This is so interesting! And it touches on a very interesting question - why do elliptical galaxies produce so few stars? And do they produce any new stars at all?
Very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown that a reasonable proportion (~25%) of these galaxies have residual gas reservoirs and low level star-formation. Researchers with the Herschel Space Observatory have speculated that the central black holes in elliptical galaxies keep the gas from cooling enough for star formation.
If 25% of elliptical galaxies show obvious star formation, that means that 75% of them do not. What sets the starforming ellipticals off from the non-starforming ones?
NGC 3077. Photo:ESA/Hubble.
Song Huang and Qiu-Sheng Gu
We find that SFEGs [starforming elliptical galaxies] have relative younger stellar population age, higher metallicity, and lower stellar mass, and that their star formation history can be well described by a recent minor and short starburst superimposed on old stellar component. We also detect 11 E+A galaxies whose stellar population properties are closer to those of quiescent (normal) ellipticals than to star-forming ones. However, from the analysis of their absorption line indices, we note that our E+A galaxies show a significant fraction of intermediate-age stellar populations, remarkably different from the quiescent galaxies. This might suggest an evolutionary link between E+As and star-forming ellipticals.
So the obviously starforming ellipticals are either relatively small, or else they contain whole populations of moderately young stars overall. A good example might be relatively small and elliptical-shaped NGC 3077, a satellite galaxy of M81, which displays a central starburst. If supermassive central black holes play a role in quenching star formation, we should note that most small galaxies have small central black holes, or none at all.
What about dusty filaments? Why do some of them form stars and others do not?
NGC 4696. NASA, ESA, Hubble, A Fabian.
I chose to place the pictures of NGC 4696 and 3597 side by side, because I find their dusty filaments interestingly similar in overall shape. I think, however, that they differ considerably in size. The filaments of NGC 4696 are small and located close to the nucleus of of the galaxy, whereas the filaments of NGC 3597, which is the product of a titanic collision between two galaxies, possibly encircle large volumes of the entire galaxy.
There can be no mistaking the fertile star formation of at least parts of the dusty filaments of NGC 3597, while those of NGC 4696 are literally red and dead. On the other hand, not all of the dusty filaments of NGC 3597 seem to produce new stars. This "limited" star formation ability is in fact typical of dusty filaments. Check out this superb but large
(1.2MB) Hubble picture of NGC 3314, processed by André van der Hoeven. NGC 3314 consists of two non-interacting galaxies superimposed on one another as seen from the Earth, and we can judge the stellar content of the spiral arms of the galaxy seen in silhouette against the other one.
2MASX J00482185-2507365 occulting pairs.
NASA, ESA, and The Hubble Heritage Team
In the occulting pair of galaxies known as 2MASX J00482185-2507365, we can see that the smaller galaxy, which has a faintly blue and seemingly staarforming inner disk, seems to sport outer arm fragments that are "all dust". It happens. Besides, this small galaxy is probably close to a supermassive black hole (in its large bully of a galactic neighbor).
Let's return to the question of why the filaments of NGC 4696 are forming no stars at all. The wikipedia link I posted at the beginning of my post suggests that the supermassive black hole prevents star formation here. The caption of today's APOD links heating from the black hole to the influence of magnetic fields to explain the appearance of the the filaments of NGC 4696:
Speculation holds that this black hole pumps out energy that heats surrounding gas, pushes out cooler filaments of gas and dust, and shuts down star formation. Balanced by magnetic fields, these filaments then appear to spiral back in toward and eventually circle the central black hole.
That seems reasonable to me.