ESO CMZ Workshop 2015

Conference Photo


The workshop is intended to discuss ongoing and future work studying the central molecular zone of our galaxy, with a particular emphasis on the relationship between the molecular gas and star formation.

The conference web page is and abstracts are listed at

This document is intended to include notes from the workshop and a starting point for future work, especially observing proposals and perhaps even papers.

The “Dense” ISM

Discussions on what would normally be considered high-density gas (\(n\sim10^4\) cm\(^{-3}\)) but in the CMZ is “typical”.

What are the true gas densities and what is the density distribution? Which clouds are bound? Are they supported by turbulence, magnetic fields, thermal pressure, shear, or are they freely collapsing?

What are the magnetic fields and temperatures of these clouds?

What is the geometry of the molecular gas in the CMZ - is there really a ring? Are the clouds on elliptical orbits? Which orbits?

Do cosmic rays, turbulence, x-rays, photons at other wavelengths, or other mechanisms heat the cloud interiors? Exteriors?

What do we still need to understand from the kinematics? Are positions every uniquely associated with a given kinematic signature? How can we describe the turbulence?

What explains the “cloud collision” kinematics often seen toward CMZ clouds (Tanaka et al., 2015; Tanaka et al., 2014; Oka et al., 2012)?

Comparison of H2CO and NH3 temperatures and morphology – large and small scales

Global properties?

Total molecular mass? \(4\times10^7\) M\(_\odot\) out to \(r<200\) pc?

Slightly less than 5% of the total molecular gas reservoir of \(\sim8.4\times10^8\) in the Galaxy is concentrated in the center of the Galaxy, in a region with a diameter of \(\sim\) 600 parsecs (Dahmen et al., 1998; Nakanishi et al., 2006). The total mass of molecular gas in the CMZ is believed to be 3\(^{+2}_{-1}\times10^7\) (Dahmen et al., 1998), with 10% of this gas concentrated in one giant molecular cloud complex, Sgr B2 (Gordon et al., 1993), located \(\sim\) 100 parsecs in projection to the east of the dynamical center of the Galaxy.

Cores in the CMZ

Protostellar cores are, by definition, the precursors to stars. What do they look like in the CMZ? Are there good chemical tracers for cores in the CMZ? Are dust-based masses accurate?

Why don’t we observe cores in The Brick? Are they absent, or invisible? Do we see cores in other clouds? How do cores in the CMZ compare with cores in the Galactic plane?

Comparison to the Galactic Disk

How does star formation differ in our Galactic center from the rest of the Galaxy?

Longmore et al. (2013): Lower SFR/dense gas

How do the isotope ratios in the center of the Milky Way compare to the disk? \(^{13}\)C/\(^{12}\)C appears lower, at least, (e.g., Riquelme et al., 2010), as do more recent estimates of the \(^{15}\)N/\(^{14}\)N ratio (Adande et al., 2011).

Comparison to other Galaxies

What are the most similar galactic centers to our own? What can we learn from comparing these galaxies to ours?

How does our galactic center compare with other galaxies? Its pressure and sensitivity are similar to high-z star-forming galaxies (Kruijssen et al., 2013).

  1. M83: similar mass galaxy, more active CMZ, face on

How do isotope ratios in the Galactic center compare to those in the (centers of) other galaxies, and what role does opacity play in measurements in both of these regions?

With ALMA it will be easily possible for galaxies at  3 Mpc to observe their centers at parsec-scale resolution, or the resolution of existing (and planned) single-dish millimeter surveys. In our Galactic center, what can then be done is to probe inside any particularly representative or “proxy” clouds at resolutions orders of magnitude more compact than can be achieved in the more distant and extreme nuclei.

Molecular gas around a black hole

How is molecular gas affected by the presence of Sgr A*? Do we see its effects anywhere beyond the circumnuclear disk? (yes) SiO has been suggested to be correlated with the Fe 6.4 KeV line in the central 200 pc (Martin-Pintado et al., 2000), with the G0.11-0.11 cloud (Handa et al., 2006) being a notable example of very high SiO abundances being associated with time-variable X-ray emission (Ponti et al., 2010).

Summary of existing and ongoing surveys

Sorted from low to high frequency. Probably should be a table.

Uniform-coverage Surveys:

Pointed & Targeted Surveys (typically toward high column-density gas):

  • 230 GHz SMA Legacy Survey of the CMZ (Battersby, Keto)

  • NH3 (1,1)-(5,5) and H2O masers, GCMS (Kauffmann et al.)

  • NH3 (1,1)-(7,7), HC3N, and CH3OH masers (Mills et al.)

  • 220 + 290 GHz APEX observations of five/seven CMZ clouds (Immer, Kauffmann, Pillai)

Missing: 50-80 GHz, 115-210 GHz

Options for filling in these frequency ranges:

  • GBT 4mm receiver : 67-93 GHz

  • ALMA Band 4: 125-163 GHz

  • APEX Band 5 (SEPIA): 158-211 GHz

Figure showing coverage for a selection of existing and in-progress surveys (Figure needs inclusion of APEX-H2CO, SWAG and SMA surveys to be up to date; such inner 2 degree surveys should probably by done with a zoomed-in secondary figure)

Rough map of existing and proposed-for ALMA observations in the inner R < 200 pc, with arcsecond (subarcsecond to 1.5 arcsecond) resolution.

Non-exhaustive map of existing and proposed-for high-frequency VLA observations at  2-3\(''\) resolution

Proposals: Ideas and plans

(remember that this is public)

Large surveys of the CMZ: More needed? How should we prioritize?

  1. LASMA band 7 APEX survey?

  2. JCMT-HARPS CO 3-2 survey?

  3. Multi-J 13CO surveys?

Followup to APEX 1.4mm survey? Bigger, deeper, complementary transitions?

Followup to SMACMZ survey? More area? Higher frequency? Multiple frequencies? ALMA, SMA?

Low frequencies: VLA large survey? Extension of Law et al. (2008)a continuum survey?

  1. (deep) S band, to include CH and recombination lines?

  2. X band, to include HC3N 1-0?

Preparing for Cycle 4

What line surveys (single dish and other) exist?

Cover a large area with a single band (3?) or a targeted cloud sample at multiple bands for, e.g., excitation?

ALMA configuration options:

  • No special modes (OTF)

  • Vanilla setup: band 3, 4, 5, or 6 (or 7)

  • At least 2 GHz continuum? Or just spectral line?


8 GHz now, 16 GHz future, but extra 8 GHz probably only good for continuum

Data to become public when they are reduced.

How can we identify star forming cores? Original idea: hot core tracers + outflows. Does that work? CH\(_3\)OH masers, but incomplete/maybe just shock tracers?

Evolutionary sequence in massive cores seen in dust ridge clouds. Steve: free-fall times come from single-dish, large scale. Diederik: position along ridge dominates spread in density. (Comment by Diederik: I have now checked this quantitatively and it turns out that the position is indeed somewhat more important, but not by much – from Table 2 in Kruijssen et al. (2015), the time-scale spread is from 0.30 Myr post-pericentre to 0.74 post-pericentre (a factor of 2.5), whereas the density spread is factor of 4 => factor of 2 in free-fall time.)

Lines mapped: H\(_2\)CO, C\(^{18}\)O, CH\(_3\)OH, \(^{13}\)CO, SiO.

Sgr B2: Dust is dominant at 1mm, but free-free is partially still present

Lots of interesting & impressive detections. PDFs - do they mean anything?

Mixed temperature components

How much gas can be cold?

Hi-Gal Compact Sources

Single-band extraction with subsequent band-merging

Inner region: 27% “protostellar” and 73% “prestellar” sources in \(|\ell|<5\). Large dispersion in latitude for both populations.

Most higal “cores” are clouds in the CMZ

Qizhou: Correlation with 24sources? Yes, included. Do all 24 sources have 70 counterparts? Yes.

Dust temperature: CMZ vs Galactic disk - essentially the same in both. 16 vs 13 K for proto/pre-stellar. L/M is also nearly the same in and out of the CMZ... slightly less difference in the CMZ than outside.

Milky Way simulation: Emsellem, Renaud, Bournaud, Teyssier

See ref paper = (Renaud 2013) and (Emsellem 2014)

Relative Milky Way BH and nuclear cluster mass may be an order of magnitude less than in simulation, BH / CMZ (MW) \(\sim0.1\) BH/CMZ(sim) but to be checked.

Clearly the mass ratio, extent, etc will decide on the fate, and cycle (if cycle there is).


Comments from Padelis:

The relevant papers, that expand the view of what such extreme conditions mean in galaxies beyond the CMZ are :

and for some of the consequences for the initial conditions of SF and the IMF:

Future Meetings?

Prior to Cycle 4 call: similar meeting? 20ish participants Where?

Lorentz Center? Sexten? Aspen?

IAU 322 Australia Reefish 25-29 July 2016?

Aspen center:

  • Summer workshop deadline end of May. Summer 2016, 40 participants. 3 weeks

  • Winter workshop deadline January. Winter 2016. 80 participants. 1 week

  • Summer 2017.... deadline May 2016?

Sexten: Deadlines? Times?! Information TBD...

(Higher energy) CMZ-focused meetings:

  • Ringberg June 5-11

  • COSPAR July 30-August 4?

  • Summer 2016 - ESO Galactic star formation meeting

Key follow-up work

  • Further proper motion studies. Interested: Everyone.

  • Large spatial scale (\(b\pm1\)) CO mapping to test Diederik’s latest theory

  • SEPIA line surveys - what is useful in Band 5?

  • keep an eye on the CO J=3-2 mapping projects (ongoing) - they may lead to followup with LASMA higher-J high-density tracers? (characterize density, temperature more accurately)


  1. G. Dahmen, S. Huttemeister, T. L. Wilson, R. Mauersberger. Molecular gas in the Galactic center region. II. Gas mass and N_, = H_2/I_^(12)CO conversion based on a C^(18)O(J = 1 - 0) survey. 331, 959-976 (1998).

  2. M. A. Gordon, U. Berkermann, P. G. Mezger, R. Zylka, C. G. T. Haslam, E. Kreysa, A. Sievers, R. Lemke. Anatomy of the Sagittarius complex. 3: Morphology and characteristics of the SGR B2 giant molecular giant molecular cloud. 280, 208-220 (1993).

  3. P. A. Jones, M. G. Burton, M. R. Cunningham, N. F. H. Tothill, A. J. Walsh. Spectral imaging of the central molecular zone in multiple 7-mm molecular lines. 433, 221-234 (2013). Link

  4. P. A. Jones, M. G. Burton, M. R. Cunningham, M. A. Requena-Torres, K. M. Menten, P. Schilke, A. Belloche, S. Leurini, J. Martín-Pintado, J. Ott, A. J. Walsh. Spectral imaging of the Central Molecular Zone in multiple 3-mm molecular lines. 419, 2961-2986 (2012).

  5. J. M. D. Kruijssen, S. N. Longmore. Comparing molecular gas across cosmic time-scales: the Milky Way as both a typical spiral galaxy and a high-redshift galaxy analogue. 435, 2598-2603 (2013).

  6. J. M. D. Kruijssen, J. E. Dale, S. N. Longmore. The dynamical evolution of molecular clouds near the Galactic Centre - I. Orbital structure and evolutionary timeline. 447, 1059-1079 (2015).

  7. C. J. Law, F. Yusef-Zadeh, W. D. Cotton. A Wide-Area VLA Continuum Survey near the Galactic Center at 6 and 20 cm Wavelengths. 177, 515-545 (2008).

  8. C. J. Law, F. Yusef-Zadeh, W. D. Cotton, R. J. Maddalena. Green Bank Telescope Multiwavelength Survey of the Galactic Center Region. 177, 255-274 (2008).

  9. S. N. Longmore, J. Bally, L. Testi, C. R. Purcell, A. J. Walsh, E. Bressert, M. Pestalozzi, S. Molinari, J. Ott, L. Cortese, C. Battersby, N. Murray, E. Lee, J. M. D. Kruijssen, E. Schisano, D. Elia. Variations in the Galactic star formation rate and density thresholds for star formation. 429, 987-1000 (2013).

  10. H. Nakanishi, Y. Sofue. Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: II. The Molecular Gas Disk. 58, 847-860 (2006).

  11. T. Oka, Y. Onodera, M. Nagai, K. Tanaka, S. Matsumura, K. Kamegai. ASTE CO J = 3-2 Survey of the Galactic Center. 201, 14 (2012).

  12. G. Ponti, R. Terrier, A. Goldwurm, G. Belanger, G. Trap. Discovery of a Superluminal Fe K Echo at the Galactic Center: The Glorious Past of Sgr A* Preserved by Molecular Clouds. 714, 732-747 (2010). Link

  13. D. Riquelme, M. A. Amo-Baladron, J. Martin-Pintado, R. Mauersberger, S. Martin, L. Bronfman. Tracing gas accretion in the Galactic center using isotopic ratios. ArXiv e-prints (2010).

  14. K. Tanaka, M. Nagai, K. Kamegai, T. Oka. CO-0.30-0.07: A Peculiar Molecular Clump with an Extremely Broad Velocity Width. ArXiv e-prints (2015).

  15. Kunihiko Tanaka, Tomoharu Oka, Shinji Matsumura, Makoto Nagai, Kazuhisa Kamegai. High Velocity Compact Clouds in the Sagittarius C Region. (2014).

  16. A. J. Walsh, S. L. Breen, T. Britton, K. J. Brooks, M. G. Burton, M. R. Cunningham, J. A. Green, L. Harvey-Smith, L. Hindson, M. G. Hoare, B. Indermuehle, P. A. Jones, N. Lo, S. N. Longmore, V. Lowe, C. J. Phillips, C. R. Purcell, M. A. Thompson, J. S. Urquhart, M. A. Voronkov, G. L. White, M. T. Whiting. The H\(_{2}\)O Southern Galactic Plane Survey (HOPS) - I. Techniques and H\(_{2}\)O maser data. 416, 1764-1821 (2011).

  17. F. Yusef-Zadeh, M. Wardle, D. Lis, S. Viti, C. Brogan, E. Chambers, M. Pound, M. Rickert. 74 MHz Nonthermal Emission from Molecular Clouds: Evidence for a Cosmic Ray Dominated Region at the Galactic Center. Journal of Physical Chemistry A 117, 9404-9419 (2013).

  18. F. Yusef-Zadeh, W. Cotton, S. Viti, M. Wardle, M. Royster. Widespread Methanol Emission from the Galactic Center. ArXiv e-prints (2013).


  20. E. Emsellem, F. Renaud, F. Bournaud, B. Elmegreen, F. Combes, J. M. Gabor. The interplay between a galactic bar and a supermassive black hole: nuclear fuelling in a subparsec resolution galaxy simulation. Monthly Notices of the Royal Astronomical Society 446, 2468–2482 Oxford University Press (OUP), 2014. Link

  21. Toshihiro Handa, M Sakano, Seiichiro Naito, M Hiramatsu, M Tsuboi. Thermal SiO and H13CO+ Line Observations of the Dense Molecular Cloud G0.11-0.11 in the Galactic Center Region. The Astrophysical Journal 636, 261 (2006).

  22. J Martin-Pintado, P de Vicente, Nemesio J Rodríguez-Fernández, A Fuente, P Planesas. A correlation between the SiO and the Fe 6.4 keV line emission from the Galactic center. Astronomy and Astrophysics 356, L5 (2000).

  23. F. Renaud, F. Bournaud, E. Emsellem, B. Elmegreen, R. Teyssier, J. Alves, D. Chapon, F. Combes, A. Dekel, J. Gabor, P. Hennebelle, K. Kraljic. A sub-parsec resolution simulation of the Milky Way: global structure of the interstellar medium and properties of molecular clouds. Monthly Notices of the Royal Astronomical Society 436, 1836–1851 Oxford University Press (OUP), 2013. Link

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