumped through absorption of far-IR radiation and the observable excitation is then sensitive to the far-IR radiation density that in turn depends on the compactness of the far-IR continuum source. In addition, these molecular observations shed light on the dominant chemistry in those nuclear regions. G-A08 reported the ISO detection of 3 high-lying H2O lines, relevant upper limits over the entire ISO spectrum, and also high-lying OH lines, indicating the ocurrence of a compact-luminous far-IR component.
With their high sensitivity, spectral resolution, and wavelength
coverage, the Herschel (Pilbratt et al. 2010) instruments are ideal for
extending our previous
study to additional key lines in the far-IR/submillimeter.
As part of the HerCULES Key Programme (see van der Werf et al. 2010, hereafter
vdW10), we report in this Letter the
Herschel SPIRE/PACS (Poglitsch et al. 2010; Griffin et al. 2010) detection and first analysis of
several H2O lines in
Mrk 231, which supports the conclusions of G-A08 and gives
additional clues to the origin of H2O in this ULIRG. We adopt a
distance to Mrk 231 of 192 Mpc (H0=70 km s-1 Mpc-1,
,
and z=0.04217).
2 Observations
The SPIRE FTS observations of Mrk 231 (vdW10) were conducted on December 9th, 2009. The PACS observation of one H2O line was conducted on November 8th, 2009, as part of the SHINING key programme, and kindly provided for the present study. Details on data reduction, calibration, and line extraction are given in vdW10 and Swinyard et al. (2010). Excerpts of the spectrum around the H2O lines are displayed in Fig. 1, and an energy level diagram indicating the lines detected with SPIRE, the one detected with PACS, and those detected with ISO is shown in Fig. 2. Table 1 lists the line fluxes. Figure 1 also shows the results of our reference model, discussed below.
Table 1: Observed and modeled line fluxes.
H218O could be marginally detected at 250.0 m (220211) (see
vdW10); however, this identification should be
confirmed as the feature is shifted by 150 km s-1 from the nominal
line wavelength. Confirmation of H218O would be important as its
presence would support the strong enhancement of 18O in Mrk 231
derived from 18OH observations (Fischer et al. 2010, hereafter
F10). A number of the H2O lines in Fig. 1
are blended with the C18O lines 9-8 (303.57
m), 10-9 (273.24
m), and 11-10 (248.43
m).
Contamination by C18O is minimal, however,
as the lower-lying 6-5, 7-6, and 8-7 lines are not
detected. PACS observations show a broad absorption feature at 121.7
m, nearly coincident with the H2O 432423 line (F10). However,
this feature is probably contaminated by HF (2-1) at the same
wavelength, as the 1-0 line is detected with SPIRE (vdW10);
HF has been previously detected towards the Galactic star forming region Sgr
B2 (Neufeld et al. 1997). Therefore the 121.7
m feature is not used for the H2O analysis below.
Of particular interest is the detection of the very high-lying 523514 H2O line at a 4
level, which we have verified by reprocessing
the data with no correction applied for the instrument spectral efficiency and
by comparing these data with reduced observations of the dark sky
over the same spectral range. The ground state lines (p-H2O 111000 in Fig. 1 and o-H2O 110101) are not detected.
The lines detected with SPIRE are all
in emission and peak at central velocities, in contrast to
the low-lying OH lines that show P-Cygni profiles characteristic of an extreme molecular outflow
(F10). The red horizontal lines indicate the
FWHM of an unresolved line, and show that the H2O lines detected with SPIRE
are barely resolved. The o-H2O 423312 line detected with the
higher spectral resolution of PACS is in absorption and well
resolved, showing a central main body and, apparently, a
relatively weak blue-shifted wing extending up to -800 km s-1 and
possible low level emission from the receding gas. These wings should be
confirmed with additional observations of
H2O
absorption lines, as the limited wavelength coverage of the 423312 line
makes the adopted baseline uncertain. The detection would indicate that
H2O also participates in the prominent outflow detected in OH (F10).
While the shape of the 423312 absorption line shows the
centroid of the main body slightly blue-shifted (by -70 km s-1),
some lines observed in emission tend to show, on the contrary, a slight
red-shift of their centroid (up to 100 km s-1). This effect could be
related to systematic motions, i.e. a low-velocity nuclear-scale outflow, and
will be explored in the future with additional high spectral resolution
observations.
3 Analysis
The observed pattern of line emission cannot be explained in terms of
pure collisional excitation (G-A08).
Adopting the collisional rates of Faure et al. (2007) with Tk > 200 K and
cm-3,
and ignoring
radiative pumping, the models that account within a factor of 2
for the high-lying 523514 and 422413 lines predict fluxes for the
low-lying lines that exceed the observed values
by factors of
10. Thus the observed line ratios indicate an
excitation mechanism that favours the emission in the high-lying (
300 K) lines at the expense of the low-lying lines. This does not imply that the H2O lines are not formed in warm/dense regions,
but just that the dominant excitation mechanism for the high-lying lines
is not collisional.
Such a mechanism is the pumping
through absorption of dust-emitted far-IR photons, which efficiently pumps the
high-lying 321312/422413/523514 lines through
absorptions at 75.4/57.6/45.1 m in the strong
321212/422313/523414 lines (Fig. 2),
of which the lower backbone levels are preferently populated. This requires a strong continuum component at 30-70
m. However, this component cannot dominate the emission at >130
m, as it would produce strong H2O absorptions in that wavelength range that are not observed (G-A08). The data then support
the occurrence of both a warm/compact component with moderate opacity, and a
second colder component naturally associated with the more extended 1-kpc
starburst that dominates the emission at long wavelengths. Our proposed SED
decomposition is shown in Fig. 3a, and defines the reference
model (
)
with parameters as listed in Table 2:
(i) a hot component (
)
with
K
dominates the emission at
m; (ii) a warm (95 K) and compact (R=120 pc) component (
)
dominates at
;
(iii) an extended 1-kpc component (
), with
K, accounts
for most of the continuum at
m.
The
,
with
,
is responsible
for the observed high-lying H2O line emission.
![]() |
Figure 3:
Comparison between observations and results for the reference model. a) Continuum emission from Mrk 231. Spitzer IRS
data (Armus et al. 2007), ISO data (G-A08), and SPIRE data (red
spectrum) are shown. Flux densities at 800
and 1100 |
Calculations for H2O were carried out using the code described in
González-Alfonso & Cernicharo (1999). In
(Table 2), line broadening is caused by microturbulence. We have adopted a ``mixed'' approach (i.e. the H2O
molecules are evenly mixed with dust, G-A08), discussed
below. An ortho-to-para H2O abundance ratio of 3 is assumed.
For those H2O lines observed in emission, Fig. 3b compares
the expected fluxes from the
(in violet) with the observed
fluxes (in red). Collisional excitation is included with gas at
Tk=150 K and
cm-3 but,
even for these shock-like conditions
, it has a low
effect on the H2O level populations and line fluxes as these are mostly
determined by the strong radiation field. The high-lying lines are reproduced
with the
,
but there is a model deficit of emission from the
low-lying lines. This deficit indicates that the
contributes to those
low-lying lines (green); both radiative and collisional excitation, the latter
significant for
K, are included in the model.
The high-lying 422413 and 523514 lines are (nearly)
optically thin in the ,
so that their expected fluxes are sensitive to
.
Table 2 shows that, despite
the strong radiation field in this region, a high
is
required to account for the observed fluxes. Assuming a gas-to-dust mass
ratio of 100, the average H2O abundance relative to H2 in this
is
.
The H2O line at 78.7 m observed by PACS is consistently predicted in
absorption, and its flux is reasonably reproduced (Table 1),
given that the blue-shifted wing is not modeled. However, the observed line
shape suggests systematic motions that are not included in our model. In
general, we expect that lines with
m are observed in absorption, while lines with
m are in emission
(with exceptions due to the pumping details and level energies).
Concerning the ISO lines (G-A08),
also matches the absorption in the 220111 line at 101
m, but underestimates the absorption in the 330221 and 331220 lines
at 66.4 and 67.1
m by a factor of 2. As mentioned above, our model
uses a mixed approach, which implies that for given values of
,
the absorbing H2O lines are relatively weak, as
molecules located deep inside the source do not contribute to the
absorption features. Conversely, if a screen approach is adopted (i.e. a
H2O shell surrounds the continuum source), the absorption lines become much
stronger. In both approaches,
the
required to match the lines observed with
SPIRE are similar. Thus an analysis that combines absorption and
emission lines is a powerful tool to establish the distribution of H2O
relative to the warm dust responsible for the excitation. The screen
version of
yields absorption in 330221, 331220, and other
transitions that overestimate the observed values or upper limits. Therefore,
our preliminary result is that a combination of both the mixed and screen
scenarios best describes that observed data, with the mixed version
favoured. Nevertheless, it remains unclear what fraction of the observed
330221 and 331220 absorption arises from the outflow detected in OH (F10).
By increasing ,
the radius of
,
to 170 pc, and
keeping LW constant, the 321312/423312 line strengths become
overestimated by 50/25%, and the 211202/523514 intensities are underestimated by
30%; given the simplicity of our spherically symmetric models,
we estimate a size for
in the range
pc. A lower
limit for the luminosity arising from
is estimated by decreasing
to 85-80 K and increasing
to
1018 cm-2,
which results in too weak 423312 and ISO
absorption lines. We estimate that the mid- and far-IR emissions from
the nuclear region account for more than 45% of the bolometric
luminosity; observations at 60-200
m are required to better constrain that value and to establish a firm upper limit. Results are more uncertain for
,
and we may expect that its contribution to the H2O emission arises from its innermost region.
Table 2:
Parameters of the reference model (
).
4 Discussion
The extreme nature of the nuclear region in Mrk 231 is well illustrated by
comparing its SPIRE spectrum with that of the Orion Bar
(Habart et al. 2010), the prototypical Galactic PDR. The Orion Bar
spectrum shows CO lines a factor of 50 stronger than the H2O lines,
while in Mrk 231 the H2O and CO lines have comparable strengths (vdW10). This contrast will be still higher in the nuclear region, provided
that a significant fraction of the CO emission in Mrk 231 arises from a more
extended region. Thus the H2O-to-CO line intensity ratios
in the SPIREwavelength range are an excellent diagnostic of
extragalactic compact/warm far-IR continuum sources with unusually high
amounts of H2O.
The above comparison also indicates that the nuclear region of Mrk 231 cannot
be interpreted as an ensemble of classical PDRs. Three main scenarios are
proposed to explain such high amounts of H2O: (i) widespread
shocks/cosmic rays: although the H2O lines peak around the
systemic velocity, outflows of 100 km s-1 are not ruled out by our data, and indeed some indications in the H2O
line shapes of systematic motions have been found; an enhanced cosmic
ray flux could also have an important impact on the nuclear chemistry.
(ii) XDR chemistry: our derived H2O abundance of
10-6 is in very good agreement with XDR model results by Meijerink & Spaans (2005, their Fig. 3, Model 3), as well as with our preliminary
estimate of the H2O spatial distribution; (iii) an undepleted chemistry, where H2O
that formed on grain mantles is released into the gas phase, as in
Galactic hot cores; in support of this scenario, the derived
in
is close to the evaporation temperature of solid H2O. All three scenarios are probably taking place, and the identification
of the dominant process requires a multi-species analysis.
The nuclear region traced by the high-lying H2O lines has a
size similar to the nuclear disk (or outflow) observed at radio wavelengths
and H I 21 cm by Carilli et al. (1998, their Figs. 3 and 7),
suggesting a close physical correspondence. From
and
,
the nuclear surface brightness (
kpc-2) exceeds the highest values attained in starburst on
spatial scales
100 pc (Meurer et al. 1997; Davies et al. 2007), while the nuclear
luminosity-to-mass ratio (
/
)
exceeds the limit
for a starburst estimated by Scoville (2003). From near-IR data, Davies et al. (2007) estimated a starburst luminosity from a similarly sized region of
;
according to the
joint luminosity of our
and
,
the AGN would account for at least 50% of the output power in Mrk 231.
We thank the SHINING consortium for proving us with the spectrum of the H2O 423312, E. Habart for providing us the SPIRE spectrum of the Orion Bar prior to its publication in this volume, and the SPIRE ICC FTS team for their great help in data reduction/analysis. E.G-A is a Research Associate at the Harvard-Smithsonian Center for Astrophysics. Dark Cosmology Centre is funded by DNRF.
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Footnotes
- ...Herschel
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
- ... 200 K
- An LTE ortho-to-para H2 ratio according to Tk is assumed.
- ... conditions
- The average
densities are much lower than those adopted for collisional excitation,
and 2000 cm-3 for
and
.
All Tables
Table 1: Observed and modeled line fluxes.
Table 2:
Parameters of the reference model (
).
All Figures
![]() |
Figure 1: Comparison between the observed spectra (black/green histograms: unapodized/apodized spectrum, see vdW10) and results for the reference model discussed in Sect. 3 (dark blue lines; a Gaussian instrumental line shape with FWHM=0.048 cm-1 is used for simplicity). The 312221 line, shown in the same panel as the 321312 line, is blended with 12CO (10-9) (vdW10). The red segment in each panel indicates the FWHM of an unresolved line. The velocity scale has been calculated with respect to the systemic redshift of z=0.04217. |
In the text |
![]() |
Figure 2: Energy level diagram for H2O, showing the detected/undetected (blue arrows/lines) lines with SPIRE, the line detected with PACS (green) and those detected by ISO (light blue). Dashed red arrows indicate the main pumping paths for the high-lying lines observed with SPIRE. Upward (downward) arrows: absorption (emission) lines. |
In the text |
![]() |
Figure 3:
Comparison between observations and results for the reference model. a) Continuum emission from Mrk 231. Spitzer IRS
data (Armus et al. 2007), ISO data (G-A08), and SPIRE data (red
spectrum) are shown. Flux densities at 800
and 1100 |
In the text |
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