Schawinski et al. Article - Evolution of Early and Late Types Through the Green Valley

Title: The Green Valley is a Red Herring: Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early- and late-type galaxies

Authors: K. Schawinski, M. Urry, B. Simmons, L. Fortson, S. Kaviraj, W. Keel, C. Lintott, K. Masters, R. Nichol, M. Sarzi, S. Ramin, E. Treister, K. Willett, I. Wong, and S. Yi

First Author's Institution: Institute for Astronomy, Department of Physics, ETH Zurich

To read the full article, please click here.  

Article Summary:

The green valley has long been thought of as the crossroads of galaxy evolution – creating a divide between the star-forming galaxies of the blue cloud and passively-evolving galaxies of the red sequence. The intermediate color of green valley galaxies is thought of as an indicator that star formation in these samples was recently quenched, and studying this region between the two main populations will lead to a better understanding of the evolutionary pathways of galaxies, possibly even predicting what may happen to our own Milky Way. Galaxies spend most of their life on what is dubbed the Main Sequence, where there is a tight correlation between stellar mass and star formation rate (SFR).  It is believed that certain processes can cause them to leave the main sequence and begin to travel through the green valley. Most galaxies were presumed to follow a similar tract through this desolate region of color space, progressing through it in a relatively short timescale to keep the scarcity of galaxies observed in this region. In recent years, the Galaxy Zoo project has brought a new tool to the galaxy evolution table – a plethora of morphological classifications of galaxies imaged by the Sloan Digital Sky Survey (SDSS), allowing astronomers to inquire as to the effects of morphology on a galaxy's transition through the green valley. A recent study done by Schawinski et al. using data from the SDSS, Galaxy Evolution Explorer (GALEX), and Galaxy Zoo has found that different morphological characteristics will alter the movement of a galaxy through the green valley into its quiescent fate.

This study acquired a sample of mass-limited galaxies in the local universe from the SDSS, with redshifts ranging from z=0.02 to z=0.05.  To gain a better understanding of star formation histories, ultraviolet photometry from the GALEX was found for 71% of their sample. Galaxy Zoo classifications were used to determine the morphology of the sample, where morphology was assigned when volunteers agreed on the classification at a rate of 80% or more. This resulted in the classification of 18% early types, 34% late types, 45% intermediate types (galaxies that did not receive at least 80% votes for early or late type), and 3% merging, where these classification follow the morphology of Hubble's Tuning Fork (see figure 1). The science team believes the relatively high number of intermediate types most likely results from the abundance of systems in which the bulge or disk do not clearly dominate rather than poor imaging data.  

Figure 1. Example gri images ordered by Galaxy Zoo classifications.  The left, center, and right columns show blue cloud, green valley, and red sequence galaxies, respectively.  The top, center, and bottom rows show early-type, intermediate-type, and late-type galaxies, respectively.  

Though most late-type galaxies of the sample inhabited the blue cloud and most early types were in the red sequence, this study found that both early-type and late-type galaxies spanned almost the entire color range, and within a given morphological class, the green valley was nothing more than a collection of outliers. 

Figure 2. Reddening-corrected u-r color-mass diagram for the sample.
This figure shows two important findings - that both early and late type
galaxies span almost the entire color range, and that the green valley
is only defined in the all-galaxy color panel.  

The bimodality of galaxy colors is a result of the superposition of the two populations; late types are mostly in the blue cloud and decrease smoothly to the red sequence, and a few early types reach all the way to the blue cloud. Figure 2 shows a stellar mass-color contoured plot after dust extinction corrections, and indicates the position of the green valley in relation to the blue cloud and red sequence.   

Using ultraviolet and optical photometry, star formation rates (SFRs) were analyzed to find that early-type galaxies are quenched much more rapidly than their late-type counterparts. Late-type galaxies were still blue in the ultraviolet through the green valley, indicating that they were still undergoing star formation as they were being quenched. Transitions through the green valley were found to be highly dependent on morphology, with early-type galaxies exhibiting quenching on timescales as short as 250 Myr. If this process took longer, like the 1-3 Gyr track estimated for late-type galaxies, there would be a build-up of early-types in the green valley that are not accounted for in observations. Perhaps early-type galaxies transition through the green valley as fast as star formation would allow.

Figure 3. UV-optical dust corrected color-color diagrams of green valley
galaxies.  Color-coded evolutionary tracks using different quenching
 timescales are overplotted on the green valley early- and late-type plots.
Emission line-selected AGN in host galaxies are marked with green
points.  These signatures would only appear after a few megayears after
the occurrence of a quenching event.  

Investigation of local environment, gas supply for star formation, and black hole activity was done to see if these factors could have contributed to the very different star formation histories of late-type and early-type galaxies. A catalogue of galaxy halo masses and information as to whether the green valley galaxies in question were central or satellites in their clusters was used to investigate the differences in galactic environments. Schawinski et al. found that there were striking differences in environments for galaxies of both populations traveling through the green valley. Though early types were found in both low- and high-mass haloes, late types had a dramatic split. Blue cloud late types were mostly in low-mass haloes, but those in the green valley and red sequence were almost exclusively in high-mass halos. This is a possible indication that late-type quenching is caused by environmental processes. In accordance with the large percentage of late types in the green valley relative to early types, it was found that late types have large gas reservoirs relative to early types to fuel star formation and slow the evolution through the green valley. Lastly, the growth of supermassive black holes was investigated to see how these culprits of quenching may differ in early and late types. Figure 3 indicates samples in the green valley that had Active Galactic Nuclei (AGN) activity.  Since several hundred Myr or more must elapse between the end of star formation in early types and the detection of an optical AGN, it is likely that AGN are not responsible for the rapid quenching of star formation in early types and rather an after-effect of the event that triggered quenching. 

Schawinski et al. concluded with discussion on the evolutionary tracks related to the end of star formation for late and early types. Morphological classifications of SDSS images in Galaxy Zoo as well as ultraviolet photometric analysis to probe star formation histories have led to conclusions on how early- and late-type galaxies transition through the green valley. Figure 4 and figure 5 show cartoons of the predicted evolutionary sequence through the green valley for early- and late-type galaxies.

This study concluded that late-type galaxies initiate their quenching processes when they are cut off from reservoirs of cosmic gas fueling their ongoing star formation. This can happen when the galactic halo reaches a critical mass that prevents further accretion or if cooling the hot halo gas becomes inefficient. Though the star formation rate begins to decline, the stellar mass may continue to increase as the remaining gas reservoirs are converted to stars. Slowly, the galaxy moves out of the blue cloud and into the green valley, with certain physical processes possibly accelerating the gas-depletion process. Black hole accretion may appear in late types after the galaxies have been quenched. This process occurs over several gigayears.  

Figure 4. Cartoon showing the evolution of late-type galaxies from the blue cloud, through the green valley, and into the red sequence.  

As for early-type galaxies, quenching of star formation is triggered by the rapid destruction of galaxy gas reservoirs, and happens too quick to be due to gas exhaustion by star formation alone. These galaxies immediately leave the main sequence as the SFR approaches zero and stellar mass ceases to increase. As fast as stellar evolution allows, these galaxies move through the green valley and into the red sequence, typically on timescales of about 1 Gyr. Since there are very few observed blue early types, it is thought this process is initiated by a merging event of two late types and the morphology transforms as the galaxy color and SFR do. After the quenching event, visible radiation from black hole accretion can be seen, and the rapid destruction of gas reservoirs suggests the involvement of unusually strong stellar processes or AGN feedback.  

Figure 5. Cartoon showing the evolution of early-type galaxies from the blue cloud, through the green valley, and into the red sequence.  

Wong et al. Article -- Galaxy Zoo: Building the Low-mass End of the Red Sequence with Local Post-starburst Galaxies

Title: Galaxy Zoo: building the low-mass end of the red sequence with local post-starburst galaxies

Authors: O. I. Wong, K. Schawinski, S. Kaviraj, K. L. Masters, R. C. Nichol, C. Lintott, W. C. Keel, D. Darg, S. P. Bamford, D. Andreescu, P. Murray, M. J. Raddick, A. Szalay, D. Thomas and J. VandenBerg

First Author’s Institution: CSIRO Astronomy & Space Science, Astronomy Department, Yale University

To read the full article, please click here.

Article Summary + Additional Background Information:

Galaxies were once believed to be isolated and unevolving systems in our universe.  In the past few decades this viewpoint has drastically changed; observations suggest that galaxies are strongly affected by gravitational interactions from the other galaxies in their nearby environment, and these interactions potentially are the main drivers of galaxy evolution.  Studying collisions, tidal interactions, and their effects are exceptionally important for further probing the important processes of galaxy evolution.

Figure 1. A color-luminosity graph indicates the location of the blue cloud, green valley, and red sequence.  The blue cloud is populated by star forming, spiral galaxies (late type) while the red sequence contains non-star forming, passively evolving elliptical galaxies (early type).  Post-starburst galaxies transitioning from the blue cloud to the red sequence inhabit the green valley.  

There are two basic types of galaxies recognized by Hubble’s “tuning fork” classification scheme: blue, star-forming spiral galaxies (late type) and red, quiescent elliptical galaxies (early type).  Spiral galaxies tend to have younger stellar populations emitting higher-energy (bluer) light, while elliptical galaxies are littered with older stars emanating lower-energy (red) light.  Through galaxy collisions, star-forming spiral galaxies in the ‘blue cloud’ are believed to develop into passively evolving elliptical galaxies in the ‘red sequence’, as illustrated in Figure 1.  During this transition galaxies occupy the sparser ‘green valley’.

During galactic interactions, the probability of a star-star collision is on the order of 1 part in a quadrillion.  The dark matter halos of galaxies, which make up about 80% of the galaxy mass, do not interact other than gravitationally.  However, the interstellar gas in galaxies does interact, causing a period of abnormally high star formation called a starburst.  This influx of gas also fuels the supermassive black holes at the center of galaxies, generating an active galactic nuclei (AGN).  The process of two galaxies colliding and merging is extremely slow by terrestrial standards, occurring over hundreds of millions to billions of years.  To study these transitional galaxies, astronomers turn to the Galaxy Zoo project for information about the aftermath of galactic interactions.  This provides a way to understand the evolutionary path from the blue cloud to the red valley.

Galaxies that have recently quenched star formation are called post-quenched or post-starburst galaxies (PSGs).  A recent study of local PSGs using the photometric and spectroscopic data from the Sloan Digital Sky Survey (SDSS) in conjunction with the results from the Galaxy Zoo project led to a better understanding of this transitional period of galaxy evolution.  PSGs are also called ‘E+A’ or ‘K+A’ galaxies, because they are galaxies that have ceased current star formation but still exhibit the spectral signature of recently formed stars (i.e., stars with stellar type 'A', as shown in Figure 2).

Figure 2. Comparison of a typical post-starburst galaxy spectrum with a normal star forming galaxy and a spectrum of a star with stellar type 'A'.

Using the SDSS, one of the largest and most complete samples of local PSGs was assembled, allowing volunteers of the Galaxy Zoo citizen science project to investigate the visual properties of these galaxies.  The galaxies selected were in the nearby universe, with redshifts of 0.02 < z < 0.05, and with a z band magnitude of Mz < -19.5 mag.  The z band was chosen for selection purposes because it is the reddest waveband provided by the SDSS and provides the closest proxy to stellar mass.  This selection reduced the Malmquist bias – which is the preferential detection of intrinsically bright objects.  Since the PSGs are galaxies with recently truncated star formation that still exhibit strong Balmer absorption from young stars, they were identified as having Hα emission line weaker than four times the rms level and Hδ equivalent width wider than 3 angstroms.  Of the 47,573 galaxies in the selected volume 80 matched the criterion of PSGs.  12 of the PSGs selected are presented in Figure 3.

Figure 3. This image shows the SDSS images of 12 PSGs in the sample.  The left-hand panel shows four examples of early-type PSGs, the center panel intermediate-type PSGs, and the right panel late-type PSGs.  

About 74% of the PSGs were neither early or late type galaxies, and were therefore classified as intermediate type.  About 16% and 10% of the PSGs were classified as early and late types, respectively.  This suggests that the PSGs in the sample are an evolutionary stepping-stone from blue, star forming spiral galaxies to red, quiescent elliptical galaxies.  Quantification of merger properties from Galaxy Zoo results concluded that most of the PSG samples did not have signs of an actively merging system, though many of the samples were asymmetrical or disturbed.

Additionally, using the SDSS modelMag tool, the u – r color was determined for these galaxies (where u and r are the bluest and middle band magnitudes used by SDSS, respectively).  The magnitudes were corrected to take out the effects of absorption by methods used in Calzetti et al. (2000), which accounted for both warm dust (T~40-55 K) and cool dust (T~20-23 K).  Most of the PSGs lay in the color range 1.8 < u - r < 2.3, which is the ‘green valley’ between the ‘blue cloud’ (late type galaxies) and the ‘red sequence’ (early type galaxies).

This study also looked at the environment around the local PSGs to correlate galaxy density with evolutionary processes.  The environment of the PSG samples was determined by measuring the number and proximity of galaxies around the point in space where the samples lay.  Half of the PSGs resided in low-density environments, while 26 and 24 percent resided in medium- and high-density environments, respectively.

Since most of the PSG sample consisted of intermediate-type morphologies, further investigation of stellar structure was required to reveal if the PSGs have intermediate-type morphologies due to past interactions or are similar in structure to the early- or late-type galaxies within the same volume.  To determine this, the SDSS fracDev parameter was used.  This gives the fraction of light fitted by a de Vaucouleurs profile, which describes how the surface brightness of an elliptical galaxy varies as a function of the radius from the galactic center.  Using this parameter, the authors found that the structural stellar morphologies of the PSGs in the ‘green valley’ more closely resemble the morphologies of low-mass early-type galaxies, even though star formation has only recently been truncated.

Stellar mass estimates for the galaxies were measured by fitting the five optical wavebands used by the SDSS to star formation history libraries created from stellar models in Maraston (1998, 2005).  A majority of the PSGs in the study had stellar masses below the transition mass that separates low-mass star-forming galaxies from the high-mass passively evolving bulge-dominated galaxies.  No PSGs were found with log stellar masses greater than 11.5 solar masses (i.e., greater than 10^11.5 Msun).  One possible reason for this lack of high-mass PSGs is that the sample was restricted to a very local volume.  These results are consistent with the idea of galaxy formation ‘downsizing’, the theory that more massive galaxies from higher density areas run through their gas quicker and evolve through the PSG phase at higher redshifts than lower mass galaxies.  Figure 4 shows the color versus stellar mass for the PSG sample.

Figure 4. The image above shows the location of the sample PSGs on u-r color vs stellar mass graphs.  The panels of the top row, from left to right, show the location of all the galaxies, early-type galaxies, intermediate-type galaxies, and late-type galaxies in the study, respectively.  The bottom row of panels shows the number fraction of the PSG sample to the galaxy sample of a particular type in a given color-stellar mass bin.  

Current models of galaxy evolution suggest that feedback from AGN could provide the means to quench and truncate the star formation history of a massive galaxy.  Mergers may induce inflows of gas that fuel star formation and the central black hole, while feedback from AGNs quench star formation by reheating cold gas and expelling much of it in AGN-driven winds.  This hypothesis suggests that AGN feedback may play a role in quenching star formation in PSGs.  However, apart from two PSGs in this study that exhibit spectral properties of AGN called LINERs (low-ionization nuclear emission-line regions) no observations of AGN spectral signatures were found in the PSG sample.  These observations coincide with the idea of 'downsizing', in which the buildup of smaller galaxies occurs at later epochs.  The low-z galaxies in this sample were most likely not massive enough to host an AGN and therefore AGN feedback was not the primary quenching mechanism.  For more information on AGN feedback, refer to Schawinski et al. 2007. 

The results of this study show that most local PSGs occupy the ‘green valley’ and are rapidly transitioning to the low-mass end of the ‘red sequence’, with duration of this transitional period on the order of 1 billion years.  The structural morphology of local ‘green valley’ PSGs is very similar to that of low-mass early-type galaxies in the ‘red sequence’, even though star formation has only recently ceased.  This study suggests that these galaxies changed their shape and became bulge-dominated prior to the cessation of star formation, and therefore the transition through the 'green valley' will take approximately as long as it takes for the last batch of recently-formed stars to fade.  These local PSGs show that galactic interactions in recent epochs lead to the growth of the low-mass end of the 'red sequence' and agree with the idea of downsizing. 

Studying galaxy collisions, starburst galaxies, active galactic nuclei, and post-starburst galaxies is giving a clearer image on how galaxies evolve, and the star formation processes that occur during this transitional phase of galaxy evolution.

Darg et al. Article -- Utilizing Galaxy Zoo to examine properties of merging galaxies

Title: Galaxy Zoo: the properties of merging galaxies in the nearby Universe – local environments, colours, masses, star formation rates, and AGN activity

Authors: D. W. Darg, S. Kaviraj, C. J. Lintott, K. Schawinski, M. Sarzi, S. Bamford, J. Silk, D. Andreescu, P. Murray, R. C. Nichol, M. J. Raddick, A. Slosar, A. S. Szalay, D. Thomas, and J. Vandenberg

First Author’s Institution:  University of Oxford, Department of Physics

To read the full article, please click here.

Article Summary: 

Examining large-scale morphological properties of galaxy mergers proved to be a trying feat until the Galaxy Zoo project was set in motion.  Because of the great variety of configurations of mergers, visually examining images of galaxies is a much better method for identifying and classifying these specimens than using structural parameters.  Using classifications on the Galaxy Zoo interface, one can determine how ‘merger-like’ a Sloan Digital Sky Survey (SDSS) image appears to be based on the percentage of volunteers that flagged the particular image as a merger.  By utilizing the morphological data from Galaxy Zoo, Darg et al. were able to delve into important properties of merging galaxies, such as the structure of progenitor galaxies, internal properties of interacting galaxies, time-scales of merger events, local environments of mergers, star formation histories, and AGN activity. 

In this study, 3003 merging pairs were classified as well as a redshift-matched control sample.  The galaxies lie in the relatively local universe, with redshift range 0.005 < z < 0.1.  Binned redshifts for the merger and control samples are shown in figure 1.  Galaxies were classified both by their morphologies (Elliptical, Spiral, Unclear but probably Elliptical, and Unclear but probably a Spiral) and their merger stages.  Mergers could either be classified as ‘separated’, ‘interacting’, or ‘approaching post-merger’.  Of the 3003 samples, ~84% were classified as interacting, ~6% as separated, and ~10% as approaching post-merger. 

Figure 1. Binned redshift distributions for the merger and control samples.  

Figure 2. Number density of galaxies within 2.0 Mpc
of the merger and control samples.  Rho symbolizes
the Adaptive-Gaussian-environment parameter.  White
background shows galaxies in the field, dark gray shows
galaxies in clusters, and light gray shows the
intermediate regime.  

Of the merging systems visually examined in this study, there were about 3 times as many spirals than ellipticals.  This is interesting given that the ratio of spirals to ellipticals in the global galaxy population is ~1.5.  One issue that Darg et al. inquired about was the reason for this discrepancy.  Does it have to do with the environment in which these mergers take place or differences in the internal properties of these galaxies?  To parameterize the environment of these mergers, a sophisticated measure of the number of galaxies per unit volume called the ‘adaptive-Gaussion-environment parameter’ was used.  This allowed the determination of whether the merging galaxies and the control were located in the low-density field, high-density clusters, or in an intermediate regime.  Figure 2 shows that both mergers and controls peaked in a region dubbed ‘intermediate environments’.  Since mergers were also found to occupy similar if not denser environments than the control (where elliptical galaxies are more prevalent), the role of environment in causing the high spiral-to-elliptical ratio in mergers can be ruled out.  Instead, the prevalence of spirals in mergers likely arises from the longer time-scales of detectability for mergers involving spirals than for mergers involving ellipticals. 

Figure 3. Volume-limited and non-volume limited
distribution of galaxies in color space.  The graphs
on the left show the u-r color versus absolute
magnitude.  The center graphs show the frequency
of ellipticals and spirals compared to the control
sample (EU and SU stand for unsure but probably
elliptical and unsure but probably spiral,
respectively).  The right graphs show the frequency
of all merging samples compared to control.  

At least one of each of the galaxies merging had spectral data, allowing this study to do a color analysis of the samples.  In accord with earlier observations, the merging galaxies had a larger spread of colors than the control sample, supporting the notion that ‘irregular’ morphologies have a greater spread in color than ‘regular’ ones.  A volume limited (where only galaxies with M_r < -20.55 were used) and a non-volume limited color-magnitude diagram for the merger and control samples can be seen in figure 3.  A clear bimodality between the elliptical and spiral regimes can be seen in the binned color plots.  Figure 4 shows the mass-distributions of galaxies in both merger and control samples.  Across almost all environments, the spiral-galaxy stellar mass distributions appear to be the same in the mergers as in the control sample.  Ellipticals mergers on the other hand appear to be slightly more massive than their control counterparts.  When morphologies are not looked at, a very similar mass distribution for merger and control samples is attained.  The fact that mergers favor spirals (which are generally less massive) yet have an overall distribution just as massive as the control sample may indicate that galaxies involved in mergers really are more massive.  

Figure 4. Mass distributions for galaxies in all
environments (top row), galaxies in the field (second
row), galaxies in intermediate regimes (third row) and
galaxies in dense clusters (bottom).  

Figure 5 shows the entire sample of merger-pairs in a mass-color-morphology graph.  Both color and morphology of the galaxies scale strongly with mass.  An interesting find is that there is a near absence of ellipticals with masses below 3 x 10¹⁰ solar masses, raising the question as to what happens to two low-mass spiral galaxies when they merge.  This may be due to low-mass galaxies retaining a sufficient amount of gas to reform a disc after a major merger event (gas content along with conservation of angular momentum is what leads to a flattened-out disc shape in galaxies).  More massive galaxies may be prone to more catastrophic angular momentum loss during a merger event, and the remaining gas supplies may plunge into the central core and transfer the angular momentum required for disc morphology into the stellar dispersion of the remnant bulge. 

Figure 5. A mass-color-morphology diagram.  The top three plots show the average stellar mass for (from left to right) spiral-spiral mergers, spiral-elliptical mergers, and elliptical-elliptical mergers.  The main plot shows the u-r color of each sample, the stellar mass of each galaxy (with the more massive galaxy's value on the x-axis) and the type of galaxies that are taking part in the merger event (given by a circle, asterisk, or triangle data point).  

Due to the importance of feedback mechanisms to a gas retention model, the study next examined active galactic nuclei (AGN) and star formation signatures of the mergers.  By examining the measured fluxes of emission lines in the samples, this study was able to determine the most likely sources of these emissions and separate their galaxies into 4 categories: star-forming, mixed (both star formation and AGN activity), AGN (either Seyfert nuclei or LINERs), and quiescent (or ‘weak emission-line’).  Figure 6 shows the locations of these four types of mergers in mass-color space.  In this plot, galaxies characterized by star-formation occupied the low-mass region, AGN occupied the intermediate-mass region, and quiescent types occupied the high-mass region.  This suggests that the fuel supply of high-mass galaxies has been exhausted (as to not fuel star formation or AGN activity) and low-mass galaxies may have insufficient mass to power AGN.  Alternatively, AGN signatures in low-mass galaxies may also be obscured by high gas content and high star formation rates (SFRs).  By comparing spectral signatures to a control sample, the study determined that mergers significantly enhance SFRs in spiral galaxies only, whereas ellipticals live up to being ‘red and dead’ and their SFRs not as affected by major mergers.  Using H-alpha emission strength, estimations of the SFRs (of galaxies that fell in the star-forming category) were measured to be ~5.2 solar masses per year, which was about twice the value of a control sample of non-merging star-forming galaxies.  The highest SFR of the merging galaxies was ~ 95 solar masses per year. 

Figure 6. Color-stellar mass relation for galaxies of differing spectral types.  Each plot highlights the samples that fall into each respective category.  

The use of galaxy zoo morphology classifications allowed this study to analyze the effects of mergers on different types of galaxies.  By estimating the environment around mergers, the prevalence of spiral galaxies in merger events was found to not be due to the density of the environments in which mergers occur.  Therefore, internal properties of galaxies may be the reason for the high number of spirals in mergers; spirals have large gas reservoirs that may result in longer time-scales of merger events, whereas when two elliptical galaxies merge one would expect them to produce comparatively faint tidal tails and little star formation, thus making them harder to detect.  Since detectability of mergers relates to their timescales and timescales relate to internal properties of galaxies, addressing the colors, stellar masses, and spectral emission of the samples is of importance.  This study found that colors of merging galaxies scale strongly with mass and morphology, and are spread over a larger area than control galaxies.  Ellipticals are rare below a mass of ~ 3 x 10¹⁰ solar masses, which may be due to low-mass spiral mergers surviving the event and having enough gas to reform their disc.  Moving to the feedback mechanisms of the merging samples, Darg et al. found that mergers induce intense star-formation only when they involve spiral galaxies, and AGN activity was not present in low-mass mergers.  In star-forming mergers, the SFRs were ~2 times greater than that of a control sample of star-forming galaxies.  This study also found that specific SFRs (star-formation rates per unit stellar mass), scale down with stellar mass, possibly due to gas supplies being continually drained as galaxies accumulate mass.  The results generally imply that mergers affect spirals much more than ellipticals, which in turn affects the time-scales of detectability for merger events.  

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.