Star Formation and the ISM Evolution of Galaxies Across Cosmic Time
Lead Research Organisation:
University College London
Department Name: Physics and Astronomy
Abstract
Galaxies consist of stellar populations, a central supermassive black hole and a so-called interstellar medium (ISM). The latter is made up of gas and dust, with the gas consisting almost entirely of atomic and molecular hydrogen except for He (~10%) and a few other much less abundant atomic and molecular species. Stars form in overdense regions of the ISM that have become gravitationally unstable, while the supermassive black hole grows via the accretion of gas in the central regions of a galaxy. In other words, the ISM provides the 'fuel' for star formation and black hole growth, while the dust - condensing out of supernovae and stellar winds - is a consequence of star formation. Clearly the ISM plays a fundamental role in the formation and evolution of galaxies. Unfortunately, the bulk of the molecular gas is not directly observable due to the symmetry of the hydrogen molecule, which means that we have to rely on observations of emission lines from other molecules and atoms. The CO molecule, being the second-most abundant molecule after hydrogen and having easily excitable rotational quantum levels, is the principal tracer of the molecular ISM. By studying the brightness of the CO lines along with those of other useful, albeit observationally more challenging, tracers such as HCO+ and HCN, one is able to constrain the density, kinetic temperature and chemical composition of the gas. While these types of observations have been carried out in our own Galaxy, there are huge gaps in our understanding of the ISM in other galaxies. The reason for this is that, for low redshift galaxies, the key diagnostic emission lines of the ISM lie at far-infrared and submillimetre wavelengths, which are difficult or impossible to observe from the ground due to the Earth's atmosphere. The main objective of this research proposal is to obtain a detailed picture of the ISM in other galaxies, from local starburst galaxies in our Cosmic backyard to the most distant galaxies at the edge of the observable Universe. Using the full rotational line spectra of CO, HCN and HCO+ as well as diagnostic lines of neutral and single-ionised carbon we will characterize the physical and chemical properties of the ISM in galaxies. By delineating how the microscopic properties of the gas (i.e. density, temperature, chemical abundances) affect macroscopic processes such as star formation we will gain vital new insight into the star formation and ISM evolution of galaxies throughout the history of the Universe. To this end we will make use of new, extremely powerful telescopes in space and on the ground such as the Herschel Space Observatory and the Atacama Large Millimeter Array, which are able to overcome many of the difficulties that have faced ground-based observations to date. These facilities represent a quantum leap in our ability to study the gas and dust in distant galaxies, and will open up a new window in the study of star formation and ISM evolution in galaxies.
People |
ORCID iD |
Thomas Greve (Principal Investigator / Fellow) |
Publications
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Tunnard R
(2016)
HOW DENSE IS YOUR GAS? ON THE RECOVERABILITY OF LVG MODEL PARAMETERS
in The Astrophysical Journal
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Tunnard R
(2015)
MODELING THE MOLECULAR GAS IN NGC 6240
in The Astrophysical Journal
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Vieira JD
(2013)
Dusty starburst galaxies in the early Universe as revealed by gravitational lensing.
in Nature
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Weiß A
(2013)
ALMA REDSHIFTS OF MILLIMETER-SELECTED GALAXIES FROM THE SPT SURVEY: THE REDSHIFT DISTRIBUTION OF DUSTY STAR-FORMING GALAXIES
in The Astrophysical Journal
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Welikala N
(2016)
Probing star formation in the dense environments of z ~ 1 lensing haloes aligned with dusty star-forming galaxies detected with the South Pole Telescope
in Monthly Notices of the Royal Astronomical Society
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Wylezalek D
(2013)
The Herschel? view of the environment of the radio galaxy 4C+41.17 at z = 3.8
in Monthly Notices of the Royal Astronomical Society
Description | Studying a sample of 25 nearby ultra-luminous infra-red galaxies observed with the Herschel Space Observatory, we were able to delineate how the star formation in these types of galaxies correlate with different phases of their interstellar gas. We used spectral lines of carbon monoxide (so-called rotational CO lines) to trace the different gas phases. We found that while the cold molecular gas traced tightly with star formation, the hotter dense gas did not. The latter result was surprising, since in more normal star forming galaxies hot dense gas is seen to be associated with newly formed stars (such as in the Orion region in our own Milky Way), where the UV light from the stars heat the gas. But in these more extreme galaxies, the hot gas is undergoing mechanical heating from shocks, possibly associated with supernovae or stellar outflows (Greve et al. 2014). In a separate research project, we have undertaken multi-line spectroscopic survey of individual nearby star forming galaxies. We developed statistical techniques which we applied to radiative transfer models of these data -- which in turn allows us to accurately disentangle the properties (density and temperature) and the mass of different gas phases (Tunnard, Greve, et al. 2016). In Tunnard & Greve (2016) we examined how robust such radiative transfer models were in inferring the gas density and temperature, showing that significant uncertainties are inherent. Also, we examined the effects of the cosmic microwave background on certain measurements of CO of distant galaxies -- showing that this is a severe effect, that could lead to an underestimate of the total gas mass in distant galaxies. Through the involvement in the SPT-SMG survey, we have established the existence of a significant population of very distant star burst galaxies (z > 4) (Greve et al. 2012; Vieira et al. 2013). Using ALMA we have studies these galaxies in great detail - determining their star formation rates, dust masses, and gas properties. Recently, we published the discovery of the most distant such galaxies: a massive starburst galaxy uncovered when the Universe was less than 800 million years old (Marrone et al. 2018, Nature). Finally, we have developed state-of-the-art numerical simulations capable of simulating the far-IR and mm molecular and atomic line emission from distant galaxies (Olsen, Greve, et al., 2015, 2016, 2017). Using this code (called SIGAME), we have been able to explain the observed relationship between star formation and the line emission from single ionised carbon ([CII]), in particular, our simulations are able to reproduce the observed 'drop' in the ratio between star formation rate and [CII] line luminosity observed in the very early Universe. We find that the most likely explanation is low metallicity, which leads to low [CII] luminosity. |
Exploitation Route | Within the SPT-SMG collaboration we will continue our ALMA studies - and were recently awarded JWST ERS time to study two of the SPT sources. We aim to obtain further JWST time in the future to study the our sources in the wavelength window opened up by JWST. We will use SIGAME to simulate the so-called fine-structure lines in distant galaxies - in particular their ability to trace the kinematics and mass of the gas, but also if they can be used as diagnostics of the chemical enrichment of the gas. |
Sectors | Education |
Title | SImulator of GAlaxy Millimeter/submillimeter Emission (SÍGAME) |
Description | We developed a code able to simulate the far-IR and millimeter line emission from snapshot zoom simulations of cosmological simulations. |
Type Of Material | Improvements to research infrastructure |
Year Produced | 2015 |
Provided To Others? | Yes |
Impact | Three papers have been published using SIGAME to date: 1) Olsen, Greve, et al. 2015, ApJ, 814, 76. 2) Olsen, Greve, et al. 2016, MNRAS, 457, 3306. 3) Olsen, Greve et al. 2017, ApJ, 846, 105. |
URL | https://github.com/kpolsen/SIGAME |
Description | A Step in the Dark: The Dense Molecular Gas (DeMoGas) in Galaxies |
Organisation | National Observatory of Athens |
Country | Greece |
Sector | Academic/University |
PI Contribution | Contributed significantly to the CIGALE SED fits to the sample of galaxies. I was responsible for collating all the continuum data that went into the fits. I did also did the first CIGALE fits and then instructed a collaborator on how to do the remaining fits in the sample. I collated the CO line data that went i to the IR-CO analysis. I subsequently did an analysis of the IR-CO luminosity relations from rotational line J=1-0 to J 13-12. I wrote the paper which was published in 2014. Greve et al. 2014). I have written and PI'ed two successful proposals for the IRAM 30m to collect dense gas tracer lines (HCN, HCO+). Currently we are writing a second paper (Leonidaki, Greve et al.) that will analyse the dense gas properties of our sample galaxies. |
Collaborator Contribution | Contributed significantly to the CIGALE SED fits to the sample of galaxies. Analysis of dense gas tracer lines. Commenting on paper Greve et al. |
Impact | The collaboration is not multi-disciplinary. |
Start Year | 2012 |
Description | South Pole Telescope Submillimetre Galaxy Consortium |
Organisation | California Institute of Technology |
Department | Caltech Astronomy |
Country | United States |
Sector | Academic/University |
PI Contribution | Obtained APEX LABOCA 870micron and SABOCA 350micron data for the SPT-SMG sample. This allowed us to infer photometric redshifts and a first estimate of the redshift distribution of these sources (Greve et al. 2012). I have also lead Herschel proposal to obtain SPIRE photometry of the sample. |
Collaborator Contribution | Provided initial sample from the SPT, and multiple ALMA and ATCA data. |
Impact | 1) Redshift distribution of ultra-bright, strongly lensed mm-selected galaxies at high redshift (Greve et al. 2012; Weiss et al. 2013; Strandet et al. 2016). 2) Molecular gas masses (Aravena et al. 2016). 3) Dense gas luminosities and masses from HCN, HCO+ and HNC detections with ALMA (Bethermin, Greve, et al. 2018). |
Start Year | 2012 |