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Central Molecular Zone in the Milky Way

ALMA Observations of Massive Clouds in the Central Molecular Zone: Ubiquitous Protostellar Outflows

  • Question:
    • CMZ is a region with a large amount of dense gas but with low star formation rates. The star-forming activities in the CMZ are still not well constrained in this special region. Further investigation of star formation in the CMZ is needed.
  • Aims:
    • Molecular outflows can trace obscured star formation. This study aims to identify outflows from YSOs in the CMZ.
  • new method:
    • ALMA 1.3 mm observations of shock tracers, SiO, SO, CH3OH, H2CO, HC3N, and HNCO, down to 2000 au resolution.
  • progress:
    • They identified 43 protostellar outflows (37 highly likely + 6 candidates) while 791 out of the 834 total cores do not have detected outflows. A large amount of starless cores in CMZ, indicate a early evolutionary phase? Outflow chemistry is similar in the CMZ and nearby clouds.
  • note:
    • The conclusion of early phase of star formation seems to contradict the galaxy evolution? or different generations of star formation?
    • They developed a code (calcu) to calculate the outflow parameters.
    • They also presented the partition functions of H2CO, CH3OH, and HNCO in their Table 2.

The Dense Gas Fraction in Galactic Center Clouds

  • Question:
    • The distribution of gas densities in the CMZ and the density structure are poorly constrained.
  • Aims:
    • HC3N is a good density probe and its transitions are usually optically thin. This study will use HC3N lines to determine the density structure in the CMZ.
  • Method:
    • They use MOPRA 22m, GBT, and APEX to observe a few HC3N lines.
  • Progress:
    • Two density component with one lower than 1e4 cm-3 and the other one between 1e5 and 1e6 cm-3. The results support the higher star-formation threshold in the CMZ. The line ratio increases sharply at R<140 pc, indicating the increase of the dense gas fraction.
  • note:
    • Actually, one should try to obtain the density distribution of the CMZ.

Chemistry

Phosphorus-bearing molecules in the Galactic Center

  • Question:
    • Phosphorus (P) is one of the essential elements for life. P-bearing molecules have already detected in star-forming regions, but their formation routines are poorly constrained.
  • Aims:
    • The aim is to investigate the formation and destruction routines of P-bearing molecules.
  • Method:
    • They observed 7 position in CMZ with IRAM 30m.
  • Progress:
    • PN is detected toward 5 position and PO is detected toward G+0.693-0.03. The two position of non-detection was attributed to the intense UV/X-ray/cosmic ray radiation. P-bearing molecules are likely formed in shocked gas as a result of dust grain sputtering.
  • note:
    • Mapping such molecules is still not possible, which may require the next-generation receivers.

high-resolution radio continuum

Detection of a dense group of hyper-compact radio sources in the central parsec of the Galaxy

  • Question:

    • THis is a so-called "missing pulsar problem" in the Galactic center. In the past ten million years, abundant massive stars are produced in the GC. However, the detection of massive stellar remnants in the GC is still small. One reason could be the scatter-broadening at longer radio wavelength. Recent studies have shown that the scatter-broadening may not be the mayor reason, but this could be due to the slected wavelength for this study. The other possiblity is that the massive stars in the GC tend to form magnetars that have rather short lifetime.

  • Aim:

    • search for radio continuum compact sources in the GC. This may facinate to use spectral index to find magnetar candidates which may be the solution to the missing pular problem.

  • Method:

    • Using JVLA high frequency to map the central parsec around the GC to determine the radio continuum compact sources.

  • Progress:

    • They detected 64 hyper-compact radio sourcs (HCRs). 38 steep with alpha<-0.5 -- nonthermal from neutron stars and stellar black holes, 10 flat-spectrum (-0.5<alpha<0.2), 17 inverted-spectrum sources with alpha>0.2. The known magnetar SGR J1745-2900 belongs to the inverted-spectrum sample, alpha_cm=-1.5+-0.6, alpha_mm=1.1+-0.2.

diffuse molecular component

The diffuse molecular component in the nuclear bulge of the Milky Way

-Questions: - Recently, a more diffuse, hotter (n 100 cm-3, T 250 K) gas component was discovered through absorption observations of H3+. This component may be widespread in the Galactic center, and low density gas detectable in absorption may be present even outside the CMZ along sightlines crossing the extended bulge of the Galaxy.

  • Aims:
    • We aim to observe and characterize diffuse and low density gas using observations of 3-mm molecular transitions seen in absorption.
  • Method:
    • ALMA absorption measurements toward the quasar J1744-312.
  • Result:
    • They detected three velocity components at 0, -153, and -192 km/s. While the 0 km/s component is associated with disk, the components at -153 and -192 km/s are likely to originate from the Galactic bulge.
  • commnets:
    • such abosrption against continuum sources should serve as a good probe to study the diffuse gas not in the halo but also across the whole galaxy.

Isotopic ratios

Tracing gas accretion in the Galactic cener using isotopic ratios

-Questions: - the origin of the giant molecular loops: from supernova of disk or the disk accretion.

  • Aim:
    • using C isotopic ratio to figure out the origin of the giant molecular loops.
  • Result:
    • they find that ratios of >40 in the halos and X1 orbits and values of 20-25 in the disks. This is suggestive of the presence of different degree of nuclear processing toward the GC.
  • comments:
    • furture resolved maps of isotopic ratios should better constrain the gas accretion scenarios.

Kinetic Temperature

Kinetic temperatures toward X1/X2 orbit interceptions regions and giant molecular loops in the Galactic center region

  • Questions:
    • Temperature of the giant molecular loops are not well constrained.
  • Aim:
    • Determine kinetic temperature and its implication of the origin. This is used to study gas cooling and heating
  • Method:
    • Ammonia observations.
  • Results:
    • They discovered two kinetic temperature components (150 K and 40 K). They argue that shocks provide heating both in the GC and high latitude. They discuss quite a few different heating mechanisms in this paper.

Extended velocity features - nice paper

The geometry of the gas surrounding the Central Molecular Zone: on the origin of localized molecular clouds with extreme velocity dispersions

  • Questions: - There are quite a few extended velocity features in the CMZ with velocity dispersion of an order of 200 km/s. Despite many possible interpretations, the origin of these features are not well constrained.
  • Aim: - They aim to determine the orgin of the EVFs.
  • Method: - Numerical simulations.
  • Results: - The EVFs arise from the cloud-cloud collisions. The l=3 and l=5 EVFs originate from the collision between material on the dust lanes and material that has overshot from the dust lane on the opposite side. - The l=1.3 EVFs originate from the collision between the accreted fresh gas and the material of the CMZ.

mass inflow rate

Mass inflow rate into the Central Molecular Zone: observational determination and evidence of episodic accretion

  • Questions: - The Galactic bar drives a gas flow into the CMZ, and such inflow happens mostly through two dust lanes. However, the infall rate has not been well determined observationally.
  • Aim: - Determine the mass infall rate from CO observations.
  • Method: - They isolate the dust lanes based on the l-v diagrams. They use X factor to determine the mass on the dust lanes. They adopted the geometric model to do deprojection.
  • Results: - They actually determine the mass inflow rate along the dust lanes (i.e., as a function of time). They found that the accretion rates are changing with time, suggesting episodic accretion. The mass infall rate is roughly 2.7 Msun/yr. They suggest that gas replenishment should take place in the region. Based on the input kinetic energy per unit time and turbulence dissipation rate, the accretion-driven turbulence appears to be feasible in this case.
  • comments: - This is a nice idea to follow, and a nice estimate of the mass infall rate. For the mass uncertainties, one might use optically thin tracers to better estimate the mass. However, the isotopic ratio and fractional abundances would be new sources of uncertainties. They can be determined with more effects. One argument on the accretion-driven turbulence, this scenario works only if the conversion efficiency is close to 100%.

Widespread Hot Ammonia in the Central Kiloparsec of the Milky Way

  • Questions:
    • The molecular gas in this region is more than an order of magnitude hotter than gas in the Galactic disk, but the mechanism responsible for heating the gas remains uncertain.
  • Aims:
    • They aim to characterize the temperature for 16 regions, extending out to a projected radius of ∼450 pc.
  • Method:
    • ATCA NH3(1,1)-(6,6) + GBT (8,8)-(14,14)
  • Results:
    • For the cool component they detect rotational temperatures ranging from 20-80 K, and for the hot component we detect temperature ranging from 210-580 K. (not Tkin, but a lower limit. Hence, the most extreme temperature). They found no correlation between line width and temperature. Non-thermal motions are dominant. Cosmic rays and turbulence should play an important role in the gas heating.
  • Note:
    • Bania's Clump 2 show hottest temperature.