The Multi-Messenger Astrophysics of the Galactic Centre
Proceedings IAU Symposium No. 322, 2016
R.M. Crocker, S.N. Longmore & G.V. Bicknell, eds.
c
International Astronomical Union 2017
doi:10.1017/S1743921316011960
Temperature Evolution of Molecular Clouds
in the Central Molecular Zone
Nico Krieger
1
,J¨urgen Ott
2
, Fabian Walter
1, 2
,J.M.Diederik
Kruijssen
3, 1
, Henrik Beuther
1
and the SWAG team
1
Max Planck Institute for Astronomy, Heidelberg, Germany
2
National Radio Astronomy Observatory, Socorro, NM, USA
3
Astronomisches Rechen-Institut, Universit¨at Heidelberg, Germany
Abstract. We infer the absolute time dependence of kinematic gas temperature along a pro-
posed orbit of molecular clouds in the Central Molecular Zone (CMZ) of the Galactic Center
(GC). Ammonia gas temperature maps are one of the results of the “Survey of Water and Am-
monia in the Galactic Center” (SWAG, PI: J. Ott); the dynamical model of molecular clouds
in the CMZ was taken from Kruijssen et al. (2015). We find that gas temperatures increase as
a function of time in both regimes before and after the cloud passes pericenter on its orbit in
the GC potential. This is consistent with the recent proposal that pericenter passage triggers
gravitational collapse. Other investigated quantities (line width, column density, opacity) show
no strong sign of time dependence but are likely dominated by cloud-to-cloud variations.
Keywords. ISM: clouds, ISM: evolution, ISM: structure, radio lines: ISM, stars: formation
The Central Molecular Zone (CMZ) in the Galactic Center (GC) contains large amounts
of dense molecular gas, but observed star formation rates are lower than expected by a
factor of 10 − 100 (Longmore et al. 2013a). Part of this gas is projected onto an ∞-like
shape and has been modeled as open-ended streams wrapping around the GC (Longmore
et al. 2013b; Kruijssen et al. 2015). The streams repeatedly pass close (60 pc) to the GC
where cloud collapse is expected to be triggered by tidal interactions. Downstream of the
near side pericenter, a sequence of advancing star formation (SF) tracers is present that
may indicate a sequence of progressing star formation states (Longmore et al. 2013b).
We aim to understand the conditions of molecular gas, which is the fuel of SF, at the
different stages of the sequence.
In the “Survey of Water and Ammonia in the Galactic Center” (SWAG, cf. overview
by Ott et al. in this volume) targeted the CMZ at six meta-stable ammonia lines among a
variety of other spectral lines. Fig. 1 (left panel) shows a peak intensity map of NH
3
(1,1)
highlighting the ring-like gas streams and other relevant objects like molecular clouds and
star clusters. The right panel of Fig. 1 represents the same view in kinetic gas temperature
T
24
which can be derived from the relative line fluxes through hyperfine structure fits of
NH
3
(2,2) and NH
3
(4,4) on a pixel by pixel basis (Krieger 2016a, 2016b).
The kinematic model of Kruijssen et al. (2015, see marked orbit in fig. 1, left) allows
us to assign time stamps to CMZ clouds if their 3D position (longitude, latitude, line-of-
sight velocity) is known. Thus, mapped quantities like temperature can be plotted as a
function of time as in Fig. 2 for kinetic gas temperature T
24
. Temperatures increase in
the dust ridge (-1.7 to -1.3 Myr) and near side pericenter passage (+1.8 to +2.0 Myr) as
shown by the fits at consistent slopes of ∼ 46 K/Myr and ∼ 42 K/Myr. At 0.0 Myr (far
side pericenter), we find a similar trend, although the statistical basis is weak because of
the short sampled time range and few measurements due to low SNR.
160
https://doi.org/10.1017/S1743921316011960 Published online by Cambridge University Press
Molecular Clouds in the CMZ 161
Figure 1. left: Overview of the central CMZ. The NH
3
(1,1) peak flux map extends from -0.65
◦
to +0.80
◦
in Galactic longitude and ±0.35
◦
in Galactic latitude. Gas streams are highlighted
(dark band) and overplotted with time (white) since the far side pericenter passage in the
model of Kruijssen et al. (2015). Pericenter passages occur at ±2.0 Myr and 0.0 Myr. The “dust
ridge” spans between the “Brick” and Sgr B2. right: NH
3
(2,2)-(4,4) kinetic temperature (T
24
in
Kelvin) map of the CMZ derived from pixel-by-pixel fitting of the ammonia hyperfine structure
(Krieger 2016a, 2016b). Blanked regions cannot be fitted due to low signal-to-noise ratios.
Figure 2. Kinetic ammonia temperature T
24
[NH
3
(2,2) to (4,4)] as function of time since far
side pericenter passage. Individual temperature measurements are overplotted with data density
contours, medians of bins of 0.01 Myr width (crosses) and linear fits to these medians.
We also find quantitatively similar results for kinetic ammonia temperatures T
45
[NH
3
(4,4) to (5,5)] and T
36
[NH
3
(3,3) to (6,6)]. The physical reasons for differences
between temperature slopes of different temperature tracers are currently unknown but
likely reflect different conditions in the clouds.
Other mapped quantities (column density, opacity, line width) are dominated by vari-
ations of cloud structure and do not show consistent evolution with time.
References
Krieger, N. 2016a, Master thesis, University of Heidelberg
Krieger, N., Ott, J., Walter, F., et al. 2016b, in prep.
Kruijssen, J. M. D., Dale, J. E., & Longmore, S. N. 2015, MNRAS, 447, 1059
Longmore, S. N., Bally, J., Testi, L. et al. 2013a, MNRAS, 429, 987
Longmore, S. N., Kruijssen, J. M. D., Bally et al. 2013b, MNRAS, 433, L15
https://doi.org/10.1017/S1743921316011960 Published online by Cambridge University Press