ABSTRACT
The disc structure of the Milky Way is marked by a chemical dichotomy, with high-α and low-α abundance sequences, traditionally identified with the geometric thick and thin discs. This ...identification is aided by the old ages of the high-α stars, and lower average ages of the low-α ones. Recent large-scale surveys such as APOGEE have provided a wealth of data on this chemical structure, including showing that an identification of chemical and geometric thick discs is not exact, but the origin of the chemical dichotomy has remained unclear. Here we demonstrate that a dichotomy arises naturally if the early gas-rich disc fragments, leading to some fraction of the star formation occuring in clumps of the type observed in high-redshift galaxies. These clumps have high star formation rate density. They therefore enrich rapidly, moving from the low-α to the high-α sequence, while more distributed star formation produces the low-α sequence. We demonstrate that this model produces a chemically defined thick disc that has many of the properties of the Milky Way’s thick disc. Because clump formation is common in high-redshift galaxies, we predict that chemical bimodalities are common in massive galaxies.
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 10r
e 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.
In this paper, we demonstrate a new method for fitting galaxy profiles which makes use of the full multiwavelength data provided by modern large optical-near-infrared imaging surveys. We present a ...new version of galapagos, which utilizes a recently developed multiwavelength version of galfit, and enables the automated measurement of wavelength-dependent Sérsic 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 Galaxy And Mass Assembly survey and simulated data made to the same specifications. We find that fitting galaxy light profiles with multiwavelength data increases the stability and accuracy of the measured parameters, and hence produces more complete and meaningful multiwavelength 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 r
e and Sérsic 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 utilizes 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.
ABSTRACT
A quantitative derivation of the intrinsic properties of galaxies related to their fundamental building blocks, gas, dust, and stars is essential for our understanding of galaxy evolution. A ...fully self-consistent derivation of these properties can be achieved with radiative transfer (RT) methods that are constrained by panchromatic imaging observations. Here, we present an axi-symmetric RT model of the UV-optical-FIR/submm spectral and spatial energy distribution of the face-on spiral galaxy M51. The model reproduces reasonably well the azimuthally averaged radial profiles derived from the imaging data available for this galaxy, from GALEX, Sloan Digital Sky Survey, 2MASS, Spitzer, and Herschel. We model the galaxy with three distinct morphological components: a bulge, an inner disc, and a main disc. We derive the length parameters of the stellar emissivity and of the dust distribution. We also derive the intrinsic global and spatially resolved parameters of M51. We find a faint ‘outer disc’ bridging M51 with its companion galaxy M51b. Finally, we present and discuss an alternative model, with dust properties that change within the galaxy.
ABSTRACT
We obtained an axisymmetric model for the large-scale distribution of stars and dust in the Milky Way (MW) using a radiative transfer code that can account for the existing near-infrared ...(NIR)/mid-infrared/submm all-sky emission maps of our Galaxy. We find that the MW has a star-formation rate of ${\rm SFR}=1.25\pm 0.2\, {\rm M}_{\odot }$ yr−1, a stellar mass $M_{*}=(4.9\pm 0.3)\times 10^{10}\, {\rm M}_{\odot }$, and a specific SFR that is relatively constant with radius (except for the inner 1 kpc). We identified an inner radius Rin = 4.5 kpc beyond which the stellar emissivity and dust distribution fall exponentially. For R < Rin the emissivities fall linearly towards the centre. The old stellar populations in the disc have an exponential scale length that increases monotonically from $h_{\rm s}^{\rm disc}(K)=2.2\pm 0.6$ kpc in the NIR, to $h_{\rm s}^{\rm disc}(B)=3.2\pm 0.9$ kpc at the shorter optical bands, and a scale height that varies with radial distance, from $z_{\rm s}^{\rm disc}(0) =140\pm 20$ pc in the centre to $z_{\rm s}^{\rm disc}(R_{\odot }) =300\pm 20$ pc at the solar radius. The young stellar populations have a scale length of $h_{\rm s}^{\rm tdisc}=3.2\pm 0.9$ kpc and a scale height that varies from $z_{\rm s}^{\rm tdisc}(0)=50\pm 10$ pc in the centre to $z_{\rm s}^{\rm tdisc}(R_{\odot })=90\pm 10$ pc at the solar radius. We discovered an inner stellar disc within the central 4.5 kpc, which we associate with the extended long bar of the MW. Most of the obscured star formation happens within this inner thin disc. The diffuse dust is mainly distributed in a disc with scale length $h_{\rm d}^{\rm disc}=5.2\pm 0.8$ kpc and scale height $z_{\rm d}^{\rm disc}=0.14\pm 0.02$ kpc. We give the first derivation of the MW attenuation curve and present it as a functional fit to the model data. We find the MW to lie in the Green Valley of the main sequence relation for spiral galaxies.
We present results of a study to quantify the effects of dust on the derived photometric parameters of disk and bulges obtained from bulge-disk decomposition: scale-length, effective radius, Sérsic ...index, disk axis-ratio, and bulge-to-disk ratio. The dust induced changes in these parameters were obtained by fitting simulated images of composite systems (containing a disk and a bulge) produced using radiative transfer calculations. The simulations were fitted with the GALFIT 3.0.2 data analysis algorithm. Fits were done with both a combination of an exponential plus a variable-index Sérsic function as well as with a combination of two variable-index Sérsic functions. We find that dust is biasing the derived exponential scale-length of decomposed disks towards smaller values than would be otherwise derived if the galaxy were to have no bulge. Similarly, the derived bulge-to-disk ratio is biased towards smaller values. However, the derived axis-ratio of the disk is not changed in the decomposition process. The derived effective radius of decomposed disks of systems having exponential bulges is found to be less affected by dust when fits are done with two variable-index Sérsic functions. For the same type of fits dust is found to bias the value of the derived effective radius of decomposed disks towards lower values for systems having de Vaucouleurs bulges.
A radiative transfer model for the spiral galaxy M33 Thirlwall, Jordan J; Popescu, Cristina C; Tuffs, Richard J ...
Monthly notices of the Royal Astronomical Society,
06/2020, Volume:
495, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT
We present the first radiative transfer (RT) model of a non-edge-on disc galaxy in which the large-scale geometry of stars and dust is self-consistently derived through the fitting of ...multiwavelength imaging observations from the ultraviolet to the submm. To this end, we used the axisymmetric RT model of Popescu et al. and a new methodology for deriving geometrical parameters, and applied this to decode the spectral energy distribution (SED) of M33. We successfully account for both the spatial and spectral energy distribution, with residuals typically within $7{{\ \rm per\ cent}}$ in the profiles of surface brightness and within $8{{\ \rm per\ cent}}$ in the spatially integrated SED. We predict well the energy balance between absorption and re-emission by dust, with no need to invoke modified grain properties, and we find no submm emission that is in excess of our model predictions. We calculate that $80\pm 8{{\ \rm per\ cent}}$ of the dust heating is powered by the young stellar populations. We identify several morphological components in M33, a nuclear, an inner, a main and an outer disc, showing a monotonic trend in decreasing star formation surface density (ΣSFR) from the nuclear to the outer disc. In relation to surface density of stellar mass, the ΣSFR of these components defines a steeper relation than the ‘main sequence’ of star-forming galaxies, which we call a ‘structurally resolved main sequence’. Either environmental or stellar feedback mechanisms could explain the slope of the newly defined sequence. We find the star formation rate to be ${\rm SFR}=0.28^{+0.02}_{-0.01}{\rm M}_{\odot }{\rm yr}^{-1}$.
We report the morphological classification of 3727 galaxies from the Galaxy and Mass Assembly survey with M
r
< −17.4 mag and in the redshift range 0.025 < z < 0.06 (2.1 × 105 Mpc3) into E, S0-Sa, ...SB0-SBa, Sab-Scd, SBab-SBcd, Sd-Irr and little blue spheroid classes. Approximately 70 per cent of galaxies in our sample are disc-dominated systems, with the remaining ∼30 per cent spheroid dominated. We establish the robustness of our classifications, and use them to derive morphological-type luminosity functions and luminosity densities in the ugrizYJHK passbands, improving on prior studies that split by global colour or light profile shape alone. We find that the total galaxy luminosity function is best described by a double-Schechter function while the constituent morphological-type luminosity functions are well described by a single-Schechter function. These data are also used to derive the star formation rate densities for each Hubble class, and the attenuated and unattenuated (corrected for dust) cosmic spectral energy distributions, i.e. the instantaneous energy production budget. While the observed optical/near-IR energy budget is dominated 58:42 by galaxies with a significant spheroidal component, the actual energy production rate is reversed, i.e. the combined disc-dominated populations generate ∼1.3 times as much energy as the spheroid-dominated populations. On the grandest scale, this implies that chemical evolution in the local Universe is currently largely confined to mid-type spiral classes like our Milky Way.
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 h
3 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.
We present an estimate of the galaxy stellar mass function and its division by morphological type in the local (0.025 < z < 0.06) Universe. Adopting robust morphological classifications as previously ...presented (Kelvin et al.) for a sample of 3727 galaxies taken from the Galaxy And Mass Assembly survey, we define a local volume and stellar mass limited sub-sample of 2711 galaxies to a lower stellar mass limit of
$\mathcal {M}=10^{9.0}\,{\rm M}_{{\odot }}$
. We confirm that the galaxy stellar mass function is well described by a double-Schechter function given by
$\mathcal {M}^{*}=10^{10.64}\,{\rm M}_{{\odot }}$
, α1 = −0.43,
$\phi _{1}^{*}=4.18\;\mathrm{dex}^{-1}\,\mathrm{Mpc}^{-3}$
, α2 = −1.50 and
$\phi _{2}^{*}=0.74\;\mathrm{dex}^{-1}\,\mathrm{Mpc}^{-3}$
. The constituent morphological-type stellar mass functions are well sampled above our lower stellar mass limit, excepting the faint little blue spheroid population of galaxies. We find approximately
$71{}_{-4}^{+3}$
per cent of the stellar mass in the local Universe is found within spheroid-dominated galaxies; ellipticals and S0-Sas. The remaining
$29{}_{-3}^{+4}$
per cent falls predominantly within late-type disc-dominated systems, Sab-Scds and Sd-Irrs. Adopting reasonable bulge-to-total ratios implies that approximately half the stellar mass today resides in spheroidal structures, and half in disc structures. Within this local sample, we find approximate stellar mass proportions for E : S0-Sa : Sab-Scd : Sd-Irr of 34 : 37 : 24 : 5.
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