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Register a new userUntangling galaxy components: full spectral bulge-disc decomposition
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To ascertain whether photometric decompositions of galaxies into bulges and disks are astrophysically meaningful, we have developed a new technique to decompose spectral data cubes into separate bulge and disk components, subject only to the constraint that they reproduce the conventional photometric decomposition. These decompositions allow us to study the kinematic and stellar population properties of the individual components and how they vary with position, in order to assess their plausibility as discrete elements, and to start to reconstruct their distinct formation histories. An initial application of this method to CALIFA integral field unit observations of three isolated S0 galaxies confirms that in regions where both bulge and disc contribute significantly to the flux they can be physically and robustly decomposed into a rotating dispersion-dominated bulge component, and a rotating low-dispersion disc component. Analysis of the resulting stellar populations shows that the bulges of these galaxies have a range of ages relative to their discs, indicating that a variety of processes are necessary to describe their evolution. This simple test case indicates the broad potential for extracting from spectral data cubes the full spectral data of a wide variety of individual galaxy components, and for using such decompositions to understand the interplay between these various structures, and hence how such systems formed.
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We introduce PHI, a fully Bayesian Markov-chain Monte Carlo algorithm designed for the structural decomposition of galaxy images. PHI uses a triple layer approach to effectively and efficiently explore the complex parameter space. Combining this with the use of priors to prevent nonphysical models, PHI offers a number of significant advantages for estimating surface brightness profile parameters over traditional optimisation algorithms. We apply PHI to a sample of synthetic galaxies with SDSS-like image properties to investigate the effect of galaxy properties on our ability to recover unbiased and well constrained structural parameters. In two-component bulge+disc galaxies we find that the bulge structural parameters are recovered less well than those of the disc, particularly when the bulge contributes a lower fraction to the luminosity, or is barely resolved with respect to the pixel scale or PSF. There are few systematic biases, apart from for bulge+disc galaxies with large bulge Sersic parameter, n. On application to SDSS images, we find good agreement with other codes, when run on the same images with the same masks, weights, and PSF. Again, we find that bulge parameters are the most difficult to constrain robustly. Finally, we explore the use of a Bayesian Information Criterion (BIC) method for deciding whether a galaxy has one- or two-components.
By applying spectroscopic decomposition methods to a sample of MaNGA early-type galaxies, we separate out spatially and kinematically distinct stellar populations, allowing us to explore the similarities and differences between galaxy bulges and discs, and how they affect the global properties of the galaxy. We find that the components have interesting variations in their stellar populations, and display different kinematics. Bulges tend to be consistently more metal rich than their disc counterparts, and while the ages of both components are comparable, there is an interesting tail of younger, more metal poor discs. Bulges and discs follow their own distinct kinematic relationships, both on the plane of the stellar spin parameter, lambda_R, and ellipticity, and in the relation between stellar mass and specific angular momentum, j, with the location of the galaxy as a whole on these planes being determined by how much bulge and disc it contains. As a check of the physical significance of the kinematic decompositions, we also dynamically model the individual galaxy components within the global potential of the galaxy. The resulting components exhibit kinematic parameters consistent with those from the spectroscopic decomposition, and though the dynamical modelling suffers from some degeneracies, the bulges and discs display systematically different intrinsic dynamical properties. This work demonstrates the value in considering the individual components of galaxies rather than treating them as a single entity, which neglects information that may be crucial in understanding where, when and how galaxies evolve into the systems we see today.
Galaxies are complex systems made up of different structural components such as bulges, discs, and bars. Understanding galaxy evolution requires unveiling, independently, their history of stellar mass and metallicity assembly. We introduce C2D, a new algorithm to perform spectro-photometric multi-component decompositions of integral field spectroscopy (IFS) datacubes. The galaxy surface-brightness distribution at each wavelength (quasi-monochromatic image) is fitted using GASP2D, a 2D photometric decomposition code. As a result, C2D provides both a characteristic one-dimensional spectra and a full datacube with all the spatial and spectral information for every component included in the fit. We show the basic steps of the C2D spectro-photometric fitting procedure, tests on mock datacubes demonstrating its reliability, and a first application of C2D to a sample of three early-type galaxies (ETGs) observed within the CALIFA survey. The resulting datacubes from C2D are processed through the PIPE3D pipeline obtaining both the stellar populations and ionised gas properties of bulges and discs. This paper presents an overview of the potential of C2D+PIPE3D to unveil the formation and evolution of galaxies.
We study the kinematics and the stellar populations of the bulge and disc of the spiral galaxy NGC 3521. At each position in the field of view, we separate the contributions of the bulge and the disc from the total observed spectrum and study their kinematics, age, and metallicities independently. Their properties are clearly distinct: the bulge rotates more slowly, has a higher velocity dispersion, and is less luminous than the disc. We identify three main populations of stars in NGC 3521: old ($geq7$ Gyr), intermediate ($approx$ 3 Gyr), and young ($leq$1 Gyr). The mass and light of NGC 3521 are dominated by the intermediate stellar population. The youngest population contributes mostly to the disc component and its contribution increases with radius. We also study the luminosity-weighed properties of the stars in NGC 3521. Along the photometric major axis, we find: i) no age gradient for the stars in the bulge, and a negative age gradient for the stars in the disc; ii) negative metallicity gradients and sub-solar $alpha$-enhancement for both the bulge and the disc. We propose the following picture for the formation of NGC 3521: initial formation a long time ago ($geq 7$ Gyr), followed by a second burst of star formation or a merger ($approx$ 3 Gyrs ago), which contributed predominantly to the mass-build up of the bulge. Recently ($leq 1$ Gyr), the disc of NGC 3521 experienced an additional episode of star formation that started in the innermost regions.
We used the spectroscopic and astrometric data provided from the GALAH DR2 and Gaia DR2, respectively, for a large sample of stars to investigate the behaviour of the [$alpha$/Fe] abundances via two procedures, i.e. kinematically and spectroscopically. With the kinematical procedure, we investigated the distribution of the [$alpha$/Fe] abundances into the high/low probability thin disc, and high/low probability thick-disc populations in terms of total space velocity, [Fe/H] abundance, and age. The high probability thin-disc stars dominate in all sub-intervals of [$alpha$/Fe], including the rich ones: [$alpha$/Fe]$>0.3$ dex, where the high probability thick-disc stars are expected to dominate. This result can be explained by the limiting apparent magnitude of the GALAH DR2 ($V<14$ mag) and intermediate Galactic latitude of the star sample. Stars in the four populations share equivalent [$alpha$/Fe] and [Fe/H] abundances, total space velocities and ages. Hence, none of these parameters can be used alone for separation of a sample of stars into different populations. High probability thin-disc stars with abundance $-1.3<{rm[Fe/H]}leq -0.5$ dex and age $9<tauleq13$ Gyr are assumed to have different birth places relative to the metal rich and younger ones. With the spectroscopic procedure, we separated the sample stars into $alpha$-rich and $alpha$-poor categories by means of their ages as well as their [$alpha$/Fe] and [Fe/H] abundances. Stars older than 8 Gyr are richer in [$alpha$/Fe] than the younger ones. We could estimate the abundance [$alpha$/Fe]=0.14 dex as the boundery separating the $alpha$-rich and $alpha$-poor sub-samples in the [$alpha$/Fe]$times$[Fe/H] plane.
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