Far-Ultraviolet and Far-Infrared Bivariate Luminosity Function of Galaxies: Complex Relation between Stellar and Dust Emission

Far-ultraviolet (FUV) and far-infrared (FIR) luminosity functions (LFs) of galaxies show a strong evolution from $z = 0$ to $z = 1$, but the FIR LF evolves much stronger than the FUV one. The FUV is dominantly radiated from newly formed short-lived OB stars, while the FIR is emitted by dust grains heated by the FUV radiation field. It is known that dust is always associated with star formation activity. Thus, both FUV and FIR are tightly related to the star formation in galaxies, but in a very complicated manner. In order to disentangle the relation between FUV and FIR emissions, we estimate the UV-IR bivariate LF (BLF) of galaxies with {\sl GALEX} and {\sl AKARI} All-Sky Survey datasets. Recently we invented a new mathematical method to construct the BLF with given marginals and prescribed correlation coefficient. This method makes use of a tool from mathematical statistics, so called "copula". The copula enables us to construct a bivariate distribution function from given marginal distributions with prescribed correlation and/or dependence structure. With this new formulation and FUV and FIR univariate LFs, we analyze various FUV and FIR data with {\sl GALEX}, {\sl Spitzer}, and {\sl AKARI} to estimate the UV-IR BLF. The obtained BLFs naturally explain the nonlinear complicated relation between FUV and FIR emission from star-forming galaxies. Though the faint-end of the BLF was not well constrained for high-$z$ samples, the estimated linear correlation coefficient $\rho$ was found to be very high, and is remarkably stable with redshifts (from 0.95 at $z = 0$ to 0.85 at $z = 1.0$). This implies the evolution of the UV-IR BLF is mainly due to the different evolution of the univariate LFs, and may not be controlled by the dependence structure.Comment: 10 pages, 7 figures, Earth, Planets and Space, in pres

Spectroscopic characterization of 250-μ\mum-selected hyper-luminous star-forming galaxies

We present near-infrared spectroscopic observations from VLT ISAAC of thirteen 250\mu m-luminous galaxies in the CDF-S, seven of which have confirmed redshifts which average to = 2.0 \pm 0.4. Another two sources of the 13 have tentative z > 1 identifications. Eight of the nine redshifts were identified with H{\alpha} detection in H- and K-bands, three of which are confirmed redshifts from previous spectroscopic surveys. We use their near-IR spectra to measure H{\alpha} line widths and luminosities, which average to 415 \pm 20 km/s and 3 \times 10^35 W (implying SFR(H{\alpha})~200 M_\odot /yr), both similar to the H{\alpha} properties of SMGs. Just like SMGs, 250 \mu m-luminous galaxies have large H{\alpha} to far-infrared (FIR) extinction factors such that the H{\alpha} SFRs underestimate the FIR SFRs by ~8-80 times. Far-infrared photometric points from observed 24\mu m through 870\mu m are used to constrain the spectral energy distributions (SEDs) even though uncertainty caused by FIR confusion in the BLAST bands is significant. The population has a mean dust temperature of Td = 52 \pm 6 K, emissivity {\beta} = 1.73 \pm 0.13, and FIR luminosity LFIR = 3 \times 10^13 L_\odot. Although selection at 250\mu m allows for the detection of much hotter dust dominated HyLIRGs than SMG selection (at 850\mu m), we do not find any >60 K 'hot-dust' HyLIRGs. We have shown that near-infrared spectroscopy combined with good photometric redshifts is an efficient way to spectroscopically identify and characterise these rare, extreme systems, hundreds of which are being discovered by the newest generation of IR observatories including the Herschel Space Observatory.Comment: 12 pages, 6 figures; Revised with minor corrections from the referee, MNRA

Molecular gas in submillimetre-faint, star-forming ultraluminous galaxies at z \gt 1

[abridged] We present interferometric CO observations of twelve z~2 submillimetre-faint, star-forming radio galaxies (SFRGs) which are thought to be ultraluminous infrared galaxies (ULIRGs) possibly dominated by warmer dust (T_dust ~> 40 K) than submillimetre galaxies (SMGs) of similar luminosities. Four other CO-observed SFRGs are included from the literature, and all observations are taken at the Plateau de Bure Interferometer (PdBI) in the compact configuration. Ten of the sixteen SFRGs observed in CO (63%) are detected at >4sigma with a mean inferred molecular gas mass of ~2*10^10 M_sun. SFRGs trend slightly above the local ULIRG L_FIR-L'_CO relation. Since SFRGs are about two times fainter in radio luminosity but exhibit similar CO luminosities to SMGs, this suggests SFRGs are slightly more efficient star formers than SMGs at the same redshifts. SFRGs also have a narrow mean CO line width, 320+-80km/s. SFRGs bridge the gap between properties of very luminous >5*10^12 L_sun SMGs and those of local ULIRGs and are consistent with intermediate stage major mergers. We suspect that more moderate-luminosity SMGs, not yet surveyed in CO, would show similar molecular gas properties to SFRGs. The AGN fraction of SFRGs is consistent with SMGs and is estimated to be 0.3+-0.1, suggesting that SFRGs are observed near the peak phase of star formation activity and not in a later, post-SMG enhanced AGN phase. This CO survey of SFRGs serves as a pilot project for the much more extensive survey of Herschel and SCUBA-2 selected sources which only partially overlap with SMGs. Better constraints on CO properties of a diverse high-z ULIRG population are needed from ALMA to determine the evolutionary origin of extreme starbursts, and what role ULIRGs serve in catalyzing the formation of massive stellar systems in the early Universe.Comment: 23 pages, 9 figures; MNRAS accepte

A survey of molecular gas in luminous sub-millimetre galaxies

We present the results from a survey of 12CO emission in 40 luminous sub-millimetre galaxies (SMGs), with 850-μm fluxes of S850 μm = 4–20 mJy, conducted with the Plateau de Bure Interferometer. We detect 12CO emission in 32 SMGs at z ∼ 1.2–4.1, including 16 SMGs not previously published. Using multiple 12CO line (Jup = 2–7) observations, we derive a median spectral line energy distribution for luminous SMGs. We report the discovery of a fundamental relationship between 12CO FWHM and 12CO line luminosity in high-redshift starbursts, which we interpret as a natural consequence of the baryon-dominated dynamics within the regions probed by our observations. We use far-infrared luminosities to assess the star formation efficiency in our SMGs, finding that the slope of the L′CO-LFIR relation is close to linear. We derive molecular gas masses, finding a mean gas mass of (5.3 ± 1.0) × 1010 M⊙. Combining these with dynamical masses, we determine the redshift evolution of the gas content of SMGs, finding that they do not appear to be significantly more gas rich than less vigorously star-forming galaxies at high redshifts. Finally, we collate X-ray observations, and study the interdependence of gas and dynamical properties of SMGs with their AGN activity and supermassive black hole masses (MBH), finding that SMGs lie significantly below the local MBH-σ relation

High-resolution CO and radio imaging of ULIRGs: extended CO structures and implications for the universal star formation law

We present high spatial resolution (0.4", ~3.5 kpc) PdBI interferometric data on three ultra-luminous infrared galaxies (ULIRGs) at z~2: two submillimetre galaxies and one submillimetre faint star forming radio galaxy. The three galaxies have been ro- bustly detected in CO rotational transitions, either 12CO(J=4-3) or 12CO(J=3-2), allowing their sizes and gas masses to be accurately constrained. These are the highest spatial resolution observations observed to date (by a factor of ~2) for intermediate-excitation CO emission in z~2 ULIRGs. The galaxies appear extended over several resolution elements, having a mean radius of 3.7 kpc. High-resolution (0.3") combined MERLIN-VLA observations of their radio continua allow an analysis of the star formation behaviour of these galaxies, on comparable spatial scales to that of the CO observations. This 'matched beam' approach sheds light on the spatial distribution of both molecular gas and star formation, and we can therefore calculate accurate star formation rates and gas surface densities: this allows us to place the three systems in the context of a Kennicutt-Schmidt (KS)-style star formation law. We find a difference in size between the CO and radio emission regions, and as such we suggest that using the spatial extent of the CO emission region to estimate the surface density of star formation may lead to error. This size difference also causes the star formation efficiencies within systems to vary by up to a factor of 5. We also find, with our new accurate sizes, that SMGs lie significantly above the KS relation, indicating that stars are formed more efficiently in these extreme systems than in other high-z star forming galaxies.Comment: 17 pages, 10 figures; revised version accepted for publication in MNRA