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Register a new userThe Global Star Formation Law of Galaxies Revisited in the Radio Continuum
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We study the global star formation law - the relation between the gas and star formation rate (SFR) in a sample of 130 local galaxies with infrared (IR) luminosities spanning over three orders of magnitude (10^9-10^12 Lsun), which includes 91 normal spiral galaxies and 39 (ultra)luminous IR galaxies [(U)LIRGs]. We derive their total (atomic and molecular) gas and dense molecular gas masses using newly available HI, CO and HCN data from the literature. The SFR of galaxies is determined from total IR (8-1000 um) and 1.4 GHz radio continuum (RC) luminosities. The galaxy disk sizes are defined by the de-convolved elliptical Gaussian FWHM of the RC maps. We derive the galaxy disk-averaged SFRs and various gas surface densities, and investigate their relationship. We find that the galaxy disk-averaged surface densities of dense molecular gas mass has the tightest correlation with that of SFR (scatter ~ 0.26 dex), and is linear in log-log space (power-law slope of N=1.03 +/- 0.02) across the full galaxy sample. The correlation between the total gas and SFR surface densities for the full sample has a somewhat larger scatter (~ 0.48 dex), and is best fit by a power-law with slope 1.45 +/- 0.02. However, the slope changes from ~ 1 when only normal spirals are considered, to ~ 1.5 when more and more (U)LIRGs are included in the fitting. When different CO-to-H2 conversion factors are used to infer molecular gas masses for normal galaxies and (U)LIRGs, the bi-modal relations claimed recently in CO observations of high-redshift galaxies appear to also exist in local populations of star-forming galaxies.
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We study the global SF law - the relation between gas and SFRs in a sample of 181 local galaxies with L_IR spanning almost five orders of magnitude, which includes 115 normal galaxies and 66 (U)LIRGs. We derive their atomic, molecular gas and dense molecular gas masses using newly available HI, CO and HCN data from the literature, and SFRs are determined both from total IR and 1.4 GHz radio continuum (RC) luminosities. In order to derive the disk-averaged surface densities of gas and SFRs, we have used high-resolution RC observations to measure the radio sizes for all galaxies. We find that dense molecular gas (as traced by HCN) has the tightest correlation with that of SFRs, and is linear in (N=1.01 +/- 0.02) across the full galaxy sample. The correlation between densities of molecular gas (traced by CO) and SFRs is sensitive to the adopted value of the alpha_CO used to infer molecular gas masses from CO luminosities. For a fixed value of alpha_CO, a slope of 1.14+/-0.02 is found. If instead we adopt values of 4.6 and 0.8 for disk galaxies and (U)LIRGs, respectively, we find the two distinct relations. If applying a continuously varying alpha_CO to our sample, we recover a single relation with slope of 1.60+/-0.03. The SFRs is a steeper function of total gas than that of molecular gas, and is tighter among low-luminosity galaxies. We find no correlation between SFRs and atomic gas.
We study the spatially resolved Radio Continuum-Star Formation Rate (RC-SFR) relation using state-of-the-art star-formation (SF) tracers in a sample of 17 THINGS galaxies. We use hybrid Sigma_SFR maps (GALEX FUV plus Spitzer 24 mu), RC maps at 22/18 cm from the WSRT SINGS survey, and H-alpha maps to correct for thermal RC emission. We compare azimuthally averaged radial profiles of the RC and FUV/MIR-based Sigma_SFR maps and study pixel-by-pixel correlations at fixed linear scales of 1.2 and 0.7 kpc. The ratio of the integrated SFRs from the RC emission to that of the FUV/MIR-based SF tracers is R_int = 0.78 +/- 0.38, consistent with Condons relation. We find a tight correlation between the radial profiles of the radio and FUV/MIR-based Sigma_SFR for the entire extent of the disk. The ratio R of the azimuthally averaged radio to FUV/MIR-based Sigma_SFR agrees with the integrated ratio with only small quasi-random fluctuations as function of radius. Pixel-by-pixel plots show a tight correlation in log-log diagrams of radio to FUV/MIR-based Sigma_SFR, with a typical standard deviation of a factor of two. Averaged over our sample we find (Sigma_SFR)_RC ~ (Sigma_SFR)_hyb^{0.63+/-0.25} implying that data points with high Sigma_SFR are relatively radio dim, whereas the reverse is true for low Sigma_SFR. We interpret this as a result of spectral ageing of CRe, which is supported by the radio spectral index: data points dominated by young CRe are relatively radio dim, those dominated by old CRe are relatively radio bright. The ratio of radio to FUV/MIR-based integrated SFR is independent of global galaxy parameters, suggesting that we can use RC emission as a universal SF tracer for galaxies, if we restrict ourselves to global or azimuthally averaged measurements. A magnetic field-SFR relation, B ~ SFR_hyb^{0.30+/-0.02}, holding both globally and locally, can explain our results. (abridged)
We present a 1.4 GHz Karl G. Jansky Very Large Array (VLA) study of a sample of early-type galaxies (ETGs) from the volume- and magnitude-limited ATLAS-3D survey. The radio morphologies of these ETGs at a resolution of 5 are diverse and include sources that are compact on sub-kpc scales, resolved structures similar to those seen in star-forming spiral galaxies, and kpc-scale radio jets/lobes associated with active nuclei. We compare the 1.4 GHz, molecular gas, and infrared (IR) properties of these ETGs. The most CO-rich ATLAS-3D ETGs have radio luminosities consistent with extrapolations from H_2-mass-derived star formation rates from studies of late-type galaxies. These ETGs also follow the radio-IR correlation. However, ETGs with lower molecular gas masses tend to have less radio emission relative to their CO and IR emission compared to spirals. The fraction of galaxies in our sample with high IR-radio ratios is much higher than in previous studies, and cannot be explained by a systematic underestimation of the radio luminosity due to the presence extended, low-surface-brightness emission that was resolved-out in our VLA observations. In addition, we find that the high IR-radio ratios tend to occur at low IR luminosities, but are not associated with low dynamical mass or metallicity. Thus, we have identified a population of ETGs that have a genuine shortfall of radio emission relative to both their IR and molecular gas emission. A number of mechanisms may conspire to cause this radio deficiency, including a bottom-heavy stellar initial mass function, weak magnetic fields, a higher prevalence of environmental effects compared to spirals and enhanced cosmic ray losses.
We study the large-scale triggering of star formation in galaxies. We find that the largest mass-scale not stabilized by rotation, a well defined quantity in a rotating system and with clear dynamical meaning, strongly correlates with the star formation rate in a wide range of galaxies. We find that this relation can be explained in terms of the threshold for stability and the amount of turbulence allowed to sustain the system in equilibrium. Using this relation we also derived the observed correlation between the star formation rate and the luminosity of the brightest young stellar cluster.
There exist strong evidence supporting the co-evolution of central supermassive black holes and their host galaxies. It is however still unclear what the exact role of nuclear activity, in the form of accretion onto these supermassive black holes, in this co-evolution is. We use a rich multi-wavelength dataset available for the North Ecliptic Pole field, most notably surveyed by the AKARI satellite infrared telescope to study the host galaxy properties of AGN. In particular we are interested in investigating star-formation in the host galaxies of radio-AGN and the putative radio feedback mechanism, potentially responsible for the eventual quenching of star-formation. Using both broadband SED modeling and optical spectroscopy, we simultaneously study the nu- clear and host galaxy components of our sources, as a function of their radio luminosity, bolo- metric luminosity, and radio-loudness. Here we present preliminary results concerning the AGN content of the radio sources in this field, while offering tentative evidence that jets are inefficient star-formation quenchers, except in their most powerful state.
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