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Register a new userThe Fine Line Between Normal and Starburst Galaxies
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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.
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We investigate the co-evolution of black-hole-accretion-rate (BHAR) and star-formation-rate (SFR) in $1.5<z<2.5$ galaxies displaying a greater diversity of star-forming properties compared to previous studies. We combine X-ray stacking and far-IR photometry of stellar mass-limited samples of normal star-forming, starburst and quiescent/quenched galaxies in the COSMOS field. We corroborate the existence of a strong correlation between BHAR (i.e. the X-ray luminosity, L_X), and stellar mass (M*) for normal star-forming galaxies, although find a steeper relation than previously reported. We find that starbursts show a factor of 3 enhancement in BHAR compared to normal SF galaxies (against a factor of 6 excess in SFR), while quiescents show a deficit of a factor 5.5 at a given mass. One possible interpretation of this is that the starburst phase does not coincide with cosmologically relevant BH growth, or that starburst-inducing mergers are more efficient at boosting SFR than BHAR. Contrary to studies based on smaller samples, we find the BHAR/SFR ratio of main sequence (MS) galaxies is not mass invariant, but scales weakly as M*^(0.43pm0.09}, implying faster BH growth in more massive galaxies at $zsim2$. Furthermore, BHAR/SFR during the starburst is a factor of 2 lower than in MS galaxies, at odds with the predictions of hydrodynamical simulations of merger galaxies that foresee a sudden enhancement of L_X/SFR during the merger. Finally, we estimate that the bulk of the accretion density of the Universe at $zsim2$ is associated with normal star-forming systems, with only 6(+/-1)% and 11(+/-1)% associated with starburst and quiescent galaxies, respectively.
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.
In this work we present new APEX/SEPIA Band-5 observations targeting the CO ($J=2text{-}1$) emission line of 24 Herschel-detected galaxies at $z=0.1-0.2$. Combining this sample {with} our recent new Valparaiso ALMA Line Emission Survey (VALES), we investigate the star formation efficiencies (SFEs = SFR/$M_{rm H_{2}}$) of galaxies at low redshift. We find the SFE of our sample bridges the gap between normal star-forming galaxies and Ultra-Luminous Infrared Galaxies (ULIRGs), which are thought to be triggered by different star formation modes. Considering the $rm SFE$ as the SFR and the $L_{rm CO}$ ratio, our data show a continuous and smooth increment as a function of infrared luminosity (or star formation rate) with a scatter about 0.5 dex, instead of a steep jump with a bimodal behaviour. This result is due to the use of a sample with a much larger range of sSFR/sSFR$_{rm ms}$ using LIRGs, with luminosities covering the range between normal and ULIRGs. We conclude that the main parameters controlling the scatter of the SFE in star-forming galaxies are the systematic uncertainty of the $alpha_{rm CO}$ conversion factor, the gas fraction and physical size.
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.
We examine a magnitude limited (M_B< -18.7) sample of post-starburst (PSB) galaxies at 0.03<z<0.11 in the different environments from the spectroscopic data set of the Padova Millennium Galaxy Group Catalog and compare their incidence and properties with those of passive (PAS) and emission line galaxies (EML). PSB galaxies have a quite precise life-time (<1-1.5 Gyr), and they hold important clues for understanding galaxy evolution. While the properties (stellar mass, absolute magnitude, color) of PSBs do not depend on environment, their frequency increases going from single galaxies to binary systems to groups, both considering the incidence with respect to the global number of galaxies and to the number of currently+recently star-forming galaxies. Including in our analysis the sample of cluster PSBs drawn from the WIde-field Nearby Galaxy-cluster Survey presented in Paccagnella et al., we extend the halo mass range covered and present a coherent picture of the effect of the environment on galaxy transformations. We find that the PSB/(PSB+EML) fraction steadily increases with halo mass going from 1% in 10^{11} M_sun$ haloes to ~15% in the most massive haloes (10^{15.5} M_sun). This provides evidence that processes specific to the densest environments, such as ram pressure stripping, are responsible for a large fraction of PSB galaxies in dense environments. These processes act on a larger fraction of galaxies than alternative processes leading to PSB galaxies in the sparsest environments, such as galaxy interactions.
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