EDP Sciences
Free Access
Issue
A&A
Volume 532, August 2011
Article Number A127
Number of page(s) 13
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201116506
Published online 04 August 2011

© ESO, 2011

1. Introduction

Young massive stars and their H ii regions are good tracers of the spiral structure of the Galaxy (Churchwell 2002). Their distances are therefore fundamentally important to outline the spiral structure of our own Galaxy. Accurate distance measurements are also needed to constrain the physical properties of H ii regions such as size, bolometric luminosity, and mass. However, optical methods of distance measurement are very difficult to apply because H ii regions are heavily obscured by interstellar dust. An alternative approach is to calculate the kinematic distances of H ii regions based on radio emission, which is not attenuated by intervening dust, such as recombination lines or molecular lines.

Kinematic distances are calculated using measured radial velocities and the rotation model of the Galaxy, if the H ii region is located in the outer Galaxy (outside the solar circle). However, if the H ii region is located in the inner Galaxy (inside the solar circle), there are two possible interpretations of the data. Our line of sight typically intersects the circular orbit of the H ii region at two points, called the near and far distances. The object has the same radial velocity at both points on its orbit, therefore, we cannot uniquely determine the location of an H ii region in the inner galaxy with radial velocity data alone. This degeneracy is referred to as the kinematic distance ambiguity (KDA). Only sources located at a tangent point will have unambiguous distances (Sewilo et al. 2004).

The method of employing H2CO absorption lines to resolve the KDA was first used by Wilson (1972). Downes et al. (1980) resolved the KDAs of 262 H ii regions by observing H2CO absorption lines with the Effelsberg 100-m telescope. More recently, Araya et al. (2002), Watson et al. (2003), and Sewilo et al. (2004) used H2CO absorption lines to resolve the KDAs of 109 sources using the 305 m Arecibo telescope and the NRAO Green Bank Telescope (GBT), respectively. Other spectral lines can be used to find the correct kinematic distance, Kolpak et al. (2003) and Fish et al. (2003) used the Very Large Array (VLA) to observe the 21 cm H I absorption spectrum and successfully obtained the kinematic distances of 49 and 20 H ii regions. Anderson & Bania (2009) resolved KDAs of 266 H ii regions using existing HI and 13CO survey data.

Compared with the distance derived from trigonometric parallaxes of massive star-formation regions, the errors of kinematic distance are larger. But determining kinematic distance is relatively easy, it is still widely used by many authors in recent years (Anderson & Bania 2009; Hou et al. 2009).

Continuing this tradition of research on the structure of our Galaxy, our study determines kinematic distances to more H ii regions. We have measured H2CO absorption lines and H110α radio recombination lines (RRLs) for 251 H ii regions. Section 2 describes our sample selection criteria and observations. Section 3 describes our data reduction procedure and presents the distribution of H ii regions perpendicular to the Galactic Plane. In Sect. 4 we discuss the relationship between the flux of H2CO and infrared 100 μm flux density. Section 5 contains a brief summary of the paper and our conclusions.

2. Observations

2.1. Sample

We selected 120 sources from the IRAS Point Source catalog that satisfy the color-color criteria for UCH ii regions: log (S60   μm/S12   μm)  ≥ 1.30 and log (S25   μm/S12   μm)  ≥  0.57 (Wood & Churchwell 1989). Sixty-one sources were selected from Quireza et al. (2006) and 70 sources were selected from Lockman (1989). In addition, we required all candidate sources to be visible with the 25-m radio telescope and lie in the galactic latitude range  ≥ −30°. Some sources of the sample are located in the outer Galaxy to use our allotted observation time efficiently.

2.2. Observations

Our observations were conducted from September 2009 to August 2010 with the 25-m radio telescope located at Nanshan station (87° E, 43° N, altitude 2080 m) of the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences. The λ 6 cm receiving system was constructed at the Max-Planck-Institut für Radioastronomie (MPIfR) in Germany and installed at the telescope in August 2004. Its half power beam width (HPBW) at 4.86 GHz is 9.5′. One digital autocorrelation spectrometer with 4096 channels was installed on the back-end, the maximum bandwidth of the spectrometer is 80 MHz. The C-band cryogenic receiver system temperature was maintained at 23 K during the observation. The pointing accuracy was 15″ in all observations and the main beam efficiency was 65%.

We observed the H2CO absorption line (ν0 = 4829.6594 MHz) and the H110α RRL (ν0 = 4874.1570 MHz) simultaneously with 4940 km s-1 bandwidth, corresponding to a spectral resolution of 1.206 km s-1. Position-switching mode was adopted. A noise-diode calibration signal was used to calibrate the spectrum. The flux error was  ~ 15%; the degrees-per-flux-unit value was 0.116 K Jy-1 at 6cm wavelength. To observe more sources during the allotted observation time, we observed every source with 6 min integration time at first. We observed those sources with a signal (H110α RRLs or H2CO absorption lines) continuously until we attained a signal-to-noise ratio (S/N)  >  3. We discarded sources without a signal. The rms for 6 min integration time lies in the range of 0.23  ~  0.47 Jy, and the actual value mainly depends on weather and elevation.

All sources were observed in “broad band mode” and the center frequency was 4800 MHz with a bandwidth 600 MHz. The flux densities were determined with a “cross-scans” approach. Every cross-scan includes eight individual subscans. half of the eight subscans were performed in azimuth and the other four in elevation. A Gaussian distribution was fitted to every subscan. By using frequently observed primary calibrators such as 3C 286, 3C 48, 0951+699, and NGC 7027, we integrated our observations into the absolute flux density scale (Baars et al. 1977; Ott et al. 1994). The flux densities of 6 cm continuum are listed in Col. 6 of Table 1.

3. Results

The H110α and H2CO absorption line parameters were derived from Gaussian fits using the CLASS software package. The sources with H2CO absorption lines or H110α RRLs or both are listed in Table 1. The fit results for H110α and H2CO lines are presented in Table 2 and Table 3, respectively. The errors quoted in the tables are the formal 1σ errors of the Gaussian fits. Figure 1 displays the H2CO spectra of those H ii regions for which only H2CO absorption lines were detected. The H2CO absorption feature of IRAS 17432-2835 has a larger line width, while the NH3 (1, 1) line width of the source is about 40 km s-1 (Hüttemeister et al. 1993), which is consistent with IRAS 17432-2835 ′s location in the Galactic center (Miyazaki & Tsuboi 2000). Figure 2 displays the H110α spectra of those H ii regions for which only H110α RRLs was detected. Sources for which both lines were detected are displayed in Fig. 3.

Among 251 H ii regions observed in this survey, 28 had H110α RRLs and 59 had H2CO absorption lines. For 43 of the regions, this work represents the first such detection. Both H2CO and H110α lines were detected toward 23 H ii regions, 9 of which were previously observed by Downes et al. (1980, see Table 1. In addition, IRAS 23116+6111 was reported by Reid et al. (2009).

3.1. Kinematic distance calculation

All kinematic distances were calculated using the Galaxy rotation curve of Reid et al. (2009), with parameters R0 = 8.4 kpc, and Θ0 = 254 km s-1.

In Table 4 we report the kinematic parameters of 16 H ii regions and 20 intervening molecular clouds. These parameters are the radial velocity of the sources (VLSR), the near (Dnear) and far (Dfar) distances, the velocity of the tangent point (VTP), the distance to the tangent point (DTP), the kinematic distance measure selected as best representing the distance to the source (DLSR), the distance from the Galactic plane (z), and the distance to the Galactic center (RGC). We followed the method of Reid et al. (2009), and calculated the errors of the kinematic distances assuming an LSR velocity uncertainty of 7 km s-1. It should be noted that the beam of the Nanshan Station 25-m radio telescope is very large and has the higher probability to detect H ii region and H2CO molecular cloud in the same beam, but they may actually not lie on the same line of sight. This is especially true when we observed the crowded regions of the inner Galaxy.

To determine kinematic distances, we made the following assumptions. (1) The H2CO absorption lines are produced by clouds that lie in the Galactic disk between us and the H ii regions; (2) the entire H2CO absorption lines can be explained by the absorption of a thermal bremsstrahlung radio continuum from a single background H ii region. In other words, we assumed that only one H ii region is located within the beam.

We were able to resolve the KDA for 14 H ii regions and 20 intervening H2CO molecular clouds following the method of Sewilo et al. (2004). Two of the H ii regions are assigned to the far distance, eight are assigned at the near distance, and four are placed at the tangent point (see Table 4). For the remaining five sources, we were unable to distinguish between the near and far distances because they do not fit our classification. Among the intervening H2CO molecular clouds, three are at the tangent point, two is at the near distance, one are unclassified and one does not have assigned distances. The other fifteen molecular clouds are associated with H ii regions: eight at the near distance, one at the far distance and two at the tangent point, four are associated with H ii region. The last two are associated with H ii region IRAS 23116+6111 (NGC 7538) and IRAS 06053-0622 (Mon R2), which have an unambiguous distance, because they lie outside the solar circle.

thumbnail Fig. 4

Panel a) is the histogram of the vertical height of 121 H ii regions (including 109 UCH ii regions) with respect to the Galactic plane, the half width at half maximum is 25.4  ±  7.1 pc. Panel b) is the histogram of the vertical height of 157 H ii regions from Boston university H ii region catalog, the half width at half maximum is 28.6  ±  8.7 pc. Positive height corresponds to positive Galactic latitude. The overlaid curve shows the Gaussian fit to the data.

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The sources near longitude 0° or 180° have radial velocities that are dominated by peculiar motions rather than circular velocities (Sewilo et al. 2004; Gómez 2006). The kinematic distances derived from this radial velocity will have large error. G1.13-0.1 and IRAS 17441-2822 (Sgr B2) are near the Galactic center and their kinematic distances are uncertain. Kinematic parameters of these two sources were not listed in Table 4.

3.2. Vertical height distribution of H ii regions

Figure 4a shows the vertical height distribution of the 121 sources (109 UCH ii regions and 12 H ii regions) above and below the Galactic plane that are taken from this survey and some similar studies (Araya et al. 2002; Watson et al. 2003; Sewilo et al. 2004; Han et al. 2011). The Gaussian fit has a half width at half maximum (HWHM) of 25.4  ±  7.1 pc and is centered at  −10.2  ±  3.0 pc. Because 109 sources of this group are UCH ii regions, this indicates that UCH ii are tightly confined to the Galactic plane and that the thickness of the star-forming layer is about 25.4 pc. Figure 4b shows the vertical height distribution of the 157 H ii regions are perpendicular to the Galactic plane, and these sources are taken from the Boston University H ii Region Catalog (Anderson & Bania 2009). The HWHM is 28.6  ±  8.7 pc and the central value is  −10.2  ±  3.5 pc. This difference is probably caused by the uncertainty of distance measurement. Evidently, a much larger sample of H ii regions with determined distance is needed before we can achieve accurate thickness of H ii regions perpendicular to the Galactic plane.

We found that more sources lie below the Galactic plane than above the Galactic plane and the distribution of H ii regions centered at −10.2  ±  3.0 pc. This bias could arise because we defined a Galactic plane centered on the Sun, parallel to, but offset by 8 pc from the symmetry plane of the Galaxy. A similar bias was obtained by Fish et al. (2003), who found the distribution of H ii regions to be centered at −7.3  ±  1.4 pc.

4. Discussions

Figure 5 shows the relationship between the flux of H2CO and the infrared 100 μm flux density of UCH ii regions. For these two quantities, 137 sources were collected from the literature (Watson et al. 2003; Sewilo et al. 2004; Han et al. 2011), and 51 sources were taken from our own data. There clearly are a horizontal branch and a vertical branch in the figure.

thumbnail Fig. 5

Relationship between log (SH2CO) and log (S100   μm), the circles represent the sources detected in H110α RRLs, and solid triangles represent the sources not detected in H110α RRLs.

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The horizontal branch contains those sources with H110α RRLs and H2CO absorption lines. It shows that log (SH2CO) is linearly correlated with log (S100   μm): log (S100   μm) = (3.73  ±  0.04) + (0.45  ±  0.04)  ×  log (SH2CO) (where the flux of H2CO is expressed as its absolute value), and the correlation coefficient of this branch is 0.65. We know that the flux of H2CO is mainly determined by 6cm continuum emission of the background source (Downes et al. 1980). Therefore, the relation above indicates that there is a correlation between the continuum fluxes at 6 cm and infrared 100 μm. Even if the H ii region and H2CO molecular clouds are not in close contact, this conclusion is valid.

The vertical branch has a weak correlation, and the correlation coefficient is 0.23. All 52 sources belonging to this branch come from our sample and Han et al. (2011). Sources of this group were not detected in H110α RRLs. The 6 cm continuum emission was detected toward all these sources except for IRAS 03271+3013, IRAS 18048-2131, and IRAS 20332+4124. Maybe those three sources are too weak to be detected by our telescope. The 6 cm continuum emissions of the sources in the vertical branch are weak, corresponding main beam brightness temperatures are usually less than 1–2 K. In this case, some other mechanism may work and affect the H2CO flux, while the 6cm continuum flux is not proportional to H2CO absolute flux.

5. Summary and conclusions

Using the Nanshan Station 25-m radio telescope of Xinjiang Astronomical Observatory, we have observed the H2CO absorption lines and H110α RRLs toward 251 H ii regions. We detected H110α RRLs in 28 sources and H2CO absorption lines in 59 sources, among which 43 H2CO sources had not previously been detected. H2CO and H110α lines were simultaneously detected toward 23 H ii regions, 9 of which had been detected by Downes et al. (1980) and one of which (IRAS 23116+6111) had been reported by Reid et al. (2009). The detection rate of H2CO absorption lines in our survey is 23.5%.

Based on the method of Sewilo et al. (2004) and a modified Galactic rotation curve, we resolved the KDAs for 14 observed H ii regions and 20 intervening molecular clouds. The thickness of vertical height distribution of UCH ii regions perpendicular to the Galactic plane was found to be 25.4  ±  7.1 pc within the solar circle.

We find that there is a strong correlation between the H2CO flux and the infrared 100 μm flux density of H ii regions, log (S100μm) = (3.73  ±  0.04) + (0.45  ±  0.04)  ×  log (SH2CO). This relation suggests that there is a correlation between the continuum fluxes at 6 cm and infrared 100 μm. Those sources without H110α features only show strong H2CO absorption lines. Their 6cm continuum fluxes are weak, some mechanism different from collision may be at work even though the 6cm continuum flux is not proportional to the H2CO absolute flux.

Acknowledgments

We thank Dr. Mark Reid for providing us the Fortran code for the estimation of kinematic distances. We also are very grateful to Jun Liu for processing the 6cm continuum data. This work was funded by The National Natural Science foundation of China under grant 10873025 and 10778703, and by The Program of the Light in China’s Western Region (LCRW) under grant No. RCPY200605 and RCPY200706.

References

Online material

Table 1

The sources with H2CO absorption lines or H110α RRLs or both of them.

Table 2

H110α RRL parameters.

Table 3

H2CO line parameters.

Table 4

Kinematic parameters.

thumbnail Fig. 1

H2CO absorption lines observed toward 36 H ii regions, the source name is given at the top of each panel.

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thumbnail Fig. 2

H110α RRLs observed toward five H ii regions, the source name is given at the top of each panel.

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thumbnail Fig. 3

Spectra of H2CO absorption lines (upper panel) and H110α RRLs (lower panel) observed toward 23 H ii regions. The source name is given at the top of each panel.

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All Tables

Table 1

The sources with H2CO absorption lines or H110α RRLs or both of them.

Table 2

H110α RRL parameters.

Table 3

H2CO line parameters.

Table 4

Kinematic parameters.

All Figures

thumbnail Fig. 4

Panel a) is the histogram of the vertical height of 121 H ii regions (including 109 UCH ii regions) with respect to the Galactic plane, the half width at half maximum is 25.4  ±  7.1 pc. Panel b) is the histogram of the vertical height of 157 H ii regions from Boston university H ii region catalog, the half width at half maximum is 28.6  ±  8.7 pc. Positive height corresponds to positive Galactic latitude. The overlaid curve shows the Gaussian fit to the data.

Open with DEXTER
In the text
thumbnail Fig. 5

Relationship between log (SH2CO) and log (S100   μm), the circles represent the sources detected in H110α RRLs, and solid triangles represent the sources not detected in H110α RRLs.

Open with DEXTER
In the text
thumbnail Fig. 1

H2CO absorption lines observed toward 36 H ii regions, the source name is given at the top of each panel.

Open with DEXTER
In the text
thumbnail Fig. 2

H110α RRLs observed toward five H ii regions, the source name is given at the top of each panel.

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In the text
thumbnail Fig. 3

Spectra of H2CO absorption lines (upper panel) and H110α RRLs (lower panel) observed toward 23 H ii regions. The source name is given at the top of each panel.

Open with DEXTER
In the text

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