Galaxy formation and evolution

Abstract:

We use galaxies from the Herschel-ATLAS (H-ATLAS) survey, and a suite of ancillary simulations based on an isothermal dust model, to study our ability to determine the effective dust temperature, luminosity and emissivity index of 250 μm selected galaxies in the local Universe (z < 0.5). As well as simple far-infrared spectral energy distribution (SED) fitting of individual galaxies based on X2 minimization, we attempt to derive the best global isothermal properties of 13 826 galaxies with reliable optical counterparts and spectroscopic redshifts. Using our simulations, we highlight the fact that applying traditional SED fitting techniques to noisy observational data in the Herschel Space Observatory bands introduces artificial anticorrelation between derived values of dust temperature and emissivity index. This is true even for galaxies with the most robust statistical detections in our sample, making the results of such fitting difficult to interpret.We apply a method to determine the best-fitting global values of isothermal effective temperature and emissivity index for z < 0.5 galaxies in H-ATLAS, deriving Teff = 22.3 ± 0.1K and β = 1.98 ± 0.02 (or Teff = 23.5 ± 0.1K and β = 1.82 ± 0.02 if we attempt to correct for bias by assuming that Teff and βeff are independent and normally distributed). We use our technique to test for an evolving emissivity index, finding only weak evidence. The median dust luminosity of our sample is log10(Ldust/L⊙) = 10.72 ± 0.05, which (unlike Teff) shows little dependence on the choice of β used in our analysis, including whether it is variable or fixed. In addition, we use a further suite of simulations based on a fixed emissivity index isothermal model to emphasize the importance of the H-ATLAS PACS data for deriving dust temperatures at these redshifts, even though they are considerably less sensitive than the SPIRE data. Finally, we show that the majority of galaxies detected by H-ATLAS are normal star-forming galaxies, though with a substantial minority (~31 per cent) falling in the Luminous Infrared Galaxy category. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We study the mid- to far-IR properties of a 24 ?m-selected flux-limited sample (S 24 5mJy) of 154 intermediate redshift (z ? 0.15), infrared luminous galaxies, drawn from the 5 Milli-Jansky Unbiased Spitzer Extragalactic Survey. By combining existing mid-IR spectroscopy and new Herschel SPIRE submm photometry from the Herschel Multi-tiered Extragalactic Survey, we derived robust total infrared luminosity (LIR) and dust mass (Mdust) estimates and infered the relative contribution of the AGN to the infrared energy budget of the sources. We found that the total (8?1000 ?m) infrared emission of galaxies with weak 6.2 ?m PAH emission (EW6.2 ? 0.2 ?m) is dominated by AGN activity, while for galaxies with EW6.2 0.2 ?m more than 50% of the LIR arises from star formation. We also found that for galaxies detected in the 250-500 ?m Herschel bands an AGN has a statistically insignificant effect on the temperature of the cold dust and the far-IR colours of the host galaxy, which are primarily shaped by star formation activity. For star-forming galaxies we reveal an anti-correlation between the LIR-to-rest-frame 8 ?m luminosity ratio, IR8 ? LIR/L8 and the strength of PAH features. We found that this anti-correlation is primarily driven by variations in the PAHs emission, and not by variations in the 5?15 ?m mid-IR continuum emission. Using the [Ne iii]/[Ne ii] line ratio as a tracer of the hardness of the radiation field, we confirm that galaxies with harder radiation fields tend to exhibit weaker PAH features, and found that they have higher IR8 values and higher dust-mass-weighted luminosities (LIR/Mdust), the latter being a proxy for the dust temperature (Td). We argue that these trends originate either from variations in the environment of the star-forming regions or are caused by variations in the age of the starburst. Finally, we provide scaling relations that will allow estimating LIR, based on single-band observations with the mid-infrared instrument, on board the upcoming James Webb Space Telescope. © ESO 2013.

Abstract:

Combining observations of multiple CO lines with radiative transfer modelling is a verypowerful tool to investigate the physical properties of the molecular gas in galaxies. Usingnew observations and literature data, we provide the most complete CO ladders ever generatedfor eight star-forming regions in the spiral arms and inter-arms of the spiral galaxyNGC 6946, with observations of the CO(1-0), CO(2-1), CO(3-2), CO(4-3), CO(6-5),13CO(1-0) and 13CO(2-1) transitions. For each region, we use the large velocity gradientassumption to derive beam-averaged molecular gas physical properties, namely the gas kinetictemperature (TK), H2 number volume density (n(H2)) and CO number column density(N(CO)). Two complementary approaches are used to compare the observations with themodel predictions: x2 minimization and likelihood. The physical conditions derived varygreatly from one region to the next: TK = 10-250 K, n(H2) = 102.3-107.0 cm-3 and N(CO) =1015.0-1019.3 cm-2. The spectral line energy distribution (SLED) in some of these extranuclearregions indicate a star formation activity that is more intense than that at the centre of ourown Milky Way. The molecular gas in regions with a large SLED turnover transition (Jmax > 4) is hot but tenuous with a high CO column density, while that in regions with a low SLEDturnover transition (Jmax ≤ 4) is cold but dense with a low CO column density. We finallydiscuss and find some correlations between the physical properties of the molecular gas ineach region and the presence of young stellar population indicators (supernova remnants, HIIregions, HI holes, etc.). © 2013 The Authors.

Abstract:

With the first ELTs around the corner it is becoming ever more important to determine observational strategies and assess the prospective success of observing programs prior to making the observations. To this end, scientific simulations need to become more refined to understand the criteria required for a specific science case. We address the science simulations for HARMONI, an AO assisted first light integral field spectrograph (IFS) for the E-ELT. AO point spread functions (PSFs) vary markedly as a function of wavelength and type of AO system used, so there is need to create detailed PSFs across all IFS wavelength channels for accurate simulations. Detailed AO simulations have shown that for LTAO on the E-ELT, Strehl ratios can vary from 0.5% in V-band up to 75% in K-band. Using a single PSF for an entire datacube (especially with large instantaneous wavelength coverage) could introduce misleading features into simulated observations using HARMONI. We have developed a method to parameterise detailed PSFs using analytical models, which can then be interpolated as a function of wavelength. This allows us to create accurate, but computationally inexpensive, AO PSF datacubes for HARMONI simulations. This shall be developed to cover LTAO, SCAO and GLAO/no-AO PSFs.

Abstract:

In the lead-up to the Square Kilometre Array (SKA) project, several next-generation radio telescopes and upgrades are already being built around the world. These include APERTIF (The Netherlands), ASKAP (Australia), e-MERLIN (UK), VLA (USA), e-EVN (based in Europe), LOFAR (The Netherlands), MeerKAT (South Africa), and the Murchison Widefield Array. Each of these new instruments has different strengths, and coordination of surveys between them can help maximise the science from each of them. A radio continuum survey is being planned on each of them with the primary science objective of understanding the formation and evolution of galaxies over cosmic time, and the cosmological parameters and large-scale structures which drive it. In pursuit of this objective, the different teams are developing a variety of new techniques, and refining existing ones. To achieve these exciting scientific goals, many technical challenges must be addressed by the survey instruments. Given the limited resources of the global radio-astronomical community, it is essential that we pool our skills and knowledge. We do not have sufficient resources to enjoy the luxury of re-inventing wheels. We face significant challenges in calibration, imaging, source extraction and measurement, classification and cross-identification, redshift determination, stacking, and data-intensive research. As these instruments extend the observational parameters, we will face further unexpected challenges in calibration, imaging, and interpretation. If we are to realise the full scientific potential of these expensive instruments, it is essential that we devote enough resources and careful study to understanding the instrumental effects and how they will affect the data. We have established an SKA Radio Continuum Survey working group, whose prime role is to maximise science from these instruments by ensuring we share resources and expertise across the projects. Here we describe these projects, their science goals, and the technical challenges which are being addressed to maximise the science return. © 2013 Astronomical Society of Australia.