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Register a new userFar-UV and deep surveys: bursting dwarfs versus normal galaxies
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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.
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The far-ultraviolet (UV) counts and the deep optical spectroscopic surveys have revealed an unexpected number of very blue galaxies (vBG). Using constraints from the UV and optical, we apply the galaxy evolution model PEGASE (Fioc & Rocca-Volmerange 1997, hereafter FRV) to describe this population with a cycling star formation. When added to normally evolving galaxy populations, vBG are able to reproduce UV number counts and color distributions as well as deep optical redshift distributions fairly well. Good agreement is also obtained with optical counts (including the Hubble Deep Field). The number of vBG is only a small fraction of the number of normal galaxies, even at faintest magnitudes. In our modelling, the latter explain the bulk of the excess of faint blue galaxies in an open Universe. The problem of the blue excess remains in a flat Universe without cosmological constant.
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.
Chandra is detecting a significant population of normal and starburst galaxies in extremely deep X-ray exposures. For example, approximately 15% of the sources arising in the 2 Ms Chandra Deep Field-North survey are fairly normal galaxies, where normal means Milky Way-type X-ray emission rather than simply exhibiting an optically normal spectrum. Many of these galaxies are being detected at large look-back times z=0.1-0.5, allowing the study of the evolution of X-ray binary populations over significant cosmological timescales. We are also detecting individual off-nuclear ultraluminous X-ray sources (e.g., X-ray binaries), providing the first direct constraints on the prevalence of lower-mass black holes at significantly earlier times. The X-ray emission from such normal galaxies may also be a useful star-formation rate indicator, based on radio/X-ray cross-identifications. We describe the contribution of normal galaxies to the populations which make up the X-ray background and present their directly measured X-ray number counts. We find that normal and starburst galaxies should dominate the 0.5--2 keV number counts at X-ray fluxes fainter than 7e-18 erg cm^{-2} s^{-1} (thus they will outnumber the mighty AGN). Finally, we look to the future, suggesting that it is important that the population of X-ray faint normal and starburst galaxies be well constrained in order to design the next generation of X-ray observatories.
Much of the far-UV emission from elliptical galaxies is thought to arise from extreme horizontal branch stars and related objects. Only about 10% of the stellar population needs to evolve through this phase even in galaxies with the strongest UV upturn. However it is not yet clear if this population represents the extreme low-metallicity or high-metallicty tail of the distribution, or rather arises from the overall population through some metallicity-insensitive mechanism that causes increased mass loss in a small fraction of RGB stars. We investigate the utility of far-UV line strengths for deciding between these possiblities. Complications include the fact that the line strengths reflect both the temperature distribution and the metallicity distribution of the stars, that there may be abundance anomalies introduced on the RGB, and that metals are likely to be redistributed by gravitational settling and radiative diffusion in the atmospheres of hot high-gravity stars. Line-strength measurements from Astro-2 HUT spectra are considered in this context.
The power mechanism and accretion geometry for low-power FR1 radio galaxies is poorly understood in comparison to Seyfert galaxies and QSOs. In this paper we use the diagnostic power of the Lya recombination line observed using the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST) to investigate the accretion flows in three well-known, nearby FR1s: M87, NGC4696, and HydraA. The Ly$alpha$ emission lines luminosity, velocity structure and the limited knowledge of its spatial extent provided by COS are used to assess conditions within a few parsecs of the super-massive black hole (SMBH) in these radio-mode AGN. We observe strong Ly$alpha$ emission in all three objects with similar total luminosity to that seen in BL Lacertae objects. M87 shows a complicated emission line profile in Lya which varies spatially across the COS aperture and possibly temporally over several epochs of observation. In both NGC 4696 and M87, the Ly$alpha$ luminosities $sim 10^{40}$ ergs/s are closely consistent with the observed strength of the ionizing continuum in Case B recombination theory and with the assumption of near unity covering factor. It is possible that the Ly$alpha$ emitting clouds are ionized largely by beamed radiation associated with the jets. Long-slit UV spectroscopy can be used to test this hypothesis. Hydra A and the several BL Lac objects studied in this and previous papers have Lya luminosities larger than M87 but their extrapolated, non-thermal continua are so luminous that they over-predict the observed strength of Ly$alpha$, a clear indicator of relativistic beaming in our direction. Given their substantial space density ($sim 4times10^{-3} Mpc^{-3}$) the unbeamed Lyman continuum radiation of FR1s may make a substantial minority contribution (~10%) to the local UV background if all FR1s are similar to M87 in ionizing flux level.
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