An observational and theoretical view of the atomic gas distribution in galaxies

Fig. 1: The top and bottom rows show two galaxies with very different gas disk morphologies. From left to right: the column density contours for neutral hydrogen overlaid on optical images from the Sloan Digital Sky Survey, the neutral hydrogen itself, and the velocity maps of the neutral hydrogen.

Fig. 2: The median radial profiles of different galaxies (blue: gas rich, green: normal, red: gas poor). For all galaxies, the radius has been scaled to R1, where the gas surface density reaches 1 solar mass / square parsec. The observed profiles in the left plot are compared to results from semi- analytical models (SAM, middle) and results from smoothed particle hydrodynamical simulations (SPH, right). 

Fig. 3: An extreme case of gas accretion in a ring-shape. This simulated galaxy at a redshift z~0.5 was the result of smoothed particle hydrodynamical simulations.  (Image provided by Michael Aumer)

How is cold gas accreted in galaxies? Observers and theorists from MPA have joined their efforts to investigate the radial distribution of atomic gas in unusually gas-rich nearby galaxies. They found a universal shape for the radial profiles of the gas in the outer regions of the observed galaxies, and obtained remarkable agreement with simulations. In half the galaxies, the atomic gas may have been accreted in the form of "rings" 

Every astronomy student learns that in galaxies stars form from huge gas clouds. However, the details of the accretion and distribution of gas in galaxies is still unclear. Therefore, an international group of scientists at MPA and ASTRON in the Netherlands joined forces and carried out the Bluedisk project to map neutral hydrogen in a sample of 25 very gas-rich galaxies as well as a similar-sized sample of “control” galaxies with similar masses, sizes and distances, but normal gas content. Their main tools were the Westerbork Synthesis Radio Telescope (WSRT) as well as elaborate computer simulations (see Research Highlight May 2013). 

There have been many efforts over the past three decades to map the distribution of cold, atomic gas in galaxies using radio synthesis telescopes. The first analyses showed that the atomic gas exhibits a wide variety of detailed features. These can be attributed to irregularities in the galaxy such as spiral arms, rings, bars, warps etc. Studies of larger samples revealed basic scaling relations that provide hints of the mechanisms regulating the evolution of galaxies. 

In contrast to the stellar surface density, which peaks in the centre of the galaxy and drops steeply with radius, the radial distribution of the atomic gas often flattens or even declines near the centre of the galaxy. In the outer regions, the gas disks usually extend to a larger distance from the centre than the stellar disks, and are well-fit by exponential functions. 

Thanks to improvements in the WSRT instrumentation and data analysis, the observations by the Bluedisk team reached significantly lower column densities than previous surveys, i.e. they were able to map the gas in regions where the gas has low density. This sample is thus also well-suited for direct comparison with theoretical models. Observers and theorists worked together closely to improve their understanding of the radial distribution of the cold, atomic gas and to find a physical explanation for its structure. 

The study revealed an interesting observational phenomenon: in the outer regions of all the galaxies, the gas exhibits a homogeneous surface density profile (if the sizes of the gas disks are properly scaled). This profile is well-fit by an exponential function with a universal scale-length. This universal profile appears to hold for all galaxies, irrespective of their stellar properties, gas masses, sizes, or morphologies (for an example see figure 1). This is remarkable, because the gas-rich galaxies contain on average 10 times more gas than the control sample. 

In addition, the team found surprising agreement between their universal profile and results from simulations, both for smoothed-particle hydrodynamical simulations and for semi-analytic models of disk galaxy formation (see figure 2). It remains something of a mystery why the agreement with the smoothed-particle hydrodynamical simulations is quite so good. 

In the semi-analytic models, the universal shape of the outer radial profiles is a direct consequence of the assumption that infalling gas is always distributed exponentially. However, there are observational indications that the atomic gas could be accreted in the form of "rings". Therefore, more work is underway on the theoretical side using smoothed particle hydrodynamical simulations to try and understand how gas settles onto the simulated galaxies in more detail (see figure 3).

Jing Wang & Guinevere Kauffmann

Further reading:

Wang, J.; Fu, J., Aumer, M., Kauffmann, G., et al., "An observational and theoretical view of radial distribution of HI gas in galaxies", 2014, submitted to MNRAS. http://arxiv.org/abs/1401.8164

Related Links:

The Bluedisk project

Source: Max Plack Institute fir Astrophysics

Solar system’s youth gives clues to planet search

Caption: Modeling results show where the injected gas and dust ended ups only 34 years after being injected at the disk’s surface. It was injected 9 astronomical units from the central prostar and is now in the disk’s midplane. The outer edge shown is 10 astronomical units from the central prostar. Mixing and transport are still underway and the underlying spiral arms that drive the mixing and transport can be seen. Image courtesy of Alan Boss. A larger version is available here.

Washington, D.C. - Comets and meteorites contain clues to our solar system's earliest days. But some of the findings are puzzle pieces that don't seem to fit well together. A new set of theoretical models from Carnegie's Alan Boss shows how an outburst event in the Sun's formative years could explain some of this disparate evidence. His work could have implications for the hunt for habitable planets outside of our solar system. It is published by The Astrophysical Journal.

One way to study the solar system’s formative period is to look for samples of small crystalline particles that were formed at high temperatures but now exist in icy comets. Another is to analyze the traces of isotopes—versions of elements with the same number of protons, but a different number of neutrons—found in primitive meteorites. These isotopes decay and turn into different, so-called daughter, elements. The initial abundances of these isotopes tell researchers where the isotopes may have come from, and can give clues as to how they traveled around the early solar system.

Stars are surrounded by disks of rotating gas during the early stages of their lives. Observations of young stars that still have these gas disks demonstrate that sun-like stars undergo periodic bursts, lasting about 100 years each, during which mass is transferred from the disk to the young star.

But analysis of particles and isotopes from comets and meteorites present a mixed picture of solar system formation, more complicated than just a one-way movement of matter from the disk to the star.

The heat-formed crystalline grains found in icy comets imply significant mixing and outward movement of matter from close to the star to the outer edges of the solar system. Some isotopes, such as aluminum, support this view. However, isotopes of the element oxygen seem to paint a different picture.

Boss' new model demonstrates how a phase of marginal gravitational instability in the gas disk surrounding a proto-sun, leading to an outburst phase, can explain all of these findings. The results are applicable to stars with a variety of masses and disk sizes. According to the model, the instability can cause a relatively rapid transportation of matter between the star and the gas disk, where matter is moved both inward and outward. This accounts for the presence of heat-formed crystalline particles in comets from the solar system's outer reaches.

According to the model, the ratios of aluminum isotopes can be explained by the parent isotope having been injected in a one-time event into the planet-forming disk by a shock wave from an exploding star and then traveling both inward and outward in the disk. The reason oxygen isotopes are present in a different pattern is because they are derived from sustained chemical reactions occurring on the surface of the outer solar nebula, rather than from a one-time event.

"These results not only teach us about the formation of our own solar system, but also could aid us in the search for other stars orbited by habitable planets," Boss said. "Understanding the mixing and transport processes that occur around Sun-like stars could give us clues about which of their surrounding planets might have conditions similar to our own."
 

* * *

This work was supported by the NASA Origins of Solar Systems program and the NASA Astrobiology Institute. Some of the calculations were performed on the Carnegie Alpha Cluster, the purchase of which was partially supported by a NSF grant.

Source: Carnegie Institution of Washington