Vol 17, No 5 (2017) / Hou

Ionized gas clouds near the Sagittarius Arm tangent

Ionized gas clouds near the Sagittarius Arm tangent

Hou Li-Gang1, , Dong Jian2, Gao Xu-Yang1, Han Jin-Lin1

National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China

† Corresponding author. E-mail: lghou@nao.cas.cn

Abstract: Abstract

Radio recombination lines (RRLs) are the best tracers of ionized gas. Simultaneous observations of multi-transitions of RRLs can significantly improve survey sensitivity. We conducted pilot RRL observations near the Sagittarius Arm tangent by using the 65-m Shanghai Tian Ma Radio Telescope (TMRT) equipped with broadband feeds and a digital backend. Six hydrogen RRLs (H96 H101α) at C band (6289 MHz–7319 MHz) were observed simultaneously toward a sky area of 2° × 1.2° by using on-the-fly mapping mode. These transitions were then stacked together for detection of ionized gas. Star forming complexes G48.6+0.1 and G49.5−0.3 were detected in the integrated intensity map. We found agreements between our measured centroid velocities and previous results for the 21 known HII regions in the mapped area. For more than 80 cataloged HII region candidates without previous RRL measurements, we obtained new RRL spectra at 30 targeted positions. In addition, we detected 25 new discrete RRL sources with spectral S/N σ, and they were not listed in the catalogs of previously known HII regions. The distances for 44 out of these 55 new RRL sources were estimated.

Keywords: Galaxy: disk;radio lines: ISM;ISM: HII regions

1 Introduction

The interstellar medium (ISM) consists of ionized gas, atomic and molecular clouds, as well as dust, cosmic rays and magnetic fields. Among them, the ionized component generally exists in three forms: individual HII regions, warm ionized gas and hot ionized gas, accounting for 20% – 50% of the total gas mass (Lequeux 2005 ; Ferrière et al. 2007 ). Radio recombination lines (RRLs) are relatively free of interstellar absorption and extinction. They are the best tracers for ionized gas because of their well-understood physics (Gordon & Sorochenko 2009 ), the line density of RRL spectra (see Thompson et al. 2015 ) and their inherent information about dynamics. There has been a wealth of surveys on interstellar atomic gas such as the HI 21cm line (e.g., Kalberla et al. 2005 , McClure-Griffiths et al. 2005 ) and molecular gas by analyzing CO (e.g., Dame et al. 2001 , Dempsey et al. 2013 ; Rigby et al. 2016 ). But to better understand the global properties of ionized gas in our Galaxy, a great deal of effort in observations at different radio frequencies is necessary (Gordon & Sorochenko 2009 ; Thompson et al. 2015 ).

Table 1 shows most surveys were at frequencies below 1.7 GHz, where is small because the Stark broadening shifts radiation energy from the line core to the line wings and the free-free continuum emission transforms from optically thin to optically thick (Gordon & Sorochenko 2009 ). Non-local thermodynamic equilibrium (LTE) effects make it not straightforward to get reliable estimates of physical parameters, e.g., electron temperature and electron density . The existing RRL surveys have not covered the entire Galactic plane, especially in the first and second Galactic quadrants. The main limitation of RRL observations is the intrinsic faintness ( a few mJy to tens of mJy, Lockman 1989 ; Anderson et al. 2011 ) at high quantum number transitions, e.g., quantum number , so that it is very time consuming to conduct a well-sampled and sensitive survey with a single RRL. Many previous papers focused on the discovery of individual bright HII regions by pointing observations of RRLs (e.g., Downes et al. 1980 ; Caswell & Haynes 1987 ; Lockman et al. 1996 ; Anderson et al. 2011 ; Bania et al. 2012 ; Du et al. 2011 ; Han et al. 2011 ).

Reference/Telescope RRLs Rest Freq. (MHz) HPBW Longitude coverage Latitude coverage Sampling
(1) (2) (3) (4) (5) (6) (7)
gc72: Onsala 26-m H157α 1683.2 9.4° − 130° separation
hp76: Mark II 38.1-m×25.4-m H166α 1424.7 5° − 70° separation /1°/2°
lock76: NRAO 43-m H166α 1424.7 358° − 0° − 50.5° separation
cahc89,cahc90: IAR 30-m H166α 1424.7 298° − 0° − 4° separation
hrk96: HCRO 26-m, NRAO 43-m H165α/167α 1350.4–1683.2 357° − 0° − 254.4° 583 positions
or H157α/158α or
acwc97: NARO 43-m H166α 1424.7 60° − 90° / separation
−4° − +5° separation
ra00, ra01: ORT 530-m×30-m H270α−273α 321.4 – 332.2 2° × 2° 332°−0°−89° separation
172°−252° 14 positions
cmf+09: Parkes 64-m H166α 1424.7 267°−302° −3.0° – +1.5° Nyquist
lmt+13: Arecibo 300-m, ongoing H163 H174α 1237.6 – 1504.6 30° – 75° Nyquist
175° – 207° −2°−+1°
acd+15: Parkes 64-m H166α-H168α 1374.6 – 1424.7 196° – 0° – 52° Nyquist
bbo+15: VLA 19 Hα RRLs 1000 – 2000 15° – 67° Nyquist

Notes: Column (1) gives the reference(s) and the telescope(s) used to conduct the survey; Col. (2) lists the observed RRLs and the corresponding line frequencies in MHz are given in Col. (3); Col. (4) is the half power beamwidth (HPBW); Cols. (5) and (6) are the Galactic longitude and latitude coverage of the survey; Col. (7) is a note about the sampling, in which “Nyquist” means that the mapping observations are Nyquist sampled. References: acd+15: Alves et al. ( 2015 ); acwc97: Azcárate et al. ( 1997 ); cahc89: Cersosimo et al. ( 1989 ); cahc90: Cersosimo ( 1990 ); cmf+09: Cersosimo et al. ( 2009 ); gc72: Gordon & Cato ( 1972 ); hp76: Hart & Pedlar ( 1976 ); hrk96: Heiles et al. ( 1996 ); lmt+13: Liu et al. ( 2013 ); lock76: Lockman ( 1976 ); ra00: Roshi & Anantharamaiah ( 2000 ); ra01: Roshi & Anantharamaiah ( 2001 ); bbo+15: Bihr et al. ( 2015 ).

Note: “†” indicates that it is a synthesized beam size.

Table 1 Previous RRL Surveys toward the Galactic Plane

With broadband feeds/receivers and a digital backend, simultaneous observations of multi-transitions of RRLs can be easily stacked to improve the survey sensitivity for ionized gas. The method has been successfully applied in searching for weak individual HII regions (e.g., Anderson et al. 2011 ; Bania et al. 2012 ) and blind RRL surveys at frequencies (e.g., Alves et al. 2015 ). For the RRLs with frequencies above 3 GHz, the observed is very close to the LTE values (Gordon & Sorochenko 2009 ), so that it is straightforward to estimate and together with radio continuum data. At present, well-sampled and sensitive surveys of higher frequency RRLs ( ) have become feasible and can be complementary to existing multi-wavelength surveys toward our Galaxy (e.g., Kalberla et al. 2005 ; Dame et al. 2001 ; Alves et al. 2015 ). The blind survey data of RRLs would be valuable in detecting new HII regions, mapping the structure of ionized gas clouds in the Milky Way and studying their properties. The newly constructed 65-m Shanghai Tian Ma Radio Telescope (TMRT) was equipped with broadband feeds/receivers and a digital backend, which enables us to conduct an RRL survey toward the northern Galactic plane. Here, we report a pilot observation near the Sagittarius Arm tangent.

2 Observations and Data Reduction

Observations were conducted from 2015 August 31 to 2015 September 2 by using the TMRT, which is located in the western suburbs of Shanghai. The C band receiver (4.0–8.0 GHz) and mode 2 1 of the digital backend system (DIBAS) were used for observations in a spectral window at a central frequency of 6660 MHz with a bandwidth of 1500 MHz. The number of channels was set to be 16384, resulting in a spectral resolution of 91.553 kHz.

The observation parameters are summarized in Table 2. The intensity is calibrated by injection of periodic noise with an accuracy of about 20% (Li et al. 2016 ). The main beam efficiency is 0.6 (Wang et al. 2015 ; Li et al. 2016 ). The conversion factor of the main beam temperature into flux density is 0.9 Jy K−1. The targeted area for the pilot survey covers the Galactic longitude range of and Galactic latitude range of near the Sagittarius Arm tangent ( , Hou & Han 2015 ). We scanned the sky area with the on-the-fly mode along the Galactic longitude and Galactic latitude directions with a speed of . Spectra were recorded every 0.8 s at intervals of . The integration time was about 10 s per pixel to achieve a sensitivity of (corresponding to a flux density of ) with a velocity resolution of about 4.4 km s−1 for the pilot observation. The off-source points were observed every five scan rows and used to calibrate the on-source observations in each scan row. We checked available data from the 1.4 GHz RRL survey (Alves et al. 2015 ) and the radio continuum survey (Stil et al. 2006 ) to ensure no RRLs have been detected in the off-source points. During observations, the continuum emission in the line-free channels were also recorded. However, we found that the continuum map suffered a serious scanning effect, i.e., having many broad and zonal emission features as shown both in the longitude and latitude scanning directions, which may be caused by radio frequency interference and/or gain variations, which cannot be removed with high confidence. The standing wave problem and the separation of non-thermal and thermal continuum emissions (e.g., Xu et al. 2013 ) make the continuum data reduction more complex. In comparison, the influence of the scanning effect and the standing wave can be easily removed for RRL data by a base-line fitting. In this work, we only focus on RRL data.

RRLs: Rest Freq. (MHz) Beam FWHM ( ) Vel. resolution (km s−1) ( , mK)
H101α 6289.144 2.57 4.4 51
H100α 6478.760 2.50 4.2 16
H99α 6676.076 2.42 4.1 16
H98α 6881.486 2.35 4.0 18
H97α 7095.411 2.27 3.9 16
H96α 7318.296 2.20 3.8 13
stacked 2.67 4.4 9
Polarizations Dual circular
Longitude coverage
Latitude coverage
Gridded beam FWHM
Gridded pixel size

Notes: “ ” means spectral rms noise for the mapped region. The spectral rms noise was estimated by using data in the line-free channels of the extracted spectrum for each gridded pixel; “ ” corresponds to the frequency of the H101α line.

Table 2 Summary of RRL Survey Parameters by the TMRT

The bandpass corrected spectra were processed by using the GILDAS software package 2 . For each of the six observed hydrogen RRLs (see Table 2), we first extracted their spectra in the velocity range ( ) from −80 km s−1 to +150 km s−1 (see Fig. 1), and then inspected possible radio frequency interference. The data were gridded using MAP in the GILDAS/CLASS software package, with a beam size of and pixel size of . A third degree polynomial base-line was usually used to fit each extracted RRL spectrum in the line-free channels of the range from −80 km s−1 to +150 km s−1 for subtraction. Examples of the observed RRL spectra for the HII region G49.204–0.345 are given in Figure 1. The signal to noise ratio (S/N) of the H101α spectrum is low, because it (6289.144 MHz) is close to the boundary of the effective bandpass ( 6300 MHz −7400 MHz).

Fig. 1 Examples of RRL (H , H , H , H , H and H ) spectra toward the HII region G49.204–0.345 with Gaussian fits for deriving line parameters. The stacked spectrum is shown in the bottom panel. The centroid velocities are marked by vertical dashed lines. The σ shown in each panel is the spectrum rms, which was estimated by using the data points in the line-free channels of the extracted spectrum, which has a range from −80 km s−1 to +150 km s−1.

Averaging the simultaneously observed RRL spectra could significantly improve sensitivity for detecting ionized gas (e.g., Balser 2006 ; Anderson et al. 2011 ; Liu et al. 2013 ; Alves et al. 2015 ). According to Menzel ( 1968 ), the oscillator strengths for RRLs we observed only differ by for transitions H101α to H96α in similar physical conditions, so that their line widths and velocities should resemble each other. To first order, it is therefore reasonable to average these transitions together to get a higher quality stacked RRL spectrum (e.g., see Fig. 1). We first gridded the data of each observed RRL by using MAP in the CLASS software package with the same beam size of and pixel size of , then, for each pixel in the mapped area, the spectra of six RRLs were resampled to the same velocity resolution of 4.4 km s−1, and stacked together by averaging the spectra weighted by their spectral root mean square (rms) noise. Here the rms was estimated by using the data in the line-free channels of each extracted spectrum in the range from −80 km s−1 to +150 km s−1. The S/N of the stacked line is higher than the individual RRL by a factor of about . The map for integrated intensity of the stacked RRL ( ) is shown in Figure 2. No beam corrections were applied to the data.

Fig. 2 Integrated RRL intensity map obtained by the TMRT observations, overlaid with contours for the spectrum S/N of 2.5σ, 3σ, 4σ, 6σ, 10σ and 18σ for the stacked line. Here, σ is the rms of the stacked spectrum in each pixel, which was estimated by using the data in the line-free channels of the stacked spectrum. The typical value of σ is about 9 mK ( , see Table 2). The beam size for the stacked data ( ) is indicated by a black filled circle in the lower-left corner.

3 Results and Discussions

The region we mapped has been covered by the HIPASS/ZOA RRL survey (Alves et al. 2010 , 2015 ) at 1.4 GHz for observations of H166α, H167α and H168α transitions with an angular resolution of about . For comparison, we made the integrated intensity map by using the data from the HIPASS 1.4 GHz RRL survey in the same area as ours.

Figure 3 also shows the smoothed TMRT 6 GHz RRL map with the same resolution as the HIPASS/ZOA RRL survey. The emission features given by our pilot observations and the HIPASS survey are consistent with each other. In the maps, there are three obvious emission features near ( , ), ( , ) and ( , ). They have been grouped into two star forming complexes, G48.6+0.1 and G49.5−0.3, by Russeil ( 2003 ). As shown in Figure 2, one can identify many more details from the 65-m TMRT observations because the resolution ( ) is about five times higher than the HIPASS/ZOA survey. The line ratios of the 6 GHz RRLs and the 1.4 GHz RRLs for regions near ( , ) and ( , ) are different, indicating different physical processes in the ISM. Sophisticated analysis and/or modeling would be necessary to reveal the physical properties in these ionized gas clouds.

Fig. 3 Left: the integrated intensity map of the stacked 1.4 GHz RRLs (H166α, H167α and H168α) that we made by using the data from the HIPASS survey (Alves et al. 2015 ). The beam size of is indicated by a black filled circle in the lower-left corner. Right: same as Fig. 2, but smoothed to the same resolution as the HIPASS survey.

In this sky area, there are 24 known HII regions with RRL measurements and one additional Sharpless HII region (SH 2−79 = G48.97−0.55) with only optical Hα detection (see Table A.1). We extracted their RRL spectra within one beam area ( ) from our stacked RRL data and obtained 21 of them detected with a spectral S/N greater than 3σ (see Fig. A.1). The other four HII regions, G49.738−0.616 (Bania et al. 2012 ), G49.998−0.125a, G49.998−0.125b (Anderson et al. 2011 ) and G50.039−0.274 (Bania et al. 2012 ), have an S/N less than 3σ. They were detected by simultaneous targeted pointing observations of four to seven RRLs by the Arecibo 300-m telescope (Bania et al. 2012 ) or GBT 100-m telescope (Anderson et al. 2011 ), which also have much longer on source integration time (e.g., minutes, Anderson et al. 2011 ) than our survey observations ( minutes per one beam area).

A Gaussian was fitted to the stacked RRL spectrum for deriving line parameters (see Table A.1). The centroid velocity and the full width at half maximum (FWHM) line width we obtained for the 21 HII regions are compared with those given in the literature as shown in Figure 4. For the HII regions having more than one measurement of or in the literature (see Table A.1), we adopted the mean value ( or ). We found that the measured by TMRT is well correlated with those given in references, with a Spearman rank correlation coefficient of 0.93. The best linear least-squares fit has a slope of 1.01±0.01 and a y-axis intercept of 0.65±0.53, which do not deviate significantly from the equal line (see Fig. 4). The measurement of line centroid velocity seems not to be influenced significantly by the observed RRL frequencies and the telescope used, which can also be seen from data collected in Table A.1. The deviations of given by different research works for an HII region are always less than 7 km s−1.

Fig. 4 Comparisons of the centroid velocity (left) and the FWHM line width (right) measured by the TMRT 65-m with those from references (see Table A.1). The 1σ errors for and are plotted as error bars. In each panel, the solid line indicates the best linear least-squares fit to the data, and the dashed line is the equal line. The Spearman rank correlation coefficient is also given in each panel. Except for SH 2−79 (G48.97−0.55), which only has optical Hα observations in the literature and is marked by a square, other HII regions shown in the plot all have RRL detections in the literature.

For the FWHM line width , values given by the TMRT are moderately correlated with those given by references, with a Spearman rank correlation coefficient of 0.61. The best linear least-squares fit to the 21 HII regions has a slope of 0.93 and a y-axis intercept of 3.73±2.54, which indicates that values given by the TMRT observations are in general slightly narrower than those given by references. As shown in Table A.1, a large number of from references were obtained by observing RRL(s) with principal quantum number n , which are higher than what are used in our observations. Theoretically, due to the Stark broadening effect, the FWHM line widths of RRLs will increase systematically with principal quantum number n (Smirnov et al. 1984 , Gordon & Sorochenko 2009 ). But in observations, things become complex. Unlike the line centroid velocity, the measurements of line width are sensitive to the spectral S/N. For the RRL spectrum with a low S/N, the weak and broad line wing will be hard to differentiate from the underlying continuum emission, hence, the fitted FWHM line width may be narrower than the true value. In addition, the line width measurements are also influenced by the observed RRL frequencies and the telescope used. As shown in Table A.1, the line widths of RRLs are in the range from 16 km s−1 to ∼40 km s−1. The deviations of line width for an HII region given by different research works could sometimes be as large as more than 10 km s−1. In the plot of the line width (Fig. 4), one may notice an “outlier” SH 2−79 (G48.97−0.55), which only has optical detection in the literature. The measured line width (41.5±2.4 km s−1, Fich et al. 1990 ) is significantly larger than (23.7±2.7 km−1). Fich et al. 1990 first found that is systematically broader than for some HII regions. Anderson et al. ( 2015 ) confirmed the finding of Fich et al. ( 1990 ), and found that could be as large as 20 km−1 for some HII regions. However, the reason is still not clear (Fich et al. 1990 , Anderson et al. 2015 ).

Anderson et al. ( 2014 ) has identified 8000 HII regions or candidates by using survey data from the all-sky Wide-field Infrared Survey Explorer (WISE) satellite. About 6 000 of them still do not have RRL measured. In the region we mapped, there are 107 WISE HII regions or candidates, and their RRL spectra can be extracted from our data. Some targets are very close to each other, with angular separation less than the beam of TMRT, and hence cannot be resolved by our observations. The 107 sources were grouped and finally we obtained 72 targeted positions. The RRL spectra within one beam ( ) for these target positions were extracted, and there are 45 target positions having spectral S/N greater than 5σ, and 30 of them are new detections (see Fig. A.2). The line parameters for these 30 new detections of RRLs are measured and listed in Table A.2.

Most of the 21 known HII regions and 30 new WISE HII regions coincide with the star forming complexes G48.6+0.1 and G49.5−0.3. Outside these two complexes, more than 90 positions with a spectral S/N (see Fig. 2) are present, and they may be candidates of new RRL sources. The RRL spectra within one beam from the targeted positions were extracted from the stacked RRL data. We found that there are 25 sources with a spectral S/N (Fig. A.3), but which are not included in the catalogs of known HII regions with RRL detections (e.g., Anderson et al. 2011 ; Bania et al. 2012 ; Hou & Han 2014 , Anderson et al. 2015 ). The measured line parameters are given in Table A.3. We made cross-identifications by using the position coincidence for SIMBAD objects with a searching radius of , and probable objects are listed in Table A.3. G48.39−0.91, G48.45−0.92 and G48.50 +0.13 coincide with Spitzer infrared bubbles (Simpson et al. 2012 ). G49.10−0.59 and G49.20+0.02 coincide with Young Stellar Objects (Kang et al. 2009 ; Spitzer Science 2009 ), which suggests that they are HII regions.

With the measured for these newly detected RRL sources, we can estimate their kinematic distances, which depend on the adopted Galaxy fundamental parameters and the solution to the kinematic distance ambiguity (e.g., Hou et al. 2009 ). How to determine the exact values of Galaxy fundamental parameters R 0, Θ0 and solar motions 3 with respect to the Local Standard of Rest (LSR) are outstanding questions. Besides the IAU standards for the distance of the Sun to the Galactic Center (GC) kpc, the circular orbital speed at the Sun , and solar motions with respect to the LSR, i.e., , and , another set of Galaxy parameters was recommended in recent research studies (see the discussions in Hou & Han 2014 for details). They are kpc, (e.g., Brunthaler et al. 2011 , Reid et al. 2014 ), and the solar motions of Schönrich et al. ( 2010 ) with , and . We noticed that most of these 55 new RRL sources have close to the tangent point speeds ( ).

For the first step, we try to identify the RRL sources located near the tangent points. According to the criteria of Reid et al. ( 2009 ), we made the identification by using two different sets of fundamental Galaxy parameters, i.e., the IAU standards and , and the solar motions of Schönrich et al. ( 2010 ). If consistent results were obtained by using the two different sets of Galaxy parameters, we place the RRL source at the tangent point. We found that 41 sources are located at the tangent points. For the other three RRL sources, we resolved the kinematic distance ambiguity by using the HI emission/absorption method and the HI self-absorption method as described in Anderson & Bania ( 2009 ). The distance parameters for these 44 RRL sources were then calculated by using a flat rotation curve (Reid et al. 2014 ) and are listed in Tables A.2 and A.3. As shown in Figure 5, they are located in the Sagittarius Arm and the Perseus Arm.

Fig. 5 Distributions of the 44 new RRL sources (blue triangles) projected into the Galactic plane, overlaid with a spiral arm model (see fig. 5 in Hou & Han 2014 ) to show the known spiral arm segments. The locations of the GC ( kpc, kpc) and the Sun ( kpc, kpc) are indicated by black stars. The known HII regions are marked by black squares. To estimate the distances, the IAU standards of kpc, and standard solar motions were adopted.

4 Conclusions

We conducted a pilot survey of RRLs toward a 2° × 1.2° sky area near the Sagittarius Arm tangent with the newly constructed TMRT. Six hydrogen RRLs (H96 H101α) were recorded simultaneously. The spectra of these transitions were first resampled to a velocity resolution of 4.4 km s−1 and then stacked together by using a weighted mean to improve the detection of the RRLs from the ionized clouds.

We showed that the major emission components in the integrated intensity map (Fig. 2) are consistent with the previous low-resolution map of 1.4 GHz RRLs. We measured the line parameters for 21 known HII regions and found that the centroid velocities are consistent with previous results. The higher resolution of this new pilot survey enables us to see detailed emission features in the area. In the region with more than 80 WISE selected HII region candidates, we obtained stacked RRLs for 30 positions with a spectral S/N greater than 5σ. We also identified 25 new discrete sources with stacked RRL S/N greater than 5σ. Among the 55 newly detected RRL sources, we estimated the distances for 44 of them. They are located in the Sagittarius Arm and the Perseus Arm. In this targeted region of only , we have detected RRLs for 55 targets for the first time. The results suggest that the 65-m TMRT has the ability to map and study Galactic ionized gas.


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Cite this article: Hou Li-Gang, Dong Jian, Gao Xu-Yang, Han Jin-Lin. Ionized gas clouds near the Sagittarius Arm tangent. Res. Astron. Astrophys. 2017; 5:047.


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