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Register a new userStructure and Evolution of Starburst and Normal Galaxies
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Bahram Mobasher
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We study the rest-frame morphology and structural properties of optically selected starburst galaxies at redshift z < 1, using multi-waveband (BViz) high resolution images taken by the Advanced Camera for Surveys (ACS) as part of the Great Observatories Origins Deep Survey (GOODS). We classify galaxies into starburst, early and late types by comparing their observed spectral energy distributions (SEDs) with local templates. We find that early-type systems have significantly higher rest-frame B -band concentration indices and AGN fraction (> 25%) than late-type spirals and optically-selected starbursts. These results are consistent with the scenario in which early-epoch (z > 1) gas-rich dissipative processes (e.g., major mergers) have played an important role in developing large central concentrations in early-type E/Sa galaxies, leading to concurrent growth of central black holes and bulge formation in some of these early merger events. The starbursts have, on average, larger asymmetries than our control sample of normal galaxies, suggesting that a significant fraction of the starburst activity is tidally triggered.
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Recent literature suggests that there are two modes through which galaxies grow their stellar mass - a normal mode characterized by quasi-steady star formation, and a highly efficient starburst mode possibly triggered by stochastic events such as galaxy mergers. While these differences are established for extreme cases, the population of galaxies in-between these two regimes is poorly studied and it is not clear where the transition between these two modes of star formation occurs. We utilize ALMA observations of the CO J=3-2 line luminosity in a sample of 20 infrared luminous galaxies that lie in the intermediate range between normal and starburst galaxies at z ~ 0.25-0.6 in the COSMOS field to examine the gas content and star formation efficiency of these galaxies. We compare these quantities to the galaxies deviation from the well-studied main sequence correlation between star formation rate and stellar mass (MS) and find that at log($SFR/SFR_{MS}$) < 0.6, a galaxys distance to the main sequence is mostly driven by increased gas content, and not a more efficient star formation process.
In systems undergoing starbursts the evolution of the young stellar population is expected to drive changes in the emission line properties. This evolution is usually studied theoretically, with a combination of evolutionary synthesis models for the spectral energy distribution of starbursts and photoionization calculations. In this paper we present a more empirical approach to this issue. We apply empirical population synthesis techniques to samples of Starburst and HII galaxies in order to measure their evolutionary state and correlate the results with their emission line properties. A couple of useful tools are introduced which greatly facilitate the interpretation of the synthesis: (1) an evolutionary diagram, whose axis are the strengths of the young, intermediate age and old components of the stellar population mix, and (2) the mean age of stars associated with the starburst, $ov{t}_{SB}$. These tools are tested with grids of theoretical galaxy spectra and found to work very well even when only a small number of observed properties (absorption line equivalent widths and continuum colors) is used in the synthesis. Starburst nuclei and HII galaxies are found to lie on a well defined sequence in the evolutionary diagram. Using the empirically defined mean starburst age in conjunction with emission line data we have verified that the equivalent widths of H$beta$ and [OIII] decrease for increasing $ov{t}_{SB}$. The same evolutionary trend was identified for line ratios indicative of the gas excitation, although no clear trend was identified for metal rich systems. All these results are in excellent agreement with long known, but little tested, theoretical expectations.
We present correlations between 9 CO transition ($J=4-3$ to $12-11$) and beam-matched far-infrared (Far-IR) luminosities ($L_{mathrm{FIR},,b}$) among 167 local galaxies, using {it{Herschel}} Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (SPIRE; FTS) spectroscopic data and Photoconductor Array Camera and Spectrometer (PACS) photometry data. We adopt entire-galaxy FIR luminosities ($L_{mathrm{FIR},,e}$) from the {it{IRAS}} Revised Bright Galaxy Sample and correct to $L_{mathrm{FIR},,b}$ using PACS images to match the varying FTS beam sizes. All 9 correlations between $L_{mathrm{CO}}$ and $L_{mathrm{FIR},,b}$ are essentially linear and tight ($sigma$=0.2-0.3 dex dispersion), even for the highest transition, $J=12-11$. This supports the notion that the star formation rate (SFR) is linearly correlated with the dense molecular gas ($n_{mathrm{H}_2}gtrsim10^{4-6},cm^{-3}$). We divide the entire sample into three subsamples and find that smaller sample sizes can induce large differences in the correlation slopes. We also derive an average CO spectral line energy distribution (SLED) for the entire sample and discuss the implied average molecular gas properties for these local galaxies. We further extend our sample to high-{it{z}} galaxies with CO $J=5-4$ data from the literature as an example, including submillimeter galaxies (SMGs) and normal star-forming BzKs. BzKs have similar FIR/CO(5-4) ratios as that of local galaxies, an follow well the locally-determined correlation, whereas SMG ratios fall around or slightly above the local correlation with large uncertainties. Finally, by including Galactic CO($J=10-9$) data as well as very limited high-{it{z}} CO $J=10-9$ data, we verify that the CO(10-9) -- FIR correlation successfully extends to Galactic young stellar objects, suggesting that linear correlations are valid over 15 orders of magnitude.
There is a consensus in the literature that starburst galaxies are triggered by inter- action events. However, it remains an open question as to what extent both merging and non-merging interactions have in triggering starbursts? In this study, we make use of the Illustris simulation to test how different triggering mechanisms can effect starburst events. We examine star formation rate, colour and environment of starburst galaxies to determine if this could be why we witness a bimodality in post-starburst populations within observational studies. Further, we briefly test the extent of quenching due to AGN feedback. From Illustris, we select 196 starburst galaxies at z = 0.15 and split them into post-merger and pre-merger/harassment driven starburst samples. We find that 55% of this sample not undergone a merger in the past 2 Gyr. Both of our samples are located in low-density environments within the filament regions of the cosmic web, however we find that pre-merger/harassment driven starburst are in higher density environments than post-merger driven starbursts. We also find that pre-merger/harassment starbursts are redder than post-merger starbursts, this could be driven by environmental effects. Both however, produce nuclear starbursts of comparable strengths.
We present the evolution of galaxy sizes, from redshift 2 to 0, for actively star forming and passive galaxies in the cosmological hydrodynamical 1003 cMpc3 simulation of the EAGLE project. We find that the sizes increase with stellar mass , but that the relation weakens with increasing redshift. Separating galaxies by their star formation activity, we find that passive galaxies are typically smaller than active galaxies at fixed stellar mass. These trends are consistent with those found in observations and the level of agreement between the predicted and observed size - mass relation is of order 0.1 dex for z < 1 and 0.2-0.3 dex from redshift 1 to 2. We use the simulation to compare the evolution of individual galaxies to that of the population as a whole. While the evolution of the size-stellar mass relation for active galaxies provides a good proxy for the evolution of individual galaxies, the evolution of individual passive galaxies is not well represented by the observed size - mass relation due to the evolving number density of passive galaxies. Observations of z approx 2 galaxies have revealed an abundance of massive red compact galaxies, that depletes below z approx 1. We find that a similar population forms naturally in the simulation. Comparing these galaxies to their z = 0 descendants, we find that all compact galaxies grow in size due to the high-redshift stars migrating outwards. Approximately 60% of the compact galaxies increase in size further due to renewed star formation and/or mergers.
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