My primary research interest revolves around investigating recent star formation activities within nearby galaxies. I specialize in the reduction and analysis of imaging and spectroscopy data spanning the ultraviolet, optical, and infrared wavelengths.
One notable accomplishment of mine is the compilation and publication of one of the most extensive catalogs to date. This catalog comprises homogenized photometry data for 1931 nearby galaxies of various morphological types, specifically focusing on ultraviolet (GALEX FUV and NUV) and infrared (Spitzer/IRAC 3.6µm) observations. The publication detailing this work is titled “The GALEX/S4G Surface Brightness and Color Profiles Catalog. I. Surface Photometry and Color Gradients of Galaxies” by Bouquin et al., published in the Astrophysical Journal Supplement Series, Volume 234, Issue 1, in 2018. The findings from this study, along with other published articles, are compiled in my Ph.D. thesis .
Video: The spatial-distribution of the GALEX/S4G sample in the Local Volume (LV) cartesian (xyz) galactic coordinates with a cubic grid shown centered on the Milky Way (green dot) and extending to ±40 Mpc in the three dimensions(positive x is the Northern-side from the Galactic plane) . The Virgo cluster is roughly delineated by a grey sphere with a blue edge. The sample selection criterion of galactic latitude l > |30˚| is seen and correspond to the zone-of-avoidance along the disk of the Milky Way galaxy where light-attenuation due to dust is severe and therefore avoided (Code credit: Erik Tollerud – Github link).
Background
Galaxies exhibit a wide range of characteristics, including diverse shapes, sizes, masses, luminosities, colors, stellar content, dust content, and dark matter content. They can have bulges or disks, show signs of active galactic nuclei (AGN), possess bars, and display varying levels of structure, from highly organized to irregular. Galaxies can be diffuse or compact, radio-loud or quiet, and exhibit different levels of brightness in the infrared (IR) or ultraviolet (UV) regions of the electromagnetic spectrum.
Furthermore, galaxies tend to congregate in groups consisting of a few tens of galaxies or clusters comprising hundreds or even thousands of galaxies. Within these groupings, galactic dynamics, including collisions and flybys, significantly impact their evolution, distinguishing them from isolated galaxies that are located far away from these groupings and clusters.
Galaxies that are believed to have undergone limited or minimal environmental influence from neighboring companions, groups, or clusters offer valuable insights into the secular evolution of galaxies. These galaxies serve as excellent test cases to study the independent evolutionary processes of galaxies, particularly in the context of secular evolution. For more information on this topic, please refer to the section on “XUV-disk galaxies” below.
There are galaxies that exhibit active star formation processes, while others do not. The ultraviolet (UV) observations provide a direct means to trace the presence of recently formed massive stars (with a spectral type of B) that have not yet gone supernova, within a time range of approximately 107 to 109 years. On the other hand, the infrared (IR) observations allow us to detect the already formed low-mass stars (with a spectral type of M). By combining UV and IR observations, we can determine whether a star-forming region represents a newly forming structure or not, by examining the underlying population of low-mass stars that are already present.
If a star-forming region appears bright in the infrared (IR) observations, indicating the presence of numerous low-mass stars, it suggests that the star formation has been ongoing for a considerable period. Conversely, if the region appears faint in the IR observations but exhibits brightness in the ultraviolet (UV), it indicates the birth of stars occurring for the first time in those specific regions of the galaxy, with a minimal presence of low-mass stars.
The bimodal distribution and GALEX GREEN VALLEY galaxies
Historically, color-magnitude and color-color diagrams have long been used to analyze galaxies. Classical optical diagrams of nearby galaxies show that the distribution is bimodal, that is, galaxies gather into two clumps in these diagrams. We find that redder and brighter galaxies are more of the early-type kind (elliptical galaxies) that are quiescent, whereas the bluer and fainter galaxies are more of the late-type kind (spiral galaxies) that are star-forming. However, classical optical diagrams were not sensitive enough to clearly distinguish between star-forming galaxies of different masses and sizes.
Using UV and IR instead, and in particular far-ultraviolet (FUV), near-ultraviolet (NUV), and near-infrared (3.6 µm) filters, greatly improves our ability to distinguish between star-forming galaxies and others that are not. From these three bands, we can effectively compute three colors, namely the (FUV – NUV), the (NUV – [3.6]), and the (FUV – [3.6]) colors.
Using the (FUV – NUV) and the (NUV – [3.6]) colors to construct the color-color diagram clearly shows the bimodality of galaxies, between those that are star-forming and those that are not. This is due to the sensitivity of the (FUV – NUV) color to recent star-formation, and the (NUV – [3.6]) color that helps to spread the distribution relative to the underlying low-mass stellar content (see Figure above). We see from this diagram that galaxies are unequivocally distributed into two sequences which we defined as follows: the GALEX Blue Sequence (GBS) mainly populated by late-type spirals and irregulars (delineated in blue), and the GALEX Red Sequence (GRS) mainly populated by early-type ellipticals and lenticulars (delineated in red).
These sequences also allow us to define what we call the GALEX Green Valley (GGV), a zone in between the GBS and the GRS (green shaded region). Galaxies found in the GGV are special in the sense that they do not belong neither to the GBS nor to the GRS, and we find that a large fraction of them are S0 or S0-a morphological types. We interpret this GGV as a zone of transition for galaxies to go from the GBS to the GRS, but we do not exclude the possibility of galaxies going from the GRS toward the GBS (for e.g. through rejuvenation). Using simulated galaxy models, we show that such transition would occur rapidly, over timescales of less than 1 billion years.
We also find, from our analysis using spatially-resolved FUV, NUV and 3.6 µm photometry, that the outskirts of the disk are redder than the inner part for some GGV and GRS galaxies. Indeed, we see this effect statistically for both groups of galaxies relative to the average GBS galaxies, with the GGV’s reddening being weaker than that for the GRS.
The integrated photometry at 3.6 µm in fixed-ellipticity and position angle elliptical-ring annuli with resolution of 6 arcseconds and width of 6 arcseconds also tells us that the specific star formation rate (sSFR) steadily decreases from the inside-out for GBS galaxies, but remain relatively constant in both GGV and GRS galaxies’ outskirts beyond µ[3.6]=20.89 mag arcsec-2.
These results altogether hint to some mechanism(s) at play, affecting the star-formation activity in the outskirts of GGV galaxies. Moreover, a larger fraction of GGV galaxies are in fact galaxies in the Virgo cluster. The environment may be driving this reddening, where such phenomenon could be explained by, and would be consistent with, a ram-pressure stripping scenario, where the gas would be stripped away from the galaxy by colliding with the intergalactic medium as the galaxy flies through the cluster, and where the stripping effect would be more efficient in the less-dense outer parts.
More detailed, multi-wavelength studies of individual GGV galaxies, as well as gathering a larger sample of them, would definitely help in better quantifying such effect. Then, we need to construct the (FUV – NUV) vs (NUV – [3.6]) color-color diagram for galaxies at larger distances (the ones we used so far are all at d < 40 Mpc or z < 0.01 or vradial < 3000 km/s) to know how this effect is changing with time if at all.
XUV-disk galaxies
Contrary to GGV galaxies that have their star formation being somewhat quenched, we also identify galaxies that sit at the opposite end of the spectrum having large bursts of recent star formation.
A significant fraction (10~20%) of GBS galaxies, have been identified as being extended-ultraviolet (XUV) disk galaxies in our sample, having excessively large and blue UV-disks compared to their IR-disk, and even having extended star-forming structures in their outskirts. We have yet to know how these star-forming structures do form (seemingly at once) at such large galactocentric distances, where densities are so low that star-formation does not occur in other galaxies
Our analysis show that XUV-disk galaxies prefer less-dense environment. These galaxies may be great cases of secular evolution. Observations at 21-cm radio wavelength reveals that these galaxies are embedded in huge clouds of neutral atomic hydrogen of a few 100 kpc in size. These reservoirs may be where the matter that “feeds” the galaxy, resulting in starbursts, comes from. However, the infall or accretion of such gas onto a galaxy remains to be observed.
Looking at the distribution of metals in such galaxies, for example by looking at star-forming HII regions (main emission lines such as Hα and NII are detectable in the optical range for nearby galaxies where the effects of redshift is minimal), and inferring the metal content with the aid of oxygen-to-hydrogen abundance ratios based on calibrators such as nitrogen, may yield hints to how pristine the gas from which the newly born stars are made of is.
We recently obtained multiple spectra of HII regions from a target galaxy with the Gran Telescopio Canarias (GTC), the world’s largest optical telescope to-date with its 10.4 meter in diameter segmented-mirror.
UV-upturn galaxies
Quiescent galaxies, found in the GRS, are known to emit in the UV as well, not because of star formation (although there could also be residual star formation) but rather due to old and evolved low-mass stars. Observations of quiescent elliptical galaxies show that their spectral energy distribution, while being relatively low at ultraviolet wavelength compared to larger ones, display an increase in the FUV at around 1500 Å. Although the very nature of the source of this FUV emission is still under debate, there is large consensus in the community that this must be the manifestation of so-called UV-upturn stars, stars in their later evolutionary stage after quitting the main sequence, having started their helium-core burning process.
We identified a subsample of early-type galaxies (ETGs) from the GALEX/S4G sample, and analyzed their (FUV – NUV) colors. We found a strong correlation between the (FUV -NUV) color and the stellar mass-to-light ratio ϒ* in ETGs, where the bluer the UV color the larger the ratio is. This can be interpreted as follows: the larger the mass, the more stars there is, the more UV-upturn stars there is, the more FUV is emitted overall. The amount of FUV is then directly related to the proportion of low-mass stars that were born.
The number of stars of different masses that are born from a giant molecular cloud can be described by what is known as the Initial Mass Function (IMF). Some people think the IMF is universal and is the same in every galaxy and every condition. Some others think the IMF may vary and is not the same from galaxy to galaxy or even from molecular cloud to molecular cloud. Suppose that we have two ETGs having the same mass and mass-to-light ratio but slightly different (FUV – NUV) color. This would indicate that the two galaxies have different numbers of UV-upturn stars, and therefore, that the IMFs could have been different (this is important because the universality of the IMF is still under debate). We conjecture that looking at the (FUV – NUV) color in galaxies can be used to differentiate between IMFs, if they vary at all.
Conclusions
The GALEX/S4G sample, comprising nearby galaxies, presents a remarkable dataset that offers extensive opportunities for exploration. It encompasses a broader range of galaxies beyond the specific subsamples that I am currently studying. Therefore, it holds great value for further investigation into various properties, such as studying the effects of bars and rings on UV-IR colors and star formation rates. The vastness of the universe provides an awe-inspiring expanse, holding countless phenomena and discoveries that await exploration by all.