Galaxy formation and evolution

Abstract:

SWIFT is a visible light (650-1000nm) integral field spectorgaph fed by the Palomar extreme adaptive optics system PALM3K. With a subaperture spacing of 8cm, PALM3K is capable of delivering diffraction limited performance even in the visible. With SWIFT providing spatially resolved spectroscopy at R=4000, this provides a truly unique facility for high resolution science in three dimensions. We present here some results from the first year of PALM3K+SWIFT science. We also report on our experience of operating a small field of view instrument (1"x0.5") with a high performance AO system, and hope the lessons learned will provide valuable input to designing successful and productive AO plus Instrument combinations for ELTs.

Abstract:

In this paper we use observations of molecular tracers inmetal-rich and a-enhanced galaxies to study the effect of abundance changes on molecular chemistry. We selected a sample of metalrich spiral and star-bursting objects from the literature, and present here new data for a sample of early-type galaxies (ETGs) previously studied by Crocker et al. We conducted the first survey of carbon monosulphide (CS) and methanol emission in ETGs, detecting seven objects in at least one CS transition, and methanol emission in five ETGs. We find that ETGs whose gas is dominated by ionization from star formation have enhanced CS emission, compared to their hydrogen cyanide (HCN) emission, supporting the hypothesis that CS is a better tracer of dense star-forming gas than HCN. We suggest that the methanol emission in these sources is driven by dust mantle destruction due to ionization from high-mass star formation in dense molecular clouds, but cannot rule out a component due to shocks dominating in some sources. We construct rotation diagrams for each early-type source where at least two transitions of a given species were detected. The rotational temperatures we derive for linear molecules vary between 3 and 9 K, with the majority of sources having rotational temperatures around 5 K. Despite the large uncertainty inherent in this method, the derived source-averaged CS and methanol column densities are similar to those found by other authors for normal spiral and starburst galaxies. This may suggest dense clouds are little affected by the differences between early-and late-type galaxies. Finally, we used the total column density ratios for both our ETG and literature galaxy sample to show for the first time that some molecular tracers do seem to show systematic variations that appear to correlate with metallicity, and that these variations roughly match those predicted by chemical models. Using this fact, the chemical models of Bayet et al. and assumptions about the optical depth we are able to roughly predict the metallicity of our spiral and ETG sample, with a scatter of ~0.3 dex. We provide the community with linear approximations to the relationship between the HCN and CS column density ratio and metallicity. Further study will clearly be required to determine if this, or any, molecular tracer can be used to robustly determine gas-phase metallically, but that a relationship exists at all suggests that in the future it may be possible to calibrate a metallicity indicator for the molecular interstellar medium. © 2013 The Authors.

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.