For some time, astronomers have known that collisions or mergers between galaxies are an integral part of cosmic evolution. In addition to causing galaxies to grow, these mergers also trigger new rounds of star formation as fresh gas and dust are injected into the galaxy. In the future, astronomers estimate that the Milky Way Galaxy will merge with the Andromeda Galaxy, as well as the Small and Large Magellanic Clouds in the meantime.
According to new results obtained by researchers at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York city, the results of our eventual merger with the Magellanic Clouds is already being felt. According to results presented at the 235th meeting of the American Astronomical Society this week, stars forming in the outskirts of our galaxy could be the result of these dwarf galaxies merging with our own.
The world’s largest airborne telescope, SOFIA, has peered into the core of the Milky Way and captured a crisp image of the region. With its ability to see in the infrared, SOFIA (Stratospheric Observatory For Infrared Astronomy) is able to observe the center of the Milky Way, a region dominated by dense clouds of gas and dust that block visible light. Those dense clouds are the stuff that stars are born from, and this latest image is part of the effort to understand how massive stars form.
Thanks to the latest generation of sophisticated telescopes, astronomers are learning things a great deal about our Universe. The improved resolution and observational power of these instruments also allow astronomers to address previously unanswered questions. Many of these telescopes can be found in the Atacama Desert in Chile, where atmospheric interference is minimal and the cosmos can be seen with greater clarity.
It is here that the European Southern Observatory (ESO) maintains many observatories, not the least of which is the Paranal Observatory where the Very Large Telescope (VLT) resides. Recently, an international team of astronomers used the VLT to study the center of the Milky Way and observed evidence of ancient starbursts. These indicate that the central region of our galaxy experienced an intense period of star birth in the past.
Neutron stars are the end-state of massive stars that have spent their fuel and exploded as supernovae. There’s an upper limit to their mass, because a massive enough star won’t become a neutron star; it’ll become a black hole. But finding that upper mass limit, or tipping point, between a star that becomes a black hole and one that becomes a neutron star, is something astronomers are still working on.
Now a new discovery from astronomers using the National Science Foundation’s (NSF) Green Bank Telescope (GBT) have found the most massive neutron star yet, putting some solid data in place about the so-called tipping point.
The Orion Nebula is one of the most observed and photographed objects in the night sky. At a distance of 1350 light years away, it’s the closest active star-forming region to Earth.
This diffuse nebula is also known as M42, and has been studied intensely by astronomers for many years. From it, astronomers have learned a lot about star formation, planetary system formation, and other bedrock topics in astronomy and astrophysics. Now a new discovery has been made which goes against the grain of established theory: stellar winds from newly-formed massive stars may prevent other stars from forming in their vicinity. They also play a much larger role in star formation, and in galaxy evolution, than previously thought.
It’s relatively easy for galaxies to make stars. Start out with a bunch of random blobs of gas and dust. Typically those blobs will be pretty warm. To turn them into stars, you have to cool them off. By dumping all their heat in the form of radiation, they can compress. Dump more heat, compress more. Repeat for a million years or so.
Eventually pieces of the gas cloud shrink and shrink, compressing themselves into a tight little knots. If the densities inside those knots get high enough, they trigger nuclear fusion and voila: stars are born.
Imagine yourself in a boat on a great ocean, the water stretching to the distant horizon, with the faintest hints of land just beyond that. It’s morning, just before dawn, and a dense fog has settled along the coast. As the chill grips you on your early watch, you catch out of the corner of your eye a lighthouse, feebly flickering through the fog.
We need to talk about the dark ages. No, not those dark ages after the fall of the western Roman Empire. The cosmological dark ages. The time in our universe, billions of years ago, before the formation of the first stars. And we need to talk about the cosmic dawn: the birth of those first stars, a tumultuous epoch that completely reshaped the face the cosmos into its modern form.
Those first stars may have been completely unlike anything we see in the present universe. And we may, if we’re lucky, be on the cusp of seeing them for the first time.
In the 1970s, astronomers discovered that a particularly large black hole (Sagittarius A*) existed at the center of our galaxy. In time, they came to understand that similar Supermassive Black Holes (SMBHs) existed in the center of most massive galaxies. The presence of these black holes was also what differentiated galaxies that had particularly luminous cores – aka. Active Galactic Nuclei (AGN) – from those that didn’t.
Since that time, astronomers and cosmologists have pondered what role SMBHs have on galactic evolution, with some venturing that they have a profound impact on star formation. And thanks to a recent study by an international team of astronomers, there is now direct evidence for a correlation between and SMBH and a galaxy’s star formation. In fact, the team demonstrated that a black hole’s mass could determine when star formation in a galaxy will end.
For the sake of their study, the team relied on data gathered the Hobby-Eberle Telescope Massive Galaxy Survey in 2015. This systematic survey used the 10m Hobby-Eberly Telescope (HET) at the McDonald Observatory to conduct an optical long-slit spectroscopic survey of over 1000 galaxies. This survey not only provided spectra for these galaxies, but also produced direct mass measurements of the central black holes for 74 of these galaxies.
Using this data, Martín-Navarro and his colleagues found the first observational evidence for a direct correlation between the mass of a galaxy’s central black hole and its history of star formation. While astrophysicists have been operating under this assumption for decades, the proof was missing until now. As Jean Brodie, professor of astronomy and astrophysics at UC Santa Cruz and a coauthor of the paper, said in a UCSC press release:
“We’ve been dialing in the feedback to make the simulations work out, without really knowing how it happens. This is the first direct observational evidence where we can see the effect of the black hole on the star formation history of the galaxy.”
Roughly 15 years ago, the correlation between a SMBHs mass and the total mass of a galaxy’s stars was discovered, which led to a major unresolved question in astrophysical circles. While this correlation appeared to be a central feature of galaxies, it was unclear as to what could have caused it. How could the mass of a comparatively small and central black hole be related to the mass of billions of stars distributed throughout a galaxy?
One possible explanation was that more massive galaxies collected larger amounts of gas, thus resulting in more stars and a more massive central black hole. However, astrophysicists also believed their was a feedback mechanism at work, where growing black holes inhibited the formation of stars in their vicinity. In short, when matter accretes on a central black hole, it sends out a tremendous amount of energy in the form of radiation and particle jets.
If this energy is transferred to gas and dust surrounding the core of the galaxy, stars will be less likely to form in this region since gas and dust need to be cold in order to undergo areas of collapse. For years, feedback of this kind has been included in cosmological simulations to explain the observed star-formation rates in galaxies. According to these same simulations, minus this mechanism, galaxies would form far more stars than have been observed.
However, no direct evidence of this phenomena had previously been available. The first step to obtaining some was to reproduce the stellar formation histories of the 74 target galaxies used for the study. Martín-Navarro and his colleagues did this by subjecting spectra obtained from each of these galaxies to computational techniques that looked for the best combination of stellar populations to fit the data.
In so doing, the team was able to reconstruct the history of star formation within the target galaxies for the past 12.5 billion years. After examining these histories, they noticed some predictable results, but also some rather significant differences. For starters, as predicted, the regions of around the galaxies’ central black holes demonstrated a clear dampening influence on the rate of star formation.
As predicted, there was also a clear correlation between the mass of the central black holes and stellar mass in these galaxies. However, the team also noted that in cases where stellar mass was slightly smaller than expected (relative to the mass of their central black holes), star formation rates were lower. In some other cases, galaxies had larger-than-expected stellar masses (again, relative to their black holes) and their star formation rates were higher.
This correlation was not only more consistent than that observed between black hole mass and stellar mass, it occurred independently of other factors (such as shape or density). As Martín-Navaro explained:
“For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. So star formation lasted longer in those galaxies with smaller central black holes.”
They also noted that this correlation extends into the deep past, where the galaxies with supermassive central black holes have been consistently producing a comparatively low rate of stars for the past 12.5 billion years. This constitutes the first strong evidence for a direct, long-term connection between star formation and the existence of a central black hole in a galaxy.
Another interesting takeaway from the study was the way it addressed possible correlations between AGN luminosity and star formation. In the past, other researchers have sought to find evidence of a link between the two, but without success. According to Martín-Navarro and his team, this may be because the time scales are incredibly different. Whereas star formation occurs over the course of eons, outbursts from AGNs occur over shorter intervals.
What’s more, AGNs are highly variable and their properties are dependent on a number of factors relating to their black holes – i.e. size, mass, rate of accretion, etc. “We used black hole mass as a proxy for the energy put into the galaxy by the AGN, because accretion onto more massive black holes leads to more energetic feedback from active galactic nuclei, which would quench star formation faster,” said Martin-Navarro.
Looking ahead, the team hopes to conduct further research and determine exactly how central black holes arrest star formation. At present, the possibility that it could be due to radiation or jets of gas heating up surrounding matter are not definitive. As Aaron Romanowsky, an astronomer at San Jose State University and UC Observatories, indicated:
“There are different ways a black hole can put energy out into the galaxy, and theorists have all kinds of ideas about how quenching happens, but there’s more work to be done to fit these new observations into the models.”
Part of determining how the Universe came to be is knowing what mechanisms were at play and the extent of their roles. With this latest study, astrophysicists and cosmologists can take comfort in the knowledge that they’ve been getting it right – at least in this case!
I didn’t notice it with the naked eye, but as soon as the time exposure ended and I looked at the camera’s back display, there it was — Saturn riding barebacked on the Galactic Dark Horse! The horse, more of a prancing pony, is a collection of dark nebulae in the southern sky beautifully placed for viewing on late June evenings. The Dark Horse is part of the Great Rift, a dark gap that splits the band of the Milky Way in half, starting at the Northern Cross and extending all the way down to the “Teapot” of Sagittarius in the south.
While appearing to be little more than empty, starless space, in reality the Rift consists of enormous clouds of cosmic dust and gas in the plane of the galaxy called dark nebulae that blot out the light of more distant stars. If you could suck it all up with a monster vacuum cleaner and expose the billions of stars otherwise hidden, the Milky Way would cast obvious shadows — even suburban skywatchers would routinely see it.
Tiny dust particles spewed by older, evolved stars and exploding supernovas have been settling in the plane of the galaxy since its birth 13.2 billion years ago. While the dust is sparse, it adds up over the light years to form a thick, dark band silhouetted against the more distant stars. Gravity has been at work on the dust since the earliest days, compressing the denser clumps into new stars and star clusters. But much raw material remains. Within the curdles of dark nebulae, astronomers use dust-penetrating infrared and radio telescopes to watch new stars in the process of incubation.
There are more obvious parts of the Rift to the naked eye but few conjure up as striking an image as the Dark Horse, located about one outstretched fist to the left of the Scorpius’ brightest star, Antares. Saturn sits astride the horse’s back or eastern side. While it’s fun to see the horse as a single figure, astronomers catalog the various body parts as individual dark nebulae with separate numbers and even names. The largest part of the horse, the hind leg, is nicknamed the Pipe Nebula and lies 600-700 light years away. The Pipe is further subdivided into B59, B72, B77 and B78, from a survey of dark nebulae by early 20th century American astronomer E.E. Barnard.
While the dark horse shows up well in time-exposure photos, you’ll need dark, rural skies to view it with the naked eye. It’s only a couple fists high for those of us living in the northern U.S. and southern Canada, but considerably higher up from the southern states and points south. The figure is large but faint, about 10° long by 7° wide, and stands due south and highest in the sky around 12:30 a.m. in late June. Allow your eyes time to fully dark adapt beforehand. Try for the dark rump and hind leg first then work from there to fill in the rest of the horse.
Once I knew what to look for, I could fleetingly see the entire horse with its various protrusions as a subtle darkness against the brighter Milky Way. Averted vision, the technique of playing your eye around the subject rather than staring directly at it, helped make it happen. Wide-field binoculars will show it easily and in greater detail against a fabulously rich star field.
The best time to horse around under the Milky Way happens from now till the end of the month, when the bright Moon sends the critter into hiding.