Yang et al. Article -- A Detailed Look at E+A Galaxy Evolution

Title: A Detailed Evolution of E+A Galaxies into Early Types

Authors: Y. Yang, A. Zabludoff, D. Zaritsky, and J. Mihos

Authors’ Institutions: Steward Observatory, University of Arizona and Department of Astronomy, Case Western Reserve University

To read the full article, please click here. 

Article Summary:

The Hubble Space Telescope captured galaxy NGC 2936 is in a celestial dance with its elliptical companion NGC 2937.  NGC 2936 used to be a flat, spiral galaxy, its stars now scrambled due to the gravitational interactions with its companion.  Compressed gas during the encounter triggers a burst of star formation, visible as bluish streams along the distorted arms.  These galactic interactions could possibly lead to the vibrant, star-forming, distorted spiral galaxy to evolve into a quiet elliptical galaxy like its companion.  Image courtesy of NASA.  

The transitional period between gas-rich, star-forming galaxies to gas-poor, passively evolving galaxies is an important phase of galaxy evolution. Post-Starburst Galaxies (PSGs) appear to inhabit this transitional period.  To determine what PSGs become after their young stellar populations fade away, Yang et al. acquired detailed morphologies of 21 PSGs using high-resolution images from the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and Wide-Field Planetary Camera 2 (WFPC2).  They used these images to measure the morphologies, color profiles, scaling relations, and star cluster characteristics of their PSG sample. Their results suggest that PSGs evolve into early-type E/SO galaxies and contribute to the building up of the ‘red sequence’.

PSGs represent the best candidates for galaxies caught in the midst of the transformation between late and early types due to their relatively young stellar population and lack of ongoing star formation, as suggested by their strong Balmer absorption lines and absence of emission lines such as [O _II] and H-alpha.  Since PSGs reside in low-density environments, the abrupt end to their star formation is likely due to galaxy-galaxy interactions rather than cluster-specific mechanisms such as ram pressure stripping and strangulation.  This theory is supported by significant fractions of PSGs having merger features. 

The 21 PSGs in the sample were spectroscopically identified from 11,113 galaxy spectra of the Las Campanas Redshift Survey (LCRS). The 21 PSGs have redshifts between 0.07 and 0.18.  The high-resolution HST images, such as those presented in figure 1, enabled small- and large-scale interaction features to be identified.  The morphologies of PSGs in this study were very diverse, including train wrecks, barred galaxies, blue cores, and relaxed-looking disky galaxies.  Over half of the PSGs had identifiable tidal or disturbed features.  Five of the galaxies in the sample had interacting, companion galaxies within ~30 kpc.  One of the samples even had a binary PSG system in which both of the galaxies were tidally disturbed.  These findings support the idea that galaxy interactions and mergers trigger the PSG phase.  In addition, morphological analysis determined that six of the PSGs had distinct, compact blue cores and seven of the galaxies had dust features such as lanes and filamentary structures. 

Figure 1.  Examples of 3 PSGs used in the study.  The left column shows dim tidal features using high-contrast R-band, the middle column shows images for the WFPC2 sample, and the right column show residual R-band images subtracted from the smooth symmetric model components

PSGs tend to be bulge-dominated systems.  The median bulge fraction (B/T), which gives the ratio of the bulge luminosity to the total luminosity of the galaxy, was 0.59.  This is consistent with that of S0 galaxies, with an average B/T of 0.63.  Sérsic profiles, represented by an r^1/n profile describing how the intensity of a galaxy varies with distance from its center, were also obtained to further investigate PSG bulge characteristics.  Disk galaxies and spheroidals generally have Sérsic indices of n=1 and n=4, respectively, yet most of the PSGs had indices with n>5 and a couple with indices n>10.  This indicates that the luminosity of PSGs is highly concentrated, potentially due to substructures near their centers such as bright nuclei, bars, and rings. 

To quantify asymmetric features, this study calculated the concentration index ‘C’ and rotational asymmetry index ‘A’ to be able to place the sample galaxies on a C-A plane, as shown in figure 2.  In general, PSGs have high concentration indices consistent with those of spheroids, but considerably larger asymmetry indices than ellipticals due to structures that arose from the starburst or recent merger.  Therefore, PSGs would be morphologically classified as early-type galaxies once the disruptions and tidal features dissipate or fade. 

Figure 2. C-A classification the the 21 PSGs (filled circles) as well as 113 local elliptical, intermediate spiral, and late-type spirals (oval, plus sign, and spiral, respectively).  The dashed line provides a rough division of early and late types on the CA plane.  

The color morphologies of the PSGs were just as diverse as their structural morphologies.  Radial color profiles can depend on dust content and spatial distributions, ages, and metallicities of stellar populations, which in turn depend on the evolutionary history of the galaxies.  For example, if galaxy-galaxy interactions are responsible for creating a PSG, then the young stellar population is expected to be concentrated in the center, yielding a positive color gradient (i.e., redder color with increasing radius).  If mechanisms such as ram pressure stripping are responsible for producing the PSG, color profiles may be more uniform. 

High-resolution color distributions in this study indicate that a significant fraction of the PSGs have positive color gradients and sometimes distinct blue cores.  Figure 3 shows that twelve PSGs had a positive color gradient, five had a negative color gradient (bluer with increasing radius), and five had flat or mixed color profiles.  Of the five with negative color gradients, three show clear dust signatures, which may mean that the red cores in these galaxies arise from increasing dust extinction toward the center.  Early-type galaxies typically have slightly negative color profiles.  E/S0s in the local universe have negative color gradients that originate from their metallicity gradients; their stellar populations become more metal-rich and redder towards the center.  Over time, the PSGs with positive color gradients may begin to exhibit negative color gradients if their young stellar populations are more metal-rich than the underlying old populations and these young stars run through their lifecycle. This possibly suggests that PSGs are the precursors of E/SO galaxies.   

Figure 3. Redshifted radial color profiles of the 20 PSGs .  A positive slope means the galaxies are bluer towards the center and a negative slope means the galaxies are redder towards the center.  

Half of the PSGs with positive color gradients also had compact, almost stellar-like, blue cores that were distinct from the other parts of the galaxy.  Though their origins are not fully understood, blue cores are common in early-type galaxies at higher redshifts (z > 0.5), when field spheroids were assembling.  Therefore, the blue-core PSGs may be the local analogs to these higher redshift blue-core spheroids.   Three of the six PSGs with blue cores also had LINER (low-ioniation emission line region) spectral signatures, indicative of Active Galactic Nuclei (AGN) activity being the potential mechanism for quenching star formation in these PSGs. 

To further determine if of the data supports the idea that PSGs evolve into early-type galaxies, this study compared scaling relations between PSG and early-type galaxies.  The fundamental plane is an empirical relation between the effective radius, the central velocity dispersion, and the mean surface brightness.  PSGs stand apart from E/S0s in the fundamental plane, which implies that the stellar populations of PSGs are different from those of E/S0s.  On average, the mass to luminosity ratio (M/L) of PSGs is 3.8 times smaller than that of E/S0s.  In PSGs, smaller or less massive galaxies appear to have a smaller M/L.  This trend arises naturally from merger scenarios, where low-mass galaxies have higher gas fractions and could produce relatively larger populations of young stars. 

Properties of newly formed star clusters in the PSGs were analyzed to determine if they are consistent with those of early-type galaxies.  High-resolution images were required to identify star clusters in the PSG sample.  At least nine of the PSGs had a population of unresolved compact sources.  The colors and luminosities of the young star clusters are consistent with the ages inferred from the PSG spectra (0.01-1 Gyr), signifying that these clusters likely arose during the interaction/starburst phase.  Though it is uncertain how many of these clusters will survive to the E/S0 phase, it is at least possible that the young star clusters in PSGs can evolve into the globular cluster systems seen in E/S0s. 

To summarize, using high spatial resolution HST ACS and WFPC2 images to derive morphological properties, color profiles, scaling relations, and characteristics of young star clusters, this study suggests that PSGs are caught in the act of transforming from gas-rich late-type galaxies to gas-poor early-type galaxies.  Further investigation of PSGs is critical to better understanding of the origin of the red sequence of galaxies.