star formation in the central molecular...
TRANSCRIPT
Star formation in the Central
Molecular Zone
Steve Longmore, Nate Bastian, Daniel Walker (Liverpool John Moores University), Jim Jackson (Boston University), Diederik Kruijssen (Heidelberg University), Joao Alves (University of Vienna), John Bally (University of Colorado), Jonathan Foster (Yale University), Guido Garay (Universidad de Chile), Leonardo Testi (ESO), Andrew Walsh (Curtin Universtity)
YaneQ Contreras – Leiden University Jill Rathborne – CSIRO Astronomy and Space Science
Our current theoretical understanding of molecular cloud structure derived from solar neighborhood
clouds also holds in extreme, high-pressure environments
Take home message
• Stars form when small, dense cores within a molecular cloud becoming self-gravitating and collapse
• Which pockets of gas collapse to form stars depends on a cloud’s internal kinematics and density structure
• Theory predicts that gravitational collapse occurs once the gas reached a critical over–density with respect to the mean cloud density
• Volume density Probability Density Function (PDF) is a statistical
measure used to describe the fraction of mass within a cloud at a given volume density
• Determine the density and rate of star formation
• Theoretical models of supersonically turbulent, isothermal media show that the volume density PDF follows a log-normal distribution
• Enhancements that are dense enough to become self-gravitating undergo runaway collapse, causing the high-density end to develop a power-law tail
Turbulence and the initial gas structure
Vazquez-‐‑Semadeni 1994; Krumholz & McKee 2005; Federrath et al. 2010; Padoan & Nordlund 1999, 2011
Lombardi et al. 2008; Goodman et al. 2009; Kainuleinen et al. 2009, 2013; Padoan et al. 2013; Schenider et al. 2014
Dust column density -‐‑ Taurus
Despite their relative simplicity, the predictions of these theoretical models match well the observed gas structure and star formation activity within solar neighborhood clouds
Theory predicts, and observations confirm, a density threshold for star formation in the solar
neighborhood of 104 cm-‐‑3
Column density PDFs for solar neighborhood clouds
Applying these theories to understand star formation in the early Universe
• Solar neighborhood : a relatively benign, low turbulent pressure environment (P/k < 105 K cm-3)
• The peak epoch of star
formation in the Universe occurred in galaxies at redshift z>2 where the turbulent pressure was several orders of magnitude higher (P/k > 107 K cm-3)
Hopkins & Beacom 2006; Swinbank et al. 2011; Krumholz et al 2012; Renaud et al. 2012; Kruijssen & Longmore 2013; Madau and Dickinson 2014
Most stars formed when the Universe was much ho4er and denser than it is today
Do these theories also describe the initial cloud structure (and, thus, star formation) in extreme, high-‐‑pressure environments, like those in the early Universe?
The Central Molecular Zone (CMZ)
Molinari et al. 2011; Walmsley et al. 1986, Ao et al. 2013
The central 500 pc of our Galaxy is extreme: the column density, gas temperature, velocity dispersion, interstellar radiation field, pressure and cosmic ray ionization rate range from being factors of a few to several orders of magnitude higher compared to the solar neighborhood
Herschel 70µm image – dust continuum emission
CMZ : an extreme, high-pressure environment
• The ISM in the Central Molecular Zone is extreme compared to the ISM in the solar neighborhood but is more similar to the ISM in high-z galaxies (gas temperature, turbulent Mach number, pressure)
• There is a modest metallicity difference between the CMZ and
rapidly star-forming, high-redshift galaxies (< a factor of 2-3) • CMZ clouds have the potential to be used as local analogues
of clouds in z > 2 galaxies
• CMZ clouds can be studied in a level of detail that is unachievable for clouds in other galaxies
Erb et al. 2006; Longmore et al. 2013a; Kruijssen & Longmore 2013
Detailed studies of CMZ clouds are relevant for understanding star formation in high-‐‑pressure environments
The Central Molecular Zone (CMZ) Herschel 70µm image – dust continuum emission
“The Brick”
G0.253+0.016: a cold, dense, high-mass clump
SpiFer 3-‐‑8µm Herschel 70µm JCMT 450µm
Lis et al. 1994; Lis & Menten 1998; Lis et al. 2001; Molinari et al. 2011; Longmore et al. 2012, Johnston et al, 2014, Pillai etal, 2015
Its low dust temperature (<20K), high mass (~105 M¤), and high volume density (>104 cm-‐‑3) combined with its lack of star-‐‑formation, makes it an excellent candidate for a high-‐‑mass cluster in a very early
stage of formation à initial conditions for cluster formation
Its location in the Central Molecular Zone makes its detailed study relevant for understanding star formation in high-‐‑pressure environments
Thermal dust emission (H2 column density)
Map coverage : 3’ x 1.5’ Resolution : 1.7” (0.07pc)
The power of ALMA
Rathborne et al. 2014, 2015
Complex morphology, chemistry
SiO HNCO C2H
Small-scale structure : testing cluster formation scenarios
End product -‐‑ Arches Precursor ?
?
Stellar distributions are smooth and centrally condensed
Is the stellar distribution observed in massive clusters set by the initial gas structure?
Small-scale structure : fragmented
The gas and dust emission is highly fractal with marginally higher values (Dp ~ 1.6) compared to solar neighborhood molecular clouds (Dp ~ 1.35)
Mandelbrot 1977; Falgarone et al. 1991; Federrath et al. 2009; Sanchez et al. 2005; Rathborne et al. 2015
Small-‐‑scale structure within G0.253+0.016
Perimeter
Area
Smooth gaussian profile
Highly fractal Dust continuum Shocked gas
Stellar distribution – Arches (>104 M⊙ in stars in 0.5pc) Protostellar distribution – Sgr
B2 Natal gas distribution – The Brick
Mass surface density profile : diffuse and extended
Walker et al. 2015
End product -‐‑ Arches
Precursor?
Initially, the gas is diffuse, with a density profile much flaQer compared to the stellar distribution If this clump is going to form an Arches-‐‑like cluster, then the final cluster must acquire its central concentrated profile as the cluster forms and evolves
Column density PDF – pinpointing star-forming cores
Vazquez-‐‑Semadeni 1994; Padoan et al. 1999; Kainulainen et al. 2009, 2013; Ballesteros-‐‑Paredes et al. 2011; Kritsuk et al. 2011; Schneider et al. 2014; Lis & Carlstrom 1994; Rathborne et al. 2014
M = 50-‐‑100 M¤ R < 0.04 pc n > few x 106 cm-‐‑3 SFR = 0.06%
Active star formation
Deviation at low column densities arises from the large-‐‑scale diffuse medium
Log-‐‑normal distribution is predicted when turbulence sets the initial gas structure
Deviation from log-‐‑normal at high column densities pinpoints self-‐‑gravitating gas where star formation can progress
• Recent observations of solar neighborhood clouds suggest a ‘universal’ column density threshold of ~1.4 x 1022 cm-2 (correspond to volume densities of ~104 cm-3)
• Although this threshold accurately describes the onset of star formation in clouds in the solar neighborhood, it does not hold for the environment within the CMZ • the gas has a much higher column density than 1.4 x 1022 cm-2, yet it is
forming stars 1-2 orders of magnitude less efficiently than predicted
• The column density PDF for G0.253+0.016 confirms this result - while the majority of the mass lies at column densities >1.4 x 1022 cm-2 only one region, corresponding to 0.06% of the total mass, shows evidence for star formation
• The derived lower limit on the volume density (>106 cm-3) is consistent with the theoretically predicted, environmentally dependent volume density threshold (108 cm-3), which is orders of magnitude higher than derived for solar neighborhood clouds
An environmentally dependent volume density threshold for star formation
Lada et al. 2010, 2012; Longmore et al. 2013; Krumholz and McKee 2005; Padoan and Nordlund 2011; Johnstone et al, 2014, Rathborne et al. 2014
Implications : star formation in the
early Universe
Predicted density threshold for star formation is orders of magnitude higher in the early Universe compared to the solar neighborhood
Cannot simply apply the same predictions derived from the solar neighborhood to describe star formation in other galaxies • Must consider : gas temperature, velocity dispersion, mean density • Specific predictions for galaxies; if resolved, at many locations within a
galaxy Star formation was harder in the early Universe
• Have to accumulate a lot more material before gravity can begin to form stars
• May explain why star formation happens in merger events
This is one example – clearly we need to extend this to include other clouds in the CMZ and those that are resolvable in nearby galaxies
Identified a high-mass clump showing little star formation that is located within the extreme, high-pressure environment of the CMZ Its internal structure appears to be highly fractal and dispersed Its lack of star formation is consistent with an environmentally dependent volume density threshold for star formation
Summary
Our current theoretical understanding of gas structure derived from solar neighborhood clouds also holds in
extreme, high-‐‑pressure environments