I work on a variety of topics related to galaxy formation and evolution. I use resolved multi-wavelength data to map the current distribution of stars, gas and dust in nearby galaxies, and then use that information to infer their past assembly and evolution. Below is a summary of some relevant results.
The Spitzer Survey of Stellar Structure in Galaxies (S4G)
S4G is an Exploration Science Legacy Program (PI: K. Sheth) that we carried out during Spitzer’s post-cryogenic mission. It is a volume, magnitude and size-limited sample of over 2300 galaxies within 40 Mpc, imaged at 3.6 and 4.5 microns. At these bands the mass-to-light ratio is only mildly affected by age, metallicity and extinction. Therefore, S4G is an ideal survey to explore the stellar structure of galaxies down to surface densities below 1 Msun pc-2.
We have publicly released a wealth of enhanced data products through IRSA. If you use them, please cite the corresponding papers below:
- Sheth et al. (2010): Main survey paper.
- Muñoz-Mateos et al. (2015): Science-ready images, object masks, surface photometry and integrated quantities (magnitudes, stellar masses, sizes, concentration indices...)
- Salo et al. (2015): GALFIT 2D structural decompositions (bulge, disk, bars...)
- Querejeta et al. (2015): contaminant-free stellar mass maps obtained from an Independent Component Analysis of the 3.6 and 4.5 microns images.
Disk breaks and stellar migration
Using deep near-IR profiles from S4G, in Muñoz-Mateos et al. (2013) we found evidence that bars can rearrange stars even out to large radii. Most disks do not have a simple exponential profile but a broken or down-bending one, with a shallow inner disk followed by an outer steeper one, and a sharp break in between. In most barred disks, the stellar mass density profile has a break at twice the bar radius, the expected locus of the Outer Lindblad Resonance. But also we found a second family of breaks at 3.5 times the bar radius, which can result from a dynamical coupling between the bar and the spiral arms if certain resonances overlap. This poses significant implications for stellar migration. Simulations show that radial mixing proceeds faster and out to larger radii under bar-spiral coupling than in the absence of such coupling. Therefore, disks with breaks at 3.5 times the bar radius may have undergone much more radial migration than other disks.
Inside-out growth of disks
Star formation proceeds at a slower pace in the outskirts of disks, over longer timescales than in the inner regions. As a result, stars are younger -on average- in the outer parts, leading to present-day color gradients that can be used to infer the growth rate of disks (but beware of stellar migration!). In Muñoz-Mateos et al. (2007, 2011) we applied this method to several dozen nearby disks. We measured multi-wavelength profiles all the way from the far-UV to the far-IR, which allowed us to map radial changes in the star formation history of these galaxies, while properly accounting for radial variations in dust extinction. We concluded that the radial scale-length of disks has increased by 20-25% since z=1. For a galaxy like our own Milky Way, this implies a radial growth rate of roughly 0.05 kpc/Gyr. This is about 180 km/h, so you could outrun the MW’s growth by driving really fast!
Dust extinction profiles
The interstellar medium is pervaded by dust grains, which absorb UV and optical photons emitted by stars and reradiate them in the IR, thereby completely modifying our view of galaxies. In Muñoz-Mateos et al. (2009a) we mapped how the internal extinction decreases with radius in galaxies with different Hubble types, which is vital to recover the intrinsic emission of stars. We also derived a spatially-resolved correlation between the total-IR/UV ratio and the UV color at each radius. This trend is similar to the one of starburst galaxies, but shifted towards redder UV colors. This is most likely an age effect, but the shape of the extinction law and the relative geometry of dust and stars also play a role.
Radial changes in the dust-to-gas ratio
As they evolve, stars synthesize elements heavier than hydrogen and helium. These elements are eventually injected into the interstellar medium when stars die. Some of these heavy atoms will be incorporated into new generations of stars, whereas others will end up locked in dust grains. Therefore, we expect the dust-to-gas ratio to be higher in regions where star formation has proceed in a faster pace. Moreover, the DGR should track the local gas-phase metallicity. In Muñoz-Mateos et al. (2009a) we measured the radial variation of the DGR in galaxies of different Hubble types. It decreases with radius, as expected from an inside-out assembly of disks, and it does indeed follow the oxygen abundance gradient.
Polycyclic Aromatic Hydrocarbons
PAHs are complex, honeycomb-shaped organic molecules that pervade the interstellar medium, characterized by prominent emission lines in the mid-IR. Analysis of galaxies as a whole have revealed that there is a paucity of PAHs in metal-poor galaxies (Draine et al. 2007). In Muñoz-Mateos et al. (2009a) we investigated how the PAH abundance changes with the local metallicity at different galactocentric distances. PAHs make up just 1-2% of the total dust mass in dwarf irregulars and the outskirts of late-type disks. It then increases up to 4-5% as we get closer to the centers of spirals, although the trend flattens and reverses at higher metallicities. This is in qualitative agreement with models were most of the carbon in PAHs has been forged in AGB stars (Galliano et al. 2008). However, we cannot neglect selective destruction of PAHs in regions with hard radiation fields.
Non-parametric multi-wavelength galaxy morphology
Visual classification of galaxies has been of tremendous importance for decades, but it has some limitations. First, visual classification becomes slow and impractical in modern surveys with several thousands or even millions of galaxies. Secondly, the classic Hubble morphological types are defined in the optical wavelength range, so comparing galaxy morphology at other wavelengths becomes problematic. Third, galaxies at high redshift deviate considerably from these predefined types. In Muñoz-Mateos et al. (2009b) we quantified in detail how galaxy morphology changes with wavelength. To do so we computed the concentration index, asymmetry, Gini coefficient and second order moment for all SINGS galaxies, in 21 photometric bands all the way from the FUV to the FIR. This constitutes an excellent local anchor for multi-wavelength morphology studies at high z.