Kaviraj et al. Article -- Ultraviolet Analysis of Post-Starburst Galaxies and Quenching Mechanisms

Title: UV properties of E+A galaxies: constraints on feedback-driven quenching of star formation

Authors: S. Kaviraj, L. A. Kirkby, J. Silk and M. Sarzi

Authors’ Institutions: University of Oxford Department of Physics and University of Hertfordshire Centre for Astrophysics Research

To read the full article, please click here. 

Article Summary: 

An image of galaxy NGC 3801 combining light from across the spectrum, ranging from ultraviolet to radio.  NASA's GALEX and other instruments caught this galaxy in the act of quenching its cold, gaseous fuel for new stars - possibly marking the transition from a star-forming spiral galaxy to a quiescent elliptical galaxy.  According to theory, star formation will soon be quenched by the shock waves from two powerful jets shooting our of NGC 3801's central supermassive black hole, as seen in the radio emission colored green.  Image courtesy of NASA/JPL-Caltech.  

The study of Post-Starburst Galaxies (PSGs) and the mechanisms that quenched their star formation provides key insights into understanding the processes that shape galaxy evolution.  PSGs offer a look at a valuable evolutionary link between gas-rich star-forming galaxies and gas-poor quiescent galaxies.  A study by Kaviraj et al. in 2007 carried out the first large-scale examination of PSGs with ultraviolet (UV) photometry.  Due to the sensitivity of the UV to young stars, this study was accurately able to reconstruct the star formation histories of 38 PSGs in the nearby Universe by combining optical and UV data from the Sloan Digital Sky Survey (SDSS) and Galaxy Evolution Explorer (GALEX) surveys. 

PSGs, also known as ‘E+A’ Galaxies, show strong Balmer absorption lines that are characteristic of recent star formation but lack the forbidden [OII] and H-alpha emission that are present during ongoing star formation.  This indicates that these galaxies have recently had a strong episode of star formation that was abruptly quenched.  Understanding the processes that ‘quench’ these galaxies is an important step to understanding this transitional period of galaxy evolution. 

At intermediate redshifts (z ~ 0.5), PSGs were found to be primarily in clusters of galaxies, as opposed to in smaller groups or in the field.  For this reason, these galaxies were believed to result from cluster-specific mechanisms such as galaxy harassment or ram-pressure stripping (the stripping of galactic gas as galaxies travel through the cluster).  However, local observations indicate that PSGs are much more common in the field.  This indicates that other channels likely exist in the production of PSGs.  Many PSGs exhibit morphological disturbances, which may mean that their evolution is linked, at least partially, to mergers and interactions.  Simulations support this hypothesis, indicating that gas-rich mergers are capable of triggering strong star formation episodes. 

The criteria used in the selection of this study’s PSG sample were similar to earlier studies: H-delta (EW) > +5.0 Å, H-alpha (EW) > -3.0 Å, and [OII] (EW) > -2.5 Å, where a positive or negative sign denotes absorption or emission lines, respectively, and EW stands for equivalent width.  To ensure accuracy, the sample was restricted to the redshift range 0 < z < 0.2, a signal-to-noise ratio greater than 10, and galaxies with evidence of an Active Galactic Nuclei (AGN) were removed. This was because the scattered light from the AGN could contaminate the UV continuum. The authors also checked the morphologies of the sample using the SDSS fracDev tool. They found that they have spheroidal morphologies, which provides support for the idea that PSGs are precursors of early-type galaxies.

Figure 1. Position of E+A galaxies used in the study (filled blue circles) compared to a sample of early-type galaxies from SDSS DR5 in (NUV – r) versus (g – r) color space.  

The seven photometric filters used were the five SDSS bands (u, g, r, i, z) and the two GALEX filters in the far-ultraviolet (FUV) and near-ultraviolet (NUV).  Figure 1 shows the position of the PSG sample in (NUV – r) versus (g – r) color space, compared to a sample of early-type galaxies.

Figure 2 shows the PSGs approximate ages, mass fractions (amount of stellar mass formed during the starburst compared to the mass of the galaxy), time-scales, and star formation rates (SFRs). They derive the SFR by dividing the stellar mass formed during the starburst by the estimated time-scale of the starburst.  While low-luminosity PSGs have implied SFRs less than 50 solar masses per year, high-luminosity PSGs exhibit SFRs greater than 300 and even as high as 2000 solar masses per year.  These SFRs are comparable to those found in Luminous Infrared Galaxies (LIRGs) and Ultra-Luminous Infrared Galaxies (ULIRGs) at low redshifts, indicating that massive LIRGs could potentially be the progenitors of massive PSGs. 

Figure 2. The top plot shows the age of the burst versus the mass fraction.  The middle plot shows the binned timescale of the starburst.  The lower plot shows the implied star formation rate versus z-band magnitude.

One of the most important aspects of starburst galaxies is the quenching mechanisms that truncate their bursts.  PSGs experience ‘negative feedback’ that causes the star formation rate to slow down.  In this study it was found that for galaxies below a mass of 10¹⁰ solar masses, the quenching efficiency decreased with an increasing galactic mass.  However, for galaxies with masses greater than ~10¹⁰ solar masses, this trend was reversed; quenching efficiency increased with an increasing galactic mass.  Figure 3 shows the relationship between galaxy mass and quenching efficiency, with a clear change at ~10¹⁰ stellar mass.  This observation suggests that there are two primary sources for negative feedback: supernovae and AGN.  In the absence of AGN, supernovae would be the primary source of negative feedback.  As galaxies become more massive, the depth of the potential well increases, making it more difficult for supernovae to eject gas from the system.  However, for galaxies greater than ~10¹⁰ solar masses, AGN begin to appear and become the dominant source of negative feedback.  Since the mass of the black hole scales with the central velocity dispersion, it is expected that AGN feedback will become more effective as the galaxy mass increases.  Because galaxies with ongoing AGN activity were excluded from the sample, it is plausible that AGN feedback processes simultaneously quench both star formation and AGN activity.  Through quantitative analysis, Kaviraj et al. were able to support these qualitative predictions. 

Figure 3. A log-log plot of galaxy mass vs time-scale ratio.  The time-scale ratio is the ratio of the time-scale of the burst to the dynamical time-scale of the galaxy, which describes the 'natural' time-scale over which processes such as star formation would take place if left unhindered.  Note that time-scale ratio and quenching efficiency are inversely correlated, so an decreasing slope is analogous to a increasing quenching efficiency.  

One last investigation in this study was to probe the migration time from gas-rich star-forming galaxies to gas-poor quiescent galaxies, also known as moving from the ‘blue cloud’ to the ‘red sequence’ (see Figure 1 in the Wong et al. article summary).  Migration times were estimated by ‘ageing’ the best-fitting star formation model of each PSG.  Most galaxies complete their migration time within 2 Gyr, with a median migration time of ~1.5 Gyr.  Figure 4 presents the migration tracks of PSGs in color-color and color-magnitude space. 

Figure 4. Migration tracks for E+A galaxies in the (NUV – r) versus (g – r) color space (top panel) and the (NUV – r) versus M(z) color-magnitude space (bottom panel).  Ages (in Gyr) along the track are shown color-coded.  

In conclusion, by combining the optical and UV data, this study was able to reconstruct the time-scales, mass fractions, SFRs, migration times, and quenching mechanisms in this sample of PSGs.  This study suggests that supernovae are the primary quenching mechanism for galaxies under 10¹⁰ solar masses, and AGN become the primary source of negative feedback for galaxies over ~10¹⁰ solar masses.  When supernovae are the primary source, quenching efficiency decreases with galaxy mass because the increasing depth of the potential well makes it more difficult to eject gas from the system.  As AGN become the dominant source of negative feedback, quenching efficiency increases with galaxy mass, due to the AGN luminosity scaling with the mass of the black hole.  The study of PSGs helps us understand the processes that shape galaxy evolution. Future comparative studies of PSGs at low and high redshifts could help provide insight into the processes that dictate galaxy evolution over cosmic time. 

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.