Excavating the fossil record of spiral galaxies

Despite spiral galaxies being extremely common in our local neighbourhood, their formation, growth, and dynamics are not fully understood. However, the spatial variation in the properties of stellar populations contained within spiral galaxies are expected to bear the imprint of some of the past and present physical processes driving global and local structure and dynamics. Combining modern integral-field spectroscopic galaxy surveys with spectral fitting methods offers an unprecedented opportunity to peer into the stellar population "fossil record" and how it varies between and within spiral galaxies. By obtaining a full star-formation history at every location, it is possible to construct images denoting the spatial distribution of stars of different ages across the galaxy. In this thesis we explore how such a "time slicing" technique can be applied to data from the SDSS-IV MaNGA survey, and show that this approach can provide insights into the formation and the internal structure of spiral galaxies. While the defining features of spiral galaxies are the beautiful arms that they display, the exact nature of such structure is still an open question. It has been widely assumed that spiral arms in "grand design" systems are the products of density waves that propagate around the disk with an approximately constant angular speed Om_P. We show that it is possible to measure an offset between young stars of a known age and the spiral arm in which they formed in a grand-design spiral galaxy, consistent with predictions of a density wave model. By measuring how this offset varies with radius, we obtain a direct measure of Om_P at a range of radii, and show that the spiral pattern in this galaxy is consistent with being quasi-stationary. We then investigate how the azimuthal structures of the barred spiral galaxy MCG+07-28-064 vary when traced by stars of different ages. Decomposing this galaxy into "time slices", we find evidence for the ongoing growth of the bar, and for the most recent star formation occurring on its leading edge. We also show that spiral arms can be traced in stellar populations as old as 2 Gyr, providing further evidence for the density wave model of spiral structure. In preparation to apply time slicing analyses to a large population of galaxies, we refine and test the spectral fitting methods. We show that the stellar population fitting techniques employed in this thesis must be carefully interpreted. For example, we find that accurately extracting the very youngest (<=30 Myr) stellar populations is not feasible, due to the limitations of modelling template spectra. However, reassuringly, we demonstrate that most populations can be reliably modelled in all of the conditions typically found in MaNGA galaxies. Finally, we perform a fossil record analysis for a large population of low-redshift spiral galaxies, thereby making use of the full power of MaNGA's sample size. By measuring the mean stellar ages and formation times as a function of galactic radius --- and also the radial profiles of different time slices --- we find evidence for inside-out growth being a generic feature of spiral galaxies, and most significant in massive galaxies. By interpreting the radial profiles of time~slices as indicative of the size of the galaxy at the time those populations had formed, we are able to use the stellar population fossil record to quantitatively trace the simultaneous growth in mass and size of the spiral galaxies over the last 10 Gyr. Despite finding that the evolution of the measured light-weighted radius is consistent with inside-out growth in the majority of spiral galaxies, we observe that an equivalent mass-weighted radius has changed little over the same time period. Since radial migration effects are likely to be small, we conclude that although the growth of disks in spiral galaxies has occurred predominantly through an inside-out mode, this has not had anywhere near as much impact on the distribution of stellar mass within spiral galaxies. These studies show that there is a wealth of untapped information on the spatial variation of the stellar populations which is now available to exploit with the current generation of integral-field spectroscopic galaxy surveys. A time-slicing approach to studying the fossil record is therefore an extremely powerful technique to answer some open questions on the structure and dynamics of spiral galaxies

SDSS-IV MaNGA: Excavating the fossil record of stellar populations in spiral galaxies

We perform a “fossil record” analysis for ≈800 low-redshift spiral galaxies, using STARLIGHT applied to integral field spectroscopic observations from the SDSS-IV MaNGA survey to obtain fully spatially-resolved high-resolution star formation histories (SFHs). From the SFHs, we are able to build maps indicating the present-day distribution of stellar populations of different ages in each galaxy. We find small negative mean age gradients in most spiral galaxies, especially at high stellar mass, which reflects the formation times of stellar populations at different galactocentric radii. We show that the youngest (109.5 years), again with a strong dependence on stellar mass. By interpreting the radial profiles of “time slices” as indicative of the size of the galaxy at the time those populations had formed, we are able to trace the simultaneous growth in mass and size of the spiral galaxies over the last 10 Gyr. Despite finding that the evolution of the measured light-weighted radius is consistent with inside-out growth in the majority of spiral galaxies, the evolution of an equivalent mass-weighted radius has changed little over the same time period. Since radial migration effects are likely to be small, we conclude that the growth of disks in spiral galaxies has occurred predominantly through an inside-out mode (with the effect greatest in high-mass galaxies), but this has not had anywhere near as much impact on the distribution of mass within spiral galaxies

Time-slicing spiral galaxies with SDSS-IV MaNGA

Spectra of galaxies contain a wealth of information about the stellar populations from which they are made. With integral field unit (IFU) surveys, such data can be used to map out stellar population properties across the face of a galaxy, allowing one to go beyond simple radial profiles and study details of non-axisymmetric structure. To-date, however, such studies have been limited by the quality of available data and the power of spectral analysis tools. We now take the next step and study the barred spiral galaxy MCG + 07-28-064 from observations obtained as part of the SDSS-IV MaNGA project. We find that we can decompose this galaxy into ‘time slices,’ which reveal the varying contributions that stars of differing ages make to its bar and spiral structure, offering new insight into the evolution of these features. We find evidence for the ongoing growth of the bar, including the most recent star formation on its leading edge, and for the underlying density wave responsible for spiral structure. This pilot study indicates that there is a wealth of untapped information on the spatial distribution of star formation histories available in the current generation of IFU galaxy survey

SDSS-IV MaNGA: the “G-dwarf problem” revisited

The levels of heavy elements in stars are the product of enhancement by previous stellar generations, and the distribution of this metallicity among the population contains clues to the process by which a galaxy formed. Most famously, the “G-dwarf problem” highlighted the small number of low-metallicity G-dwarf stars in the Milky Way, which is inconsistent with the simplest picture of a galaxy formed from a “closed box” of gas. It can be resolved by treating the Galaxy as an open system that accretes gas throughout its life. This observation has classically only been made in the Milky Way, but the availability of high-quality spectral data from SDSS-IV MaNGA and the development of new analysis techniques mean that we can now make equivalent measurements for a large sample of spiral galaxies. Our analysis shows that high-mass spirals generically show a similar deficit of low-metallicity stars, implying that the Milky Way’s history of gas accretion is common. By contrast, low-mass spirals show little sign of a G-dwarf problem, presenting the metallicity distribution that would be expected if such systems evolved as pretty much closed boxes. This distinction can be understood from the differing timescales for star formation in galaxies of differing masses

SDSS-IV MaNGA: The link between bars and the early cessation of star formation in spiral galaxies

Bars are common in low-redshift disk galaxies, and hence quantifying their influence on their host is of importance to the field of galaxy evolution. We determine the stellar populations and star formation histories of 245 barred galaxies from the MaNGA galaxy survey, and compare them to a mass-and morphology-matched comparison sample of unbarred galaxies. At fixed stellar mass and morphology, barred galaxies are optically redder than their unbarred counterparts. From stellar population analysis using the full spectral fitting code Starlight, we attribute this difference to both older and more metal-rich stellar populations. Dust attenuation however, is lower in the barred sample. The star formation histories of barred galaxies peak earlier than their non-barred counterparts, and the galaxies build up their mass at earlier times. We can detect no significant differences in the local environment of barred and un-barred galaxies in this sample, but find that the HI gas mass fraction is significantly lower in high-mass (M > 10 10 M) barred galaxies than their non-barred counterparts. We speculate on the mechanisms that have allowed barred galaxies to be older, more metal-rich and more gas-poor today, including the efficient redistribution of galactic fountain byproducts, and a runaway bar formation scenario in gas-poor disks. While it is not possible to fully determine the effect of the bar on galaxy quenching, we conclude that the presence of a bar and the early cessation of star formation within a galaxy are intimately linked

SDSS-IV MaNGA: when is morphology imprinted on galaxies?

It remains an open question as to how long ago the morphology that we see in a present-day galaxy was typically imprinted. Studies of galaxy populations at different redshifts reveal that the balance of morphologies has changed over time, but such snapshots cannot uncover the typical time-scales over which individual galaxies undergo morphological transformation, nor which are the progenitors of today’s galaxies of different types. However, these studies also show a strong link between morphology and star formation rate (SFR) over a large range in redshift, which offers an alternative probe of morphological transformation. We therefore derive the evolution in SFR and stellar mass of a sample of 4342 galaxies in the SDSS-IV MaNGA survey through a stellar population ‘fossil record’ approach, and show that the average evolution of the population shows good agreement with known behaviour from previous studies. Although the correlation between a galaxy’s contemporaneous morphology and SFR is strong over a large range of lookback times, we find that a galaxy’s present-day morphology only correlates with its relatively recent (⁠∼2Gyr⁠) star formation history. We therefore find strong evidence that morphological transitions to galaxies’ current appearance occurred on time-scales as short as a few billion years

A direct test of density wave theory in a grand-design spiral galaxy

The exact nature of the arms of spiral galaxies is still an open question1. It has been widely assumed that spiral arms in galaxies with two distinct symmetrical arms are the products of density waves that propagate around the disk, with the spiral arms being visibly enhanced by the star formation that is triggered as the passing wave compresses gas in the galaxy disk1,2,3. Such a persistent wave would propagate with an approximately constant angular speed, its pattern speed ΩP. The quasi-stationary density wave theory can be tested by measuring this quantity and showing that it does not vary with radius in the galaxy. Unfortunately, this measurement is difficult because ΩP is only indirectly connected to observables such as the stellar rotation speed4,5,6. Here, we use the detailed information on stellar populations of the grand-design spiral galaxy UGC 3825, extracted from spectral mapping, to measure the offset between young stars of a known age and the spiral arm in which they formed, allowing a direct measurement of ΩP at a range of radii. The offset in this galaxy is found to be as expected for a pattern speed that varies little with radius, indicating consistency with a quasi-stationary density wave, and lending credence to this new method.PostprintPeer reviewe

SDSS-IV MaNGA: spatially resolved dust attenuation in spiral galaxies

Dust attenuation in star-forming spiral galaxies affects stars and gas in different ways due to local variations in dust geometry. We present spatially resolved measurements of dust attenuation for a sample of 232 such star-forming spiral galaxies, derived from spectra acquired by the SDSS-IV MaNGA survey. The dust attenuation affecting the stellar populations of these galaxies (obtained using full spectrum stellar population fitting methods) is compared with the dust attenuation in the gas (derived from the Balmer decrement). Both of these attenuation measures increase for local regions of galaxies with higher star formation rates; the dust attenuation affecting the stellar populations increases more so than the dust attenuation in the gas, causing the ratio of the dust attenuation affecting the stellar populations to the dust attenuation in the gas to decrease for local regions of galaxies with higher star formation rate densities. No systematic difference is discernible in any of these dust attenuation quantities between the spiral arm and inter-arm regions of the galaxies. While both the dust attenuation in the gas and the dust attenuation affecting the stellar populations decrease with galactocentric radius, the ratio of the two quantities does not vary with radius. This ratio does, however, decrease systematically as the stellar mass of the galaxy increases. Analysis of the radial profiles of the two dust attenuation measures suggests that there is a disproportionately high concentration of birth clouds (incorporating gas, young stars and clumpy dust) nearer to the centres of star-forming spiral galaxies.Comment: 17 pages, 8 figures, accepted for publication in Monthly Notices of the Royal Astronomical Societ

SDSS IV MaNGA: full spectroscopic bulge-disc decomposition of MaNGA early-type galaxies

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, λR, and ellipticity, ϵ, and in the relation between stellar mass, M*, 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