scholarly journals The life cycle of the Central Molecular Zone – II. Distribution of atomic and molecular gas tracers

2020 ◽  
Vol 493 (4) ◽  
pp. 5273-5289 ◽  
Author(s):  
Lucia Armillotta ◽  
Mark R Krumholz ◽  
Enrico M Di Teodoro
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Time Window ◽  
Thermal State ◽  
Formation Rate ◽  
Galactic Centre ◽  
Molecular Gas ◽  
Emission Data ◽  
Data Cubes ◽  
Intense Star Formation

ABSTRACT We use the hydrodynamical simulation of our inner Galaxy presented in Armillotta et al. to study the gas distribution and kinematics within the Central Molecular Zone (CMZ). We use a resolution high enough to capture the gas emitting in dense molecular tracers such as NH3 and HCN, and simulate a time window of 50 Myr, long enough to capture phases during which the CMZ experiences both quiescent and intense star formation. We then post-process the simulated CMZ to calculate its spatially dependent chemical and thermal state, producing synthetic emission data cubes and maps of both H i and the molecular gas tracers CO, NH3, and HCN. We show that, as viewed from Earth, gas in the CMZ is distributed mainly in two parallel and elongated features extending from positive longitudes and velocities to negative longitudes and velocities. The molecular gas emission within these two streams is not uniform, and it is mostly associated with the region where gas flowing towards the Galactic Centre through the dust lanes collides with gas orbiting within the ring. Our simulated data cubes reproduce a number of features found in the observed CMZ. However, some discrepancies emerge when we use our results to interpret the position of individual molecular clouds. Finally, we show that, when the CMZ is near a period of intense star formation, the ring is mostly fragmented as a consequence of supernova feedback, and the bulk of the emission comes from star-forming molecular clouds. This correlation between morphology and star formation rate should be detectable in observations of extragalactic CMZs.

scholarly journals CO Observations of Bright IRAS Galaxies

1987 ◽  
Vol 115 ◽  
pp. 653-653
Author(s):  
D. B. Sanders
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Formation Rate ◽  
Far Infrared ◽  
Molecular Gas ◽  
Infrared Excess ◽  
Luminous Galaxies ◽  
Be Star ◽  
Co Emission ◽  
Co Observations

CO emission has been detected from 75 bright infrared galaxies with CZ = 2 000 – 16 000 km/s. These include the most distant and the most luminous galaxies (Arp 55, IR 1713+63) yet detected in CO. All of these galaxies are rich in molecular gas with Mtotal (H2) = 2 × 109 −6x1010 M⊙, and they have a strong far-infrared excess, with LFIR/LB = 2-40 and LFIR (40-400μ) = 1010 – 3 × 1012 L⊙. The primary luminosity source appears to be star formation in molecular clouds. A strong correlation is found between the FIR and 21-cm continuum flux, implying that the IMF is independent of the star formation rate. The ratio LFIR/M(H2) provides a measure of the current rate of star-formation, which is found to be a factor 3-20 larger in these galaxies than for the ensemble of molecular clouds in the Milky Way. VLA maps plus a few high resolution (14″-30″) CO (1-0) and CO (2-1) maps suggest that most of the luminosity comes from core regions 1-3 kpc in size. The abnormal concentration of molecular gas in these galactic cores is presumably the result of a collision or strong interaction with a nearby companion.

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

2019 ◽  
Vol 488 (4) ◽  
pp. 4663-4673 ◽  
Author(s):  
Mattia C Sormani ◽  
Robin G Treß ◽  
Simon C O Glover ◽  
Ralf S Klessen ◽  
Ashley T Barnes ◽  
...  
Keyword(s):  
Molecular Clouds ◽  
Gas Flow ◽  
Milky Way ◽  
Line Of Sight ◽  
Galactic Centre ◽  
Molecular Gas ◽  
Distinct Population ◽  
Data Cubes ◽  
Velocity Dispersions

ABSTRACT Observations of molecular gas near the Galactic Centre (|l| < 10°, |b| < 1°) reveal the presence of a distinct population of enigmatic compact clouds that are characterized by extreme velocity dispersions ($\Delta v \gt 100\, {\rm km\, s^{-1}}$). These extended velocity features are very prominent in the data cubes and dominate the kinematics of molecular gas just outside the Central Molecular Zone (CMZ). The prototypical example of such a cloud is Bania Clump 2. We show that similar features are naturally produced in simulations of gas flow in a realistic barred potential. We analyse the structure of the features obtained in the simulations and use this to interpret the observations. We find that the features arise from collisions between material that has been infalling rapidly along the dust lanes of the Milky Way bar and material that belongs to one of the following two categories: (i) material that has ‘overshot’ after falling down the dust lanes on the opposite side; (ii) material which is part of the CMZ. Both types of collisions involve gas with large differences in the line-of-sight velocities, which is what produces the observed extreme velocity dispersions. Examples of both categories can be identified in the observations. If our interpretation is correct, we are directly witnessing (a) collisions of clouds with relative speeds of $\sim 200\, {\rm km\, s^{-1}}$ and (b) the process of accretion of fresh gas onto the CMZ.

scholarly journals Star Formation in the Local Milky Way

2015 ◽  
Vol 10 (S314) ◽  
pp. 8-15
Author(s):  
Charles J. Lada
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Milky Way ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Young Stars ◽  
Molecular Gas ◽  
Physical Processes ◽  
Star Forming ◽  
Star Forming Regions

AbstractStudies of molecular clouds and young stars near the sun have provided invaluable insights into the process of star formation. Indeed, much of our physical understanding of this topic has been derived from such studies. Perhaps the two most fundamental problems confronting star formation research today are: 1) determining the origin of stellar mass and 2) deciphering the nature of the physical processes that control the star formation rate in molecular gas. As I will briefly outline here, observations and studies of local star forming regions are making particularly significant contributions toward the solution of both these important problems.

scholarly journals FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45 m telescope (FUGIN). V. Dense gas mass fraction of molecular gas in the Galactic plane

2019 ◽  
Vol 71 (Supplement_1) ◽  
Author(s):  
Kazufumi Torii ◽  
Shinji Fujita ◽  
Atsushi Nishimura ◽  
Kazuki Tokuda ◽  
Mikito Kohno ◽  
...  
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Galactic Plane ◽  
Formation Rate ◽  
Molecular Gas ◽  
Galactic Rotation ◽  
Dense Gas ◽  
Gas Formation ◽  
Star Formation In Galaxies ◽  
Fractional Mass

Abstract Recent observations of the nearby Galactic molecular clouds indicate that the dense gas in molecular clouds has quasi-universal properties on star formation, and observational studies of extra-galaxies have shown a galactic-scale correlation between the star formation rate (SFR) and the surface density of molecular gas. To reach a comprehensive understanding of both properties, it is important to quantify the fractional mass of dense gas in molecular clouds, fDG. In particular, for the Milky Way (MW) there are no previous studies resolving fDG disk over a scale of several kpc. In this study, fDG was measured over 5 kpc in the first quadrant of the MW, based on the CO J = 1–0 data in l = 10°–50° obtained as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) project. The total molecular mass was measured using 12CO, and the dense gas mass was estimated using C18O. The fractional masses, including fDG, in the region within ±30% of the distances to the tangential points of the Galactic rotation (e.g., the Galactic Bar, Far-3 kpc Arm, Norma Arm, Scutum Arm, Sagittarius Arm, and inter-arm regions) were measured. As a result, an averaged fDG of $2.9^{+2.6}_{-2.6}$% was obtained for the entirety of the target region. This low value suggests that dense gas formation is the primary factor in inefficient star formation in galaxies. It was also found that fDG shows large variations depending on the structures in the MW disk. In the Galactic arms, fDG was estimated to be ∼4%–5%, while in the bar and inter-arm regions it was as small as ∼0.1%–0.4%. These results indicate that the formation/destruction processes of the dense gas and their timescales are different for different regions in the MW, leading to differences in Star formation efficiencies.

scholarly journals A panchromatic spatially resolved analysis of nearby galaxies – II. The main sequence – gas relation at sub-kpc scale in grand-design spirals

2020 ◽  
Vol 496 (4) ◽  
pp. 4606-4623 ◽  
Author(s):  
L Morselli ◽  
G Rodighiero ◽  
A Enia ◽  
E Corbelli ◽  
V Casasola ◽  
...  
Keyword(s):  
Star Formation ◽  
Main Sequence ◽  
Formation Rate ◽  
Molecular Gas ◽  
Spatially Resolved ◽  
Grand Design ◽  
Nearby Galaxies ◽  
Formation Efficiency ◽  
H2 Molecule ◽  
Intense Star Formation

ABSTRACT In this work, we analyse the connection between gas availability and the position of a region with respect to the spatially resolved main-sequence (MS) relation. Following the procedure presented in Enia et al. (2020), for a sample of five face-on, grand design spiral galaxies located on the MS we obtain estimates of stellar mass and star formation rate surface densities (Σ⋆ and ΣSFR) within cells of 500 pc size. Thanks to H i 21cm and 12CO(2–1) maps of comparable resolution, within the same cells we estimate the surface densities of the atomic (ΣH i) and molecular ($\Sigma _{\rm {H_2}}$) gas and explore the correlations among all these quantities. Σ⋆, ΣSFR, and $\Sigma _{\rm {H_2}}$ define a 3D relation whose projections are the spatially resolved MS, the Kennicutt–Schmidt law and the molecular gas MS. We find that $\Sigma _{\rm {H_2}}$ steadily increases along the MS relation and is almost constant perpendicular to it. ΣH i is nearly constant along the MS and increases in its upper envelope. As a result, ΣSFR can be expressed as a function of Σ⋆ and ΣH i, following the relation log ΣSFR = 0.97log Σ⋆ + 1.99log ΣH i − 11.11. We show that the total gas fraction significantly increases towards the starburst regions, accompanied by a weak increase in star formation efficiency. Finally, we find that H2/H i varies strongly with the distance from the MS, dropping dramatically in regions of intense star formation, where the UV radiation from newly formed stars dissociates the H2 molecule, illustrating the self-regulating nature of the star formation process.

scholarly journals Using the Milky Way as a template for understanding star formation in extreme environments across cosmological timescales

2012 ◽  
Vol 8 (S292) ◽  
pp. 333-333
Author(s):  
Steven N. Longmore
Keyword(s):  
Star Formation ◽  
Milky Way ◽  
Formation Rate ◽  
Volume Density ◽  
Galactic Centre ◽  
Molecular Gas ◽  
Dense Gas ◽  
Star Forming ◽  
Star Formation Activity ◽  
The Galaxy

AbstractRecent surface- and volume-density star formation relations have been proposed which potentially unify our understanding of how gas is converted into stars, from the nearest star forming regions to ultra-luminous infrared galaxies. The inner 500 pc of our Galaxy – the Central Molecular Zone – contains the largest concentration of dense, high-surface density molecular gas in the Milky Way, providing an environment where the validity of these star-formation prescriptions can be tested.We have used recently-available data from HOPS, MALT90 and HiGAL at wavelengths where the Galaxy is transparent, to find the dense, star-forming molecular gas across the Milky Way [Longmore et al. (2012a), Longmore et al. (2012b)]. We use water and methanol maser emission to trace star formation activity within the last 105 years and 30 GHz radio continuum emission from the Wilkinson Microwave Anisotropy Satellite (WMAP) to estimate the high-mass star formation rate averaged over the last ∼ 4 × 106 years.We find the dense gas distribution is dominated by the very bright and spatially-extended emission within a few degrees of the Galactic centre [Purcell et al. (2012)]. This region accounts for ∼80% of the NH3(1,1) integrated intensity but only contains 4% of the survey area. However, in stark contrast, the distribution of star formation activity tracers is relatively uniform across the Galaxy.To probe the dense gas vs SFR relationship towards the Galactic centre region more quantitatively, we compared the HiGAL column density maps to the WMAP-derived SFR across the same region. The total mass and SFR derived using these methods agree well with previous values in the literature. The main conclusion from this analysis is that both the column-density threshold and volumetric SF relations over-predict the SFR by an order of magnitude given the reservoir of dense gas available to form stars. The region 1° < l < 3.5°, |b| < 0.5° is particular striking in this regard. It contains ∼107 M⊙ of dense molecular gas — enough to form 1000 Orion-like clusters — but the present-day star formation rate within this gas is only equivalent to that in Orion. This implication of this result is that any universal column/volume density relations must be a necessary but not sufficient condition for SF to occur.Understanding why such large reservoirs of dense gas deviate from commonly assumed SF relations is of fundamental importance and may help in the quest to understand SF in more extreme (dense) environments, like those found in interacting galaxies and at earlier epochs of the Universe.

scholarly journals Molecular gas kinematics of the CMZ: Great oaks from little acorns grow

2016 ◽  
Vol 11 (S322) ◽  
pp. 85-89
Author(s):  
Jonathan D. Henshaw
Keyword(s):  
Star Formation ◽  
Velocity Field ◽  
Molecular Clouds ◽  
Extreme Environment ◽  
Galactic Centre ◽  
Molecular Gas ◽  
Evolutionary Sequence ◽  
Gas Kinematics ◽  
Gravitational Instabilities ◽  
The Galaxy

AbstractThe central molecular zone (CMZ) hosts some of the most massive and dense molecular clouds and star clusters in the Galaxy, offering an important window into star formation under extreme conditions. Star formation in this extreme environment may be closely linked to the 3-D distribution and orbital dynamics of the gas. Here I discuss how our new, accurate description of the {l,b,v} structure of the CMZ is helping to constrain its 3-D geometry. I also present the discovery of a highly-regular, corrugated velocity field located just upstream from the dust ridge molecular clouds (which include G0.253+0.016 and Sgr B2). The extremes in this velocity field correlate with a series of massive (~ 104 M⊙) cloud condensations. The corrugation wavelength (~23 pc) and cloud separation (~8 pc) closely agree with the predicted Toomre (~17 pc) and Jeans (~6 pc) lengths, respectively. I conclude that gravitational instabilities are driving the formation of molecular clouds within the Galactic Centre gas stream. Furthermore, I suggest that these seeds are the historical analogues of the dust ridge molecular clouds – possible progenitors of some of the most massive and dense molecular clouds in the Galaxy. If our current best understanding for the 3-D geometry of this system is confirmed, these clouds may pinpoint the beginning of an evolutionary sequence that can be followed, in time, from cloud condensation to star formation.

scholarly journals Molecular Clouds and Star Formation Rate in Disk Galaxies

2016 ◽  
Vol 25 (3) ◽  
Author(s):  
E. O. Vasiliev ◽  
S. A. Khoperskov ◽  
A. V. Khoperskov
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Disk Galaxies

AbstractWe use

scholarly journals Quenching by gas compression and consumption

2019 ◽  
Vol 624 ◽  
pp. A81 ◽  
Author(s):  
Allison W. S. Man ◽  
Matthew D. Lehnert ◽  
Joël D. R. Vernet ◽  
Carlos De Breuck ◽  
Theresa Falkendal
Keyword(s):  
Star Formation ◽  
Formation Rate ◽  
Relative Strength ◽  
Star Formation History ◽  
Molecular Gas ◽  
Host Galaxies ◽  
Massive Galaxies ◽  
Star Forming ◽  
Gas Compression ◽  
Formation History

The objective of this work is to study how active galactic nuclei (AGN) influence star formation in host galaxies. We present a detailed investigation of the star-formation history and conditions of a z = 2.57 massive radio galaxy based on VLT/X-shooter and ALMA observations. The deep rest-frame ultraviolet spectrum contains photospheric absorption lines and wind features indicating the presence of OB-type stars. The most significantly detected photospheric features are used to characterize the recent star formation: neither instantaneous nor continuous star-formation history is consistent with the relative strength of the Si IIλ1485 and S Vλ1502 absorption. Rather, at least two bursts of star formation took place in the recent past, at 6+1-2 Myr and ≳20 Myr ago, respectively. We deduce a molecular H2 gas mass of (3.9 ± 1.0) × 1010 M⊙ based on ALMA observations of the [C I] 3P2−3P1 emission. The molecular gas mass is only 13% of its stellar mass. Combined with its high star-formation rate of (1020-170+190 M⊙ yr-1, this implies a high star-formation efficiency of (26 ± 8) Gyr−1 and a short depletion time of (38 ± 12) Myr. We attribute the efficient star formation to compressive gas motions in order to explain the modest velocity dispersions (⩽55 km s−1) of the photospheric lines and of the star-forming gas traced by [C I]. Because of the likely very young age of the radio source, our findings suggest that vigorous star formation consumes much of the gas and works in concert with the AGN to remove any residual molecular gas, and eventually quenching star formation in massive galaxies.

scholarly journals The global star formation law: from dense cores to extreme starbursts

2006 ◽  
Vol 2 (S237) ◽  
pp. 331-335
Author(s):  
Yu Gao
Keyword(s):  
Star Formation ◽  
Linear Correlation ◽  
Molecular Clouds ◽  
High Redshift ◽  
Gas Content ◽  
Molecular Gas ◽  
Giant Molecular Clouds ◽  
Active Star ◽  
Dense Cores ◽  
The Galaxy

AbstractActive star formation (SF) is tightly related to the dense molecular gas in the giant molecular clouds' dense cores. Our HCN (measure of the dense molecular gas) survey in 65 galaxies (including 10 ultraluminous galaxies) reveals a tight linear correlation between HCN and IR (SF rate) luminosities, whereas the correlation between IR and CO (measure of the total molecular gas) luminosities is nonlinear. This suggests that the global SF rate depends more intimately upon the amount of dense molecular gas than the total molecular gas content. This linear relationship extends to both the dense cores in the Galaxy and the hyperluminous extreme starbursts at high-redshift. Therefore, the global SF law in dense gas appears to be linear all the way from dense cores to extreme starbursts, spanning over nine orders of magnitude in IR luminosity.

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