arXiv:0911.2673v1 [astro-ph.CO] 13 Nov 2009
Photodissociation chemistry footprints in the Starburst galaxy
Sergio Mart´ ın
Harvard-Smithsonian Center for Astrophysics, 60 Garden St., 02138, Cambridge, MA, USA
J. Mart´ ın-Pintado
Centro de Astrobiolog´ ıa (CSIC-INTA), Ctra de Torrej´ on a Ajalvir, km 4, 28850 Torrej´ on
de Ardoz, Madrid, Spain
Physics and Astronomy Department, University College London, Gower Street, London,
WC1E 6BT, UK
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UV radiation from massive stars is thought to be the dominant heating mech-
anism of the nuclear ISM in the late stages of evolution of starburst galaxies, cre-
ating large photodissociation regions (PDRs) and driving a very specific chem-
istry. We report the first detection of PDR molecular tracers, namely HOC+,
and CO+, and confirm the detection of the also PDR tracer HCO towards the
starburst galaxy NGC253, claimed to be mainly dominated by shock heating and
in an earlier stage of evolution than M82, the prototypical extragalactic PDR.
Our CO+detection suffers from significant blending to a group of transitions of
13CH3OH, tentatively detected for the first time in the extragalactic interstellar
medium. These species are efficiently formed in the highly UV irradiated outer
layers of molecular clouds, as observed in the late stage nuclear starburst in M82.
The molecular abundance ratios we derive for these molecules are very similar
to those found in M82. This strongly supports the idea that these molecules
are tracing the PDR component associated with the starburst in the nuclear re-
gion of NGC253. The presence of large abundances of PDR molecules in the
ISM of NGC253, which is dominated by shock chemistry, clearly illustrates the
potential of chemical complexity studies to establish the evolutionary state of
starbursts in galaxies. A comparison with the predictions of chemical models for
PDRs shows that the observed molecular ratios are tracing the outer layers of
UV illuminated clouds up to two magnitudes of visual extinction. We combine
the column densities of PDR tracers reported in this paper with those of easily
photodissociated species, such as HNCO, to derive the fraction of material in
the well shielded core relative to the UV pervaded envelopes. Chemical models,
which include grain formation and photodissociation of HNCO, support the sce-
nario of a photo-dominated chemistry as an explanation to the abundances of the
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observed species. From this comparison we conclude that the molecular clouds in
NGC253 are more massive and with larger column densities than those in M82,
as expected from the evolutionary stage of the starbursts in both galaxies.
Subject headings: galaxies: abundances — galaxies: ISM — galaxies: starburst —
galaxies: individual(NGC 253)
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Intense UV radiation from massive stars is one of the main mechanisms responsible
for the heating of the interestelar medium in the nuclear region of starburst galaxies. This
mechanism is particularly important in the latest stages of starburst (SB) galaxies where
the newly formed massive star clusters are responsible for creating large photodissociation
regions (PDRs). This is the case for the prototypical SB galaxy M82, where the large
observed abundances of molecular species such as HCO, HOC+, CO+, and H3O+are
claimed to be probes of the high ionization rates in large PDRs formed as a consequence
of its extended evolved nuclear starburst (Garc´ ıa-Burillo et al. 2002; Fuente et al. 2006;
van der Tak et al. 2008).
Observational evidences point to a significant enhancement in the abundance of HOC+
in regions with large ionization fractions. The abundance ratio [HCO+]/[HOC+]= 270 is
found in the prototypical Galactic PDRs of the Orion Bar (Apponi et al. 1999). Similar
or even lower abundance ratios are observed in the PDRs NGC7023 (50-120, Fuente et al.
2003), SgrB2(OH) and NGC2024 (360-900, Ziurys & Apponi 1995; Apponi & Ziurys 1997),
and the Horsehead (75-200 Goicoechea et al. 2009), as well as in diffuse clouds (70-120,
Liszt et al. 2004). This is in contrast with the much larger ratios of ≫ 1000 found in dense
molecular clouds well shielded from the UV radiation. However, these low HCO+/HOC+
ratios are not found in other galactic PDRs. Large values of this ratio of ? 2000 are found
in the PDRs M17-SW, S140, and NGC2023 (Apponi et al. 1999; Savage & Ziurys 2004).
The HCO molecule has also been observed to be a particularly good tracer of the PDR
interfaces. Low ratios of [HCO+]/[HCO]∼ 2.5 − 30 are found in prototypical Galactic
PDRs (Schenewerk et al. 1988; Schilke et al. 2001). The large HCO abundance (> 10−10)
altogether with the low ratio [HCO+]/[HCO]∼ 1 in the Horsehead PDR is claimed to be
a diagnostic for an ongoing FUV-dominated photochemistry (Gerin et al. 2009). CO+is
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also claimed to be particularly prominent in the chemical modeling of PDRs and high
abundances of this molecule appear to be correlated to similar enhancements of HOC+
(Sternberg & Dalgarno 1995; Savage & Ziurys 2004). [CO+]/[HOC+] ratios in the range of
1-10 are observed in a number of PDRs (Savage & Ziurys 2004), but only of ? 0.1. towards
the Horsehead PDR (Goicoechea et al. 2009).
As mentioned above, this set of PDR probes has been extensively studied towards
M82. However, no such complete studies have been carried out towards other prototypical
galaxies, but for the detection of HCO and HOC+towards NGC1068 (Usero et al. 2004)
and H3O+in Arp220 (van der Tak et al. 2008). M82 and NGC253 are the brightest
prototypes of nearby SB galaxies, at a similar distance and showing very similar IR
luminosities and star formation rates of about ∼ 3M⊙yr−1(Ott et al. 2005; Minh et al.
2007). However, both galaxies show very different chemical composition. The chemistry and
to a large extend the heating in the central region of NGC253 is believed to be dominated
by large scale low velocity shocks (Mart´ ın et al. 2006b). The similar chemical composition
found in the nuclear region of NGC253 to that in Galactic star forming molecular complexes
points to an earlier evolutionary stage of the starburst in this galaxy than that in M82
(Mart´ ın et al. 2003, 2005, 2006b).
Furthermore, our recent observations of the PDR component as traced by the easily
photodissociated HNCO molecule towards a sample of galaxies (Mart´ ın et al. 2008)
showed the non-detection of HNCO in M82, at a very low abundance limit. This low
HNCO abundance supports the scenario that the PDR chemistry dominates the molecular
composition of the ISM in this galaxy. However, from the HNCO measured abundance in
NGC253, it would be placed in an intermediate stage of evolution where photodissociation
should be starting to play a significant role in driving a UV-dominated chemistry which has
not been yet identified towards this galaxy. The presence of a significant PDR component
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Fig. 3.— JCMT observations of the CO+emission blended with transitions of13CH3OHJ =
5 − 4 and observed in the same window as HC3NJ = 25 − 24. This is the first detection
of the13C isotopologue of methanol. The overall spectral fitting to all the lines is shown
in grey. The contribution of the CO+emission is shown with dashed line. The position of
the three brighter transitions of the J = 5 − 4 group of13CH3OH are shown indicated with
vertical dashed lines. See Sect. 2 for details on the fitting to the spectra. Velocity resolution
has been degraded to ∼30kms−1.
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Fig. 4.— Theoretical predictions for the fractional abundances relative to H2(Upper panels)
and abundance ratios (Lower Panels) for the observed species as derived from the two dif-
ferent PDR models: a pure gas-phase model A (Left panels) and a coupled dense core-PDR
model B (Right panels). Details are given in Section 4.2. The vertical position of the key for
each molecule and ratio shown only in the plots for model A correspond to the actual derived
parameters from the observations. Additionally, the fractional abundances of CH3OH and
HNCO are shown for model B.
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Table 1: Parameters derived from the observed line profiles.
C18O1 − 03.1 ± 0.6183 ± 13100 ± 9116.75
6.0 ± 0.8283 ± 7100 ± 9a
HCO+1 − 021.98 ± 1.1177.9 ± 0.4 118.1 ± 0.3174.9
35.79 ± 1.4289.0 ± 0.3118.1 ± 0.3a
HOC+1 − 00.8 ± 0.2170 ± 10100 ± 207.6
1.0 ± 0.2282 ± 9100 ± 209.2
HOC+3 − 2< 0.8b
0.41 ± 0.08183 ± 14102 ± 4a
0.43 ± 0.08297 ± 13102 ± 4a
H13CO+1 − 01.26 ± 0.09176 ± 6 102 ± 4a
1.36 ± 0.10285 ± 5102 ± 4a
SiO 2 − 11.52 ± 0.11 182 ± 5 102 ± 4a
1.59 ± 0.11 292 ± 5 102 ± 4a
HC3N J = 5 − 41.05 ± 0.15 191 ± 795 ± 1810.5
CO+5/2 − 3/2F = 2 − 10.30 ± 0.10 191c
CO+3/2 − 1/2F = 2 − 10.17 ± 0.10 191c
13CH3OH 50,5− 40.4
0.10 ± 0.07191c
13CH3OH 5−1,5− 4−1.4
0.16 ± 0.07191c
aLinewidths forced to have the same value in the Gaussian fit.
b3σ upper limit assuming a 200 kms−1linewidth.
cParameters forced to equal those derived from HC3N Gaussian fit.
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Table 2: Derived fractional abundances and H13CO+ratios for each velocity component
1.6 ± 0.54.8 ± 1.01
1.7 ± 0.62.7 ± 0.41
0.8 ± 0.32.4 ± 0.72.0 ± 0.8
1.1 ± 0.41.7 ± 0.41.6 ± 0.4
HCO12.3 ± 5.837 ± 100.13 ± 0.04
12.9 ± 6.1 20 ± 40.14 ± 0.03
1.7 ± 0.85 ± 20.9 ± 0.4
aWith N(H2) = 3.3±1.2×1022cm−2and 6.3±2.1×1022cm−2for each velocity component, respectively, as
derived from C18O with16O/18O = 150 (Harrison et al. 1999) and a CO/H2= 10−4.
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Table 3: Abundance ratios of HCO+vs HOC+and HCO.
NGC25380 ± 305.2 ± 1.838 ± 15
63 ± 175.4 ± 1.3...
M8260 ± 28a
9.6 ± 2.8b
32 ± 16a
NGC1068 128 ± 28c
3.2 ± 1.2d
NGC4945...2.4 ± 1.2e
GC prototypical PDRs
Horsehead75 − 200f
Orion Bar< 166 − 270h,i
< 83 − 140i,k
NGC702350 − 120i
3.5− > 62j,l
aDerived from single dish data (Mauersberger & Henkel 1991; Fuente et al. 2006). See Sect. 4.1 for details.
bAverage ratio from the interferometric maps by (Garc´ ıa-Burillo et al. 2002)
cAverage over the whole line profile in the CND position (Usero et al. 2004).
dAverage value over the three positions in the circunnuclear starburst ring with HCO detections (Usero et al.
eFrom Wang et al. (2004).
fGoicoechea et al. (2009)
gGerin et al. (2009)
hApponi et al. (1999)
iOrion Bar ionization front and PDR-peak in NGC7023 Fuente et al. (2003)
jSchilke et al. (2001)
kSavage & Ziurys (2004)
lSchenewerk et al. (1988)
mSt¨ orzer et al. (1995)