Undergraduate ALFALFA Team Groups Project
Introduction to Gas Loss, Quenching of Star Formation, and Role of the Environment

The Undergraduate ALFALFA Team Groups project is targeting nearby groups of galaxies. At the January 2014 workshop, we will be following up galaxies in our groups that show evidence of star formation, but were not detected by ALFALFA.

Star Formation Indicators

How do we know if a galaxy has star formation? There are several tracers used; we will explore two: (1) Halpha emission, which can be measured via imaging or via spectra and (2) color. (You can explore more tracers here.)

(1) Hα emission originates in star formation regions. UV photons from hot, massive stars ionizes neutral hydrogen. Recombination radiation is emitted at many wavelengths as electrons recombine with protons. The Balmer n=3 to n=2, or Hα, at 656.3nm is one of the strongest recombination lines. We can map star formation by obtaining an image of a galaxy in a narrow filter around Hα. An example for a spiral galaxy is shown to the right. We can also measure this using a spectrum, also shown at right for the same galaxy. Note that the SDSS spectrum comes from a fiber optic spanning 3 arcsec on the sky; this corresponds to 7 pixels in the image. You can judge pixel size by looking at the variation in the background.)

Questions: what is an advantage of imaging? spectra? What is a disadvantage of imaging? spectra?

(2) Color is a commonly-used although cruder measure of star formation in a galaxy. It is based on the idea that hot, massive, young stars are very bright at blue wavelengths and therefore a substantial star-forming population will affect the galaxy color. It is convenient to use color because observations are so simple - all you need are two filters. For example, many studies based on SDSS use the g-i color, where g is centered at green wavelengths and i at near-infrared.

Example of spiral galaxy with "normal" Hα emission. The stellar distribution (as measured in a red optical filter) is shown at left and the Hα image is shown at right. Click on image for larger view.

SDSS Spectrum for the same spiral galaxy. Note the Hα indicating star formation.

ALFALFA HI spectrum show healthy HI content.


Star Formation and Gas Content

Galaxies that form stars have gas reservoirs. See for example this summary of a classic paper describing the Kennicutt-Schmidt Law. A plot from this paper is reproduced at right. (The units are in surface density.) Thus we could trace gas in galaxies by looking for HI or by looking for Hα, which occurs in gas near star formation regions. You can think of the gas as tracing the future, star formation the present, and stars the past of a galaxy. Galaxies that have star formation, but are not detected in ALFALFA (review 1st telecon), are thus likely to have HI. Why don't we see it? Because the amount of gas lies beneath the limit for the ALFALFA survey.

Review Question: What are two reasons a galaxy's HI content would lie beneath the ALFALFA limit?

Classic Kennicutt-Schmidt Correlation

Galaxies and Their Environment

Galaxies tend to clump together in groups and clusters, which are themselves associated with larger structures called superclusters. (Review Large Scale Structure of the Universe and What's in an Environment. ) These structures formed as the Universe evolved in time due to graviational attraction. A schematic is shown at right and this site shows an animation. Clusters form at the crossing points of filaments.

Galaxies are members of a group or a cluster if they are bound by gravity to the structure. (What does this mean?) Clusters have been more extensively studied than groups. One reason is that clusters have many galaxies in a small area of the sky, so that observations of more galaxies can be obtained for the same amount of observing time.

We have learned a lot about the star formation and gas properties of clusters. For example:

  • Many galaxies are depleted of gas in cluster environments. See the HI maps of galaxies in the Virgo Cluster at right.
  • Galaxies in clusters have less star formation than more isolated counterparts.
  • There is a higher proportion of elliptical and lenticular galaxies in clusters, which is known as the Morphology Density Relation.
Thus galaxies that are in clusters have lost gas, their star formation has been quenched, and they may have undergone a morphological transformation. These changes may be caused by the galaxy's environment.

HI maps of galaxies in the Virgo Cluster superposed on X-ray observations.

Environmental Effects and Gas Loss

Interactions of galaxies with their environment can remove gas and change their stellar distributions (i.e., morphological change). Here are the two main types of interactions:

(1) Galaxy tidal interactions with other members
Galaxies in groups and clusters may interact gravitationally with one another. There are many observations of this and it has been simulated via computer. Check out this site for descriptions of tidal effects and links to simulations and this site for some more simulations. The outcome of a tidal interaction depends on many factors, including the masses of the two galaxies (merger vs accretion/cannibalism) and the relative velocities of the galaxies. In clusters, galaxies move quickly and so mergers are not likely, but a galaxy may have many brief encounters over its lifetime, leading to a cumulative effect called galaxy harassment. In groups, velocities are slower and mergers are more common. Tidal interactions are capable of changing the distributions of gas and stars, e.g., through long tidal tails. Gas is also funnelled to the center and the stellar distribution can be churned up, leading to a galaxy with a larger stellar concentration (bigger bulge). In an ongoing tidal interaction, stars and gas are likely to be affected. Because galaxies have extended gas disks, it's often the gas that shows evidence of tidal interaction first.

(2) Galaxy Interactions with the Intracluster Medium
When X-ray telescopes were first launched into space, observations showed that galaxy clusters contain a huge amount of hot, low density gas, known as the Intracluster Medium or ICM. See the image at right and read more at the Chandra Summary of Clusters of Galaxies. Galaxies traveling through this gas will tend to lose gas from the outside in, an effect known as ram-pressure stripping. See this site for simulations. There is at least circumstantial evidence that this is going on in clusters. Note that the HI maps in the Virgo Cluster discussed above show smaller HI disks in the location of the X-ray emission. There are a few cases of galaxies that show current stripping. Note that the stars would not be affected by this interaction: it is only the gas that is stripped. Thus this explains gas loss and star formation quenching, but not morphological transformation.

While we have learned a great deal about environmental interactions, unfortunately none of the studies of clusters to date has succeeded in explaining all the observations by an environmental model. For example, while ram-pressure stripping appears to be happening, it cannot explain morphological transformation of galaxies (why?).

Optical Image of Interacting Galaxies

Optical (left) and HI map (right) of the M81 Group of Galaxies

Optical (left) and X-ray (right) in the Hydra A Cluster.


The Role of Groups

Some astronomers have suggested that the real action of environmental alteration is going on in group environments rather than clusters. Some groups show X-ray emission, indicating an intra-group medium (IGM, also known as inter-galactic medium) that may cause ram-pressure stripping, or a milder form known as starvation or strangulation. Tidal interactions are likely to be more important in groups because of lower galaxy velocities.

The Undergraduate ALFALFA Team Groups Project is investigating these issues via ALFALFA observations, optical observations of star formation at Kitt Peak National Observatory, and by examining public archives of optical and x-ray observations. However given the selection effect of the ALFALFA observing time, we do not have good values of HI masses for a large number of galaxies that are known to be star-forming from the Hα imaging or SDSS spectra. We hope to probe deeper with L-band Wide observations so that we can compare gas contents and star formation rates across a wide variety of environments and establish the local density at which star formation quenching becomes important.

Question: Can you think of another reason why populations of galaxies may be different in different environments? Hint: what assumptions have we made when we talk about "transformation" of galaxies?

This page created by and for the members of the ALFALFA Survey Undergraduate team

Last modified: Thu Nov 7 18:37:07 EST 2013 by becky