Investigating the Edge of High Mass Star Formation in the Milky Way Galaxy
Nicholas Gregory Ferraro Dr. Dana Balser (NRAO), Trey V. Wenger (UVa)
This thesis is submitted in partial completion of the requirements of the BS Astronomy-Physics Major.
Department of Astronomy University of Virginia May 10th, 2018 DRAFTVERSION MAY 11, 2018 Typeset using LATEX twocolumn style in AASTeX61
INVESTIGATING THE EDGE OF HIGH MASS STAR FORMATION IN THE MILKY WAY GALAXY
NICHOLAS G.FERRARO1
1Department of Astronomy, University of Virginia
ABSTRACT The Outer Scutum-Centaurus arm (OSC) is the most distant molecular spiral arm known in the Milky Way. The OSC was first revealed in 2011 by Dame and Thaddeus using H I and CO data as a possible extension of the Scutum-Centaurus arm. Here we use the Arizona Radio Observatory (ARO) 12m telescope to observe the 12CO (J = 1-0) and 13CO (J = 1-0) transitions towards 78 H II region and H II region candidates in the First Galactic quadrant, and towards 63 in the Second and Third Galactic quadrants, all selected from the WISE Catalog of Galactic H II Regions. Within the First Galactic quadrant, we detect 17 CO clouds associated with H II regions in the OSC’s locus as defined by H I emission. Using the same techniques in the Second and Third Galactic quadrants, we did not detect any CO clouds definitively associated with H II Regions past the Outer arm. CO clouds have also been observed in the First and Second Galactic quadrant coincident with a potential extension of the OSC as defined by H I emission by the Milky Way Imaging Scroll Painting (MWISP) project. Examining these data with the same techniques we used to reduce our observational data produces similar findings, 4 CO clouds associated with H II regions in the first Galactic quadrant and none in the Second or Third. The relative lack of CO molecular clouds associated with H II regions in the Second and Third Galactic quadrants compared with the First Galactic quadrant indicates that star formation in the OSC spiral arm does not follow a log-spiral fit, consistent with the H I emission. Based on our data, we estimate the edge of high high mass star formation in the Milky Way occurs on the OSC, near the border of the First and Second Galactic quadrant, at a Galactocentric distance of approximately 14.5kpc. NICHOLAS FERRARO SENIOR THESIS 3
1. INTRODUCTION cluding the intensity peak (T), full width at half-maximum Since the OSC is the most recently discovered, distant spi- (FWHM) line width, local standard of rest velocity (VLSR), ral arm known in the Milky Way, it is still relatively unchar- and their respective errors. acterized. However since its initial classification by Dame During the reduction process for the first set of observa- & Thaddeus in 2011, more researchers are beginning to use tional data, we compared each individual’s resulting analysis various star formation tracers in an attempt to define the lo- for every target and created a “master file” with the best ap- cus, structure, and composition of the OSC (Sun et al. 2015, pearing fit taken from one of the individual’s analysis. Dur- Sun et al. 2017, Koo et al. 2017). Wenger et al. (2018) sum- ing the second reduction process, we instead used the com- marizes some of these recent experiments that are relevant to parisons to find mistakes in one data set which were then the scope of this paper (see Appendix A). corrected, allowing the use of just one reconciled analysis. We target 12CO molecular clouds toward H II regions be- The same Quality Factor ranking system was implemented cause alone, 12CO is not a robust enough tracer to guarantee to visually deduce the likelihood of a detection, taking extra star formation through its detection. However when paired precautions to assign the lowest values of ‘C’ to any source’s with a second tracer that is more strongly correlated with star line that appeared to be blended with background gas and ‘D’ formation, such as H II, the dual detection provides data that for any non-detections. can be used to map Galactic spiral structure. We used this 2.3. Archival Data strategy in Wenger et al. (2018) to trace the OSC in the First Galactic quadrant. In addition to our CO observations, we also used archived We continue our previous work by extending the scope of data from previous observations, namely the Milky Way our observations to include CO in the Second and Third Imaging Scroll Painting (MWISP; Sun et al. 2015, Sun et Galactic quadrants. Following the precedent set by our al. 2017) and the WISE Catalog of Galactic H II Regions previous paper, all targets are H II regions and H II region (Anderson et al. 2014). We summarize the CO observations candidates chosen from the WISE Catalog of Galactic H II used to study star formation in the OSC in Table 1. Regions. Additionally we use archival observations of CO detections from the Milky Way Imaging Scroll Painting Table 1. Observational Parameters (MWISP) project in the First and Second Galactic quadrants (Sun et al. 2015, Su et al. 2016, Du et al. 2016, Sun et al. Project Telescope Tracer Coverage Dates Reference 2017). Filtering these data based on proximity to sources in Our work ARO1 12m 12CO, 13CO 20 ◦ < ` < 70 ◦ Feb ’16 Wenger et al. 2018 the WISE Catalog provides more data that can be compared -2 ◦ < b < 4 ◦ directly with our own observations. 70 ◦ < ` < 240 ◦ Mar ’17 -2 ◦ < b < 5 ◦ 2. OBSERVATIONS AND DATA REDUCTION MWISP2 PMO3 13.7m 12CO, 13CO, 18CO 100 ◦ < ` < 150 ◦ 2011 - 2014 Sun et al. 2015 ◦ ◦ 2.1. WISE H II Region Candidates in the First, Second, and -3 < b < 4 ◦ ◦ Third Galactic Quadrants 35 < ` < 45 Nov ’11 - Mar ’15 Su et al. 2016 -5 ◦ < b < 5 ◦ 12 13 We observed CO (J = 1-0) and CO (J = 1-0) transi- 140 ◦ < ` < 150 ◦ Sep ’13 - Dec ’15 Du et al. 2017 tions towards 141 H II regions or H II region candidates se- -5 ◦ < b < 5 ◦ lected from the WISE Catalog of H II Regions using the Ari- 35 ◦ < ` < 45 ◦ Nov ’11 - Mar ’15 Sun et al. 2017 zona Radio Observatory (ARO) 12m telescope in February -5 ◦ < b < 5 ◦ of 2016 (First Galactic quadrant) and March of 2017 (Sec- 1 2 3 NOTE— Arizona Radio Observatory, Milky Way Imaging Scroll Painting, Purple Mountain Ob- ond and Third Galactic quadrants). The First Galactic quad- servatory rant observations are documented in Wenger et al. (2017), including a more thorough description of the observing pro- cedures.
2.2. Data Reduction 3. ANALYSIS We reduced data following the same procedures described 12 in Wenger et al (2018), attached in the appendix. Essentially 3.1. CO Distribution in the First, Second, and Third this entailed multiple independent reductions of the data us- Galactic Quadrants ing an IDL software package, TMBIDL, to model third-order We use 12CO and H II data to characterize star formation polynomial spectral baselines and fit Gaussian profiles to and spiral structure in the outer Galaxy. To best view the each potential detection line. This process provided infor- distribution of observed 12CO molecular clouds, we choose mation for each potential molecular cloud detection line in- to plot each detection on a face-on map of the Milky Way 4 N. FERRARO disk. We include log-spiral fits and H I trace data (Koo et al. Figure 1 demonstrates that our First Quadrant 12CO detec- 2017) to provide a reference of outer Galactic structure. To tions (Our Project 2016) cover a wider variety of Galacto- accomplish this, we use a kinematic distance code package centric distances than those in the Second and Third Quad- pdf_kd.py (Wenger et al. 2018). The code takes the Galactic rant (Our Project 2017). Additionally, more of the 12CO longitude of each source, a rotation curve model (Reid 2014), detections from the First Quadrant are near the log-spiral a number of re-samples (here chosen to be 10,000) as well fit and OSC trace as defined by H I emission than those in as the measured velocity and respective uncertainty provided the Second and Third Quadrant. In comparison, the Second from the data reduction as inputs. Using Monte Carlo resam- and Third Quadrant 12CO detections all appear to be located pling, this code produces the far kinematic distance and tan- much closer to the Sun, with only two data points being lo- gent point for each detection line as well as a variety of other cated further than the Outer spiral arm. outputs. From this information, we can use trigonometry to convert the far or tangent distance, and galactic longitude to Cartesian coordinates, allowing us to create the face-on map seen in Figure 1. NICHOLAS FERRARO SENIOR THESIS 5
Figure 1. Face-On Map: All 12CO Data
Figure 1. Face-on map of 12CO detections in the First, Second, and Third Galactic quadrants. The red, green, yellow, pink, blue, purple and maroon points are CO detections from projects listen in the legend, the solid black line is the solar orbit, the dashed black line is the tangent point line, the orange line is the logarithmic spiral fit of pitch angle 12.4 ◦ to OSC data, the lime-green line is the OSC locus in the first quadrant (Dame & Thaddeus 2011), the cyan and purple dots represent the OSC and Outer arms, respectively, as defined by H I emission (Koo et al. 2017). 6 N. FERRARO
3.2. Comparison of 12CO and H I Data the two 12CO detections from the "2017 Data" nearest the To better view the 12CO detections compared to the OSC H I trace (G101.663+2.820 and G122.769+2.528), Figures 3 trace as defined by H I emission, we plot the 12CO detections, and 4 respectively, revealed that both contained two detection H I trace, and logarithmic spiral (pitch angle = 12.4 ◦) on a lines at very different local standard of rest velocities which Galactocentric distance (R) vs. azimuthal angle (Φ) graph, makes it impossible to say definitively if either detection is as shown in Figure 2. actually associated with an H II region at that distance.
12 Figure 2. Azimuthal Angle vs. Galactocentric Distance 12CO Figure 3. CO Spectrum Map
Figure 3. Spectrum of 12CO observed at (` = 101.663, b = 2.820) with two different detection lines clearly made visible by the red −1 −1 Gaussian fit at VLSR = -102.35 kms , -21.46 kms . Figure 2. 12CO detections plotted as function of Galactocentric dis- tance (R) and azimuthal angle (Φ). The red dots are 12CO detection 12 lines observed in 2016, the green dots are CO detection lines ob- 12 served in 2017, the blue line is the logarithmic spiral fit of pitch Figure 4. CO Spectrum angle 12.4 ◦ to OSC data, and the cyan dots represent a trace of the OSC as defined by H I emission (Koo et al. 2017).
In Figure 2, the H I trace follows the logarithmic spiral fairly closely in the first Galactic quadrant, but it deviates sig- nificantly in the other quadrants. While no detections from the “2017 Data” appear to be close to the logarithmic spi- ral fit, there are two points that seem to match the H I trace in quadrant two. Thus if the OSC does extend into the sec- ond Galactic quadrant, it appears likely it would deviate there from the logarithmic spiral. It is also still notable that the per- centage of detections close to either the logarithmic spiral or H I trace is much higher in the “2016 Data” than the “2017 Data”. Figure 4. Spectrum of 12CO observed at (` = 122.769, b = 2.528) 12 3.3. Comparison of CO and H II Data with two different detection lines clearly made visible by the red −1 −1 All of our observational targets were specifically chosen to Gaussian fit at VLSR = -106.52 kms , -65.68 kms . The quality factors in this figure should be ‘A’ and ‘B’ respectively. be spatially coincident with H II regions or H II region candi- dates from the WISE catalog. Therefore any 12CO detections in the locus of the OSC associated with only local standard To repeat the experiment with the archival survey data from of rest velocity are definitely associated with an H II region. MWISP (Sun et al. 2015, Su et al. 2016, Du et al. 2016 and This methodology produced 17 12CO emission lines associ- Sun et al. 2017), we first had to filter the 12CO emission lines ated with H II regions and consistent with the OSC (`, b) lo- to just those that were spatially coincident with HII regions cus in the first Galactic quadrant. Checking the spectra for or H II region candidates from the WISE catalog. We defined NICHOLAS FERRARO SENIOR THESIS 7 a 12CO cloud as being spatially coincident with an H II region All of our findings are summarized in Table 2, demonstrat- if the 12CO cloud and H II region were within two diameters ing a much higher likelihood of observing a 12CO emission of the H II cloud or within an arcminute. This criteria was line associated with an H II region in the First Galactic quad- chosen because an arcminute is roughly the HPBW for the rant than in the Second or Third. PMO 13.7m telescope. The results of this spatially coinci- dent filtering are displayed in the face-on map in Figure 5. 8 N. FERRARO
12 Figure 5. Face-On Map: CO Spatially Coincident with H II Regions
Figure 5. Face-on map of 12CO detections in the First, Second, and Third Galactic quadrants that are spatially coincident with H II Regions or H II Region Candidates chosen from the WISE catalog. The red, green, yellow, pink, blue, purple and maroon points are 12CO detections from projects listen in the legend, the solid black line is the solar orbit, the dashed black line is the tangent point line, the orange line is the logarithmic spiral fit of pitch angle 12.4 ◦ to OSC data, the lime-green line is the OSC locus in the first quadrant (Dame & Thaddeus 2011), the cyan and purple dots represent the OSC and Outer arms, respectively, as defined by H I emission (Koo et al. 2017) NICHOLAS FERRARO SENIOR THESIS 9
Table 2. MWISP1 vs. WISE Data The presence of 12CO emission lines coincident with H II regions in the Second quadrant at lower Galactocentric dis- Reference Galactic Spatial Spatial Matches with Kinematic tances appear to indicate this is not a coincidence but rather 2 Quadrant Matches H II VLSR Information Matches the OSC as measured by H II regions may end in the First
Su et al. 2016 First 187 88 17 Galactic quadrant. Sun et al. 2017 First 62 33 4 Sun et al. 2015 Second 31 12 0 Du et al. 2016 Second 80 23 4 1 2 NOTE— Milky Way Imaging Scroll Painting, not all H II Regions in the WISE cata- log have associated values for VLSR
3.4. Comparison of 12CO Detections in the First vs. Second & Third Galactic Quadrants We used the same procedures to examine five different sets of observational data, all summarized in Table 3. Most no- tably, we find a complete lack of any 12CO emission lines associated with H II regions outside of the Outer spiral arm despite finding them in the First quadrant for both sets of in- dependently observed data, as shown in Figure 6.
12 Table 3. CO Detections vs. H II Regions
Reference Galactic Number of Percentage of Targets Spatially and
Quadrant Targets Kinematically Associated with H II Regions
Su et al. 2016 First 187 88 Sun et al. 2017 First 62 33 Wenger et al. 2017 First 78 7 Sun et al. 2015 Second 31 12 Du et al. 2016 Second 80 23 Our work Second/Third 63 18 10 N. FERRARO
12 Figure 6. Face-On Map: CO Spatially & Kinematically Coincident with H II Regions
Figure 6. Face-on map of 12CO detections in the First, Second, and Third Galactic quadrants that are spatially and kinematically coincident with H II Regions or H II Region Candidates chosen from the WISE catalog. The red, green, yellow, pink, blue, purple, and maroon points are 12CO detections from projects listen in the legend, the solid black line is the solar orbit, the dashed black line is the tangent point line, the orange line is the logarithmic spiral fit of pitch angle 12.4 ◦ to OSC data, the lime-green line is the OSC locus in the first quadrant (Dame & Thaddeus 2011), the cyan and purple dots represent the OSC and Outer arms, respectively, as defined by H I emission (Koo et al. 2017). NICHOLAS FERRARO SENIOR THESIS 11
4. CONCLUSION prevent us from conclusively determining if the OSC does 4.1. Summary indeed extend from the First Galactic Quadrant to the Sec- ond. Based on our data, we estimate the edge of high high In this paper, we discuss high mass star formation tracers, mass star formation in the Milky Way occurs on the OSC, 12 specifically CO molecular clouds spatially and kinemati- near the border of the First and Second Galactic quadrant, at cally coincident with known H II Regions, with a focus on the a Galactocentric distance of approximately 14.5kpc. Outer Scutum-Centaur spiral arm. Using data from two in- 4.2. Future Work dependent 12CO observations in the outer First, Second, and Third Galactic quadrants, we find at least 17 12CO emission While our initial results appear to suggest a quantitative es- lines associated with H II regions in the locus of the Outer timate for the edge of high mass star formation in the Galaxy, Scutum-Centaurus spiral arm and none in the Second Galac- further work is required to make this thesis a publishable re- tic quadrant. While we searched extensively in the Second search paper. The inclusion of more star formation tracers, Galactic quadrant, as demonstrated by the number of 12CO especially in the Third and Fourth Galactic quadrants, as well emission lines spatially coincident with H II Regions or H II as some recently confirmed H II Regions in the outer Second Region Candidates, the lack of sources with matches in VLSR Galactic quadrant will increase the statistical robustness of this estimation.
REFERENCES
Dame, T. M., & Thaddeus, P. 2011, ApJL, 734, L24 Sun, Y., Su, Y., Zhang, S., et al. 2017, ApJS, 230, 17 Du, X., Xu, Y., Yang, J., et al. 2016, ApJS, 224, 7 Sun, Y., Xu, Y., Yang, J., et al. 2015, ApJL, 798, L27 Wenger, T., Khan, A., Ferraro, N., et al. 2018, ApJ, 852, 1 Su, Y., Sun, Y., Li, C., et al. 2016, ApJ, 828, 59 12 N. FERRARO
APPENDIX arXiv:1711.06235v1 [astro-ph.GA] 16 Nov 2017 htaecnitn ihtevlcte fteOC hr sn appar non no and is clouds There molecular OSC OSC. these the of of density) velocities column 17 the molecular Most are (e.g., with orbit. there consistent Solar but are the Arm, that beyond Outer originating the line with emission associated one least at have msinin emission r ptal onietwt h aatclniuelttd ( longitude-latitude Galactic the with coincident spatially are rniin oad7 H 78 toward transitions eew s h rzn ai bevtr AO 2mtlsoeto telescope m 12 (ARO) Observatory th (C Radio monoxide in Arizona carbon seen the and is use formation, uncharacteriz we OSC largely star Here the remains and distance OSC gas Scutum-Centaur the this molecular well-known in At between tracers the Galaxy. formation of star outer end of the distant to very bar the Galactic be may OSC 5 6 4 3 2 1 USA. USA. duc srnmra h re akOsraoy ..Bx2 Box P.O. Observatory, Bank Green the at Astronomer Adjunct nttt o srpyia eerh eateto Astro of Department Research, Astrophysical for Institute etrfrGaiainlWvsadCsooy etVirgini West Cosmology, and Waves Univers Gravitational Virginia for West Center Astronomy, and Ch Rd., Physics Edgemont of Department 520 Observatory, 400 Astronomy Box Radio P.O. National Virginia, of University Department, Astronomy h ue ctmCnarsam(S)i h otdsatmolecu distant most the is (OSC) arm Scutum-Centaurus Outer The yee sn L using 2017 Typeset 17, November version Draft ryV Wenger, V. Trey ABNMNXD BEVTOSTWR TRFRIGRGOSI REGIONS FORMING STAR TOWARD OBSERVATIONS MONOXIDE CARBON Keywords: ∼ 0 fortres nttl edtc 117 detect we total, In targets. our of 80% A T E X aay tutr,IM oeue,rdolns S,surveys ISM, lines: radio molecules, ISM: structure, Galaxy: twocolumn ii ,2 1, eincniae hsnfo the from chosen candidates region sdA Khan, A. Asad tl nAASTeX61 in style RvsdNvme 7 07Sbitd–TVW) – Submitted 2017 17, November (Revised CTMCNARSSIA ARM SPIRAL SCUTUM-CENTAURUS .D Anderson, D. L. 1 ihlsG Ferraro, G. Nicholas 12 oy otnUiest,75Cmowat vne Boston, Avenue, Commonwealth 725 University, Boston nomy, Oeiso ie n 8 and lines emission CO ABSTRACT nvriy hsntRdeRsac ulig Morgantow Building, Research Ridge Chestnut University, a 2,Caltevle A29442,USA. 22904-4325, VA Charlottesville, 325, ,5 4 5, 3, t,P o 35 ognonW 60,USA. 26506, WV Morgantown 6315, Box PO ity, rotsil,V 20,USA. 22903, VA arlottesville, re akW 44,USA. 24944, WV Bank Green , 12 Oad40 and CO n .M Bania M. T. and ,b ℓ, WISE S ou sdfie yH by defined as locus OSC ) d xrglci tde hwasrn correlation strong a show studies Extragalactic ed. 1 )eiso a eetydsoee nteOSC. the in discovered recently was emission O) aao fGlci H Galactic of Catalog aaS Balser, S. Dana n ieec ewe h hsclproperties physical the between difference ent 13 bev the observe sam hc tece rmteedo the of end the from stretches which arm, us a prlamkoni h ik a.The Way. Milky the in known arm spiral lar OCmlclrcod ihnorsample. our within clouds molecular –OSC Oeiso ie.Aot23o u targets our of 2/3 About lines. emission CO ftedtcin eodteSlrobtare orbit Solar the beyond detections the of rtGlci udat h population The quadrant. Galactic first e 6 13 Oeiso ie ihLRvelocities LSR with lines emission CO 12 OJ=10and 1–0 = J CO 2 .P Armentrout, P. W. ii i msin edtc CO detect We emission. H OUTER THE N ein.Teetargets These Regions. 13 OJ=1–0 = J CO A 02215, MA, ,W 26505, WV n, ,4 3, 2 Wenger et al.
1. INTRODUCTION suming a log-periodic spiral arm model. First quadrant ◦ data from the MWISP for the longitude range of 35 < Galactic structure in the Milky Way is difficult to ◦ determine because of our location within the disk ℓ< 45 reveal 168 molecular clouds consistent with the − ± −1 and the difficulties in deriving accurate distances. OSC (ℓ, v) locus, defined by VLSR = 1.6 ℓ 13.2kms . There is strong evidence, however, that the Milky These molecular clouds have typical masses and sizes of × 3 Way is a barred spiral galaxy (e.g., Churchwell et al. 3 10 M⊙ and 5 pc, respectively (Sun et al. 2017). 2009). Most barred spiral galaxies are Grand Design Detection of CO does not necessarily imply star for- galaxies, with two prominent symmetric spiral arms mation. Observations of high-mass star formation trac- (Elmegreen & Elmegreen 1982). Understanding the ers are therefore critical for characterizing the OSC. H ii Milky Way structure is important since bars and spiral regions are the archetypical tracers of star formation arms have dynamical effects (e.g., radial migration) and and spiral structure. As part of the H ii Region Discov- influence star formation (e.g., shock gas). ery Survey, Anderson et al. (2015) discovered six H ii To our knowledge, Dame & Thaddeus (2011) were the regions whose radio recombination line (RRL) emission first to connect the Scutum-Centaurus (SC) spiral arm velocities are consistent with the OSC (ℓ, v) locus de- − ± −1 from its beginning at the end of the bar in the in- fined by VLSR = 1.6 ℓ 15kms . There are four ner Galaxy to the outer Galaxy in the first quadrant. additional H ii regions in the WISE Catalog of Galactic They called this outermost section the Outer Scutum- H ii Regions consistent with this definition of the OSC Centaurus (OSC) arm. In the first quadrant, the OSC (Anderson et al. 2012). is at a distance between 15kpc and 19kpc kpc from the RRL emission is the best tracer of high-mass star for- Galactic Center (GC) and between 20kpc and 25kpc mation, but it is faint at the large distances of the OSC. from the Sun (Armentrout et al. 2017). The connection Armentrout et al. (2017) took a different approach and between the SC and OSC arm segments, however, re- used the Green Bank Telescope (GBT) to observe the mains hypothetical. Toward the GC, velocity crowding dense molecular gas tracers NH3 (J,K) = (1,1), (2,2), → and complex structure prohibits clear identification of (3,3) and H2O 6(1,6) 5(2,3) toward 75 WISE H ii re- spiral structure. The H i emission that has now been gions and H ii region candidates located within the OSC associated with the OSC arm was detected long ago. (ℓ,b) locus as defined by H i (Dame & Thaddeus 2011). The arm appears in the early 21 cm maps of both Kerr Because these targets were identified as H ii region can- (1969) and Weaver (1974). Yet it took decades to trace didates based on their mid-infrared morphology, these the SC spiral arm from the bar to the outer Galaxy. molecular lines trace dense gas that is more likely to be Dame & Thaddeus (2011) carefully traced the OSC associated with star formation. Any detected spectral in the Leiden/Argentine/Bonn (LAB) H i 21 cm line all- line probably indicates an active star formation region. sky survey data (Hartmann & Burton 1997; Arnal et al. About 20% of the targets were detected in either ammo- 2000; Bajaja et al. 2005; Kalberla et al. 2005) and then, nia or water maser emission, but only two have velocities using the Center for Astrophysics 1.2 m telescope, de- consistent with the OSC. tected molecular gas in the arm for the first time, at 10 Armentrout et al. (2017) also observed a similar sam- locations coincident with HI emission peaks. A CO map ple of OSC targets in radio continuum at 8–10 GHz with was made of one location revealing a molecular cloud the Jansky Very Large Array (JVLA). About 60% of 4 the targets were detected in radio continuum with the with mass and radius of 5 × 10 M⊙ and 47pc, respec- tively. Koo et al. (2017) recently re-analyzed the LAB JVLA. Five of these are associated with the locus of survey data using a peak-finding algorithm and identi- the OSC. Together, RRL and continuum emission al- fied the OSC arm as a 20kpc long H i structure coinci- lowed various H ii region physical properties to be de- dent with several H ii regions and molecular clouds. rived. Associating the radio continuum with the molec- Sun et al. (2015) suggested a possible extension of the ular transition, however, is less secure since the molec- OSC from the first Galactic quadrant into the second ular cloud may not be associated with the H ii region quadrant. Their results are part of the Milky Way (Anderson et al. 2009). Nonetheless, assuming they are Imaging Scroll Painting (MWISP) project to map 12CO related yields a distance. If the hydrogen-ionizing pho- ◦ ◦ and 13CO from Galactic longitudes −10 <ℓ< 250 ton flux is produced by a single star, then the observed ◦ ◦ and Galactic latitudes −5 15kpc) high-mass star for- 2 4 ◦ ◦ mation is ongoing in the Milky Way. 10 − 10 M⊙ that lie between 120 <ℓ< 150 and are roughly consistent with an extension of the OSC as- CO in the OSC 3
G026.418+1.683 G021.541+1.676 − − 25 43.32 km s 1, QF = A 6 31.07 km s 1, QF = B . −1 5 33 66 km s , QF = B 20 4 15
(K) (K) 3 ∗ ∗ A A
T 10 T 2 5 1 0 0 30 40 50 60 70 80 20 30 40 50 60 70 −1 −1 VLSR (km s ) VLSR (km s )
G054.491+1.579 G042.210+1.081 − . −1 − . −1 8 38 82 km s , QF = C 0.20 57 02 km s , QF = B −36.25 km s−1, QF = C 0.15 6 0.10
(K) (K) 0.05
∗ 4 ∗ A A
T T 0.00 2 −0.05 −0.10 0 −0.15 −50 −40 −30 −20 −10 0 −70 −60 −50 −40 −30 −20 −1 −1 VLSR (km s ) VLSR (km s )
Figure 1. Representative 12CO spectra for different quality factors (QF). Plotted is the antenna temperature as a function of the LSR velocity. The black curves are the data and the red curves are Gaussian fits to the data. The LSR velocity of each Gaussian profile, together with the QF, is shown in the right-hand corner of the plot. Here we seek to explore the molecular content in selected all H ii region candidates in the WISE Catalog the OSC using 12CO and 13CO transitions, which are of Galactic H ii Regions (Anderson et al. 2012) that lay brighter than the molecular line transitions observed by near the OSC (ℓ,b) locus defined as b =0.375◦+0.075×ℓ Armentrout et al. (2017). Since molecular clouds need within the range 20◦ <ℓ< 70◦. Many of these tar- not be forming high-mass stars, carbon monoxide is not gets have measured radio continuum emission and are as good a tracer of high-mass star formation as RRLs, most likely bona fide H ii regions (Armentrout et al. NH3,orH2O, but detection is much more likely at these 2017). In addition, we included 10 H ii regions from large distances. Following Armentrout et al. (2017) we (Armentrout et al. 2017) with RRL velocities within the −1 only target H ii regions and H ii region candidates from OSC (ℓ, V ) locus defined as VLSR = −1.6 ℓ ± 15kms . the WISE Catalog of Galactic H ii Regions. These tar- Since 12CO is often optically thick in Galactic molecu- gets all have the same characteristic infrared morphol- lar clouds, we observed the optically thin 13CO J=1–0 ogy which increases the probability that the molecular transition at 110.20132 GHz in a subset of targets with gas is associated with high-mass star formation. bright 12CO detections to provide a more accurate mea- sure of the molecular column density. 2. OBSERVATIONS AND DATA REDUCTION The ARO 12m telescope is the European ALMA pro- We used the Arizona Radio Observatory (ARO) 12 m totype antenna that began operation on Kitt Peak in telescope to observe the 12CO J=1–0 transition at 2014. The telescope’s half-power beam-width (HPBW) 115.27120GHz toward 78 H ii regions and H ii region is 54′′ and 57′′ at 115GHz and 110GHz , respectively. candidates located in the first Galactic quadrant. We 4 Wenger et al.
G050.901+1.056 G061.154+2.170 4 . 2 0 12CO 12CO 13CO 3 13CO 1.5
(K) 1.0 (K) 2 ∗ ∗ A A T T 0.5 1
0.0 0 −100 −90 −80 −70 −60 −50 −40 −100 −90 −80 −70 −60 −50 −40 −1 −1 VLSR (km s ) VLSR (km s )
G062.194+1.715 G066.608+2.061 7 5 12CO 12CO 6 13CO 13CO 4 5 3 4 (K) (K) ∗ ∗ A A 3
T 2 T 2 1 1 0 0 −100 −90 −80 −70 −60 −50 −40 −100 −90 −80 −70 −60 −50 −40 −1 −1 VLSR (km s ) VLSR (km s )
Figure 2. 12CO (black) and 13CO (blue) spectra of targets where the association of 12CO emission and 13CO emission was not straightforward. In G050.901+1.056 and G062.194+1.715 there are two 13CO components that correspond to one 12CO component. In G066.608+2.061 there is one 13CO component that corresponds to two 12CO components. Finally, in G061.154+2.170 there are 12CO components at velocities where the 13CO emission is corrupted by emission in the Off position. The main beam efficiency is & 90% at these frequencies. total power pairs were sufficient to detect 12CO emission Our observations were performed between February 11– but in some cases we integrated longer. The telescope 16, 2016. The receiver consisted of the ALMA band 3, pointing and focus were corrected every 1–2 hours by dual polarization, sideband-separating mixers with typ- peaking on Jupiter, Venus, or Saturn. The typical point- ical on-sky system temperatures of ∼ 300K. We em- ing accuracy was ∼ 2′′ rms. At the start of each session ployed both the filter bank spectrometer with 256 chan- we checked the tuning of the spectrometers by observing nels at 2MHz spectral resolution (5kms−1 at 115 GHz), the test source M17SW. and the millimeter autocorrelator (MAC) with 4096 The data were reduced and analyzed using TM- channels at 195kHz spectral resolution (0.5kms−1 at BIDL, an IDL single-dish software package (Bania et al. 115 GHz). Both spectrometers accept two intermedi- 2016).1 The data reduction and analysis were performed ate frequency signals consisting of the two orthogonal independently by three of the authors. Each spectrum polarizations. Here we only consider data from the was visually inspected. We discarded ∼1% of the spec- MAC spectrometer given the narrow line widths of CO tra due to poor baseline structure. For each target the (∼ 1kms−1). The typical atmospheric optical depth at data were averaged over all total power pairs and po- zenith was τ0 ∼ 0.2. larizations to produce a single, averaged spectrum. We We made total power, position switched observations modeled the spectral baselines with a third-order poly- where the reference position (Off) is offset by 20′ in az- nomial function that was subtracted from the data to imuth from the source (On). The On and Off positions were observed for 5 minutes each with a switching rate of 1 V7.1, see https://github.com/tvwenger/tmbidl 30 seconds, for a total time of 10 minutes. Typically 1–2 CO in the OSC 5
G028.319+1.243 G033.008+1.151 0.4 G038.627+0.813 5 −45.75 km s−1, QF = A −46.84 km s−1, QF = A −56.25 km s−1, QF = B 4 0.3 −52.76 km s−1, QF = B 4 3 . 3 0 2 (K) (K) (K)
∗ ∗ ∗ . A A 2 A 0 1
T 2 T T . 1 1 0 0
0 0 −0.1 −60 −50 −40 −30 −20 −10 −60 −50 −40 −30 −20 −10 −70 −60 −50 −40 −30 −20 −1 −1 −1 VLSR (km s ) VLSR (km s ) VLSR (km s )
G039.185-1.421 G040.955+2.473 4 G041.304+1.997 −55.67 km s−1, QF = A −58.54 km s−1, QF = A −57.09 km s−1, QF = A − 2.5 − 7.17 km s 1, QF = A 6.87 km s 1, QF = A 1.5 3 2.0 1.0 1.5
(K) (K) (K) 2 ∗ ∗ ∗ A A A
T T 1.0 T 0.5 1 0.5 0.0 0.0 0 −60 −40 −20 0 20 40 −70 −60 −50 −40 −30 −20 −60 −40 −20 0 20 40 −1 −1 −1 VLSR (km s ) VLSR (km s ) VLSR (km s )
G041.810+1.503 G042.210+1.081 G046.368+0.802 0.25 − . −1 − . −1 − . −1 . 61 55 km s , QF = B 0.20 57 02 km s , QF = B 5 59 92 km s , QF = A 0 20 −48.87 km s−1, QF = B −35.60 km s−1, QF = A . 0.15 0 15 4 0.10 0.10 3 (K) 0.05 (K) 0.05 (K) ∗ ∗ ∗ A A A T 0.00 T 0.00 T 2 −0.05 −0.05 1 −0.10 −0.10 0 −0.15 −0.15 −80 −70 −60 −50 −40 −30 −20 −10 −70 −60 −50 −40 −30 −20 −80 −70 −60 −50 −40 −30 −20 −10 0 −1 −1 −1 VLSR (km s ) VLSR (km s ) VLSR (km s )
G046.375+0.897 G054.094+1.749 G061.085+2.502 6 −63.49 km s−1, QF = C 0.6 −85.29 km s−1, QF = A −84.29 km s−1, QF = A − . 5 −61.79 km s 1, QF = C 0.5 2 0 − −59.44 km s 1, QF = C . 4 0 4 1.5 0.3
(K) 3 (K) (K)
∗ ∗ ∗ . A A 0.2 A 1 0 T 2 T T 0.1 0.5 1 0.0 0 −0.1 0.0 −80 −70 −60 −50 −40 −30 −20 −100 −90 −80 −70 −60 −50 −100 −90 −80 −70 −60 −50 −1 −1 −1 VLSR (km s ) VLSR (km s ) VLSR (km s )
G061.180+2.447 G062.578+2.387 5 −84.81 km s−1, QF = A −89.09 km s−1, QF = A 10 −69.20 km s−1, QF = C 4 −1 8 −65.33 km s , QF = C 3 6 (K) (K) ∗ ∗ A A
T 2 T 4 1 2 0 0 −100 −90 −80 −70 −60 −50 −100 −90 −80 −70 −60 −50 −40 −30 −1 −1 VLSR (km s ) VLSR (km s )
Figure 3. 12CO spectra of emission lines originating within the OSC. Plotted is the antenna temperature as a function of the LSR velocity. The black curves are the data and the red curves are Gaussian fits to the data. The LSR velocity of each Gaussian profile, together with the QF, is shown in the right-hand corner of the plot. The vertical black line indicates the RRL velocity, when available. 6 Wenger et al.
G033.008+1.151 G039.185-1.421 G040.955+2.473 0.25 − . −1 − . −1 − . −1 0.4 47 04 km s , QF = A 55 30 km s , QF = A . 58 60 km s , QF = A 0.3 0 20 0.3 0.15 . 0 2 0.10 0.2 (K) (K) . (K) 0.05 ∗ ∗ ∗ A . A 0 1 A T 0 1 T T 0.00 . 0.0 0 0 −0.05 −0.10 −0.1 −0.1 −0.15 −60 −50 −40 −30 −20 −10 −70 −60 −50 −40 −30 −20 −70 −60 −50 −40 −30 −20 −1 −1 −1 VLSR (km s ) VLSR (km s ) VLSR (km s )
G041.304+1.997 G046.368+0.802 G046.375+0.897 − − − . −57.11 km s 1, QF = A −60.13 km s 1, QF = A −63.51 km s 1, QF = C 0 3 −1 −1 6.95 km s , QF = A 0.6 0.6 −59.42 km s , QF = C 0.2 0.4 0.4 (K) 0.1 (K) (K) ∗ ∗ ∗ A A A T T 0.2 T 0.2 0.0 0.0 0.0 −0.1 −60 −40 −20 0 20 40 −80 −70 −60 −50 −40 −30 −80 −70 −60 −50 −40 −30 −20 −1 −1 −1 VLSR (km s ) VLSR (km s ) VLSR (km s )
G061.180+2.447 . − 0 7 −84.74 km s 1, QF = A 0.6 0.5 0.4
(K) 0.3 ∗ A
T 0.2 0.1 0.0 −0.1 −100 −90 −80 −70 −60 −50 −1 VLSR (km s )
Figure 4. 13CO spectra of emission lines originating within the OSC. Plotted is the antenna temperature as a function of the LSR velocity. The black curves are the data and the red curves are Gaussian fits to the data. The LSR velocity of each Gaussian profile, together with the QF, is shown in the right-hand corner of the plot. The vertical black line indicates the RRL velocity, when available. remove any sky continuum emission or residual baseline sion, we eliminated these components from any further structure in the spectrum. We fitted a Gaussian func- analysis. The 13CO spectra were analyzed in the same tion to each profile using a Levenberg-Markwardt least manner. squares method (Markwardt 2009) to derive the peak We assigned a Quality Factor (A, B, or C) to every CO intensity, the full width at half-maximum (FWHM) line emission line. This judgment was based on the signal- width, and the local standard of rest (LSR) velocity. to-noise ratio and expected line properties (e.g., the line We compared the results of each independent analy- widths should be a few kms−1 at most). A quality sis. The results were similar for ∼90% of the spectra. factor of “A” was given to profiles with a signal-to-noise Any differences typically involved how best to fit com- ratio larger than 10 with no blending and flat spectral plex, blended profiles. Multiple 12CO emission compo- baselines. Targets with either lower signal-to-noise ratio nents were detected in about 2/3 of the targets. These or some blending of lines were given a quality factor directions thus contain several 12CO clouds along the “B”. Here the blending should not be so severe that line-of-sight. Furthermore, because CO is pervasive in the individual peaks could not be visually detected. A the inner Galactic plane, there was often 12CO emis- quality factor of “C” was given to the remaining targets sion detected in the Off position. This creates appar- which were often blended and more difficult to fit. A ent absorption lines in the processed position-switched quality factor of D was assigned to spectra with no CO spectrum. Since any emission components near these detections. absorption features are contaminated by this Off emis- CO in the OSC 7