4 Applications

In this section we present the results of three studies that made extensive use of the IRAS-CO association table (Table 1). The purpose of this section is twofold - first it will give the reader a sense of how Table 1 can be utilized in practice, and second it presents some interesting results from our initial application of the table to some astronomical studies.

  
4.1 ZOAGs and Galactic Nebulae

The Galactic zone-of-avoidance (ZOA) is the low galactic latitude portion of the sky where extinction due to dust in our Galaxy makes the optical detection of galaxies very difficult. A common, and surprisingly productive, way to detect galaxies in this region is to visually inspect the Palomar Observatory Sky Survey (POSS) plates for non-stellar objects at low galactic latitude. As a result of such studies there now exists a large compilation of optically identified zone-of-avoidance galaxies (ZOAGs) in the literature (Weinberger et al. 1999; Seeberger & Saurer 1998, and references therein). It has been recognized though that the observed nebulosities cannot be easily differentiated from Galactic nebulosities (Weinberger et al. 1999) thus the possibility that the ZOAG identification is spurious is more likely when there is an associated IRAS source (expected for a Galactic star forming region). To investigate the occurrence of false ZOAG identifications we compiled a list of all of the ZOAGs with IRAS sources contained in the OGS survey region. From this list we looked at which of the ZOAGs had associated CO. In total there are 56 ZOAGs with IRAS associations in the OGS region. Each of the objects was investigated using SIMBAD to determine if there was any more information on the object beyond the optical identification on the POSS plates. We found that 17 of the ZOAGs had another identification; four of the ZOAGs were definitively shown to be Galactic regions and 13 of the ZOAGs were definitively shown to be extra-galactic (primarily from a measured H I velocity). This extra piece of information allowed us to calibrate the $N_{\rm E}$ values for the remaining objects. All of the known Galactic objects have $N_{\rm E} \leq 0.0028$ while all of the known extra-galactic objects have $N_{\rm E} \geq 0.0253$ (see Table 9).

Of the remaining 39 objects (see Tables 10 and 11), 12 have no CO associations and thus we conclude they are good ZOAG candidates. The remaining 27 objects with CO associations were sorted by $N_{\rm E}$. Seven of these putative ZOAGs had $N_{\rm E} \leq 0.0028$ and are probably Galactic nebulae, while 17 ZOAGs had $N_{\rm E} \geq 0.0253$ and are probably correctly identified as extragalactic objects. Three of the objects fell between these two limits defined by the known object sample. While their $N_{\rm E}$ values tend to be closer to the Galactic sample we looked for further information from other datasets to help decide if the objects were Galactic or not.

First we looked at whether or not the 56 ZOAGs were MSX Band A ($\sim$8.3 $\mu$m) sources and if they had any associated 21 cm radio continuum emission using the continuum images from the CGPS. All of the known Galactic sources were strong MSX sources with no 21 cm continuum emission except for some weak emission from the distant H II region IRAS 02421+6233. In contrast all of the known extragalactic sources have associated 21cm continuum emission and all but one (the nearby Maffei 2 galaxy) have either no or very weak MSX Band A emission. This criterion (no MSX but 21 cm continuum emission) is seen in all but one of the 17 probable extragalactic sources lending weight to the conclusion that these objects are indeed extragalactic. The pattern is also seen in most of the 12 objects that had no CO association to start with and thus were automatically assumed to be extragalactic. The three intermediate $N_{\rm E}$ sources have MSX and 21 cm continuum emission properties consistent with them being Galactic objects.

In summary, of the 56 original ZOAGs 14 (25%) of them are Galactic objects (4 previously known, 10 identified as such from the $N_{\rm E}$analysis), and 42 (75%) of them are extragalactic (13 previously known and 29 identified in this analysis).

 

 

Table 9: ZOAGs with IRAS counterparts in the OGS - known objects.

ZOAG	IRAS	BKP	$N_{\rm E}$	MSX	C21	Notes
Galactic
131.86+01.33	02071+6235	10684	0.0028	Y	N	H II Region (1)
135.63+02.77	02421+6233	10578	0.0009	Y	Y	H II Region (1)
136.39+02.27	02461+6147	7537	0.0003	Y	N	-42.4 (2)
118.97+01.89	00117+6412	6687	0.0002	Y	N	-36.2 (2)
Extragalactic with CO association
136.50-00.33	02381+5923	12354	0.4952	Y	Y	-17, Maffei 2
132.78+03.68	02217+6430	11399	0.3833	N	Y	+12145 (3)
129.83+03.28	01542+6500	11733	0.2389	N	Y	+10493 (3)
134.22+04.05	02354+6418	11680	0.1590	N	Y	+5306 (3)
136.27-01.91	02317+5801	12271	0.1267	N	Y	+5650, Wein 20 (4)
133.83+03.40	02297+6351	11872	0.1228	N	Y	+4202 (3)
138.96+02.66	03067+6055	12924	0.0921	Y(weak)	Y	+2350 (5)
127.05-02.58	01211+5946	13339	0.0663	N	Y	+17678 (3)
107.13+03.41	22287+6137	2088	0.0253	Y (weak)	Y	+3503 (6)
Extragalactic without CO association
129.64+02.58	01509+6423	...	...	N	Y	+10461 (3)
130.20+03.90	01591+6531	...	...	N	Y	+9593 (3)
135.64+02.43	02410+6215	...	...	Y (weak)	Y	+13191 (1) Seyfert 1
138.52-00.11	02530+5843	...	...	N	Y	Dwingeloo 1

(1) - Rudolph et al. (1996).

(2) - Bronfman et al. (1996), CS(2-1) LSR velocity shown.

(3) - Nakanishi et al. (1997), redshift (cz, in km s^-1) shown.

(4) - Pfleiderer et al. (1981), redshift (cz, in km s^-1) shown.

(5) - Hau et al. (1995), redshift (cz, in km s^-1) shown.

(6) - Weinberger et al. (1995), redshift (cz, in km s^-1) shown.

 

 

Table 10: ZOAGs with IRAS counterparts in the OGS - Unknown Objects - 1.

ZOAG	IRAS	BKP	$N_{\rm E}$	MSX	C21	Notes
No CO Association - Probable Extragalactic
102.24-01.89	22184+5432	...	...	N	Y	Wein 7 (1)
124.21-02.87	00585+5942	...	...	N	N	...
129.90-01.32	01455+6031	...	...	N	Y	...
131.18+03.40	02068+6445	...	...	N	N	...
131.42+05.27	02146+6627	...	...	...	Y	...
131.48+02.72	02075+6401	...	...	N	Y	...
135.90-02.25	02280+5751	...	...	N	Y	...
137.17-02.92	02348+5644	...	...	N	N	...
139.52+04.53	03189+6213	...	...	N	Y	...
141.52+02.97	03256+5949	...	...	N	Y	...
141.85+04.95	03370+6115	...	...	N	Y	...
141.96-02.65	03061+5450	...	...	N	Y	...
CO Association - Probable extragalactic from $N_{\rm E}$ value
134.02+04.67	02359+6457	11606	0.5307	N	Y	...
131.32+04.61	02115+6552	11749	0.4935	N	Y	...
140.41-00.39	03046+5733	12063	0.4813	N	Y	...
133.19+02.76	02222+6330	11422	0.4493	N	Y	...
125.95+02.72	01170+6509	11765	0.3921	N	Y	...
123.99+04.29	00590+6652	11301	0.3444	N	Y	...
129.98+04.96	01599+6636	11216	0.3042	N	Y	...
133.39+04.10	02283+6440	11345	0.2862	N	Y	...
129.93+03.60	01559+6518	11697	0.2528	N	Y	...
139.30+04.83	03188+6236	12987	0.2410	N	Y	...
125.73-02.19	01111+6018	13334	0.2012	N	N	...
138.24+04.06	03074+6230	13181	0.1748	N	Y	...
121.84-02.76	00397+5949	5954	0.1532	N	Y	...
115.43+05.10	23319+6633	11157	0.1418	N	Y	...
126.16-00.37	01159+6203	13272	0.0740	Y (weak)	Y	...
135.92-01.66	02299+5823	12277	0.0563	N	Y	...
138.62-00.86	02510+5759	7866	0.0338	N	Y	...

(1) - Pfleiderer et al. (1981).

 

 

Table 11: ZOAGs with IRAS counterparts in the OGS - Unknown Objects - 2.

ZOAG	IRAS	BKP	$N_{\rm E}$	MSX	C21	Notes
CO Association - Probable Galactic from $N_{\rm E}$ value
130.17+00.49	01511+6213	8175	0.0140	Y	N	...
116.77+01.56	23527+6328	9536	0.0089	Y	N	Wein 8 (1)
137.06+03.12	02546+6214	8779	0.0050	Y (weak)	Y (weak)	...
CO Association - Galactic from $N_{\rm E}$ value
139.97+02.59	03134+6021	8437	0.0027	Y	N	...
134.27-01.90	02175+5845	8120	0.0026	Y	N	...
130.29+01.65	01546+6319	9296	0.0023	Y	Y (weak)	Wein 17 (1)
118.44+01.25	00080+6329	7063	0.0019	Y	N	Wein 9 (1)
133.88+02.53	02272+6302	6403	0.0009	Y	N	...
118.63-00.40	00117+6153	8501	0.0005	Y	N
137.24+05.36	03054+6407	6073	0.0002	...	N	...

(1) - Pfleiderer et al. (1981).

  
4.2 Far outer Galaxy CO

The study of molecular clouds and star formation in the far outer Galaxy is intriguing as it allows us to examine a region of the Galaxy with different properties (metallicity, pressure) than our local region. It is not within the scope of this paper to perform a detailed analysis of all of the far outer Galaxy molecular clouds within the OGS so for purposes of illustration we will restrict the analysis to those clouds with $V_{\rm lsr} < -100$ km s^-1. Over the range of the OGS this corresponds to Galactocentric distances ranging from >20 kpc at the high longitude end to $\sim$16 kpc at the low longitude end (e.g., see Fig. 4 of Heyer et al. 1998). We find a total of 24 BKP clouds meeting this criteria, five of which have associated IRAS sources. Included in this sample are Clouds 1 and 2 of Digel et al. (1994) and the molecular cloud associated with WB89 288, mapped in various CO lines by Brand & Wouterloot (1994). In Table 12 we list the basic properties of the clouds and Fig. 12 shows some representative images of the BKP clouds. Most of the more massive clouds in our sample have associated IRAS sources and thus have been previously investigated; however, BKP 10873 appears to be a previously unexamined large far outer Galaxy molecular cloud with $M \sim 10^4~M_\odot$. There is clearly sufficient mass in this object that star-formation is certainly possible and BKP 10873 would make a worthwhile target for further higher resolution and sensitivity observations in both the millimeter and infrared.

 

 

Table 12: Far outer Galaxy CO clouds with $V_{\rm lsr} < -100$ km s^-1.

BKP	$N_{{\rm pix}}$a	lb	bb	$V_{\rm lsr}^{b}$	$T_{\rm p}$	$R_\odot$c	$R_{\rm G}$c	Md	Notes
		$(\hbox{$^\circ$ })$	$(\hbox{$^\circ$ })$	(km s-1)	(K)	(kpc)	(kpc)	(103 $M_\odot$)
10872	86	114.338	0.780	-101.22	3.54	11.8	17.15	2	IRAS 23338+6207 WB89 288
10873	37	114.212	0.836	-102.05	1.83	12.0	17.32	10
10877	5	131.162	1.394	-101.22	1.09	15.3	21.85	0.2	D94 Cloud 1
10878	27	109.288	2.078	-101.22	1.72	11.7	16.67	0.7
10879	5	107.726	2.943	-101.05	1.01	11.8	16.51	0.1
10880	6	105.243	3.012	-100.22	1.02	11.7	16.17	0.1
10881	10	137.286	-1.159	-101.22	1.21	19.7	26.58$^{\rm e}$	0.6	assoc. with D94 Cloud 2
10882	7	114.282	0.655	-101.22	1.13	11.9	17.24	0.2
10883	7	123.489	1.478	-102.05	1.04	13.1	19.15	0.2
10884	4	121.690	2.050	-101.22	1.01	12.6	18.54	0.1
10885	74	137.760	-0.963	-103.70	2.83	21.5	28.37$^{\rm e}$	8	D94 Cloud 2, IRAS 02450+5816 = YSO $\sim$ B star (KT00)
10886	27	137.774	-1.061	-102.87	1.83	20.9	27.79$^{\rm e}$	2	D94 Cloud 2, IRAS 02447+5811 = PDR (KT00)
10887	4	122.289	1.687	-102.87	1.01	13.0	18.96	0.1
10888	18	122.373	1.771	-102.87	1.14	13.0	18.96	0.5
10889	16	104.991	3.305	-102.87	1.97	12.1	16.49	0.5	IRAS 22143+6023
10890	34	123.364	1.659	-103.70	1.37	13.4	19.42	1
10891	6	122.345	1.924	-102.87	0.98	13.2	19.15	0.2
10892	36	121.815	3.054	-104.52	1.35	13.2	19.10	1
10893	6	122.736	2.371	-104.52	1.01	13.6	19.55	0.2
10894	5	122.750	2.454	-104.52	1.02	13.5	19.46	0.1
10895	13	122.778	2.524	-106.99	1.91	14.3	20.21	0.6
10896	8	117.588	3.961	-106.17	1.16	13.1	18.63	0.2
10897	4	118.146	3.417	-106.99	0.96	13.4	18.96	0.1
10898	9	116.723	3.543	-107.82	1.27	13.3	18.73	0.3	IRAS 23482+6524

^a Total number of lbv pixels in the cloud.

^b Refers to CO cloud peak.

^c Kinematic distances using $R_\odot = 8.5$ kpc and $V_\odot = 220$ km s^-1.

^d Calculated using $X=1.9 \times 10^{20}$ (Strong & Mattox 1996).

^e Smartt et al. (1996) suggest 15 kpc < $R_{\rm G} < 19$ kpc.

D94 - Digel et al. (1994); KT00 - Kobayashi & Tokunaga (2000).

Of particular interest are two distant CO clouds that have associated IRAS sources and have not been previously studied (see Fig. 12). IRAS 22143+6023 is associated with BKP 10889 and has a kinematically derived Galactocentric distance of $R_{\rm G} = 16.49$ kpc. The IRAS-CO association is very good with $N_{\rm E} = 0.0009$, and the cloud is the only significant CO emission along this line of sight. The IRAS flux densities are low as one would expect for a distant source: 0.22, 0.25, 1.45 and 14.27 Jy at 12, 25, 60 and 100 $\mu$m respectively. While there are upper limits at 25 and 100 $\mu$m, the 12 and 60 $\mu$m flux densities do indicate that the spectrum is rising into the far-infrared. We thus identify IRAS 22143+6023 as a new far outer Galaxy star forming region.

Figure 12: Far outer Galaxy molecular clouds with and without IRAS sources. Each panel shows CO emission at the peak emission channel of the cloud (indicated in parentheses above each panel). Contours are also CO emission at the following levels: BKP 10890 - 1, 2, and 3 K; BKP 10873 - 2, 4, and 6 K; BKP 10898 - 1 and 2 K; BKP 10889 - 1, 2, and 4 K. In the two right-hand panels the crosses indicate the position of the associated IRAS source: IRAS 23482+6524 for BKP 10898, and IRAS 22143+6023 for BKP 10889.

IRAS 23482+6524 is another possible far outer Galaxy star forming region. In this case there are no IRAS flux density upper limits and the spectrum is rising through to 100 $\mu$m: 0.37, 0.35, 4.04, and 32.16 Jy at 12, 25, 60 and 100 $\mu$m respectively. The association with BKP 10898 has an $N_{\rm E}$ of 0.0035. There is also another possible CO association at -60 km s^-1 but with a very high $N_{\rm E} = 0.1601$.

In brief, our initial exploration of the BKP catalog and the IRAS-CO association table has led to the discovery of two new candidate far outer Galaxy star forming regions and a large distant molecular cloud that shows no signs of massive star formation.

  
4.3 Bright CO clouds with no IRAS sources

An interesting question one can ask is whether or not there are any very bright CO clouds in the survey region that do not have any associated IRAS sources and thus are not currently associated with star formation. For this example, we define a "very bright'' CO cloud as one having a peak CO brightness temperature $T_{\rm p} \geq 10$ K. This is a very small subset of the CO clouds we find in the outer Galaxy. Of the 14592 BKP CO clouds only 120 (0.8%) have $T_{\rm p} \geq 10$ K and, of these, 114 (95%) have IRAS associations at the $\rm rank=1$ (111 clouds) or $\rm rank=2$ (three clouds) level. This is clearly a special population as only 28% of the entire BKP sample have associated IRAS sources at any rank. The properties of the six very bright clouds with no IRAS sources are tabulated in Table 13. The cloud masses range from a few hundred to a few thousand $M_\odot$.

A careful examination of the positions of these sources reveals that they are not randomly distributed in the OGS region, rather we find that these objects are found at the edges of H II regions and are thus associated with PDRs. BKP 7782 is found at the edge of the large W4 continuum loop, BKP 11016 and 11018 are both found at the edge of the western part of Sh 2-171, BKP 13900 is on the edge of Sh 2-159, BKP 12547 is near Sh 2-155 and BKP 12127 is near Sh 2-140 (FG S140 18) (Falgarone & Gilmore 1991). High resolution ($\sim$20'') MSX Band A images were inspected to search for any signs of point-like infrared emission. In all six cases none was found; rather, clearly elongated structures associated with the ionized-molecular gas interface were seen. We suggest that these six clouds are CO clouds that are being externally heated by the nearby H II region and do not have an internal source of heat as would be suggested by the presence of an IRAS point source. We note that if $T_{\rm p}$ is substantially raised in this manner then the masses in Table 13 will be overestimates of the true mass. The location of these clouds, at the edge of various H II regions, makes them ideal targets to examine the earliest stages of star formation induced by the expansion of H II regions.