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Register a new userHigh-J CO Versus Far-Infrared Relations in Normal and Starburst Galaxies
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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.
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We report the detection of far-infrared (FIR) CO rotational emission from nearby active galactic nuclei (AGN) and starburst galaxies, as well as several merging systems and Ultra-Luminous Infrared Galaxies (ULIRGs). Using Herschel-PACS, we have detected transitions in the J$_{upp}$ = 14 - 20 range ($lambda sim$ 130 - 185 $mu$m, $ u sim$ 1612 - 2300 GHz) with upper limits on (and in two cases, detections of) CO line fluxes up to J$_{upp}$ = 30. The PACS CO data obtained here provide the first well-sampled FIR extragalactic CO SLEDs for this range, and will be an essential reference for future high redshift studies. We find a large range in the overall SLED shape, even amongst galaxies of similar type, demonstrating the uncertainties in relying solely on high-J CO diagnostics to characterize the excitation source of a galaxy. Combining our data with low-J line intensities taken from the literature, we present a CO ratio-ratio diagram and discuss its potential diagnostic value in distinguishing excitation sources and physical properties of the molecular gas. The position of a galaxy on such a diagram is less a signature of its excitation mechanism, than an indicator of the presence (or absence) of warm, dense molecular gas. We then quantitatively analyze the CO emission from a subset of the detected sources with Large Velocity Gradient (LVG) radiative transfer models to fit the CO SLEDs. Using both single-component and two-component LVG models to fit the kinetic temperature, velocity gradient, number density and column density of the gas, we derive the molecular gas mass and the corresponding CO-to-H$_2$ conversion factor, $alpha_{CO}$, for each respective source. For the ULIRGs we find $alpha$ values in the canonical range 0.4 - 5 M$_odot$/(K kms$^{-1}$pc$^2$), while for the other objects, $alpha$ varies between 0.2 and 14.} Finally, we compare our best-fit LVG model ..
We revisit the nature of the FIR/Radio correlation by means of the most recent models for star forming galaxies. We model the IR emission with our population synthesis code, GRASIL (Silva et al. 1998). As for the radio emission, we revisit the simple model of Condon & Yin (1990). We find that a tightFIR/Radio correlation is natural when the synchrotron mechanism dominates over the inverse Compton, and the electrons cooling time is shorter than the fading time of the supernova rate. Observations indicate that both these conditions are met in star forming galaxies. However since the radio non thermal emission is delayed, deviations are expected both in the early phases of a starburst, when the radio thermal component dominates, and in the post-starburst phase, when the bulk of the NT component originates from less massive stars. This delay allows the analysis of obscured starbursts with a time resolution of a few tens of Myrs, unreachable with other star formation indicators. We suggest to complement the analysis of the deviations from the FIR/Radio correlation with the radio slope to obtain characteristic parameters of the burst. The analysis of a sample of compact ULIRGs shows that they are intense but transient starbursts, to which one should not apply usual SF indicators devised for constant SF rates. We also discuss the possibility of using the q- radio slope diagram to asses the presence of obscured AGN. A firm prediction of the models is an apparent radio excess during the post-starburst phase, which seems to be typical of a class of star forming galaxies in rich cluster cores. We discuss how deviations from the correlation, due to the evolutionary status of the starburst, affect the technique of photometric redshift determination widely used for high-z sources.
We use Herschel 70 to 160um images to study the size of the far-infrared emitting region in about 400 local galaxies and quasar (QSO) hosts. The sample includes normal `main-sequence star-forming galaxies, as well as infrared luminous galaxies and Palomar-Green QSOs, with different levels and structures of star formation. Assuming Gaussian spatial distribution of the far-infrared (FIR) emission, the excellent stability of the Herschel point spread function (PSF) enables us to measure sizes well below the PSF width, by subtracting widths in quadrature. We derive scalings of FIR size and surface brightness of local galaxies with FIR luminosity, with distance from the star-forming main-sequence, and with FIR color. Luminosities L_FIR ~ 10^11 L_Sun can be reached with a variety of structures spanning 2 dex in size. Ultraluminous L_FIR >~ 10^12 L_Sun galaxies far above the main-sequence inevitably have small Re_70um ~ 0.5 kpc FIR emitting regions with large surface brightness, and can be close to optically thick in the FIR on average over these regions. Compared to these local relations, first ALMA sizes for the dust emission regions in high redshift galaxies, measured at somewhat longer rest wavelengths, suggest larger sizes at the same IR luminosity. We report a remarkably tight relation with 0.15 dex scatter between FIR surface brightness and the ratio of [CII] 158um emission and FIR emission -- the so-called [CII]-deficit is more tightly linked to surface brightness than to FIR luminosity or FIR color. Among 33 z <~ 0.1 PG QSOs with typical L_FIR/L_Bol,AGN ~ 0.1, 19 have a measured 70um half light radius, with median Re_70um = 1.1kpc. This is consistent with the FIR size for galaxies with similar L_FIR but lacking a QSO, in accordance with a scenario where the rest FIR emission of these types of QSOs is, in most cases, due to host star formation.
Galaxy counts from bright ultraviolet (UV) and deep optical spectroscopic surveys have revealed an unexpectedly large number of very blue galaxies. The colors and luminosities of these objects indicate that they are dwarf galaxies undergoing bursts of star formation. We use a galaxy evolution model (PEGASE, Fioc & Rocca-Volmerange 1997) to describe this population as galaxies undergoing cyclical bursts of star formation, thereby determining the luminosity function of these galaxies. When these bursting galaxies are added to normally evolving populations, the combination reproduces the UV number counts, color distributions and deep optical redshift distributions fairly well. Optical (including the Hubble Deep Field) and near-infrared number counts are fitted assuming an open or a flat, Lambda-dominated, Universe. The high amplitude of the angular correlation function of very blue galaxies discovered by Landy et al. (1996) is also recovered in this modelling. The number of bursting galaxies is only a small fraction of the total number of galaxies at optical and near-infrared wavelengths, even at faintest magnitudes. In our evolution modelling, normal galaxies explain most of the blue excess in a low-Omega Universe. The problem of the blue excess remains in a flat Universe without a cosmological constant.
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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