scholarly journals What is the physics behind the Larson mass–size relation?

2019 ◽  
Vol 490 (2) ◽  
pp. 2648-2655 ◽  
Author(s):  
J Ballesteros-Paredes ◽  
C Román-Zúñiga ◽  
Q Salomé ◽  
M Zamora-Avilés ◽  
M J Jiménez-Donaire
Keyword(s):  
Power Law ◽  
Molecular Clouds ◽  
Line Of Sight ◽  
Random Factor ◽  
Dense Cores ◽  
The Galaxy ◽  
Mass Size ◽  
Gas Contamination ◽  
Theoretical Evidence ◽  
Size Relation

ABSTRACT Different studies have reported a power-law mass–size relation M ∝ Rq for ensembles of molecular clouds. In the case of nearby clouds, the index of the power-law q is close to 2. However, for clouds spread all over the Galaxy, indexes larger than 2 are reported. We show that indexes larger than 2 could be the result of line-of-sight superposition of emission that does not belong to the cloud itself. We found that a random factor of gas contamination, between 0.001 per cent and 10 per cent of the line of sight, allows to reproduce the mass–size relation with q ∼ 2.2–2.3 observed in Galactic CO surveys. Furthermore, for dense cores within a single cloud, or molecular clouds within a single galaxy, we argue that, even in these cases, there is observational and theoretical evidence that some degree of superposition may be occurring. However, additional effects may be present in each case, and are briefly discussed. We also argue that defining the fractal dimension of clouds via the mass–size relation is not adequate, since the mass is not necessarily a proxy to the area, and the size reported in M−R relations is typically obtained from the square root of the area, rather than from an estimation of the size independent from the area. Finally, we argue that the statistical analysis of finding clouds satisfying the Larson’s relations does not mean that each individual cloud is in virial equilibrium.

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.

scholarly journals The distribution of cosmic-ray ionization rates in diffuse molecular clouds as probed by H 3 +

2012 ◽  
Vol 370 (1978) ◽  
pp. 5142-5150 ◽  
Author(s):  
Nick Indriolo
Keyword(s):  
Molecular Clouds ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Line Of Sight ◽  
Interstellar Clouds ◽  
Upper Limits ◽  
The Galaxy ◽  
Cosmic Ray Flux ◽  
Made In ◽  
Ionization Rates

Owing to its simple chemistry, H is widely regarded as the most reliable tracer of the cosmic-ray ionization rate in diffuse interstellar clouds. At present, H observations have been made in over 50 sight lines that probe the diffuse interstellar medium (ISM) throughout the Galaxy. This small survey presents the opportunity to investigate the distribution of cosmic-ray ionization rates in the ISM, as well as any correlations between the ionization rate and line-of-sight properties. Some of the highest inferred ionization rates are about 25 times larger than the lowest upper limits, suggesting variations in the underlying low-energy cosmic-ray flux across the Galaxy. Most likely, such variations are caused predominantly by the distance between an observed cloud and the nearest site of particle acceleration.

scholarly journals Compact, bulge dominated structures of spectroscopically confirmed quiescent galaxies at z ≈ 3

2020 ◽  
Author(s):  
Peter Lustig ◽  
Veronica Strazzullo ◽  
Chiara D’Eugenio ◽  
Emanuele Daddi ◽  
Maurilio Pannella ◽  
...  
Keyword(s):  
Structural Properties ◽  
Surface Brightness ◽  
Cosmic Time ◽  
Stellar Populations ◽  
Statistical Estimates ◽  
The Galaxy ◽  
Mass Size ◽  
Formation And Evolution ◽  
Size Relation ◽  
Good Agreement

Abstract We study structural properties of spectroscopically confirmed massive quiescent galaxies at z ≈ 3 with one of the first sizeable samples of such sources, made of ten 10.8 < log (M⋆/M⊙) < 11.3 galaxies at 2.4 < z < 3.2 in the COSMOS field whose redshifts and quiescence are confirmed by HST grism spectroscopy. Although affected by a weak bias toward younger stellar populations, this sample is deemed to be largely representative of the majority of the most massive and thus intrinsically rarest quiescent sources at this cosmic time. We rely on targeted HST/WFC3 observations and fit Sérsic profiles to the galaxy surface brightness distributions at ≈4000Årestframe. We find typically high Sérsic indices and axis ratios (medians ≈4.5 and 0.73, respectively) suggesting that, at odds with some previous results, the first massive quiescent galaxies may largely be already bulge-dominated systems. We measure compact galaxy sizes with an average of ≈1.4kpc at log (M⋆/M⊙) ≈ 11.2, in good agreement with the extrapolation at the highest masses of previous determinations of the stellar mass - size relation of quiescent galaxies, and of its redshift evolution, from photometrically selected samples at lower and similar redshifts. This work confirms the existence of a population of compact, bulge dominated, massive, quiescent sources at z ≈ 3, providing one of the first statistical estimates of their structural properties, and further constraining the early formation and evolution of the first quiescent galaxies.

scholarly journals Gravity, turbulence and the scaling “laws” in molecular clouds

2015 ◽  
Vol 11 (A29B) ◽  
pp. 716-716
Author(s):  
Javier Ballesteros-Paredes
Keyword(s):  
Molecular Clouds ◽  
Column Density ◽  
Scaling Laws ◽  
Cloud Formation ◽  
List Type ◽  
Present Contribution ◽  
Scale Free ◽  
Dense Cores ◽  
Star Formation Histories ◽  
Size Relation

AbstractThe so-called Larson (1981) scaling laws found empirically in molecular clouds have been generally interpreted as evidence that the clouds are turbulent and fractal. In the present contribution we discussed how recent observations and models of cloud formation suggest that: (a)these relations are the result of strong observational biases due to the cloud definition itself: since the filling factor of the dense structures is small, by thresholding the column density the computed mean density between clouds is nearly constant, and nearly the same as the threshold (Ballesteros-Paredes et al. 2012).(b)When accounting for column density variations, the velocity dispersion-size relation does not appears anymore. Instead, dense cores populate the upper-left corner of the δ v-R diagram (Ballesteros-Paredes et al. 2011a).(c)Instead of a δ v-R relation, a more appropriate relation seems to be δ v2 / R = 2 GMΣ, which suggest that clouds are in collapse, rather than supported by turbulence (Ballesteros-Paredes et al. 2011a).(d)These results, along with the shapes of the star formation histories (Hartmann, Ballesteros-Paredes & Heitsch 2012), line profiles of collapsing clouds in numerical simulations (Heitsch, Ballesteros-Paredes & Hartmann 2009), core-to-core velocity dispersions (Heitsch, Ballesteros-Paredes & Hartmann 2009), time-evolution of the column density PDFs (Ballesteros-Paredes et al. 2011b), etc., strongly suggest that the actual source of the non-thermal motions is gravitational collapse of the clouds, so that the turbulent, chaotic component of the motions is only a by-product of the collapse, with no significant “support" role for the clouds. This result calls into question if the scale-free nature of the motions has a turbulent, origin (Ballesteros-Paredes et al. 2011a; Ballesteros-Paredes et al. 2011b, Ballesteros-Paredes et al. 2012).

scholarly journals THE MASS-SIZE RELATION FROM CLOUDS TO CORES. I. A NEW PROBE OF STRUCTURE IN MOLECULAR CLOUDS

2010 ◽  
Vol 712 (2) ◽  
pp. 1137-1146 ◽  
Author(s):  
J. Kauffmann ◽  
T. Pillai ◽  
R. Shetty ◽  
P. C. Myers ◽  
A. A. Goodman
Keyword(s):  
Molecular Clouds ◽  
Mass Size ◽  
Size Relation

scholarly journals HST/WFC3 grism observations of z ∼ 1 clusters: evidence for evolution in the mass–size relation of quiescent galaxies from post-starburst galaxies

2020 ◽  
Vol 493 (4) ◽  
pp. 6011-6032 ◽  
Author(s):  
J Matharu ◽  
A Muzzin ◽  
G B Brammer ◽  
R F J van der Burg ◽  
M W Auger ◽  
...  
Keyword(s):  
Rapid Change ◽  
Driving Mechanism ◽  
Cluster Galaxies ◽  
Star Forming ◽  
Large Size ◽  
The Galaxy ◽  
Size Growth ◽  
Mass Size ◽  
Minor Mergers ◽  
Size Relation

ABSTRACT Minor mergers have been proposed as the driving mechanism for the size growth of quiescent galaxies with decreasing redshift. The process whereby large star-forming galaxies quench and join the quiescent population at the large size end has also been suggested as an explanation for this size growth. Given the clear association of quenching with clusters, we explore this mechanism by studying the structural properties of 23 spectroscopically identified recently quenched (or ‘post-starburst’ (PSB)) cluster galaxies at z ∼ 1. Despite clear PSB spectral signatures implying rapid and violent quenching, 87 per cent of these galaxies have symmetric, undisturbed morphologies in the stellar continuum. Remarkably, they follow a mass–size relation lying midway between the star-forming and quiescent field relations, with sizes 0.1 dex smaller than z ∼ 1 star-forming galaxies at log(M*/M⊙) = 10.5. This implies a rapid change in the light profile without directly effecting the stellar distribution, suggesting changes in the mass-to-light ratio gradients across the galaxy are responsible. We develop fading toy models to explore how star-forming galaxies move across the mass–size plane as their stellar populations fade to match those of the PSBs. ‘Outside-in’ fading has the potential to reproduce the contraction in size and increase in bulge-dominance observed between star-forming and PSB cluster galaxies. Since cluster PSBs lie on the large size end of the quiescent mass–size relation, and our previous work shows cluster galaxies are smaller than field galaxies, the sizes of quiescent galaxies must grow both from the quenching of star-forming galaxies and dry minor mergers.

scholarly journals Molecular clouds in galaxies as seen from NIR extinction studies

2006 ◽  
Vol 2 (S237) ◽  
pp. 204-207
Author(s):  
João Alves
Keyword(s):  
Mass Spectrum ◽  
Power Law ◽  
Molecular Cloud ◽  
Molecular Clouds ◽  
Column Density ◽  
Large Scale ◽  
Near Infrared ◽  
Mass Function ◽  
Core Mass ◽  
Dense Cores

AbstractNear infrared dust extinction mapping is opening a new window on molecular cloud research. Applying a straightforward technique to near infrared large scale data of nearby molecular complexes one can easily construct density maps with dynamic ranges in column density covering, 3σ~ 0.5 < AV< 50 mag or 1021<N<1023 cm−2. These maps are unique in capturing the low column density distribution of gas in molecular cloud complexes, where most of the mass resides, and at the same time allow the identification of dense cores (n~104cm−3) which are the precursors of stars. For example, the application of this technique to the nearby Pipe Nebula complex revealed the presence of 159 dense cores (the largest sample of such object in one single complex) whose mass spectrum presents the first robust evidence for a departure from a single power-law. The form of this mass function is surprisingly similar in shape to the stellar IMF but scaled to a higher mass by a factor of about 3. This suggests that the distribution of stellar birth masses (IMF) is the direct product of the dense core mass function and a uniform star formation efficiency of 30%±10%, and that the stellar IMF may already be fixed during or before the earliest stages of core evolution. We are now extending this technique to extra-galactic mapping of Giant molecular Clouds (GMCs), and although a much less straightforward task, preliminary results indicate that the GMC mass spectrum in M83 and Centaurus A is a power-law characterized by α~−2 unlike CO results which suggest α~−1.

scholarly journals The effects of the cluster environment on the galaxy mass-size relation in MACS J1206.2-0847

2017 ◽  
Vol 604 ◽  
pp. A54 ◽  
Author(s):  
U. Kuchner ◽  
B. Ziegler ◽  
M. Verdugo ◽  
S. Bamford ◽  
B. Häußler
Keyword(s):  
The Galaxy ◽  
Cluster Environment ◽  
Mass Size ◽  
Size Relation

Evolution of the number of communicative civilizations in the Galaxy: implications on Fermi paradox

2020 ◽  
Vol 19 (4) ◽  
pp. 314-319
Author(s):  
Giorgio Spada ◽  
Daniele Melini
Keyword(s):  
Power Law ◽  
Slow Rate ◽  
Rate Of Growth ◽  
Long Run ◽  
Rate Of Increase ◽  
The Galaxy ◽  
Zero Number

AbstractIt has been recently proposed DeVito [(2019) On the meaning of Fermi's paradox. Futures, 389–414] that a minimal number of contacts with alien radio-communicative civilizations could be justified by their logarithmically slow rate of growth in the Galaxy. Here we further develop this approach to the Fermi paradox, with the purpose of expanding the ensemble of the possible styles of growth that are consistent with the hypothesis of a minimal number of contacts. Generalizing the approach in DeVito (2019), we show that a logarithmic style of growth is still found. We also find that a style of growth following a power law would be admissible, however characterized by an exponent less than one, hence describing a sublinear increase in the number of communicative civilizations, still qualitatively in agreement with DeVito (2019). No solutions are found indicating a superlinear increase in the number of communicative civilizations, following for example an exponentially diverging law, which would cause, in the long run, an unsustainable proliferation. Although largely speculative, our findings corroborate the idea that a sublinear rate of increase in the number of communicative civilizations in the Galaxy could constitute a further resolution of Fermi paradox, implying a constant and minimal – but not zero – number of contacts.

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