2.2 The data
2.2.1 CO data
Here we present the CO data coming from our own observations or from the literature, for a sample of the 86 galaxies that belong to the selected 20 HCGs. The aim was to achieve data for all the galaxies in the selected HCGs. CO data are missing for only 2 galaxies in these 20 groups:
HCG67d and 92f. The CO(1-0) line was detected for 45 galaxies out of these 86 galaxies.
2.2.1.1 IRAM CO(1-0) and CO(2-1) observations and data reduction
We observed 47 galaxies belonging to 14 different HCGs. The observations of CO rotational transition lines J(1→0) and J(2→1) (at 115.271 and 230.538 GHz, respectively) were carried out with the IRAM 30m radiotelescope at Pico Veleta2 during June, October and December 2006.
We performed single-pointing observations using the wobbler switch mode, with a switch fre- quency of 0.5 Hz and a throw of 200′′. As we observed the center of the galaxies, the CO intensi- ties were corrected assuming an exponential distribution (see Sec. 2.2.1.3). We checked for all the objects that the off-position did not coincide with a neighbor galaxy.
The dual polarization receivers A100 and B100 were used to observe in parallel the CO(1-0) and CO(2-1) lines. The median system temperature was 231 K for the CO(1-0) observations, with∼80% of the galaxies observed with system temperatures between 150 and 350 K. In the case of CO(2-1), the median system temperature was 400 K, with a temperature range between 230 and 800 K for 85% of the galaxies. For CO(1-0) the 1 MHz filterbank was used, and for CO(2- 1) the 4 MHz filterbank. The corresponding velocity resolutions were 2.6 kms−1and 5.3 kms−1 for CO(1-0) and CO(2-1), respectively. The total bandwidth was 1 GHz. The Half Power Beam Width (HPBW) is 22′′and 11′′for 115 and 230 GHz, respectively. All CO spectra and intensities are presented on the main beam temperature scale (Tmb) which is defined asTmb= (Feff/Beff)× TA∗. The IRAM forward efficiency,Feff, was 0.95 and the beam efficiency,Beff, 0.75.
The data reduction and analysis was performed using CLASS, while further analysis was per- formed using GREG, both part of the GILDAS3package developed by IRAM. First we inspected and discarded bad spectral scans. The baseline of each spectrum was subtracted and the scans were individually revised to discard those with a bad quality baseline. Any spike on the scans was removed. The scans were then averaged to achieve a single spectrum for each galaxy and transition. These spectra were smoothed to a velocity resolution of 21 to 27 km s−1, depending on the rms.
A total of 23 galaxies were detected in CO(1-0), 22 in CO(2-1) and 18 in both transitions.
The CO(1-0) spectra of the 47 galaxies, together with the spectra of the 22 detected galaxies are shown in Appendix A.
For each spectrum, we integrated the intensity along the velocity interval where emission was detected. In the case of no-detections we set an upper limit as:
ICO<3×r m s×δV Ç∆V
δV (2.2)
2IRAM is supported by CNRS/INSU (France), the MPG (Germany) and the IGN (Spain).
3http://www.iram.fr/IRAMFR/GILDAS
2.2 The data 17
whereδV is the width of each channel and∆V is the total line width. We used a value of∆V
=300 kms−1when there was no detection in CO(1-0) nor in CO(2-1). In those cases were the source was detected in only one transition, this line width was used to calculate the upper limit in the other transition.
The results of our CO(1-0) and CO(2-1) observations are displayed in Table 2.2 as follows:
• Column 1 Galaxy: galaxy designation.
• Column 2ICO(1−0): integrated intensity of the CO(1-0) emission in K kms−1.
• Column 3 rms : rms of the CO(1-0) spectrum (when available) in mK.
• Column 4 Ref.: reference of the CO(1-0) data, detailing whether data come from our ob- servations or from the literature (see 2.2.1.2).
• Column 5 Beam: HPBW of the telescope in arcsec.
• Column 6∆VCO(1−0): line width of the CO(1-0) emission (if detected) in kms−1.
• Column 7ICO(2−1): integrated intensity of the CO(2-1) emission (when observed) in K km s−1.
• Column 8 rms: rms of the CO(2-1) spectrum (when observed) in mK.
• Column 9∆VCO(2−1): line width of the CO(2-1) emission (if detected) in kms−1.
• Column 10 log(MH2observed): logarithm of theH2 mass (in solar masses) calculated from the observedICO(Sec.2.2.1.3).
• Column 11 log(MH2): logarithm of theH2 mass (solar masses) extrapolated to the emis- sion from the full disk (see Sec.2.2.1.3).
Table 2.2: Observed and derived molecular gas properties
Galaxy ICO(1−0) rms Ref. Beam ∆V ICO(2−1) rms ∆V logMH2obs logMH2extrapol
(K kms−1) (mK) (arcsec) (kms−1) K kms−1 (mK) (kms−1) (logM⊙) (logM⊙)
7a 7.20 3 43 500 9.51 9.71
7b <0.70 3 55 ... <8.71 <8.80
7c 1.40 3 55 183 9.01 9.17
7d <0.60 3 43 ... <8.43 <8.52
10a 2.72±0.49 2 22 339 8.79 9.51
10b <0.90 3 55 ... <9.00 <9.27
10c 7.14±0.36 2 22 359 9.21 9.54
10d <1.60 3 55 ... <9.25 <9.30
15a <0.42 3.68 1 22 ... <1.45 6.97 ... <8.62 <8.90
15b <0.75 4.41 1 22 ... <2.45 11.5 ... <8.87 <9.12
15c <0.52 6.12 1 22 ... <0.64 3.78 ... <8.71 <8.92
15d 0.62±0.16 3.94 1 22 160 <0.35 6.99 ... 8.31 8.56
15e <0.95 5.59 1 22 ... <2.03 9.79 ... <8.97 <9.14
15f <0.54 4.35 1 22 ... 1.29±0.42 7.88 180 <8.73 <8.84
16a 42.86±1.29 2 22 ... 9.79 10.00
16b 2.30 3 43 560 8.99 9.12
16c 9.20 3 43 270 9.59 9.73
16d 32.96±1.10 2 22 ... 9.68 9.96
23a 0.90±0.20∗ 3.26 1 22 360 <1.20 5.05 ... 8.11 8.39
Table 2.2: Observed and derived molecular gas properties (continued)
Galaxy ICO(1−0) rms Ref.(1) Beam ∆V ICO(2−1) rms ∆V logMH2obs logMH2
(K kms−1) (mK) (arcsec) (kms−1) K kms−1 (mK) (kms−1) (logM⊙) (logM⊙)
23b 9.70±0.60 2 22 318 9.26 9.68
23c <1.95 3.82 1 22 ... <0.63 7.44 ... <8.92 <9.08
23d 2.15±0.20 2 22 83 8.60 8.76
25a 2.56±0.20 2 22 193 8.96 9.24
25b 2.61±0.25 3.37 1 22 510 0.84±0.16 2.63 225 8.86 9.06
25d <0.40 2.38 1 22 ... <0.59 2.85 ... <8.52 <8.58
25f <0.89 5.26 1 22 ... <1.35 6.52 ... <8.87 <8.93
30a <0.70 3 55 ... <8.77 <8.89
30b <0.70 3 55 ... <8.77 <8.85
30c <0.80 3 55 ... <8.82 <8.83
30d <0.80 3 55 ... <8.82 <8.83
31a <0.40 3 55 ... <8.47 <8.55
31b <0.70 3 55 ... <8.72 <8.78
31c 1.53±0.30 2 22 148 8.38 8.48
31ac(2) <8.73 <8.82
31g <0.40 3 55 ... <8.47 <8.73
37a 0.38±0.13∗ 2.85 1 22 185 <0.79 3.81 ... 8.08 8.41
37b 8.65±0.40 2 22 552 9.56 9.80
37c 0.56±0.10 1.94 1 22 240 <0.60 8.08 ... 8.25 8.31
37d 0.50±0.13 2.80 1 22 200 1.19±0.16 3.09 170 8.20 8.27
37e <0.71 4.20 1 22 ... <1.21 5.85 ... <8.83 <8.83
40a <1.32 2 22 ... <8.69 <8.97
40b 1.04±0.13 3.80 1 22 110 0.96±0.22 4.79 130 8.47 8.70
40c 9.70±0.65 2 22 233 9.56 9.84
40d 5.09±0.43 2 22 295 9.28 9.44
40e 1.62±0.40 2 22 480 8.78 8.88
44a 10.50±0.60 2 22 269 8.30 8.87
44b <0.70 4.15 1 22 ... <0.63 3.06 ... <7.48 <8.14
44c 7.63±0.50 2 22 165 8.16 8.70
44d 2.33±0.16 4.00 1 22 155 2.12±0.12 2.65 125 7.53 8.00
58a 29.36±0.19 2.55 1 22 550 39.65±0.26 2.75 545 9.92 10.23
58b 3.97±0.44 5.34 1 22 625 7.92±0.61 6.21 605 9.05 9.40
58c <0.49 2.88 1 22 ... <1.03 4.97 ... <8.62 <8.93
58d <0.48 2.83 1 22 ... <0.88 4.25 ... <8.61 <8.79
58e 2.98±0.12 2.61 1 22 195 2.93±0.14 2.71 180 8.92 9.08
67a <0.90 3 55 ... <9.31 <9.48
67b 2.30 3 55 234 9.72 9.77
67c <0.80 3 55 ... <9.26 <9.32
68a <0.47 4.34 1 22 ... 0.95±0.25 6.07 105 <7.62 <8.03
68b <0.70 3 55 ... <8.10 <8.35
68c 7.02±0.42 2 22 214 8.43 9.11
68d <0.30 3 55 ... <7.75 <7.83
68d3 <0.81 1 22 ... 1.31±0.17 2.38 328
68e <1.11 6.59 1 22 ... 1.34±0.33∗ 5.52 230 <7.99 <8.18
79a <1.70 2 22 ... <8.47 <9.06
79b 0.91±0.13 2.24 1 22 320 1.42±0.23∗ 2.68 460 8.09 8.56
79c 0.42±0.12 3.68 1 22 105 <0.36 8.79 ... 7.75 8.13
79d <0.66 3.93 1 22 ... <1.28 6.21 ... <8.43 <8.62
88a 6.39±0.23 3.06 1 22 550 5.41±0.31 3.25 575 9.22 9.55
88b 3.17±0.13 2.16 1 22 355 3.11±0.22 2.45 505 8.92 9.30
88c 2.00 3 55 251 9.52 9.60
88c3 3.83±0.29 7.62 1 22 135 4.78±0.58 11.8 150
88d 1.06±0.17 2.96 1 22 330 0.79±0.25∗ 4.49 190 8.44 8.64
92b <0.50 3 55 ... <8.97 <9.17
2.2 The data 19
Table 2.2: Observed and derived molecular gas properties (continued)
Galaxy ICO(1−0) rms Ref.(1) Beam ∆V ICO(2−1) rms ∆V logMH2obs logMH2
(K kms−1) (mK) (arcsec) (kms−1) K kms−1 (mK) (kms−1) (logM⊙) (logM⊙)
92c 0.60 3 55 195 9.05 9.19
92d <0.50 3 55 ... <8.97 <9.10
92e <0.49 2.91 1 22 ... <1.06 5.09 ... <8.64 <8.87
93a 1.64±0.10 1.99 1 22 255 3.02±0.17 2.46 295 8.41 8.83
93b 11.26±0.35 5.98 1 22 325 13.11±0.48 7.27 270 9.25 9.65
93c 3.83±0.25 3.34 1 22 520 3.76±0.23 2.67 465 8.78 9.01
93d <0.75 4.45 1 22 ... <1.27 6.14 ... <8.55 <8.68
97a <1.35 7.94 1 22 ... <2.82 13.60 ... <9.06 <9.34
97b 3.55±0.43 6.33 1 22 430 <1.04 14.20 ... 9.00 9.25
97c <0.23 2.90 1 22 ... <0.36 3.71 ... <8.29 <8.49
97d <0.91 5.39 1 22 ... <4.06 19.50 ... <8.89 <9.17
97e <0.55 3.27 1 22 ... <1.34 6.48 ... <8.67 <8.76
100a 4.34±0.28 4.14 1 22 435 3.35±0.29 4.44 275 8.95 9.11
100b 0.83±0.13 2.90 1 22 185 1.60±0.23 4.28 180 8.23 8.37
100c 1.02±0.18 3.16 1 22 310 1.41±0.28 8.75 190 8.32 8.46
100d <0.72 4.27 1 22 ... <0.79 3.78 ... <8.65 <8.72
(1)CO reference: 1: Our observations. 2: Leon’98. 3: Verdes-Montenegro et al. (1998).
(2)The value of the observed and extrapolatedMH2for 31ac are the sum of their respective values (see Sec. 2.2.2).
(3)Galaxies observed by us for which we have selected the data from Verdes-Montenegro et al. (1998).
Galaxies flagged with∗present tentative detection.
2.2.1.2 CO(1-0) data from the literature
We have searched for the available CO(1-0) data for the 20 HCGs of our sample, compiling from the literature the data corresponding to the integrated CO(1-0) intensities and line widths of the following galaxies:
• Verdes-Montenegro et al. (1998): 24 galaxies corresponding to 9 different HCGs. 20 of these galaxies were observed with the NRAO 12 m telescope at Kitt Peak with a beam size of 55′′. The data from the other 4 galaxies are from Boselli et al. (1996), observed with the SEST 15m telescope, with a 43′′beam.
• Leon et al. (1998): 17 galaxies corresponding to 10 different HCGs. Observed with the IRAM 30m telescope with a similar setting as in our observations (see Sec. 2.2.1.1).
In total, we have CO data for 86 galaxies. There are 16 galaxies in common between the sample we observed and the literature samples. Another 14 out of the 86 galaxies were observed by both Verdes-Montenegro et al. (1998) and Leon et al. (1998). In order to choose between the different existing spectra (either from our observations or from the literature), we applied the following criteria: if available, we chose the spectrum with detected emission. If more than one detected spectrum existed, we chose the one with the lower rms or -in case of comparable rms- the spectrum observed with a larger beam, in order to probe a larger fraction of the disk.
There are 2 galaxies in common between our observations and the sample of Verdes-Montenegro et al. (1998) for which we have chosen the data from the bibliography:
HCG68d and 88c. For the other 14 galaxies that we observed, with data available in the liter- ature, we have selected the data coming from our observations.
2.2.1.3 Molecular gas mass
We calculate the molecular gas mass,MH2 using the following equation:
MH2=75×D2ICO(1−0)Ω (2.3)
whereΩis the area covered by the observations in arcsec2(i.e.Ω=1.13θ2for a single pointing with a gaussian beam whereθ is the HPBW). The former equation assumes a ratio X=NH2/ICO
=2×1020cm−2(K km s−1)−1(e.g. Dickman et al. 1986). TheMH2 calculated for the 86 galaxies are listed in Table 2.2.
In both the observations we carried out and the data obtained from the literature, a single position at the center of the galaxy was observed. Because of this and the different beams used by us and in the literature we need to correct for possible emission outside the beam. To correct the observed CO intensities we need to know the distribution and extension of the CO emission.
Different authors (Nishiyama et al. 2001; Regan et al. 2001; Leroy et al. 2008) have found that the integrated CO intensity in spiral galaxies follows an exponential distribution as a function of radius with a scale lenghtre:
ICO(r) =I0∝e(r/re) (2.4)
We adopt a scale length ofre =0.2×r25, wherer25is the isophotal radius at an optical sufrace brightness of 25 mag arcsec−1isophotal radius, following Lisenfeld et al. (in prep.), who derived this scale length from studies of the above mentioned authors and from their own CO data.
We have used this distribution to calculate the expected CO emission from the entire disk, tak- ing into account the galaxy inclination. The expected emission has been compared with the observed emission in the central pointing to calculate a correction factor.
The correction factor to the CO(1-0) intensities is shown in Figure 2.1. The ratio between the corrected and uncorrected intensities is below 2 in most of the galaxies (66 out of 86, or 77 %), with an average value of 1.78. To check the consistency of the correction, we have also performed the analysis presented in this paper for a sample restricted to the galaxies with cor- rections lower than 1.6 (45 galaxies), finding no significant differences with respect to the full sample, so we can conclude that the aperture correction does not introduce a bias in the results.
The values for the extraoplated molecular gas mass are shown in Table 2.2. Here, and in the following, we always use the extrapolated molecular gas mass and denote itMH2 for simplicity.
TheMH2 distribution is shown in Fig. 2.2. The average value for the full sample and the spiral galaxies is shown in Table 2.3. The distribution and average values ofMH2, as well as the sta- tistical distributions and average values throughout this work, have been calculated using the Kaplan-Meier estimator implemented in ASURV4, to take into account the upper limits in the data.