Photon Dominated Chemistry in the Nucleus of
M82: Detection of HOC
in a Starburst Galaxy.
A. Fuente
£, S. García-Burillo
†, M. Gerin
££, A. Usero
†, D. Teyssier
‡and
J.R. Rizzo
§
Observatorio Astronómico Nacional (OAN), Apdo. 112, E-28800 Alcalá de Henares (Madrid,Spain)
†Observatorio Astronómico Nacional (OAN), Alfonso XII 3, E-28014 Madrid (Spain)
LERMA, UMR 8112, CNRS, Ecole Normale Superieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
‡SRON, PO Box 800, 9700 AV Groningen, The Netherlands
§Departamento de Física, UEM, Urb. El Bosque, E-28670 Villaviciosa de Odón (Madrid, Spain)
Abstract. The nucleus of M82 has been mapped in 3mm and 1mm lines of CN, HCN, C2H,
c-C3H2, CH3C2H, HC3N and HOC
using the IRAM 30m telescope. These species have been selected for constituting a good diagnostic of photon-dominated chemistry. We have measured [CN]/[HCN] ratios 5 in the inner 650 pc galaxy disk. Furthermore, we have detected the HOC
10 line with an intensity similar to that of the H
13CO
10 line. This implies a
[HCO
]/[HOC
] ratio of40. This low HCO
/HOC
abundance ratio unambiguously shows that the chemistry of the molecular gas reservoir in the nucleus of M82 is determined by the intense interstellar UV flux (G010
4in units of Habing field). Finally, the detection of the hydrocarbons
c-C3H2and CH3C2H in the nucleus of M82 suggests that a complex carbon chemistry is developing
in this giant photo-dissociation region (PDR).
INTRODUCTION
M82 is one of the nearest and brightest starburst galaxies. Located at a distance, d=3.9 Mpc, and with a luminosity, L=3.7 1010 L¬, it has been extensively studied in
many molecules. Compared to other prototypical starburst galaxies like NGC 253 and IC 342, M82 presents systematically low abundances of molecules like NH3, CH3OH, CH3CN, HNCO and SiO [1]. Different explanations have been proposed to account for this peculiar chemistry. Since all these molecules are related to dust grain chemistry, [1] proposed that the formation of molecules on dust and/or evaporation to gas phase is not efficient in M82. On the other hand, recent results suggest that the UV flux is dominating the chemistry of the molecular gas in M82. [2] obtained a high-angular-resolution im-age showing widespread HCO emission in this galaxy. The enhanced HCO abundance (HCO/H13CO
3.6) measured across the whole M82 disk was interpreted in terms of a giant PDR of 650 pc size.
CN, the reactive ions (CO
, HOC
) and small hydrocarbons (C2H, c-C3H2, C4H.) present significant abundances in PDRs. In particular, the [CN]/[HCN] ratio has been successfully used as a PDR indicator in regions with very different physical conditions in our Galaxy [3, 4]. A value of the [CN]/[HCN] ratio 1 is commonly considered as an evidence for the existence of a PDR. The detection of the reactive ion HOC
is almost unambiguously associated to PDRs with a high ionizing flux. Values of the
[HCO
]/[HOC
] ratio lower than 300 have only been found in a handful of prototypical galactic PDRs and in the active nucleus of NGC 1068 [5, 6, 7]. Finally, recent works have revealed that the abundances of some hydrocarbons (c-C3H2, C4H) in PDRs are an order of magnitude larger than those predicted by gas-phase models [8]. The failure of chemical models in explaining the hydrocarbons abundances in PDRs, as well as the spatial coincidence between the distribution of small hydrocarbons and PAHs in these regions, have led several authors to propose a formation process for small carbon chains linked to the PAHs chemistry.
OBSERVATIONS AND RESULTS
The observations were carried out in June 2004 with the IRAM 30 m radiotelescope at Pico de Veleta (Spain). The observed transitions are: CN 10 and 21,HCN 10,
C2H 10, c-C
3H2 21 21
0 1 and 61 65
0 5, CH3C2H 5k4
k, HC3N 98 and the
HOC
10. We observed three positions across the M82 disk: the nucleus (RA(2000):
09h55m51.9s, Dec(2000): 69o4¼
47.11¼¼
) and the two peaks in the HCO emission [offsets (14
¼¼
,5 ¼¼
) and (14 ¼¼
,5 ¼¼
) hereafter referred to as E and W knot respectively] [2]. We have not detected HC3N in the M82 disk. All the other species have been detected towards the observed positions. In particular, we have detected the emission of the
HOC
10 line. In Fig. 1, we show the observed HOC
spectra compared with those obtained from the interferometric H13CO
image obtained by [2]. As it is clearly seen in the figure, the intensity of the HOC
10 line is similar to that of the H 13CO
line. Column densities have been estimated using the LTE (C2H and CH3C2H) and LVG (CN,c-C3H2,HC3N,HOC
, H13CO
) codes. In Table 1, we show the obtained column density ratios.
Discussion
The fast hydrogen-catalyzed isomerization of HOC
into HCO
keeps the HOC
abundance very low in molecular clouds ([HCO
]/[HOC
] 1000). Only in regions with a high ionization degree the reactive ion HOC
can reach significant abundances [7]. Thus far, values of the [HCO
]/[HOC
] ratio300 have only been measured in the
harsh environment of the HI/H2transition layer of a PDR [5] and in the XDR associated to the active nucleus of NGC 1068 [7]. The PAH emission is expected to be very weak in XDRs since the doubly ionized PAHs produced by X-rays are very unstable [9]. The intense PAH emission observed in M82 [10] shows that X-rays are not dominating the chemistry in M82. Thus, we can conclude the high ionization degree of the molecular
FIGURE 1. In the top panels we show the HOC
10 spectra observed with the 30m telescope towards
the positions East (E), Center and West (W) in M82. For comparison, we also show the H13CO
10
spectra obtained from the interferometric image reported by [2] assuming the same beam as that of the HOC
observations. In the bottom panels, we show the H13CO
and HCO/H13CO
images (see [2]) in which we have drawn the beam of the 30m telescope at the frequency of the HOC
10 line.
gas in M82 is mainly due to the intense UV flux produced by the massive stars. Thus, the low [HCO
]/[HOC
] ratio derived from our observations unambiguously shows that the PDR chemistry is propagating in the M82 disk.
TABLE 1. Relative fractional abundances
Molecule M82 Orion Bar NGC 7023 Horsehead TMC1
E. (0,0) W. IF
PDR peak† IR peak
CP‡
C2H (1013cm 3) 58 41 30 57 3.8 16 7.2
c-C3H2/C2H 0.02 0.04 0.02 0.02 0.03 0.06 0.8 CH3C2H/C2H 0.33 0.70 0.50 ... ... 0.07 1.4 c-C3H2/HC3N 5 7 3 10 ... 19 0.4
CN/HCN 6 8 7 3 4–8 ... 0.07
CN/HC3N 150 200 150 180 ... ... 0.05
HCO
/HOC
44 36 30 166 50–120 ... ...
Fuente et al. 1996,2003
†Fuente et al. 1993,2003
Teyssier et al. 2004
‡Teyssier et al. 2004 and references therein
towards M82 is only consistent with the c-C3H2emission arising in a giant PDR.
ACKNOWLEDGMENTS
We are grateful to the technical staff in Pico de Veleta for their supporting during observations. This paper has been partially funded by the Spanish MCyT under projects DGES/AYA2000-927, ESP2001-4519-PE and ESP2002-01693, and European FEDER funds.
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