H-α

 

Our Hα imaging is tuned to detect light emitted from the group galaxies at a specific single wavelength -- the Hα emission line at a rest-frame wavelength of 656.3nm, i.e. located in the red part of the visible spectrum. The Hα emission line is a nebular recombination line, produced when previously ionized hydrogen gas recombines to form neutral atomic hydrogen. As the electron is recaptured to form the new hydrogen atom, it cascades down through the fixed energy levels permitted by quantum physics, emitting photons with each transition, until it reaches the ground state (n=1). The Hα emission is a member of the Balmer series and occurs when the electrons transition from the n=3 energy level down to the n=2 level.

Hα emission marks the location of ionized hydrogen gas in a galaxy. Since it takes almost as much energy to excite a hydrogen atom's electron from the ground state to the n=3 state as it does to fully ionize the hydrogen atom, it is highly unlikely that the electron in the hydrogen atom had remained bound. The dominant source of Hα emission in galaxies are star forming regions. These regions contain one or more OB stars, more than 10 times more massive than our sun, which shine extremely brightly with luminosities 10,000 times that of our sun, but as a consequence have total lifetimes of only a few million years. These stars are also much hotter than normal main sequence stars like our sun, having temperatures of 30,000K or more. Hence instead of emitting in the optical and infrared like our sun, most of the energy from these OB stars is radiated at ultraviolet wavelengths, including most significantly at energies sufficient to ionize the surrounding interstellar hydrogen gas.

This ionized gas later recombines, emitting photons as the electrons cascade back down to the ground state, often including one at the Hα emission line. These nebular recombination lines effectively re-emit the integrated UV luminosity of these young massive stars shortward of the Lyman limit, providing a direct measure of the rate at which these stars are forming. Over the years much work has gone into establishing robust and well understood calibrations to derive the current star formation rate of individual HII regions or entire galaxies from their total Hα luminosities.

These stellar nurseries filled with hydrogen gas partially ionized by the UV emission from massive OB stars are known as H II regions, and have sizes of 1-800 light-years across. These include many of the most famous and spectacular astronomical objects in the sky including the Orion, Eagle and Rosette Nebulae in our Galaxy and the Tarantula Nebula in the Large Magellanic Cloud. When viewed in the optical they are embued with a characteristic red glow due to the diffuse Hα emission from the ionized hydrogen gas which fills them.

   
  Caption: The Orion Nebula as viewed by the Hubble Space Telescope. The characteristic red glow is diffuse Hα emission produced when hydrogen gas ionized by ultraviolet radiation from the massive OB stars in the Trapezium cluster (the compact group of 4 stars in the central white-colored region) later recombines back into hydrogen atoms consisting of a bound electron and proton. It is this Hα emission associated with ongoing star formation that we wish to detect. Credit: NASA, ESA, M. Robberto (STScI/ESA) and the Hubble Space Telescope Orion Treasury Project Team.  

Normal spiral galaxies such as our Milky Way may contain hundreds or thousands of H II regions associated with ongoing star formation. They are found throughout the galaxy, but are most abundant within the spiral arms where the interstellar medium is densest allowing molecular clouds to form and collapse under gravity into the star forming regions which delineate the spiral structure. Hα imaging of galaxies allows us to map the distribution of H II regions at both high resolution and high sensitivity, telling us precisely where stars are forming within the galaxy and at what rate.

   
  Caption: The grand-design spiral galaxy M51 as viewed by the Hubble Space Telescope. The red colors in this image come from the Hα emission produced within the H II regions embedded along the full extent of its spiral arms, marking out in incredible detail where new stars are forming in this galaxy. Some of this star formation may be being triggered by the interaction of M51 with its smaller neighbor NGC 5195. Credit: NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA).  

The ability of galaxies to form stars within H II regions depends fundamentally on their gas contents. Elliptical galaxies contain little or no interstellar hydrogen gas and so no H II regions can form. Environmental processes are known to strongly impact the gas contents of galaxies and by consequence their abilities to form stars. Spiral galaxies in local clusters are found to be strongly deficient in hydrogen gas with respect to isolated galaxies of the same Hubble type, indicating that gas removal mechanisms related to the cluster environment are efficient. There are two types of gas removal mechanisms believed to be active in dense environments (i.e. galaxy groups or clusters): ram-pressure stripping and starvation.

Ram-pressure due to the passage of the galaxy though the intra-cluster medium selectively strips the least-bound gas from the outskirts of the spiral disk, producing truncated gas disks within the spiral galaxy, and in extreme cases completely removing the gas from the galaxy. This results in star formation being shut down from the outside-in, resulting in characteristic Hα profiles which are truncated with respect to the stellar disk. Hα images of Virgo cluster galaxies have revealed such truncated Hα profiles characteristic of ram-pressure stripping. The ongoing stripping of gas from the galaxy may also be directly detected via Hα imaging, as the gas is ionized by heating from compression, turbulence of in-situ star-formation, forming spectacular tails of extra-galactic Hα emission.

   
  Caption: A massive spiral galaxy undergoing ram-pressure stripping. The red colored clumps trailing below the galaxy disk are knots of Hα emitting gas which has been stripped from the galaxy by its passage through the dense ICM. The Hα image (red colors) was obtained using the Maryland-Magellan Tunable Filter on the Magellan 6.5m telescope as part of a parallel survey of galaxies in the Shapley supercluster undergoing environmental transformation.  

The truncated Hα profiles cannot be produced by the starvation mechanism which, rather than actively removing gas from the galaxy disk itself, simply acts to prevent galaxies from accreting new gas from their surroundings. They then slowly consume their remaining gas by forming stars until their gas runs out. This process produces a gentle global decrease in gas and star formation as the gas is consumed over many Gyr, leading to anemic spirals where star formation is occuring throughout the disk but at much reduced levels.

The power of Hα imaging to distinguish between gas removal mechanisms.
Three spiral galaxies in the Virgo cluster which show normal stellar disks (left panels) but have clearly truncated Hα disks (center panels) indicative of ongoing ram-pressure stripping shutting down star-formation from the outside-in (right panels). Three anemic spirals in the Virgo cluster, which show reduced levels of star formation across their entire disks, consistent with starvation models in which new gas is prevented from being accreted onto the galaxy, producing a gentle global decrease in the gas and star formation densities, as the remaining gas contents are slowly consumed.
[Figures from Koopmann & Kenney 2004, ApJ, 613, 866] These images were taken using the same KPNO 0.9m telescope we are using in CHANGES.

Hα imaging also allows us to identify galaxies in which star formation is being triggered by their interaction with the local environment. Low-velocity galaxy encounters and mergers produce gravitational torques capable of rapidly channelling large quantities of gas inwards inducing a nuclear starburst. Ram-pressure may also induce localized starbursts within galaxies by compressing the gas and molecular clouds on the leading edge of the galaxy.


 
       

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