Determining Star Formation Thresholds from Observations

Abstract Most gas in giant molecular clouds is relatively low-density and forms star inefficiently, converting only a small fraction of its mass to stars per dynamical time. However, star formation models generally predict the existence of a threshold density above which the process is efficient and most mass collapses to stars on a dynamical timescale. A number of authors have proposed observational techniques to search for a threshold density above which star formation is efficient, but it is unclear which of these techniques, if any, are reliable. In this paper we use detailed simulations of turbulent, magnetised star-forming clouds, including stellar radiation and outflow feedback, to investigate whether it is possible to recover star formation thresholds using current observational techniques. Using mock observations of the simulations at realistic resolutions, we show that plots of projected star formation efficiency per free-fall time εff can detect the presence of a threshold, but that the resolutions typical of current dust emission or absorption surveys are insufficient to determine its value. In contrast, proposed alternative diagnostics based on a change in the slope of the gas surface density versus star formation rate surface density (Kennicutt-Schmidt relation) or on the correlation between young stellar object counts and gas mass as a function of density are ineffective at detecting thresholds even when they are present. The signatures in these diagnostics sometimes taken as indicative of a threshold in observations, which we generally reproduce in our mock observations, do not prove to correspond to real physical features in the 3D gas distribution.

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Giant star-forming clumps?

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa046 ◽

2020 ◽

Vol 495 (1) ◽

pp. L1-L6 ◽

Cited By ~ 2

Author(s):

R J Ivison ◽

J Richard ◽

A D Biggs ◽

M A Zwaan ◽

E Falgarone ◽

...

Keyword(s):

Star Formation ◽

Spatial Scale ◽

Gravitational Lensing ◽

Spatial Scales ◽

Formation Rate ◽

Dust Emission ◽

Giant Molecular Clouds ◽

Giant Star ◽

Star Forming ◽

Dusty Galaxies

ABSTRACT With the spatial resolution of the Atacama Large Millimetre Array (ALMA), dusty galaxies in the distant Universe typically appear as single, compact blobs of dust emission, with a median half-light radius, ≈1 kpc. Occasionally, strong gravitational lensing by foreground galaxies or galaxy clusters has probed spatial scales 1–2 orders of magnitude smaller, often revealing late-stage mergers, sometimes with tantalizing hints of sub-structure. One lensed galaxy in particular, the Cosmic Eyelash at z = 2.3, has been cited extensively as an example of where the interstellar medium exhibits obvious, pronounced clumps, on a spatial scale of ≈100 pc. Seven orders of magnitude more luminous than giant molecular clouds in the local Universe, these features are presented as circumstantial evidence that the blue clumps observed in many z ∼ 2–3 galaxies are important sites of ongoing star formation, with significant masses of gas and stars. Here, we present data from ALMA which reveal that the dust continuum of the Cosmic Eyelash is in fact smooth and can be reproduced using two Sérsic profiles with effective radii, 1.2 and 4.4 kpc, with no evidence of significant star-forming clumps down to a spatial scale of ≈80 pc and a star formation rate of <3 M⊙ yr−1.

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Turbulence, feedback, and slow star formation

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307001767 ◽

2006 ◽

Vol 2 (S237) ◽

pp. 378-383

Author(s):

Mark R. Krumholz

Keyword(s):

Star Formation ◽

Milky Way ◽

Star Formation Rate ◽

Formation Rate ◽

Free Fall ◽

Giant Molecular Clouds ◽

Star Forming ◽

Striking Result ◽

Wide Range ◽

Individual Cluster

AbstractOne of the outstanding puzzles about star formation is why it proceeds so slowly. Giant molecular clouds convert only a few percent of their gas into stars per free-fall time, and recent observations show that this low star formation rate is essentially constant over a range of scales from individual cluster-forming molecular clumps in the Milky Way to entire starburst galaxies. This striking result is perhaps the most basic fact that any theory of star formation must explain. I argue that a model in which star formation occurs in virialized structures at a rate regulated by supersonic turbulence can explain this observation. The turbulence in turn is driven by star formation feedback, which injects energy to offset radiation from isothermal shocks and keeps star-forming structures from wandering too far from virial balance. This model is able to reproduce observational results covering a wide range of scales, from the formation times of young clusters to the extragalactic IR-HCN correlation, and makes additional quantitative predictions that will be testable in the next few years.

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Testing the star formation scaling relations in the clumps of the North American and Pelican nebulae cloud complex

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3222 ◽

2020 ◽

Vol 500 (3) ◽

pp. 3123-3141

Author(s):

Swagat R Das ◽

Jessy Jose ◽

Manash R Samal ◽

Shaobo Zhang ◽

Neelam Panwar

Keyword(s):

Star Formation ◽

Surface Density ◽

Star Formation Rate ◽

Formation Rate ◽

Free Fall ◽

Scaling Relations ◽

Time Model ◽

Fall Time ◽

The North ◽

Crossing Time

ABSTRACT The processes that regulate star formation within molecular clouds are still not well understood. Various star formation scaling relations have been proposed as an explanation, one of which is to formulate a relation between the star formation rate surface density $\rm \Sigma _{SFR}$ and the underlying gas surface density $\rm \Sigma _{gas}$. In this work, we test various star formation scaling relations, such as the Kennicutt–Schmidt relation, the volumetric star formation relation, the orbital time model, the crossing time model and the multi free-fall time-scale model, towards the North American Nebula and Pelican Nebula and in the cold clumps associated with them. Measuring stellar mass from young stellar objects and gaseous mass from CO measurements, we estimate the mean $\rm \Sigma _{SFR}$, the star formation rate per free-fall time and the star formation efficiency for clumps to be 1.5 $\rm M_{\odot}\, yr^{-1}\, kpc^{-2}$, 0.009 and 2.0 per cent, respectively, while for the whole region covered by both nebulae (which we call the ‘NAN’ complex) the values are 0.6 $\rm M_{\odot}\, yr^{-1}\, kpc^{-2}$, 0.0003 and 1.6 per cent, respectively. For the clumps, we notice that the observed properties are in line with the correlation obtained between $\rm \Sigma _{SFR}$ and $\rm \Sigma _{gas}$, and between $\rm \Sigma _{SFR}$ and $\rm \Sigma _{gas}$ per free-fall time and orbital time for Galactic clouds. At the same time, we do not observe any correlation with $\rm \Sigma _{gas}$ per crossing time and multi free-fall time. Even though we see correlations in the former cases, however, all models agree with each other within a factor of 0.5 dex. It is not possible to discriminate between these models because of the current uncertainties in the input observables. We also test the variation of $\rm \Sigma _{SFR}$ with the dense gas but, because of low statistics, a weak correlation is seen in our analysis.

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SDSS-IV MaNGA: effects of morphology in the global and local star formation main sequences

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1894 ◽

2019 ◽

Vol 488 (3) ◽

pp. 3929-3948 ◽

Cited By ~ 16

Author(s):

M Cano-Díaz ◽

V Ávila-Reese ◽

S F Sánchez ◽

H M Hernández-Toledo ◽

A Rodríguez-Puebla ◽

...

Keyword(s):

Star Formation ◽

Surface Density ◽

Main Sequence ◽

Formation Rate ◽

Stellar Mass ◽

Star Forming ◽

Spatially Resolved ◽

Star Formation Activity ◽

Global And Local ◽

Universal Mechanism

ABSTRACT We study the global star formation rate (SFR) versus stellar mass (M*) correlation, and the spatially resolved SFR surface density (ΣSFR) versus stellar mass surface density (Σ*) correlation, in a sample of ∼2000 galaxies from the MaNGA MPL-5 survey. We classify galaxies and spatially resolved areas into star forming and retired according to their ionization processes. We confirm the existence of a star-forming main sequence (SFMS) for galaxies and spatially resolved areas, and show that they have the same nature, with the global as a consequence of the local one. The latter presents a bend below a limit Σ* value, ≈3 × 107 M$\odot$ kpc−2, which is not physical. Using only star-forming areas (SFAs) above this limit, a slope and a scatter of ≈1 and ≈0.27 dex are determined. The retired galaxies/areas strongly segregate from their respective SFMSs, by ∼−1.5 dex on average. We explore how the global/local SFMSs depend on galaxy morphology, finding that for star-forming galaxies and SFAs, there is a trend to lower values of star formation activity with earlier morphological types, which is more pronounced for the local SFMS. The morphology not only affects the global SFR due to the diminish of SFAs with earlier types, but also affects the local SF process. Our results suggest that the local SF at all radii is established by some universal mechanism partially modulated by morphology. Morphology seems to be connected to the slow aging and sharp decline of the SF process, and on its own it may depend on other properties as the environment.

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Mapping Obscuration to Reionization with ALMA (MORA): 2 mm Efficiently Selects the Highest-redshift Obscured Galaxies

The Astrophysical Journal ◽

10.3847/1538-4357/ac2eb4 ◽

2021 ◽

Vol 923 (2) ◽

pp. 215

Author(s):

Caitlin M. Casey ◽

Jorge A. Zavala ◽

Sinclaire M. Manning ◽

Manuel Aravena ◽

Matthieu Béthermin ◽

...

Keyword(s):

Star Formation ◽

Near Infrared ◽

Formation Rate ◽

Dust Emission ◽

Volume Density ◽

Drop Out ◽

Spectral Energy ◽

Photometric Redshifts ◽

Star Forming ◽

Dusty Galaxies

Abstract We present the characteristics of 2 mm selected sources from the largest Atacama Large Millimeter/submillimeter Array (ALMA) blank-field contiguous survey conducted to date, the Mapping Obscuration to Reionization with ALMA (MORA) survey covering 184 arcmin2 at 2 mm. Twelve of 13 detections above 5σ are attributed to emission from galaxies, 11 of which are dominated by cold dust emission. These sources have a median redshift of 〈 z 2 mm 〉 = 3.6 − 0.3 + 0.4 primarily based on optical/near-infrared photometric redshifts with some spectroscopic redshifts, with 77% ± 11% of sources at z > 3 and 38% ± 12% of sources at z > 4. This implies that 2 mm selection is an efficient method for identifying the highest-redshift dusty star-forming galaxies (DSFGs). Lower-redshift DSFGs (z < 3) are far more numerous than those at z > 3 yet are likely to drop out at 2 mm. MORA shows that DSFGs with star formation rates in excess of 300 M ⊙ yr−1 and a relative rarity of ∼10−5 Mpc−3 contribute ∼30% to the integrated star formation rate density at 3 < z < 6. The volume density of 2 mm selected DSFGs is consistent with predictions from some cosmological simulations and is similar to the volume density of their hypothesized descendants: massive, quiescent galaxies at z > 2. Analysis of MORA sources’ spectral energy distributions hint at steeper empirically measured dust emissivity indices than reported in typical literature studies, with 〈 β 〉 = 2.2 − 0.4 + 0.5 . The MORA survey represents an important step in taking census of obscured star formation in the universe’s first few billion years, but larger area 2 mm surveys are needed to more fully characterize this rare population and push to the detection of the universe’s first dusty galaxies.

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Fast molecular outflow from a dusty star-forming galaxy in the early Universe

Science ◽

10.1126/science.aap8900 ◽

2018 ◽

Vol 361 (6406) ◽

pp. 1016-1019 ◽

Cited By ~ 37

Author(s):

J. S. Spilker ◽

M. Aravena ◽

M. Béthermin ◽

S. C. Chapman ◽

C.-C. Chen ◽

...

Keyword(s):

Star Formation ◽

High Redshift ◽

Large Fraction ◽

Formation Rate ◽

Free Fall ◽

Big Bang ◽

Star Forming ◽

Molecular Outflow ◽

The Galaxy ◽

The Big Bang

Galaxies grow inefficiently, with only a small percentage of the available gas converted into stars each free-fall time. Feedback processes, such as outflowing winds driven by radiation pressure, supernovae, or supermassive black hole accretion, can act to halt star formation if they heat or expel the gas supply. We report a molecular outflow launched from a dust-rich star-forming galaxy at redshift 5.3, 1 billion years after the Big Bang. The outflow reaches velocities up to 800 kilometers per second relative to the galaxy, is resolved into multiple clumps, and carries mass at a rate within a factor of 2 of the star formation rate. Our results show that molecular outflows can remove a large fraction of the gas available for star formation from galaxies at high redshift.

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Star Formation in Isolated LIRGs: Clues to Star-forming Processes at Higher z

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311022769 ◽

2010 ◽

Vol 6 (S277) ◽

pp. 195-198

Author(s):

Isaura Fuentes-Carrera ◽

Lorenzo Olguín ◽

Patricia Ambrocio-Cruz ◽

Simon Verley ◽

Margarita Rosado ◽

...

Keyword(s):

Star Formation ◽

Large Fraction ◽

Formation Rate ◽

Local Universe ◽

Star Forming ◽

Luminous Infrared Galaxies ◽

History Of ◽

Observational Techniques ◽

Intermediate Redshift ◽

Very High

AbstractLuminous infrared galaxies (LIRGs) are galaxies with LIR > 1011 L⊙. For a star-forming galaxy to emit at a LIRG level, it must have a very high star formation rate (SFR). In the local Universe, the star formation (SF) is primarily triggered by interactions. However, at intermediate redshift, a large fraction of LIRGs are disk galaxies with little sign of recent merger activity. The question arises whether the intermediate redshift LIRGs are “triggered” or experiencing “normal”, if elevated, SF. Understanding these SF processes is important since this type of systems may have contributed to 20% or more of the cosmic SFR in the early Universe. In order to address this issue we study similar systems in the Local Universe, that is isolated late-type galaxies displaying LIRG activity. We use different observational techniques in order to trace the star-forming history of these systems. Here we present preliminary results.

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The rocky road to quiescence: compaction and quenching of quasar host galaxies at z ∼ 2

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3433 ◽

2020 ◽

Vol 500 (3) ◽

pp. 3667-3688

Author(s):

H R Stacey ◽

J P McKean ◽

D M Powell ◽

S Vegetti ◽

F Rizzo ◽

...

Keyword(s):

Star Formation ◽

Formation Rate ◽

Dust Emission ◽

Host Galaxies ◽

Gas Reservoirs ◽

Gas Reservoir ◽

Star Forming ◽

Black Hole Accretion ◽

The One ◽

Hole Accretion

ABSTRACT We resolve the host galaxies of seven gravitationally lensed quasars at redshift 1.5–2.8 using observations with the Atacama Large (sub)Millimetre Array. Using a visibility plane lens modelling technique, we create pixellated reconstructions of the dust morphology, and CO line morphology and kinematics. We find that the quasar hosts in our sample can be distinguished into two types: (1) galaxies characterized by clumpy, extended dust distributions (Reff ∼ 2 kpc) and mean star formation rate (SFR) surface densities comparable to sub-mm-selected dusty star-forming galaxies (ΣSFR ∼ 3 M⊙ yr−1 kpc−2 ) and (2) galaxies that have sizes in dust emission similar to coeval passive galaxies and compact starbursts (Reff ∼ 0.5 kpc), with high mean SFR surface densities (ΣSFR = 400–4500 M⊙ yr−1 kpc−2 ) that may be Eddington-limited or super-Eddington. The small sizes of some quasar hosts suggest that we observe them at a stage in their transformation into compact spheroids via dissipative contraction, where a high density of dynamically unstable gas leads to efficient star formation and black hole accretion. For the one system where we probe the bulk of the gas reservoir, we find a gas fraction of just 0.06 ± 0.04 and a depletion time-scale of 50 ± 40 Myr, suggesting it is transitioning into quiescence. In general, we expect that the extreme level of star formation in the compact quasar host galaxies will rapidly exhaust their gas reservoirs and could quench with or without help from active galactic nucleus feedback.

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From peculiar morphologies to Hubble-type spirals: the relation between galaxy dynamics and morphology in star-forming galaxies at z ∼ 1.5

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3576 ◽

2019 ◽

Vol 492 (1) ◽

pp. 1492-1512

Author(s):

S Gillman ◽

A L Tiley ◽

A M Swinbank ◽

C M Harrison ◽

Ian Smail ◽

...

Keyword(s):

Angular Momentum ◽

Star Formation ◽

Galaxy Evolution ◽

Surface Density ◽

Rest Frame ◽

Star Formation Rate ◽

High Redshift ◽

Formation Rate ◽

Stellar Mass ◽

Star Forming

ABSTRACT We present an analysis of the gas dynamics of star-forming galaxies at z ∼ 1.5 using data from the KMOS Galaxy Evolution Survey. We quantify the morphology of the galaxies using HSTcandels imaging parametrically and non-parametrically. We combine the H α dynamics from KMOS with the high-resolution imaging to derive the relation between stellar mass (M*) and stellar specific angular momentum (j*). We show that high-redshift star-forming galaxies at z ∼ 1.5 follow a power-law trend in specific stellar angular momentum with stellar mass similar to that of local late-type galaxies of the form j* ∝ M$_*^{0.53\, \pm \, 0.10}$. The highest specific angular momentum galaxies are mostly disc-like, although generally both peculiar morphologies and disc-like systems are found across the sequence of specific angular momentum at a fixed stellar mass. We explore the scatter within the j* – M* plane and its correlation with both the integrated dynamical properties of a galaxy (e.g. velocity dispersion, Toomre Qg, H α star formation rate surface density ΣSFR) and its parametrized rest-frame UV / optical morphology (e.g. Sérsic index, bulge to total ratio, clumpiness, asymmetry, and concentration). We establish that the position in the j* – M* plane is strongly correlated with the star-formation surface density and the clumpiness of the stellar light distribution. Galaxies with peculiar rest-frame UV / optical morphologies have comparable specific angular momentum to disc- dominated galaxies of the same stellar mass, but are clumpier and have higher star formation rate surface densities. We propose that the peculiar morphologies in high-redshift systems are driven by higher star formation rate surface densities and higher gas fractions leading to a more clumpy interstellar medium.

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