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

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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.