A POSSIBLE EXTENSION OF THE SCUTUM–CENTAURUS ARM INTO THE OUTER SECOND QUADRANT

Our knowledge of the outer Galaxy to date is largely dependent on H i emission surveys. The 21 cm line observations outline a spiral structure out to at least 25 kpc, implying a minimum radius for the gas disk (Levine et al. 2006). Through the International Galactic Plane Survey, a widespread presence of cool H i (traced by H i absorption) was found between Galactocentric radii of 12 and 25 kpc (Strasser et al. 2007). It was not until 2011, however, that a segment of a spiral arm at radii of ∼15 kpc was identified by Dame & Thaddeus (2011) beyond the Outer Arm in the first Galactic quadrant. This was accomplished by using the Leiden/Argentine/Bonn (LAB) 21 cm survey (Kalberla et al. 2005) and new CO observations with the 1.2 m telescope at the Harvard-Smithsonian Center for Astrophysics (CfA).

Because of the far distances involved, high-sensitivity CO observations are essential for delineating spiral structures in the extreme outer Galaxy. Digel et al. (1994) were the first to detect molecular clouds in this region. Due to the relatively low sensitivity of the Five College Radio Astronomy Observatory CO survey (Heyer et al. 1998), only a few bright molecular clouds were identified there (Brunt et al. 2003). Until now, no coherent, large-scale structures beyond the Outer Arm have been identified in the second quadrant.

We have carried out large-scale CO mapping in the second quadrant of the Galactic plane, which is part of the Milky Way Imaging Scroll Painting (MWISP; http://www.radioast.nsdc.cn/yhhjindex.php) project. One of the goals of this project is to probe the spiral structures farther than the Outer Arm. This mapping has completely covered the regions between 120° ⩽ l ⩽ 144° and −225 ⩽b ⩽ 375, and between 144° ⩽ l ⩽ 150° and −375 ⩽ b ⩽ 375. In the longitude range of 100°–120°, the observations so far were guided by the latitude distribution of extreme outer molecular clouds previously reported by Brunt et al. (2003) and Kerton & Brunt (2003) rather than an unbiased survey.

2.1. CO Observations

2.2. Archival Data of Atomic Hydrogen

We created b–v maps of the H i emission in the CGPS (66°<l < 150°) and the CO emission in the MWISP (100°<l < 150°) by obtaining average emissions over a 5° wide window in longitude. The data of the CGPS in the range of 150° < l < 175° were excluded from this analysis because of the degeneracy of the radial velocity. The Outer Arm has been widely accepted as the outermost arm in the second quadrant (Russeil 2003; Russeil et al. 2007). However, all b–v maps reveal a clear and distinct feature distributed beyond the Outer Arm at a maximum negative velocity. This outermost feature varies in latitude and velocity. In Figure 1, we show one of these b–v distributions from both the CO (image) and the H i (contours) emissions in longitude from 127° to 132°. The Outer Arm and the outermost feature are clearly seen as strong, separate features.

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To highlight the outermost feature, a l–v map of integrated H i emission over a window that follows the feature in latitude is shown in Figure 2(a). The high-mass star forming regions (HMSFRs) assigned to the Outer and Perseus Arms are marked with squares and diamonds, respectively (Reid et al. 2014). Similarly, Figure 2(b) focuses on the outermost feature, obtained by integrating the H i emission over a window that follows the feature in velocity. It is worth noting that at large negative velocities, the H i emission forms an arc from l ∼ 75°, b ∼ 3°, v ∼ −110 km s^-1, through l ∼ 110°, b ∼ 35, v ∼ −105 km s^-1, to l ∼ 150°, b ∼ −2°, v ∼ −70 km s^-1, running roughly parallel to the locus of the Outer Arm but shifted by 20–30 km s^-1to more negative velocities. This suggests that it may be a segment of a major spiral arm since spiral arms have long been recognized as presenting coherent arcs and loops in Galactic l–v plots of atomic and molecular emissions (Weaver 1974; Dame et al. 1986; Dame & Thaddeus 2008, 2011). The present feature was largely overlooked in the past since only short segments of it appear in l–v diagrams at any specific latitude or even in an l–v diagram integrated over all latitudes covered by the CGPS (e.g., in Figure 3 of Strasser et al. 2007).

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Before giving an explicit claim of a new spiral arm, we investigated the characteristics of the outermost feature in more detail. So far a total of 72 molecular clouds that are probably attributed to the outermost feature were detected by MWISP. These 72 clouds are marked with circles in Figures 2 and 3 and also summarized in Table 1. More than 10 clouds show clear ¹³CO detections while many of the other clouds were only marginally detected. However, none of them show C¹⁸O detections. The isotope abundance ratios in the extreme outer Galaxy will be studied in the forthcoming deep observations. Actually, all known clouds in the extreme outer Galaxy reported by Digel et al. (1994) and Brunt et al. (2003) were detected by the present survey. We labeled all these known clouds in Column 1 of Table 1. Note that the clouds reported by Digel et al. (1994) in the higher longitude range of 145° < l < 151° are not included in Table 1 since these clouds are partially blended in velocity with emission at less negative velocities. Besides, they might lie in the Outer arm or in the inter-arm on the assumption of the rotation curve of Reid et al. (2014).

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Most of the clouds are newly discovered, which suggests that we may still miss some molecular clouds in the interval 100° < l < 120° due to the narrow latitude coverage. The derived masses, mainly on the order of 10³–10⁴ M_☉, fall in the typical range for the masses of molecular clouds that are usually confined to spiral arms. Note that the mass is a lower limit since the adopted CO-to-H₂ X factor 1.8 × 10²⁰ cm⁻² is measured in the solar neighborhood. However, a recent study suggested that the X factor rises by a factor of 2–3 between the solar circle and the circle at R_Gal = 14 kpc (Abdo et al. 2010). In addition, the beam dilution effects lower the signal-to-noise ratio and hence also reduce the derived masses. We have selected one example with the largest negative velocities we recently detected (molecular cloud 24 marked with cross) and presented the velocity-integrated intensity and spectrum of the peak emission in Figure 2(b). Although these molecular clouds are more sparsely distributed than the atomic gas, both explicitly trace the same coherent, large-scale structure, i.e., the new arm well.

The warp of the Galactic disk is obvious (Figure 2(b)), which exhibits a clear positive warp between l = 66° and 135°, and a negative warp between l = 135° and 150°. Assuming a mean distance of 12 kpc, the new arm with thickness in z of about 2°–3° correspond to about 400–600 pc. Generally, the new arm is more severely warped and thicker than the Outer Arm. This is in good agreement with the theory for both external spiral galaxies and our Galaxy, which suggests that the warping and thicknesses of the disks increase with increasing distance from the galactic center (Wouterloot et al. 1990; Russeil 2003; Bottema 1996). Moreover, the new arm is distinct from the Outer Arm with a spatial separation of about 3–4 kpc, which is consistent with the typical span of the spiral arms outside the Perseus Arm (Reid et al. 2014; Vallée 2014).

Locations of all available molecular clouds in the far outer Galaxy are compared with the spiral arm model in Figure 3, superposed on: (1) CO l–v diagram from Dame et al. (2001), and (2) an artist's conception of the Milky Way (R. Hurt: NASA/JPL-Caltech/SSC). The artist's image has been scaled to place the HMSFRs in the spiral arms. These clouds are indicated by filled stars, triangles, and circles, which cover Galactic longitudes from l = 13° to 55° detected by Dame & Thaddeus (2011) and from l = 100° to 150° detected by us. The squares indicate the locations of the HMSFRs associated with the Outer Arm (Reid et al. 2014). The white dashed line in Figure 3 is a log spiral that was fit to the Scutum–Centaurus arm in the inner Galaxy (Vallée 2008, 2014). Generally, our data are in agreement with the far extension of the Scutum–Centaurus Arm in the second quadrant. However, confirmation of this hypothesis would be a challenge as suggested by Dame & Thaddeus (2011).

We attempted to fit the distribution of these outermost clouds, adopting a log-periodic spiral model defined by ln(R/R_ref) = −(β − β_ref) tan ψ, where R is the Galactocentric radius at a Galactocentric azimuth β for an arm with a radius R_ref at reference azimuth β_ref and pitch angle ψ. The Bayesian Markov Chain Monte Carlo procedure was adopted to estimate the parameters R_ref and ψ (see Reid et al. 2014 for more details). The best-fitting spiral model for the CO data on the interval 120° ⩽ l ⩽ 150° is R_ref = 17.8 ± 1.0 kpc at β_ref = 270 and ψ = 108 ± 76 (indicated by the red dashed line in Figure 3). The best-fitting spiral model for all of the available CO data including clouds detected by Dame & Thaddeus (2011) is R_ref = 16.1 ± 1.0 kpc at β_ref = 439 and ψ = 93 ± 07 (indicated by the cyan solid line in Figure 3). The uncertainties give a 68% confidence range.

The consistent fitting results confirm that the new arm can be interpreted as the far extension of the distant arm recently discovered by Dame & Thaddeus (2011). Apparently, both models fit the data well over a wide longitude range except in l = 100°–116°, where the model deviates from the data. Such departures might be caused by the kinematic distance uncertainty. Indeed, any systematic velocity departures can lead to distance uncertainty. The velocity departures, including both positive and negative values have already been observed, e.g., in the Perseus Arm between 90° and 150° and the Sagittarius–Carina Arm (Russeil 2003). Another possibility is the bias introduced by the incompleteness of the sample due to the narrow latitude coverage in this longitude interval, as mentioned above. The parallax measurements and MWISP projects in progress will be required to solve the problem and determine the new arm more accurately.

We are grateful to all of the staff of the 14 m telescope of the PMO for their dedicated assistance. We would like to thank Dr. Mark Reid for use of his fitting procedure. We thank Dr. Yuan-Wei Wu for useful discussions. We also thank the anonymous referee for very helpful suggestions and comments that help improved the Letter. Research for this project is supported by the National Natural Science Foundation of China (grant nos. 11003046, 11233007, 11133008, 11403104), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB09000000), and the Key Laboratory for Radio Astronomy, CAS.