Physics Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801; and National Radio Astronomy Observatory, P.O. ...
The Astrophysical Journal Supplement Series, 154:541–552, 2004 October # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.
STUDIES OF EXTRAGALACTIC FORMALDEHYDE AND RADIO RECOMBINATION LINES Esteban Araya Physics Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801
Willem A. Baan ASTRON, Netherlands Foundation for Research in Astronomy, and Westerbork Observatory, Postbus 2, 7990AA Dwingeloo, The Netherlands
and Peter Hofner Physics Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801; and National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801 Receivved 2004 March 5; accepted 2004 June 9
ABSTRACT We present the most sensitive and extensive survey yet performed of extragalactic H2CO 6 cm (4.829 GHz) emission /absorption. Sixty-two sources were observed with the C-band system of the Arecibo Telescope to a 1 rms noise level of 0.3 mJy. We report a new detection of H2CO 6 cm absorption toward NGC 520 and the confirmation of H2CO 6 cm absorption toward several sources. We report confirmation of H2CO 6 cm emission toward the OH megamasers Arp 220, IC 860, and IRAS 15107+0724. At present these are the only extragalactic H2CO 6 cm emitters independently confirmed. A characterization of the properties of formaldehyde absorbers and emitters based on infrared properties of the galaxies is discussed. We also conducted a simultaneous survey of the H110 hydrogen recombination line toward a sample of 53 objects. We report the detection of H110 toward the giant extragalactic H ii region NGC 604 in M33. Subject headingg s: galaxies: ISM — ISM: molecules — radio lines: galaxies Among the extragalactic H2CO 6 cm absorption lines, there are six that are broad and reflect the large-scale motion of the molecular disks near the center of the galaxies. Four of the extragalactic absorption lines (including a tentative detection toward IRAS 17468+1320, Baan et al. 1993) are narrow and might represent foreground molecular structures in the outer disk of the galaxy. On the other hand, the widths of the H2CO 6 cm emission lines detected toward extragalactic objects range between 100 and 800 km s1 and typical line intensities are 2 mJy. Thermal excitation cannot explain the detected extragalactic H2CO 6 cm line intensities, and therefore a maser mechanism must be responsible for the observed emission lines (e.g., Baan et al. 1986). The pumping of the formaldehyde 6 cm transition was initially attributed to the presence of radio continuum that pumped the J ¼ 2 K-doublet and after radiative decay resulted in an inversion of the J ¼ 1 K-doublet, allowing low-gain amplification of the radio continuum (e.g., Baan et al. 1986). However, the similarity between the formaldehyde emission and the NICMOS NIR emission in Arp 220 could be a strong indication that the infrared radiation field is associated with the pumping of the molecules ( Baan & Haschick 1995; Scoville et al. 1998). The low-gain amplification scenario ( Baan et al. 1986) has also been invoked to explain the powerful line emission observed in extragalactic OH and H2O megamaser galaxies ( Baan 1985, 1989; Henkel & Wilson 1990). The most sensitive and extensive survey for extragalactic H2CO 6 cm emission /absorption reported prior to the search presented here was conducted by Baan et al. (1993). They used the Mini-Gregorian subreflector system of the Arecibo Observatory 2 (the prototype of the current Gregorian system)
1. INTRODUCTION More than 20 molecular species have been detected toward extragalactic regions during the last decades (e.g., Henkel & Mauersberger 1992; Takano et al. 1995), formaldehyde ( H2CO) being one of them. H2CO is a slightly asymmetric top molecule and therefore presents K-doublet splitting of rotational states. The most commonly observed formaldehyde K-doublet is the JKa Kc ¼ 110 111 transition (0 ¼ 4:829 GHz) in the 6 cm band, which is easily detected toward galactic molecular clouds where it absorbs background emission from discrete radio continuum sources (e.g., Araya et al. 2002a), or the diffuse cosmic background radiation (e.g., Palmer et al. 1969). Galactic and extragalactic H2CO 6 cm emission is a rare phenomenon. Reliable detections of Galactic H2CO 6 cm emission have been reported toward only five sources (e.g., Araya et al. 2002b). Emission in the H2CO 6 cm line has been interpreted as thermal emission toward Orion-KL (Zuckerman et al. 1975), as maser emission toward IRS 1 in NGC 7538 (Rots et al. 1981), toward H ii regions in Sgr B2 (Whiteoak & Gardner 1983), in G29.960.02 (Pratap et al. 1994), and recently toward IRAS 18566+0408 (Araya et al. 2004). On the other hand, extragalactic detections of the 6 cm line have been reported for a total of 20 sources, 10 in absorption (including the detection toward the LMC) and 121 in emission (see Baan et al. 1986, 1990, 1993). However, due to the weakness of the H2CO 6 cm detections, mostly in cases of emission, observational confirmation is necessary. Prior to the survey presented here, extragalactic H2CO emission had only been independently confirmed toward Arp 220, and independent confirmation of H2CO 6 cm absorption had been reported toward NGC 253, NGC 3628, and NGC 5128.
2 Arecibo Observatory is part of the National Astronomy and Ionosphere Center ( NAIC), which is operated by Cornell University under contract with the National Science Foundation.
1
Two galaxies ( NGC 253 and NGC 3079) were reported to show both emission and absorption.
541
TABLE 1 Observed Sources
Source (1)
VHel ( km s1) (4)
S6 cm a (mJy) (5)
rmsH2 CO; OH; H i b (mJy) (6)
rmsH110b (mJy) (7)
25 36 41 37
4507 18330 9270 2281
(?) 6.4(?) 11.6 79.2
( BP) 0.17 0.18 0.14 ( RFI ) 0.680 0.834 0.07d 0.69 ( RFI ) 0.22 0.987h 2.039 ( RFI ) 0.26 0.22 ( RFI ) 0.17 0.13 0.20 0.34 0.21 0.20 0.20 0.14 0.22 0.48 ( RFI ) 0.11 0.17 0.27 0.13 0.21 0.22 0.58 0.19 0.37 0.59 0.34 0.18 0.40 0.21 0.26 0.15 0.15 0.16 0.38 0.17 0.25 ( BP) 0.34 ( BP) 0.30 ( BP) 0.27 0.26 0.27 0.19 0.15 0.19 0.25 0.13 ( RFI ) 0.26 0.26 0.14 0.16 0.20 ( RFI )
0.47 ( RFI ) 0.17 0.17 0.05d
(J2000) (2)
(J2000) (3)
Mrk 348 ......................... IR 00509+1225 .............. MCG 0204025 ............... NGC 520........................
00 00 01 01
+31 +12 +14 +03
NGC 604........................
01 34 33.0
+30 47 00
226
53.2
NGC 660........................
01 43 01.7
+13 38 36
826
139.5
IR 01418+1651 .............. NGC 0925...................... UGC 02369.................... IR 03056+2034 .............. IR 03359+1523 .............. NGC 2339...................... UGC 03747.................... UGC 03870.................... IR 07329+1149 .............. NGC 2623...................... IR 08071+0509 .............. NGC 2903...................... NGC 3094...................... IR 10173+0828 .............. NGC 3227...................... NGC 3351...................... NGC 3504...................... NGC 3627...................... NGC 3628...................... IR 12112+0305 .............. NGC 4418...................... NGC 4569...................... NGC 4826...................... NGC 4922...................... IC 860 ............................ IC 883 ............................ NGC 5248...................... NGC 5363...................... NGC 5775...................... UGC 09618.................... NGC 5859...................... IR 15107+0724 .............. IR 15250+3609 .............. Arp 220 .......................... NGC 6181...................... NGC 6240...................... IR 172080014 ............. IR 17468+1320 .............. IR 17526+3253 .............. IR 18470+3233 .............. IR 20248+1734 .............. IR 20286+1846 .............. IR 20450+2140 .............. IR 20491+1846 .............. IR 20550+1656 .............. IR 21272+2514 .............. UGC 11905.................... IR 22055+3024 .............. NGC 7331...................... UGC 12289.................... NGC 7469...................... IR 23135+2516 ..............
01 02 02 03 03 07 07 07 07 08 08 09 10 10 10 10 11 11 11 12 12 12 12 13 13 13 13 13 14 14 15 15 15 15 16 16 17 17 17 18 20 20 20 20 20 21 22 22 22 22 23 23
+17 06 09 +33 34 43 +14 58 15 +20 46 18 +15 32 53 +18 47 12 +12 15 58 +33 49 08 +11 42 35 +25 45 17 +05 01 10 +21 30 06 +15 46 15 +08 13 34 +19 52 46 +11 42 14 +27 58 21 +12 59 30 +13 35 20 +02 48 39 00 52 50 +13 09 46 +21 41 00 +29 18 39 +24 37 01 +34 08 22 +08 53 08 +05 15 17 +03 32 41 +24 37 03 +19 35 06 +07 13 27 +35 58 37 +23 30 11 +19 49 36 +02 24 02 00 17 00 +13 19 54 +32 53 12 +32 37 31 +17 44 20 +18 56 46 +21 51 07 +18 58 05 +17 07 44 +25 27 49 +20 38 22 +30 39 34 +34 24 56 +24 04 28 +08 52 26 +25 33 11
8101 553 9262 8014 10507 2289 1877 4060 4881 5535 15650 554 2409 14390 1111 778 1534 727 847 21703 2040 235 408 7071 3866 7000 1153 1191 1681 10470 4860 3897 16403 5475 2375 7295 12834 4400 7644 23626 36538 40471 38398 8538 10482 45032 7400 37965 816 10160 4892 8154
48 53 20 24
44 27 54 08 38 08 13 28 35 38 09 32 01 19 23 43 03 20 20 13 26 36 56 01 15 20 37 56 53 57 07 13 26 34 32 52 23 49 54 48 27 30 47 51 57 29 05 07 37 59 03 15
47.1 34.9 02.7 35.3
30.5 17.0 01.8 30.8 47.1 19.6 12.1 53.6 42.7 24.1 47.3 09.7 25.2 59.9 30.7 57.8 11.2 15.0 17.2 45.8 56.0 49.8 43.8 24.9 04.1 35.3 32.0 07.2 57.5 00.6 34.0 13.1 59.4 57.1 21.0 59.0 21.9 06.7 29.6 54.2 09.0 55.6 13.9 26.0 23.7 29.5 54.6 48.7 04.1 41.6 15.6 58.7
57 41 21 47
28.2 (?) 10.9(?) 36.5 (?) 26.0(?) 13.3(?) 26.7 27.5 64.2 18.3 40.5 14.1 19.5 8.2 23.2 87.0 40.1(?) 130.8 12.2 30.2 13.0 (?) 27.9 15.6(?) 46.9 (?) 102.3 (?) 29.6 15.4(?) 30.8 9.7 171.9 200.0 (?) 62.3 20.0(?) 22.1 (?) (?) 19.2 (?) 2.6 8.8 (?) 22.6 (?) 40.4 (?) 68.5 2.5
0.23g 0.23
0.20 0.40 ( BP) 0.21 0.55 ( BP) 0.34 ( RFI ) 0.21 0.23 0.37 0.21 0.22 ( RFI ) 0.21 0.21 0.22 0.30 0.12 ( RFI ) 0.20 ( RFI ) 0.31 0.25 ... 0.33 0.72 0.19 ... ... ... 0.20 ... 0.31 ... 0.29 ... 0.15 0.38 0.34 ... 0.25 ( BP) 0.49 0.14 0.23 ... 0.22 0.29 0.20 0.15 0.43 ( RFI ) 0.19 0.13 0.32 0.28 0.14 0.18 ( RFI ) 0.16
Setupc (8) III III IV III OHe H if III OHe III OH H if III III III III IV IV & V IV IV IV III IV & V IV IV III & IV IV & V IV III & IV IV II IV & V IV & V IV V V II IV & V V IV & V V IV & V V IV IV I, IV, & V V IV & V IV & V III, IV, &V IV V III III III III III III III III III III III III
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Source (1) IR 23233+0946 ................. P06-1 ................................. NGC 7674......................... Mrk 331 ............................
(J2000) (2)
(J2000) (3)
23 23 23 23
+10 +08 +08 +20
25 23 27 51
55.6 32.6 56.7 26.7
02 37 46 35
45 25 45 10
VHel ( km s1) (4)
S6 cm a (mJy) (5)
rmsH2 CO; OH; H i b (mJy) (6)
rmsH110b (mJy) (7)
Setupc (8)
38240 10882 8671 5422
26.2 4.7 75.3(?) 24.8
0.32 0.18 0.26 0.13
0.32 0.17 0.29 ( BP ) 0.13 ( RFI )
IV III III III
Notes.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. ( BP)/(RFI) Bandpass structures/radio interference detected in the spectrum. (?) Continuum emission in reference position, bandpass structures, and/or RFI preclude reliable determination of the 6 cm radio continuum. We report the value of the spectral baseline followed by a question mark if radio continuum emission is found within two primary beams from the reference position based on the NVSS (Condon et al. 1998) and/or the GB6 (Gregory et al. 1996) catalog. a 6 cm radio continuum determined from the spectral baselines. b One sigma statistical noise. Channel width in the spectra is 0.20 MHz (12 km s1) except when indicated otherwise. c See Table 2. d Channel width 0.78 MHz (48 km s1). e Channel width 0.20 MHz (35.12 km s1). f Channel width 48.8 kHz (10.31 km s1). g The H110 spectrum was smoothed to a resolution of 97.6 kHz (6.00 km s1). h Only subcorrelators 1 and 3 were used for the average due to polarized RFI present in subcorrelators 2 and 4. Channel width 97.7 kHz (17.6 km s1).
to observe a large sample of objects during several epochs between 1988 and 1991. Including tentative detections, they reported H2CO 6 cm emission toward nine extragalactic sources in addition to the well-known emitter Arp 220. They also summarized the extragalactic H2CO 6 cm emitters and absorbers known at that time. The work presented here is a continuation of the Baan et al. (1993) effort using the improved C-band system of the Arecibo Telescope and covering a larger number of sources. The goals of this survey were to confirm the detections reported by Baan et al. (1993) and to extend the search for H2CO 6 cm emitters to a variety of extragalactic objects. The flexibility of the current spectral line correlator of the Arecibo Telescope allowed us to obtain simultaneous observations of the H110 radio recombination line ( RRL) while observing the H2CO 6 cm line. Hydrogen RRLs have been detected toward more than 10 extragalactic objects (e.g., Zhao et al. 1996; Anantharamaiah et al. 1993; Mohan et al. 2001; Phookun et al. 1998; Rodriguez-Rico et al. 2002). These studies show that extragalactic RRL emission originates from starburst and/or merger galaxies and is enhanced by internal stimulated emission (e.g., Phookun et al. 1998). The measured RRL peak intensities are of the order of half a mJy to a few mJy. Contrary to an early prediction by Shaver (1978), the observed RRL line strengths appear to increase with frequency in the majority of cases (e.g., Phookun et al. 1998). A ‘‘many H ii regions’’ (MHIIR) model has been used to predict the variation of peak line flux densities and total continuum flux as a function of frequency (Phookun et al. 1998). This model is based on the idea that extragalactic RRL emission originates from a cluster of H ii regions in the nuclear region of a galaxy. Depending on the parameters of the model, nearly constant to several times lower peak flux densities may be expected from recombination lines at 5 GHz with respect to RRL at 8.4 GHz (e.g., Phookun et al. 1998), where most RRL detections have been made so far. In x 2 we describe our observations and data reduction method. In x 3 we summarize the observational results of this survey, and in x 4 we discuss these results. We conclude the paper with a summary in x 5.
2. OBSERVATIONS AND DATA REDUCTION We observed a sample of 62 extragalactic objects in this survey. With the goals of confirming previously detected sources and finding new H2CO 6 cm sources we selected sources from a variety of extragalactic objects: (1) galaxies with previous tentative or weak detections of H2CO 6 cm emission/absorption, (2) OH megamaser/absorber galaxies, (3) AGNs and/or ultraluminous far-infrared galaxies with starburst nuclei (SBNs), and (4) gas-rich, nearby spiral galaxies. The sample was selected to search for H2CO 6 cm emission/absorption from nuclear regions (e.g., AGNs and SBNs) as well as from more extended molecular disks (e.g., spiral galaxies). In Table 1 (cols. [1]–[4]) we list the source names, equatorial coordinates (J2000), and heliocentric velocities adopted for Doppler correction. We observed the JKa Kc ¼ 110 111 transition of formaldehyde (0 ¼ 4829:6664 MHz, F ¼ 1 2; Kukolich 1975) with the C-band (4–6 GHz) Gregorian system of the 305 m Arecibo Telescope. A subsample of 53 sources was also observed at the frequency of the H110 recombination line (0 ¼ 4874:1570 MHz; Lilley & Palmer 1968). In addition, the L-Wide (1.12–1.73 GHz) Gregorian system was used to observe the main lines of OH 2 3=2 J ¼ 3=2 at 1.665 GHz and 1.667 GHz (ter Meulen & Dymanus 1972; hereafter OH–1.66 GHz lines) toward NGC 604, NGC 520, and NGC 660, and the 21 cm H i line toward NGC 520 and NGC 660. The survey was conducted during four different epochs: 2001 May, 2001 October, 2002 January, and 2002 April. The observing procedure was position switching with a sequence of 5 minutes spent at both ON and OFF source positions, followed by a 10 s integration at the OFF position with and without noise-diode calibration signal. The basic integration time per record was set to 1, 10, or 60 s depending on the presence of variable RFI within the bandpass. To account for different line widths, and to investigate instrumental bandpass structures and the presence of standing waves, we used seven different correlator setups during the four observing sessions of this project (details are listed in Table 2). The system
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TABLE 2 Correlator Setup Subcorrelator 1 Setup
Date
I ......................... II ........................ III....................... IV ...................... V........................ OHc ................... H id ....................
2001 2001 2001 2002 2002 2002 2002 2002
May May Sep Jan Apr Apr Apr Apr
Subcorrelator 2
Subcorrelator 3
Subcorrelator 4
0 ( MHz)
Pol
BW a
0 ( MHz)
Pol
BW a
0 ( MHz)
Pol
BW a
0 ( MHz)
Pol
BW a
4829.6594 4829.6594 4829.6664 4829.6664
A A A A
25 25 25 25
4829.6594 4829.6594 4829.6664 4874.1570
B B B A
25 25 25 25
4829.6594 4835.6594b 4874.1570 4829.6664
A A A B
25 50 25 25
4829.6594 4835.6594b 4874.1570 4874.1570
B B B B
25 50 25 25
4829.6664 1665.4018 1420.4058
A A A
25 25 25
4829.6665 1665.4018 1420.4058
A B B
50 25 25
4829.6664 1667.3590 1420.4058
B A A
25 25 25
4829.6665 1667.3590 1420.4058
B B B
50 25 25
Note.—The subcorrelators 1 to 4 were used simultaneously during the observations; 2048 channels per subcorrelator were used except for the cases pointed out by footnote b. a Bandwidth ( MHz); 25 MHz bandwidth equals 1:55 ; 103 km s1 at 4.8 GHz. b 4096 channels used instead of 2048. c Bandwidth: 4:5 ; 103 km s1, channel width: 12 kHz (2.20 km s1). d Bandwidth: 5:3 ; 103 km s1, channel width: 12 kHz (2.58 km s1).
temperatures measured during the observations were about 32 K for the C band and about 40 K for the L band. We used nine-level sampling for our observations except for those done with a bandwidth of 50 MHz, for which threelevel sampling was used. Several standard Arecibo calibrators were observed throughout the four observation epochs to check the pointing accuracy, determine the beam size, and measure the point source telescope gain. We measured a beamwidth of 5700 at 6 cm, and the pointing accuracy was better than 1500 for all observing runs. In the Appendix we give further
details about the telescope calibration for each of the seven setups. The data reduction was done using the CLASS software package.3 We used the telescope gains as derived in the Appendix to convert the unaveraged spectra, i.e., individual Tsys ; (ON OFF)=OFF records, from antenna temperature to flux density units. Based on the observation of standard Arecibo calibrators (see Appendix), we estimate that the flux 3
CLASS is part of the GILDAS software developed by IRAM.
TABLE 3 Line Parameters of Detected Sources
Source
Line
S (mJy)
NGC 520....................
H2CO OH1.665 GHz OH1.667 GHz H i aa H i ba H i ca H110 b H2COb H110 C110 c H2CO OHe Hi H2CO H2CO H2CO H2CO
1.45 (0.13) 5.88 (0.68) 6.89 (0.68) 25.06 (0.83) 18.44 (0.83) 14.21 (0.83) 0.29 (0.05) 0.34 (0.07) 1.36 (0.19) 0.55 (0.19) 1.56 (0.22) 12.37 (0.99) 238.6 (2.0) 2.97 (0.21) 1.65 (0.34) 1.91 (0.16) 2.92 (0.17)
NGC 604....................
NGC 660....................
NGC 3628.................. IC 860 ........................ IR 15107+0724 .......... Arp 220 ......................
VHel ( km s1) 2332 (12) 2286 (8) 2308 (7) 2188.7 (1.2) 2331.4 (0.6) 2437.8 (0.1) 2167 (11) 370 (48) 243 (6) 234 (4) 681 (12)d 995 (18) 852.1 (0.3) 764 (12) 3891 (12) 3922 (12) 5378 (12)
R
Width ( km s1)
S dv (mJy km s1)
Notes
571 (24) 166 (17) 252 (18) 182.8 (2.9) 35.2 (1.5) 71.1 (2.7) 158 (21) 291 (96) 67 (12) 14 (7) 450 (24) 1197 (36) 190.6 (0.6) 400 (24) 158 (24) 320 (24) 668 (24)
242.0 (69.8) 1037 (91) 1847 (109) 4875 (65) 691 (32) 1075 (36) 49 (6) 75.8 (11.1) 49.6 (4.2) 8.0 (3.7) 283 (77) 4910 (405) 48407 (137) 600 (55) 117 (50) 283 (18) 806 (70)
V G G G G G G? V? V G? V V G V V V V
Notes.—One statistical errors presented, except when (V ) is indicated. (?) Doubtful detection due to bandpass problems and /or weak emission or absorption. (G) Line parameters obtained from Gaussian fit. FWHM is reported in the ‘‘Width’’ column. (V ) Zero power width of the line is presented. The reported flux density and VHel correspond to the channel with deepest absorption or higher emission. The VHel error presented is the channel width. The reported line width error is twice the channel width. The integrated flux density is obtained from a multiple Gaussian fit of the line. a See Fig. 2. The Arecibo beam covered the central region of the H i edge-on disk mapped by Stanford (1990). b The spectrum was smoothed to a velocity resolution of 48 km s1 (0.78 MHz). c The rest frequency of the C110 line is 4876.589 MHz. This line also could be due to the He110 transition (4876.144 MHz). In that case, the velocity of the helium gas would be 261.5 km s1. d Channel with deepest absorption nearby the edge of the line. The heliocentric velocity and flux density near the center of the line are 852 (12) km s1 and 0.66 (0.22) mJy, respectively. e Complex line shape. OH1.665 GHz blended with OH1.667 GHz.
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Fig. 2.—Tentative detection of H110 toward NGC 520. Upper panel, spectrum without baseline subtraction; lower panel, spectrum after baseline subtraction. The channel width is 48 km s1 (0.78 MHz).
Fig. 1.—H2CO (upper), H i (middle), and OH–1.66 GHz (lower) spectra toward NGC 520. Artifacts in the spectra caused by RFI are marked with plus symbols. The reference frequency of the OH spectrum is 1.667 GHz. Multiple Gaussian profiles were used to obtain the line parameters of the OH spectrum as well as of the H i line (the line parameters of the Gaussian fits labeled ‘‘a,’’ ‘‘b,’’ and ‘‘c’’ are listed in Table 3). The H2CO centroid velocity is 2272 km s1, which agrees with the systemic velocity and H i emission centroid.
density calibration is accurate within 30%. In column (5) of Table 1 we list the 6 cm radio continuum derived from the baseline level of the H2CO spectra. In the case of setups I to V, we subtracted a linear baseline to each record and subsequently averaged the records weighted by 2 using the ‘‘SUM’’ routine in CLASS. We smoothed the H2CO and H110 data to a channel width of 0.20 MHz (12.17 km s1)4 in order to detect weak and broad emissions. In the case of the OH and H i setups, we applied constant gain values for the calibration as described in the Appendix. Finally, we subtracted a cubic polynomial baseline. In columns (5) and (6) of Table 1 we list the 1 statistical noise obtained from the baseline fit. 3. RESULTS The line parameters of detections and tentative detections are presented in Table 3. The main results of our observations are summarized as follows. 4
NGC 604 and NGC 520 are exceptions, see Table 1.
1. New extragalactic formaldehyde absorber in NGC 520.— NGC 520 is a starburst galaxy located at a distance of 29 Mpc that shows evidence of a nearly complete merger of two galaxies based on optical images (Sanders et al. 1998). We report the first detection of H2CO 6 cm absorption toward this galaxy with a signal-to-noise ratio (SNR) of 11 (see Table 3). Unfortunately, RFI was present in the high-velocity wing of the H2CO line. However, the use of nine-level sampling kept the effects of the RFI localized within a velocity interval of 108 km s1 ( Fig. 1). H i and OH–1.66 GHz lines had been previously detected toward this galaxy ( Baan et al. 1985). We reobserved these lines using the L-band Arecibo system. The H2CO, H i, and OH spectra are shown in Figure 1. The H2CO line shows broad wings of width identical to the width of the H i emission line and a centroid velocity that approximately agrees with that of the H i and OH emission centroids and the systemic velocity. The similarities between the H2CO detection and the H i and OH lines suggest that the RFI shown in Figure 1 is not significantly affecting the H2CO line beyond the 108 km s1 interval. 2. Tentative detection of H110 emission toward NGC 520.—In Figure 2 we show the tentative detection of H110 emission toward NGC 520. The spectrum is possibly affected by bandpass structures, and therefore we report this detection as tentative. The SNR of this tentative detection is 5.8 (see Table 3). 3. First detection of the H110 RRL toward NGC 604.— We report the detection of H110 toward the giant H ii region NGC 604 in M33 with a SNR of 7 (see Table 3). We also report a tentative detection of the carbon recombination line C110 ( Fig. 3, lower panel ) toward this region. The velocity is consistent with the H110 velocity within the reported uncertainty (see Table 3). The SNR of this tentative detection is 2.9. Furthermore, an absorption feature (SNR 4:9) in the H2CO bandpass suggests the presence of H2CO 6 cm as well ( Fig. 3, upper panel ). 4. Confirmation of previous detections.—We confirm the H2CO absorption lines toward NGC 660 ( Fig. 4) and NGC 3628 ( Fig. 5). We confirm also the megamaser emission lines toward IC 860, IR 15107+0724, and Arp 220 ( Fig. 5) reported by Baan et al. (1993). We note that these three galaxies are the only regions where extragalactic H2CO 6 cm emission has been independently confirmed. The velocity resolution of these
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Fig. 3.—H2CO (upper panel ) and H110 (lower panel ) spectra toward NGC 604. Note the clear detection of H110 emission and the tentative C110 and H2CO 6 cm detections.
detections is the same than the NGC 520 H2CO spectrum (see Table 3), and the SNRs are also similar, i.e., SNR for the confirmations range from 5 to 17 (see Table 3). 5. Nondetections.—Detection limits for H2CO 6 cm absorption/emission have been established toward 49 sources and are given in Table 1, column (5). Among them are five galaxies for which Baan et al. (1993) reported H2CO detections (see x 4.2). For five sources no useful H2CO detection limits were obtained due to strong RFI or bandpass structures, among them are IRAS 014118+1651, NGC 6240, and IRAS 17468+1320 for which Baan et al. (1993) reported H2CO detections. We report upper limits for H110 emission toward 51 objects in Table 1, column (6). 4. DISCUSSION 4.1. Infrared Characterization Although the small number of H2CO emitting or absorbing galaxies precludes a detailed statistical study, we briefly describe below their infrared properties in order to continue the initial analysis of the data by Baan et al. (1993). In Figure 6 (upper panel) we show the IRAS flux-difference (color-color) ratio ¼ (F100 m F60 m )=(F60 m F25 m ) versus heliocentric velocity for all sources in this survey. The sources observed cover a redshift range of z ¼ 0 to z ¼ 0:16. The position of observed galaxies in this diagram is indicated by square symbols. Galaxies with H2CO emission are high-
Fig. 4.—H2CO (upper panel ), H i (middle panel ), and OH–1.66 GHz (lower panel ) spectra toward NGC 660. The reference frequency of the OH spectrum is 1.667 GHz.
lighted with four-pointed star symbols, whereas galaxies with H2CO absorption are marked with a cross. The filled squares represent those spiral galaxies with polar inclination angles above 70, i.e., edge-on galaxies, according to the HYPERLEDA catalog.5 This figure shows that the galaxies with H2CO absorption detected in our survey are nearby (VHel < 5000 km s1) edge-on galaxies6 characterized by 0 < < 1:5. In addition, the three H2CO absorbers are the only edge-on spiral galaxies in our sample with F60 m > 30 Jy. In the inset of Figure 6 (upper panel ), we show from our sample only those edge-on galaxies with 0 < < 1:5 and VHel < 5000 km s1. It should be noted that only the three H2CO absorbers detected in our survey ( NGC 520, NGC 660, and NGC 3628) are edge-on galaxies within this parameter region. In the inset, we also included (triangles) the galaxies with H2CO 5 CRAL Observatoire de Lyon mirror: http://www-obs.univ-lyon1.fr/ hypercat /. 6 The type and inclination of the galaxies is known for all observed galaxies with VHel < 10; 000 km s1 with the sole exception of IR 17468+1320.
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Fig. 5.—H2CO spectra of NGC 3628, IC 860, IRAS 15107+0724, and Arp 220. Artifacts in the spectra caused by RFI are marked with a plus sign.
6 cm absorption detected or tentatively detected reported in the literature.7 As it is shown in the inset of the upper panel Figure 6, all other detections and tentative detections of extragalactic H2CO 6 cm absorption also lie within the region defined by 0 < < 1:5 and VHel < 5000 km s1. Moreover, excluding the tentative H2CO 6 cm detection toward IRAS 17468+1320 ( Baan et al. 1993), all other H2CO absorbers included in the inset of Figure 6 are also edge-on galaxies and/or show prominent dust lanes. Baan et al. (1993) reported that H2CO emitters in their sample showed larger values of the color index log (F60 m =F100 m ) and greater 60 m luminosities than the H2CO absorbers. In Figure 6 (lower panel ) we present the 60 m luminosity versus log (F60 m =F100 m ) for the sources in our sample. Although our survey has removed a number of sources (see above), the relative distribution of emitters and absorbers in this diagram agrees with what was reported by Baan et al. (1993), i.e., H2CO emitters occur in galaxies with warmer and more luminous FIR emission. We found that the H2CO emitters are located within the IRAS color ranges 0:18 < log (F60 m =F100 m ) < 0:0 and 2:0 < log (F25 m =F60 m ) < 1:1. Three nondetections are also located within the same infrared color range (see Fig. 7, upper panel ). Plotting these six sources in a log (LFIR )8 versus F100 m diagram, we find that emitters lie on a straight line (hereafter line of detections, see Fig. 7, upper panel ). In Figure 7 (upper panel ) we explicitly mark the OH emitter galaxies with hexagons (note that NGC 2623 is an OH absorber; see Baan et al. 1993). The three H2CO emitters in our sample also show OH 7 M31 and the LMC are not included in the inset since they are special nearby cases (e.g., Baan et al. 1993 and references therein). 8 The quantity LFIR is the far-infrared luminosity of the galaxies as reported by Sanders et al. (2003).
emission. As reported by Baan et al. (1993), the formaldehyde emission detected toward these three sources covers nearly the same velocity range as for the OH detection9 and therefore the H2CO emission could arise from the same molecular region as the OH emission. Hence, if one assumes that OH and H2CO megamaser emission arise under similar circumstances and have a similar relation to the infrared characteristics of the host galaxies, then the non-H2CO detection toward the OH absorber NGC 2623 would be expected. The nondetections toward IRAS 12112+0305 and IRAS 172080014 will be discussed at the end of this subsection. 2 ) versus log (LFIR ) is shown in A diagram of ¼ log (S VHel the middle panel of Figure 7 for the three galaxies with confirmed H2CO emission. In the expression for , S is the peak flux density and VHel is the heliocentric velocity of the H2CO line. If one assumes that the distance to the H2CO emitters is given by D ¼ VHel =H0 (H0 ¼ 75 km s1Mpc1), then 10 is proportional to the H2CO peak line luminosity. Figure 7 (middle panel ) shows that, for the H2CO emitters in our survey, the galaxies with greater infrared luminosity exhibit a stronger intrinsic H2CO emission, characterized by a power law of 0.54. This tendency is consistent with the pumping mechanism of the H2CO megamasers and may be related to the infrared radiation field in a similar fashion than in the case of OH megamasers (see for example Baan et al. 1993). Also, a monotonic behavior is observed when is plotted versus the logarithm of the 6 cm continuum luminosity ( Fig. 7, lower panel ), similar to what was reported by Baan et al. (1993). If the (far-) infrared properties and the existence of OH emission were necessary and sufficient observational 9 IC 860 shows OH emission and absorption. In this case, the H2CO emission covers approximately the same velocity width as the OH absorption plus emission structure (see Baan et al. 1993).
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Fig. 6.—Infrared properties of the sources in the sample. The galaxies observed in this survey are marked with squares in both panels. Those with H2CO absorption are highlighted with crosses, whereas the sources that exhibit H2CO emission are highlighted with four-pointed stars. Upper panel: IRAS flux-difference ratio ¼ (F100 m F60 m )=(F60 m F25 m ) as a function of heliocentric velocity (redshifts from z ¼ 0 to z ¼ 0:16). Spiral galaxies with inclination angles above 70 are marked with filled squares. The morphology and inclination of the galaxies is known for all sources in our sample with VHel < 10; 000 km s1, with the sole exception of IR 17468+1320. Upper panel inset: We highlight that all edge-on galaxies in our sample that fulfill the conditions 0 < < 1:5 and VHel < 5000 km s1 are H2CO 6 cm absorbers. With the exception of M31 and the LMC, all other H2CO absorbers reported in the literature (triangles) also fulfill the same conditions. [(1) NGC 3628; (2) NGC 4945; (3) NGC 3079; (4) NGC 5128; (5) NGC 660; (6) NGC 520; (7) NGC 253; (8) IR 17468+1320; (9) M82]. Lower panel: Logarithm of L60 m vs. the color index log (F60 m =F100 m ). As reported by Baan et al. (1993) the H2CO emitters are located toward the upper right in this diagram with respect to the H2CO absorbers.
characteristics to find H2CO 6 cm emission in galaxies, then one would also expect formaldehyde emission for IRAS 12112+0305 and IRAS 172080014. However, these galaxies are located in the upper left part of the log (LFIR ) F100 m diagram, i.e., above the line of detections ( Fig. 7, upper panel ). Therefore, they suffer from stronger geometric dilution effects in comparison with Arp 220, which has a similar luminosity but is located at a smaller distance (i.e., it lies along the line of detections). In fact, using the empirical versus LFIR relation ( Fig. 7, middle panel ) and assuming that such a relation holds for IRAS 12112+0305 and IRAS 172080014, one obtains that the formaldehyde emission expected from these galaxies is indeed below our 3 detection limit. We rechecked the H2CO spectra of these two sources and found weak ( 30 Jy. We confirmed H2CO 6 cm emission toward IC 860, IR 15107+0724, and Arp 220. At present, these three galaxies are the only extragalactic H2CO 6 cm emitters with independent confirmation. We find that the emitters have IRAS colors 0:18 < log (F60 m =F100 m ) < 0 and 2 < log (F25 m =F60 m ) 8 ; 3:88; za 8 ; GPolB (za) ¼ ðA2Þ 5:15 0:16za; za > 8 ; where G is the telescope gain in K Jy1, za is the zenith angle in degrees, and ‘‘PolA’’ and ‘‘PolB’’ are the linear polarizations A and B. Setup II.—This setup was used only for NGC 3628 and IC 860. We used observations of B1040+123 (S ¼ 1:80 Jy at 6 cm) 11 to calibrate the NGC 3628 spectra. The declination of B1040+123 is within 1 from that of NGC 3628. The telescope gain was parameterized as follows: 2
0:033za
;
ðA3Þ
2
0:035za
:
ðA4Þ
GPolA (za) ¼ 6:70 e0:0049za GPolB (za) ¼ 6:00 e0:0046za
For calibration of the IC 860 data we used equations (A1) and (A2) above, because they were derived from observations of B1615+212, which is within 3 from the declination of IC 860 and hence covers the same projected path on the main reflector of the Arecibo Telescope during the observations. We note that the Arecibo telescope gain at 6 cm is mainly characterized by instrumental effects, e.g., the position of the Gregorian dome during the observations, and not by atmospheric effects. Moreover, no primary surface adjustments of the main reflector were made between the observations using setups I and II. 11
AO standard calibrators list.
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Setup III, IV, and V.—These setups were used for the observations of most sources. Eighteen calibrators were observed giving a total of 30 measurements of the telescope gain at different elevations and zenith angles. This data allowed a characterization of the telescope gain as a function of both zenith angle (za) and azimuth (az): 8 za 6N8; > < E1 (za); ðA5Þ G(za; az) ¼ E2 (za); 10 za > 6N8; > : E2 (za)A(az); za >10 ; where az is the dome azimuth in degrees and E1 (za) ¼ 0:025za þ 8:90; 3
2
ðA6Þ
E2 (za) ¼ 0:0031za þ 0:12za 1:58za þ 15:03;
ðA7Þ
2 2 2 A(az) ¼ 1 0:20e(4 ln =35 )(az150) :
ðA8Þ
The function A(az) describes a region of particularly low gain in the northwest sky due to a deformation of the primary reflector in the southeast. Setup OH and H i.—The calibrator B0124+189 was used to obtain the gain values at 1415 MHz (10.8 K Jy1) and at 1.66 GHz (7.8 K Jy1). These constant gain values were used to calibrate NGC 660 (OH and H i) and NGC 604 (OH). The calibration of NGC 520 (OH and H i) was established relative to the flux density reported by Baan et al. (1985) because a reliable measurement of the system temperature was not obtained during the observations toward this source, hence the standard calibration procedure was not applicable. REFERENCES Anantharamaiah, K. R., Zhao, J., Goss, W. M., & Viallefond, F. 1993, ApJ, Kewley, L. J., Geller, M. J., Jansen, R. A., & Dopita, M. A. 2002, ApJ, 419, 585 124, 3135 Araya, E., Hofner, P., Churchwell, E., & Kurtz, S. 2002a, ApJS, 138, 63 Kim, M., Kim, E., Lee, M. G., Sarajedini, A., & Geisler, D. 2002, AJ, 123, 244 Araya, E., Hofner, P., Churchwell, E., Sewilo, M., Watson, C., Linz, H., & Kukolich, S. G. 1975, J. Am. Chem. Soc., 97, 5704 Kurtz, S. 2002b, BAAS, 201.7709A Lilley, A. E., & Palmer, P. 1968, ApJS, 16, 143 Araya, E., et al. 2004, ApJL, submitted Mezger, P. G., & Schraml, J. 1969, ApJ, 150, 807 Baan, W. A. 1985, Nature, 315, 26 Mohan, N., Anantharamaiah, K. R., & Goss, W. M. 2001, ApJ, 557, 659 ———. 1989, ApJ, 338, 804 Palmer, P., Zuckerman, B., Buhl, D., & Snyder, L. E. 1969, ApJ, 156, L147 Baan, W. A., Bragg, A. E., Henkel, C., & Wilson, T. 1997, ApJ, 491, 134 Phookun, B., Anantharamaiah, K., R., & Goss, W. M. 1998, MNRAS, 295, 156 Baan, W. A., & Goss, W. M. 1992, ApJ, 385, 188 Pratap, P., Menten, K. M., & Snyder, L. E. 1994, ApJ, 430, L129 Baan, W. A., Gu¨sten, R., & Haschick, A. D. 1986, ApJ, 305, 830 Rodriguez-Rico, C. A., Goss, W. M., & Zhao, J. H. 2002, BAAS, 200.4508 Baan, W. A., & Haschick, A. D. 1995, ApJ, 454, 745 Rohlfs, K., & Wilson, T. L. 2000, Tools of Radio Astronomy (3rd ed.; Berlin: Baan, W. A., Haschick, A. D., Buckley, D., & Schmelz, T. 1985, ApJ, 293, 394 Springer) Baan, W. A., Haschick, A. D., & Uglesich, R. 1993, ApJ, 415, 140 Rots, A. H., Dickel, H. R., Forster, J. R., & Goss, W. M. 1981, ApJ, 245, L15 Baan, W. A., Henkel, C., Schilke, P., Mauersberger, R., & Gu¨sten, R. 1990, Sanders, D. B., Mazzarella, J. M., Kim, D.-C., Surace, J. A., & Soifer, B. T. ApJ, 353, 132 2003, AJ, 126, 1607 Churchwell, E., & Goss, W. M. 1999, ApJ, 514, 188 Sanders, D. B., Scoville, N. Z., Sargent, A. I., & Soifer, B. T. 1998, ApJ, Cohen, R. J., Few, R. W., & Booth, R. S. 1979, MNRAS, 187, 35 324, L55 Condon, et al. 1998, AJ, 115, 1693 Scoville, N. Z., et al. 1998, ApJ, 492, L107 Downes, D., Wilson, T. L., Bieging, J., & Wink, J. 1980, A&AS, 40, 379 Seaquist, E. R., & Bell, M. B. 1990, ApJ, 364, 94 Gardner, F. F., & Whiteoak, J. B. 1974, Nature, 247, 526 Shaver, P. A. 1978, A&A, 68, 97 ———. 1976a, MNRAS, 175, 9P Stanford, S. A. 1990, ApJ, 358, 153 ———. 1976b, Proc. Astron. Soc. Australia, 3, 36 Takano, S., Nakai, N., & Kawaguchi, K. 1995, PASJ, 47, 801 Graham, A. A., Emerson, D. T., Weiler, K. W., Wielebinski, R., & de Jager, G. Tan, J. 2000, ApJ, 536, 173 1978, A&A, 70, L69 ter Meulen, J. J., & Dymanus, A. 1972, ApJ, 172, L21 Gregory, P. C., Scott, W. K., Douglas, K., & Condon, J. J. 1996, ApJS, 103, Watson, C., Araya, E., Sewilo, M., Churchwell, E., Hofner, P., & Kurtz, S. 427 2003, ApJ, 587, 714 Henkel, C., & Mauersberger, R. 1992, IAU Symp. 150, Astrochemistry of Whiteoak, J. E., & Gardner, F. F. 1983, MNRAS, 205, 27 Cosmic Phenomena, ed. P. D. Singh (Dordrecht: Kluwer), 111 Zhao, J. H., Anantharamaiah, K. R., Goss, W. M., & Viallefond, F. 1996, ApJ, Henkel, C., & Wilson, T. L. 1990, A&A, 229, 431 472, 54 Hughes, S. M. G., et al. 1998, ApJ, 501, 32 Zuckerman, B., Palmer, P., & Rickard, L. J. 1975, ApJ, 197, 571
