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[show abstract]
[hide abstract]
ABSTRACT: We present results of a study to quantify the effects of dust on the derived
photometric parameters of disks (old stellar disks and young stellar disks) and
bulges: disk scale-lengths, axis-ratios, central surface-brightness, bulge
effective radii and Sersic indexes. The changes in the derived photometric
parameters from their intrinsic values (as seen in the absence of dust) were
obtained by fitting simulated images of disks and bulges produced using
radiative transfer calculations and the model of Popescu et al. (2011). The
fits to the simulations were performed using GALFIT 3.0.2 data analysis
algorithm and the fitted models were the commonly used infinitely thin disks
described by exponential, general Sersic and de Vaucouleurs distributions. We
find the young stellar disks to suffer the most severe variation in the
photometric parameters due to dust effects. In this context we also present
corrections for narrow line (Balmer line) images. Old stellar disks are also
significantly affected by dust, in particular when fits are performed with
exponential functions. The photometric parameters of bulges are to a lesser
extent affected by dust. We also find that the variation of dust corrections
with face-on dust opacity and inclination is similar for bulges with different
intrinsic stellar emissivities (different Sersic index), with differences
manifesting only close to edge-on orientations of the disk. Dust corrections
for bulges are found to be insensitive to the choice of the truncation radius
and ellipticity of the bulge. All corrections are listed in the Appendix and
made available in electronic format.
Astronomy and Astrophysics 05/2013; 553(1):A80. · 4.59 Impact Factor
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Boris Häußler,
Steven P. Bamford,
Marina Vika,
Alex L. Rojas,
Marco Barden,
Lee S. Kelvin,
Mehmet Alpaslan,
Aaron S. G. Robotham,
Simon P. Driver,
I. K. Baldry,
Sarah Brough,
Andrew M. Hopkins,
Jochen Liske,
Robert C. Nichol, Cristina. C. Popescu,
Richard J. Tuffs
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we demonstrate a new method for fitting galaxy profiles which
makes use of the full multi-wavelength data provided by modern large
optical-near-infrared imaging surveys. We present a new version of GALAPAGOS,
which utilises a recently-developed multi-wavelength version of GALFIT, and
enables the automated measurement of wavelength dependent S\'ersic profile
parameters for very large samples of galaxies. Our new technique is extensively
tested to assess the reliability of both pieces of software, GALFIT and
GALAPAGOS on both real ugrizY JHK imaging data from the GAMA survey and
simulated data made to the same specifications. We find that fitting galaxy
light profiles with multi-wavelength data increases the stability and accuracy
of the measured parameters, and hence produces more complete and meaningful
multi-wavelength photometry than has been available previously. The improvement
is particularly significant for magnitudes in low S/N bands and for structural
parameters like half-light radius re and S\'ersic index n for which a prior is
used by constraining these parameters to a polynomial as a function of
wavelength. This allows the fitting routines to push the magnitude of galaxies
for which sensible values can be derived to fainter limits. The technique
utilises a smooth transition of galaxy parameters with wavelength, creating
more physically meaningful transitions than single-band fitting and allows
accurate interpolation between passbands, perfect for derivation of rest-frame
values.
12/2012;
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[show abstract]
[hide abstract]
ABSTRACT: The goal of the present study is to establish the physical origin of dust
heating and emission based on radiation transfer models, which
self-consistently connect the emission components from diffuse dust and the
dust in massive star forming regions. NGC 4214 is a nearby dwarf galaxy with a
large set of ancillary data, ranging from the ultraviolet (UV) to radio,
including maps from SPITZER, HERSCHEL and detections from PLANCK. We mapped
this galaxy with MAMBO at 1.2 mm at the IRAM 30 m telescope. We extract
separate dust emission components for the HII regions (plus their associated
PDRs on pc scales) and for the diffuse dust (on kpc scales). We analyse the
full UV to FIR/submm SED of the galaxy using a radiation transfer model which
self-consistently treats the dust emission from diffuse and SF complexes
components, considering the illumination of diffuse dust both by the
distributed stellar populations, and by escaping light from the HII regions.
While maintaining consistency with the framework of this model we additionally
use a model that provides a detailed description of the dust emission from the
HII regions and their surrounding PDRs on pc scales. Due to the large amount of
available data and previous studies for NGC 4214 very few free parameters
remained in the model fitting process. We achieve a satisfactory fit for the
emission from HII+PDR regions on pc scales, with the exception of the emission
at 8\mi, which is underpredicted by the model. For the diffuse emission we
achieve a good fit if we assume that about 30-70% of the emission escaping the
HII+PDR regions is able to leave the galaxy without passing through a diffuse
ISM, which is not an unlikely scenario for a dwarf galaxy which has recently
undergone a nuclear starburst. We determine a dust-to-gas mass ratio of 350-390
which is close to the expected value based on the metallicity.
10/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We present a theoretical study that quantifies the effect of dust on the
derived Sersic indexes of disks and bulges. The changes in the derived
parameters from their intrinsic values (as seen in the absence of dust) were
obtained by fitting Sersic distributions on simulated images of disks and
bulges produced using radiative transfer calculations and the model of Popescu
et al. 2011. We found that dust has the effect of lowering the measured Sersic
index in most cases, with stronger effects for disks and bulges seen through
more optically thick lines of sight.
Proceedings of the International Astronomical Union 08/2012; 284:306-308.
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Lee S. Kelvin,
Simon P. Driver,
Aaron S. G. Robotham,
David T. Hill,
Mehmet Alpaslan,
Ivan K. Baldry,
Steven P. Bamford,
Joss Bland-Hawthorn,
Sarah Brough,
Alister W. Graham,
Boris Häussler,
Andrew M. Hopkins,
Jochen Liske,
Jon Loveday,
Peder Norberg,
Steven Phillipps, Cristina C. Popescu,
Matthew Prescott,
Edward N. Taylor,
Richard J. Tuffs
[show abstract]
[hide abstract]
ABSTRACT: We present single-Sérsic two-dimensional (2D) model fits to 167
600 galaxies modelled independently in the ugrizYJHK bandpasses using
reprocessed Sloan Digital Sky Survey Data Release Seven (SDSS DR7) and
UKIRT Infrared Deep Sky Survey Large Area Survey imaging data available
from the Galaxy And Mass Assembly (GAMA) data base. In order to
facilitate this study we developed Structural Investigation of Galaxies
via Model Analysis (SIGMA), an R wrapper around several contemporary
astronomy software packages including SOURCE EXTRACTOR, PSF EXTRACTOR
and GALFIT 3. SIGMA produces realistic 2D model fits to galaxies,
employing automatic adaptive background subtraction and empirical point
spread function measurements on the fly for each galaxy in GAMA. Using
these results, we define a common coverage area across the three GAMA
regions containing 138 269 galaxies. We provide Sérsic magnitudes
truncated at 10re which show good agreement with SDSS
Petrosian and GAMA photometry for low Sérsic index systems (n
< 4), and much improved photometry for high Sérsic index
systems (n > 4), recovering as much as Δm= 0.5 mag in the r
band. We employ a K-band Sérsic index/u-r colour relation to
delineate the massive (n > ˜2) early-type galaxies (ETGs) from
the late-type galaxies (LTGs). The mean Sérsic index of these
ETGs shows a smooth variation with wavelength, increasing by 30 per cent
from g through K. LTGs exhibit a more extreme change in Sérsic
index, increasing by 52 per cent across the same range. In addition,
ETGs and LTGs exhibit a 38 and 25 per cent decrease, respectively, in
half-light radius from g through K. These trends are shown to arise due
to the effects of dust attenuation and stellar population/metallicity
gradients within galaxy populations.
Monthly Notices of the Royal Astronomical Society 03/2012; 421:1007-1039. · 4.90 Impact Factor
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Lee S. Kelvin,
Simon P. Driver,
Aaron S. G. Robotham,
David T. Hill,
Mehmet Alpaslan,
Ivan K. Baldry,
Steven P. Bamford,
Joss Bland-Hawthorn,
Sarah Brough,
Alister W. Graham,
Boris Häussler,
Andrew M. Hopkins,
Jochen Liske,
Jon Loveday,
Peder Norberg,
Steven Phillipps, Cristina C. Popescu,
Matthew Prescott,
Edward N. Taylor,
Richard J. Tuffs
[show abstract]
[hide abstract]
ABSTRACT: We present single-S\'ersic two-dimensional model fits to 167,600 galaxies
modelled independently in the ugrizYJHK bandpasses using reprocessed Sloan
Digital Sky Survey Data Release Seven (SDSS DR7) and UKIRT Infrared Deep Sky
Survey Large Area Survey (UKIDSS-LAS) imaging data available from the GAMA
database. In order to facilitate this study we developed SIGMA, an R wrapper
around several contemporary astronomy software packages including Source
Extractor, PSF Extractor and GALFIT 3. SIGMA produces realistic 2D model fits
to galaxies, employing automatic adaptive background subtraction and empirical
PSF measurements on the fly for each galaxy in GAMA. Using these results, we
define a common coverage area across the three GAMA regions containing 138,269
galaxies. We provide S\'ersic magnitudes truncated at 10 re which show good
agreement with SDSS Petrosian and GAMA photometry for low S\'ersic index
systems (n < 4), and much improved photometry for high S\'ersic index systems
(n > 4), recovering as much as \Delta m = 0.5 magnitudes in the r band. We
employ a K band S\'ersic index/u - r colour relation to delineate the massive
(n > ~2) early-type galaxies (ETGs) from the late-type galaxies (LTGs). The
mean S\'ersic index of these ETGs shows a smooth variation with wavelength,
increasing by 30% from g through K. LTGs exhibit a more extreme change in
S\'ersic index, increasing by 52% across the same range. In addition, ETGs and
LTGs exhibit a 38% and 25% decrease respectively in half-light radius from g
through K. These trends are shown to arise due to the effects of dust
attenuation and stellar population/metallicity gradients within galaxy
populations.
12/2011;
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David T. Hill,
Lee S. Kelvin,
Simon P. Driver,
Aaron S. G. Robotham,
Ewan Cameron,
Nicholas Cross,
Ellen Andrae,
Ivan K. Baldry,
Steven P. Bamford,
Joss Bland-Hawthorn, [......],
Steven Phillipps,
Kevin A. Pimbblet, Cristina C. Popescu,
Matthew Prescott,
Mark Seibert,
Rob G. Sharp,
Will J. Sutherland,
Daniel Thomas,
Richard J. Tuffs,
Elco van Kampen
[show abstract]
[hide abstract]
ABSTRACT: In order to generate credible 0.1–2 μm spectral energy distributions, the Galaxy and Mass Assembly (GAMA) project requires many gigabytes of imaging data from a number of instruments to be reprocessed into a standard format. In this paper, we discuss the software infrastructure we use, and create self-consistent ugrizYJHK photometry for all sources within the GAMA sample. Using UKIDSS and SDSS archive data, we outline the pre-processing necessary to standardize all images to a common zero-point, the steps taken to correct for the seeing bias across the data set and the creation of gigapixel-scale mosaics of the three 4 × 12 deg2 GAMA regions in each filter. From these mosaics, we extract source catalogues for the GAMA regions using elliptical Kron and Petrosian matched apertures. We also calculate Sérsic magnitudes for all galaxies within the GAMA sample using sigma, a galaxy component modelling wrapper for galfit 3. We compare the resultant photometry directly and also calculate the r-band galaxy luminosity function for all photometric data sets to highlight the uncertainty introduced by the photometric method. We find that (1) changing the object detection threshold has a minor effect on the best-fitting Schechter parameters of the overall population (M*± 0.055 mag, α± 0.014, ϕ*± 0.0005 h3 Mpc−3); (2) there is an offset between data sets that use Kron or Petrosian photometry, regardless of the filter; (3) the decision to use circular or elliptical apertures causes an offset in M* of 0.20 mag; (4) the best-fitting Schechter parameters from total-magnitude photometric systems (such as SDSS modelmag or Sérsic magnitudes) have a steeper faint-end slope than photometric systems based upon Kron or Petrosian measurements; and (5) our Universe’s total luminosity density, when calculated using Kron or Petrosian r-band photometry, is underestimated by at least 15 per cent.
Monthly Notices of the Royal Astronomical Society 03/2011; 412(2):765 - 799. · 4.90 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present a self-consistent model of the spectral energy distributions (SEDs) of spiral galaxies from the ultraviolet (UV) to the mid-infrared (MIR)/far-infrared (FIR)/submillimeter (submm) based on a full radiative transfer calculation of the propagation of starlight in galaxy disks. This model predicts not only the total integrated energy absorbed in the UV/optical and re-emitted in the infrared/submm, but also the colours of the dust emission based on an explicit calculation of the strength and colour of the UV/optical radiation fields heating the dust, and incorporating a full calculation of the stochastic heating of small dust grains and PAH molecules. The geometry of the translucent components of the model is empirically constrained using the results from the radiation transfer analysis of Xilouris et al. on spirals in the middle range of the Hubble sequence, while the geometry of the optically thick components is constrained from physical considerations with a posteriori checks of the model predictions with observational data. These geometrical constraints enable the dust emission to be predicted in terms of a minimum set of free parameters: the central face-on dust opacity in the B-band tau^f_B, a clumpiness factor F for the star-forming regions, the star-formation rate SFR, the normalised luminosity of the old stellar population old and the bulge-to-disk ratio B/D. We show that these parameters are almost orthogonal in their predicted effect on the colours of the dust/PAH emission. The results of the calculations are made available in the form of a large library of simulated dust emission SEDs spanning the whole parameter space of our model, together with the corresponding library of dust attenuation calculated using the same model. (see full abstract in the paper) Comment: 39 pages; accepted for publication in Astronomy & Astrophysics; For a higher resolution version of Fig.1 and Fig.20 see http://www.star.uclan.ac.uk/~ccp/index.shtml
11/2010;
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David T. Hill,
Lee S. Kelvin,
Simon P. Driver,
Aaron S. G. Robotham,
Ewan Cameron,
Nicholas Cross,
Ellen Andrae,
Ivan K. Baldry,
Steven P. Bamford,
Joss Bland-Hawthorn, [......],
Steven Phillipps,
Kevin A. Pimbblet, Cristina C. Popescu,
Matthew Prescott,
Mark Seibert,
Rob G. Sharp,
Will J. Sutherland,
Daniel Thomas,
Richard J. Tuffs,
Elco van Kampen
[show abstract]
[hide abstract]
ABSTRACT: In order to generate credible 0.1-2 {\mu}m SEDs, the GAMA project requires many Gigabytes of imaging data from a number of instruments to be re-processed into a standard format. In this paper we discuss the software infrastructure we use, and create self-consistent ugrizYJHK photometry for all sources within the GAMA sample. Using UKIDSS and SDSS archive data, we outline the pre-processing necessary to standardise all images to a common zeropoint, the steps taken to correct for seeing bias across the dataset, and the creation of Gigapixel-scale mosaics of the three 4x12 deg GAMA regions in each filter. From these mosaics, we extract source catalogues for the GAMA regions using elliptical Kron and Petrosian matched apertures. We also calculate S\'ersic magnitudes for all galaxies within the GAMA sample using SIGMA, a galaxy component modelling wrapper for GALFIT 3. We compare the resultant photometry directly, and also calculate the r band galaxy LF for all photometric datasets to highlight the uncertainty introduced by the photometric method. We find that (1) Changing the object detection threshold has a minor effect on the best-fitting Schechter parameters of the overall population (M* +/- 0.055mag, {\alpha} +/- 0.014, {\Phi}* +/- 0.0005 h^3 Mpc^{-3}). (2) An offset between datasets that use Kron or Petrosian photometry regardless of the filter. (3) The decision to use circular or elliptical apertures causes an offset in M* of 0.20mag. (4) The best-fitting Schechter parameters from total-magnitude photometric systems (such as SDSS modelmag or S\'ersic magnitudes) have a steeper faint-end slope than photometry dependent on Kron or Petrosian magnitudes. (5) Our Universe's total luminosity density, when calculated using Kron or Petrosian r-band photometry, is underestimated by at least 15%. Comment: 38 pages, 10 Tables, 26 figures. Submitted to MNRAS (revised once). Image resolution has been lowered. For higher resolution, see http://www.gama-survey.org/
09/2010;
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[show abstract]
[hide abstract]
ABSTRACT: Understanding the infrared emission of galaxies is critical to observational and theoretical investigations of the condensation of galaxies out of the intergalactic medium and the conversion of gas into stars over cosmic time. From an observational perspective, about half of all photons emitted within galaxies are locally absorbed by dust grains, necessitating a self-consistent analysis of the panchromatic emission of galaxies to quantify star-formation and AGN activity as a function of epoch and environment. From a theoretical perspective, physical processes involving dust are expected to play a major role in regulating the accumulation of baryons in galaxies and their condensation into stars on scales ranging from Mpc down to sub-pc. All this requires a quantitative analysis of the interaction between dust, gas and radiation. Here we review progress in the modelling of some of these processes. Comment: 12 pages, invited review in "Hunting for the dark: the hidden side of galaxy formation", eds. Victor P. Debattista and Cristina C. Popescu, AIP Conf. Ser. 1240, p. 35
06/2010;
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Simon P. Driver,
Peder Norberg,
Ivan K. Baldry,
Steven P. Bamford,
Andrew M. Hopkins,
Jochen Liske,
Jon Loveday,
John A. Peacock,
David T. Hill,
Lee S. Kelvin, [......],
Barry F. Madore,
Mark Seibert,
Martin J. Meyer,
Lister Staveley-Smith,
Steven Phillipps, Cristina C. Popescu,
Ann E. Sansom,
Will J. Sutherland,
Richard J. Tuffs,
Steven J. Warren
[show abstract]
[hide abstract]
ABSTRACT: The Galaxy And Mass Assembly (GAMA) project is the latest in a tradition of large galaxy redshift surveys, and is now underway on the 3.9m Anglo-Australian Telescope at Siding Spring Observatory. GAMA is designed to map extragalactic structures on scales of 1kpc - 1Mpc in complete detail to a redshift of z~0.2, and to trace the distribution of luminous galaxies out to z~0.5. The principal science aim is to test the standard hierarchical structure formation paradigm of Cold Dark Matter (CDM) on scales of galaxy groups, pairs, discs, bulges and bars. We will measure (1) the Dark Matter Halo Mass Function (as inferred from galaxy group velocity dispersions); (2) baryonic processes, such as star formation and galaxy formation efficiency (as derived from Galaxy Stellar Mass Functions); and (3) the evolution of galaxy merger rates (via galaxy close pairs and galaxy asymmetries). Additionally, GAMA will form the central part of a new galaxy database, which aims to contain 275,000 galaxies with multi-wavelength coverage from coordinated observations with the latest international ground- and space-based facilities: GALEX, VST, VISTA, WISE, HERSCHEL, GMRT and ASKAP. Together, these data will provide increased depth (over 2 magnitudes), doubled spatial resolution (0.7"), and significantly extended wavelength coverage (UV through Far-IR to radio) over the main SDSS spectroscopic survey for five regions, each of around 50 deg^2. This database will permit detailed investigations of the structural, chemical, and dynamical properties of all galaxy types, across all environments, and over a 5 billion year timeline. Comment: GAMA overview which appeared in the October 2009 issue of Astronomy & Geophysics, ref: Astron.Geophys. 50 (2009) 5.12
10/2009;
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[show abstract]
[hide abstract]
ABSTRACT: In this paper, we combine the stellar spectral synthesis code STARBURST99, the nebular modeling code MAPPINGS IIIq, a one-dimensional dynamical evolution model of H II regions around massive clusters of young stars, and a simplified model of synchrotron emissivity to produce purely theoretical self-consistent synthetic spectral energy distributions (SEDs) for (solar metallicity) starbursts lasting ~108 yr. These SEDs extend from the Lyman limit to beyond 21 cm. We find that two ISM parameters control the form of the SED: the pressure in the diffuse phase of the ISM (or, equivalently, its density), and the molecular cloud dissipation timescale. In particular, the shape of the far-infrared (dust re-emission) bump is strongly dependent on the mean pressure in the star-forming or starburst galaxy. This can explain the range of far-infrared (FIR) colors seen in starburst galaxies. In the case of objects of composite excitation, such diagrams potentially provide a means of estimating the fraction of the FIR emission that is contributed by an active nucleus. We present detailed SED fits to Arp 220 and NGC 6240, and we give the predicted colors for starburst galaxies derived from our models for the IRAS and the Spitzer Space Telescope MIPS and IRAC instruments. Our models reproduce the spread in observed colors of starburst galaxies. From both the SED fits and the color : color diagrams, we infer the presence of a population of compact and ultracompact H II regions around single OB stars or small OB clusters. Finally, we present absolute calibrations to convert observed fluxes into star formation rates in the UV (GALEX), at optical wavelengths (Hα), and in the IR (IRAS or Spitzer). We show that 25 μm fluxes are particularly valuable as star formation indicators, since they largely eliminate one of the parameters controlling the IR SED.
The Astrophysical Journal 12/2008; 619(2):755. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The dominant source of electromagnetic energy in the universe today (over ultraviolet, optical, and near-infrared wavelengths) is starlight. However, quantifying the amount of starlight produced has proved difficult due to interstellar dust grains that attenuate some unknown fraction of the light. Combining a recently calibrated galactic dust model with observations of 10,000 nearby galaxies, we find that (integrated over all galaxy types and orientations) only 11% ± 2% of the 0.1 μm photons escape their host galaxies; this value rises linearly (with log λ) to 87% ± 3% at 2.1 μm. We deduce that the energy output from stars in the nearby universe is (1.6 ± 0.2) × 1035 W Mpc−3, of which (0.9 ± 0.1) × 1035 W Mpc−3 escapes directly into the intergalactic medium. Some further ramifications of dust attenuation are discussed, and equations that correct individual galaxy flux measurements for its effect are provided.
The Astrophysical Journal 12/2008; 678(2):L101. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present deep diffraction-limited far-infrared (FIR) strip maps of a sample of 63 galaxies later than S0 and brighter than BT = 16.8, selected from the Virgo Cluster Catalogue of Binggeli, Sandage, & Tammann. The ISOPHOT instrument on board the Infrared Space Observatory was used to achieve sensitivities typically an order of magnitude deeper than IRAS in the 60 and 100 μm bands and to reach the confusion limit at 170 μm. The averaged 3 σ upper limits for integrated flux densities of point sources at 60, 100, and 170 μm are 43, 33, and 58 mJy, respectively. A total of 63.5% are detected at all three wavelengths. The highest detection rate (85.7%) is in the 170 μm band. In many cases the galaxies are resolved, allowing the scale length of the infrared disks to be derived from the oversampled brightness profiles in addition to the spatially integrated emission. The data presented should provide the basis for a variety of statistical investigations of the FIR spectral energy distributions of gas-rich galaxies in the local universe spanning a broad range in star formation activity and morphological types, including dwarf systems and galaxies with rather quiescent star formation activity.
The Astrophysical Journal Supplement Series 12/2008; 139(1):37. · 13.46 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We examine, from a theoretical viewpoint, how the physical parameters of H II regions are controlled in both normal galaxies and in starburst environments. These parameters are the H II region luminosity function, the time-dependent size, the covering fraction of molecular clouds, the pressure in the ionized gas, and the ionization parameter. The factors that control them are the initial mass function (IMF) of the exciting stars, the cluster mass function, the metallicity, and the mean pressure in the surrounding interstellar medium. We investigate the sensitivity of the Hα luminosity to the IMF, and find that this can translate to more than a factor 2 variation in derived star formation rates. The molecular cloud dissipation timescale is estimated from a case study of M17 to be ~1 Myr for this object. Based on H II luminosity function fitting for nearby galaxies, we suggest that the H II region cluster mass function is fitted by a lognormal form peaking at ~100 M☉. The cluster mass function continues the stellar IMF to a higher mass regime. The pressure in the H II regions is controlled by the mechanical luminosity flux from the central cluster. Since this is closely related to the ionizing photon flux, we show that the ionization parameter is not a free variable, and that the diffuse ionized medium may be composed of many large, faint, and old H II regions. Finally, we derive theoretical probability distributions for the ionization parameter as a function of metallicity and compare these to those derived for SDSS galaxies.
The Astrophysical Journal 12/2008; 647(1):244. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: This paper presents both stellar mass and H II region diagnostics based on dusty, radiation-pressure-dominated photoionization models for compact and ultracompact H II regions, and compares these with observational constraints. These models successfully reproduce the observed relationship between the density and the thickness of the ionized layer. The absorption of ionizing photons in the dusty ionized plasma makes denser ionized regions thinner than simple photoionization models would predict, improving the fit with the observations. The models provide a good fit to observed diagnostic plots involving ratios of infrared emission lines, all accessible with the IRS instrument of the Spitzer Space Telescope. These give the effective temperature to an accuracy of about 2500 K and the mass of the ionizing star to a precision of about ±30%. The S IV/S III ratio is sensitive to foreground extinction as well as to stellar effective temperature or mass. From this ratio, we determine that the mean extinction to observed compact H II regions is typically AV ~ 30 mag. The electron temperature depends on the chemical abundances, the pressure, and the effective temperature of the exciting star. We use these models to rederive the slope of the Galactic abundance gradient, with the result that d log(O/H)/dRG = 0.06 ± 0.01 dex kpc-1, bringing the Galactic abundance gradient derived from compact H II regions into closer agreement with those based on other techniques. The shape of the far-IR SED of compact H II regions can be used to constrain the mean pressure or density in the H II region. The Spitzer MIPS instrument should be very helpful in this regard.
The Astrophysical Journal 12/2008; 639(2):788. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We have statistically analyzed the spatially integrated far-infrared (FIR) emissions of the complete volume- and luminosity-limited sample of late-type (later than S0) Virgo Cluster galaxies measured using the Infrared Space Observatory by Tuffs and coworkers in bands centered on 60, 100, and 170 μm. Thirty of 38 galaxies detected at all three wavelengths contain a cold dust emission component, present within all morphological types of late-type systems ranging from early giant spiral galaxies to blue compact dwarfs (BCDs) and which could not have been recognized by IRAS. We fitted the data with a superposition of two modified blackbody functions, physically identified with a localized warm dust emission component associated with H II regions (whose temperature was constrained to be 47 K), and a diffuse emission component of cold dust. The cold dust temperatures were found to be broadly distributed, with a median of 18 K, some 8-10 K lower than would have been predicted from IRAS. The derived total dust mass is correspondingly increased by factors of typically 6-13. A good linear correlation is found between the "warm FIR" luminosities and the Hα equivalent widths (EWs), supporting the assumptions of our constrained spectral energy distribution fit procedure. We also found a good nonlinear correlation between the "cold FIR" luminosities and the Hα EWs, consistent with the prediction of Popescu and coworkers that the FIR-submillimeter emission should mainly be due to diffuse nonionizing UV photons. Both the "warm" and the "cold" FIR luminosity components are nonlinearly correlated with the (predominantly nonthermal) radio luminosities. There is a tendency for the temperatures of the cold dust component to become colder and for the cold dust surface densities (normalized to optical area) to increase for later morphological types. A particularly significant result concerns the low dust temperatures (ranging down to less than 10 K) and large dust masses associated with the Im and BCD galaxies in our sample. We propose two scenarios to account for the FIR characteristics of these systems.
The Astrophysical Journal 12/2008; 567(1):221. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We build, as far as theory will permit, self-consistent model H II regions around central clusters of aging stars. These produce strong emission line diagnostics applicable to either individual H II regions in galaxies or to the integrated emission line spectra of disk or starburst galaxies. The models assume that the expansion and internal pressure of individual H II regions is driven by the net input of mechanical energy from the central cluster, be it through winds or supernova events. This eliminates the ionization parameter as a free variable, replacing it with a parameter that depends on the ratio of the cluster mass to the pressure in the surrounding interstellar medium. These models explain why H II regions with low abundances have high excitation and demonstrate that at least part of the warm ionized medium is the result of overlapping faint, old, large, and low-pressure H II regions. We present line ratios (at both optical and IR wavelengths) that provide reliable abundance diagnostics for both single H II regions or for integrated galaxy spectra, and we find a number that can be used to estimate the mean age of the cluster stars exciting individual H II regions.
The Astrophysical Journal Supplement Series 12/2008; 167(2):177. · 13.46 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The dominant source of electromagnetic energy in the Universe today (over ultraviolet, optical and near-infrared wavelengths) is starlight. However, quantifying the amount of starlight produced has proven difficult due to interstellar dust grains which attenuate some unknown fraction of the light. Combining a recently calibrated galactic dust model with observations of 10,000 nearby galaxies we find that (integrated over all galaxy types and orientations) only (11 +/- 2)% of the 0.1 micron photons escape their host galaxies; this value rises linearly (with log(lambda)) to (87 +/- 3)% at 2.1 micron. We deduce that the energy output from stars in the nearby Universe is (1.6+/-0.2) x 10^{35} W Mpc^{-3} of which (0.9+/-0.1) x 10^{35} W Mpc^{-3} escapes directly into the inter-galactic medium. Some further ramifications of dust attenuation are discussed, and equations that correct individual galaxy flux measurements for its effect are provided. Comment: Accepted by ApJ Letters
03/2008;
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ABSTRACT: Modelling the UV/optical - infrared/submm SEDs of spiral galaxies observed with Herschel will be an essential tool to quantitatively interpret these observations in terms of the present and past star-formation activity of these systems. In this lecture we describe the SED modelling technique we have developed, its applications and tests of its predictions. We show that both the panchromatic SED modelling of individual galaxies and the B-band attenuation-inclination relation of large statistical samples suggest that spiral galaxies in the nearby Universe behave as optically thick systems in their global properties and large-scale distribution of light (central face-on B-band opacity of approx. 4). However disk galaxies are very inhomogeneous systems, having both optically thick components (e.g. spiral arms), and optically thin components (e.g. the interarm regions), the latter making galaxies transparent to background galaxies. Comment: invited lecture; to appear in Proceedings of the lectures given at the Les Houche Winter School "Astronomy in the submillimeter and far infrared domains with the Herschel Space Observatory"
09/2007;
