# Could a dwarf galaxy host a star at the center instead of a SMBH?

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I don't think this would be possible, but I'm curious, and as what ever query I googled I only found posts about black holes indeed existing, what black holes are, that even the Milky Way has a black hole in its center or that We can't know if dwarf galaxies have non-SMBH at their center.

So sorry if it is a stupid question but I didn't want to go on with assuming something without knowing: So is it theoretically possible for a Galaxy to have a star at its gravitational center of mass?

A galaxy's center of gravity is not determined by the most massive object, but by all objects in the galaxy. Even supermassive black holes (SMBHs) do not dominate the gravitational field except very, very close to the center. By far, most of the stars in a galaxy couldn't care less about the SMBH.

The region within which a BH dominates over that of the stars (the "sphere of influence"$^dagger$) is given by (e.g. Peebles 1972) $$r = frac{G M_mathrm{BH}}{sigma^2},$$ where $G$ is the gravitational constant, $M_mathrm{BH}$ is the mass of the black hole, and $sigma$ is the velocity dispersion.

In the Milky Way (MW), there's a SMBH (Sagittarius A*) of roughly $M_mathrm{BH} simeq 4 imes10^6,M_odot$. In that region, the stellar velocity dispersion is roughly $50$-$100,mathrm{km},mathrm{s}^{-1}$ (e.g. Genzel et al. 2010). Plugging in those values, you'll find that Sgr A* dominates the kinematic out to roughly 3 pc, or 10 lightyears, which is nothing compared to MW's radius of $sim10^5$ lightyears.

If you take the most massive conceivable stars (a hypothesized Pop III star of $Msim10^3,M_odot$) in Willman 1, the smallest known dwarf galaxy - which has stellar velocity dispersion of the order $5$-$10,mathrm{km},mathrm{s}^{-1}$ - you'll find that such a star will dominate the gravitational potential out to a distance of only $lesssim0.1,mathrm{pc}$, again completely negligible compared to the galaxy's radius of $sim25,mathrm{pc}$.

In other words, although it's possible for a dwarf galaxy to host a massive star, it will just be a star like all the others, and will in no way define the galaxy's gravitational center.

$^dagger$Not to be confused with the event horizon which is even smaller.

## A Cosmic Mystery: Disappearance of a Massive Star Surprises Astronomers

Using the European Southern Observatory’s Very Large Telescope (VLT), astronomers have discovered the absence of an unstable massive star in a dwarf galaxy. Scientists think this could indicate that the star became less bright and partially obscured by dust. An alternative explanation is that the star collapsed into a black hole without producing a supernova. “If true,” says team leader and PhD student Andrew Allan of Trinity College Dublin, Ireland, “this would be the first direct detection of such a monster star ending its life in this manner.”

Between 2001 and 2011, various teams of astronomers studied the mysterious massive star, located in the Kinman Dwarf galaxy, and their observations indicated it was in a late stage of its evolution. Allan and his collaborators in Ireland, Chile and the US wanted to find out more about how very massive stars end their lives, and the object in the Kinman Dwarf seemed like the perfect target. But when they pointed ESO’s VLT to the distant galaxy in 2019, they could no longer find the telltale signatures of the star. “Instead, we were surprised to find out that the star had disappeared!” says Allan, who led a study of the star published today in Monthly Notices of the Royal Astronomical Society.

Astronomers have discovered the absence of an unstable massive star in a dwarf galaxy with ESO’s Very Large Telescope. This video offers a summary of the research.

Located some 75 million light-years away in the constellation of Aquarius, the Kinman Dwarf galaxy is too far away for astronomers to see its individual stars, but they can detect the signatures of some of them. From 2001 to 2011, the light from the galaxy consistently showed evidence that it hosted a ‘luminous blue variable’ star some 2.5 million times brighter than the Sun. Stars of this type are unstable, showing occasional dramatic shifts in their spectra and brightness. Even with those shifts, luminous blue variables leave specific traces scientists can identify, but they were absent from the data the team collected in 2019, leaving them to wonder what had happened to the star. “It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion,” says Allan.

Image of the Kinman Dwarf galaxy, also known as PHL 293B, taken with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 in 2011, before the disappearance of the massive star. Located some 75 million light-years away, the galaxy is too far away for astronomers to clearly resolve its individual stars, but in observations done between 2001 and 2011, they detected the signatures of the massive star. These signatures were not present in more recent data. Credit: NASA, ESA/Hubble, J. Andrews (U. Arizona)

The group first turned the ESPRESSO instrument toward the star in August 2019, using the VLT’s four 8-meter telescopes simultaneously. But they were unable to find the signs that previously pointed to the presence of the luminous star. A few months later, the group tried the X-shooter instrument, also on ESO’s VLT, and again found no traces of the star.

We may have detected one of the most massive stars of the local Universe going gently into the night,” says team-member Jose Groh, also of Trinity College Dublin. “Our discovery would not have been made without using the powerful ESO 8-meter telescopes, their unique instrumentation, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO.” Ireland became an ESO member state in September 2018.

This animation shows what the luminous blue variable star in the Kinman Dwarf galaxy could have looked like before its mysterious disappearance.

The team then turned to older data collected using X-shooter and the UVES instrument on ESO’s VLT, located in the Chilean Atacama Desert, and telescopes elsewhere.“The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009,” says Andrea Mehner, a staff astronomer at ESO in Chile who participated in the study. “The comparison of the 2002 high-resolution UVES spectra with our observations obtained in 2019 with ESO’s newest high-resolution spectrograph ESPRESSO was especially revealing, from both an astronomical and an instrumentation point of view.”

This wide-field view shows the region of the sky, in the constellation of Aquarius, where the Kinman Dwarf galaxy can be found. This view was created from images forming part of the Digitized Sky Survey 2. Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin

The old data indicated that the star in the Kinman Dwarf could have been undergoing a strong outburst period that likely ended sometime after 2011. Luminous blue variable stars such as this one are prone to experiencing giant outbursts over the course of their life, causing the stars’ rate of mass loss to spike and their luminosity to increase dramatically.

This video starts by showing a wide-field view of a region of the sky in the constellation of Aquarius. It then zooms in to show the Kinman Dwarf galaxy, where a mysterious luminous blue variable star disappeared. The end of the video shows an artistic animation of what the star could have looked like before it disappeared.

Based on their observations and models, the astronomers have suggested two explanations for the star’s disappearance and lack of a supernova, related to this possible outburst. The outburst may have resulted in the luminous blue variable being transformed into a less luminous star, which could also be partly hidden by dust. Alternatively, the team says the star may have collapsed into a black hole, without producing a supernova explosion. This would be a rare event: our current understanding of how massive stars die points to most of them ending their lives in a supernova.

Future studies are needed to confirm what fate befell this star. Planned to begin operations in 2025, ESO’s Extremely Large Telescope (ELT) will be capable of resolving stars in distant galaxies such as the Kinman Dwarf, helping to solve cosmic mysteries such as this one.

This chart shows the location of the Kinman Dwarf galaxy, where a mysterious luminous blue variable star disappeared. This map shows most of the stars visible to the unaided eye under good conditions and the system itself is marked with a red circle. Credit: ESO, IAU and Sky & Telescope

Reference: “The possible disappearance of a massive star in the low metallicity galaxy PHL 293B”by Andrew P. Allan, Jose H. Groh, Andrea Mehner, Nathan Smith, Ioana Boian, Eoin J. Farrell and Jennifer E. Andrews, 30 June 2020, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/staa1629

The team is composed of Andrew Allan (School of Physics, Trinity College Dublin, Ireland [TCD]), Jose J. Groh (TCD), Andrea Mehner (European Southern Observatory, Chile), Nathan Smith (Steward Observatory, University of Arizona, USA [Steward Observatory]), Ioanna Boian (TCD), Eoin Farrell (TCD), Jennifer E. Andrews (Steward Observatory).

ESO is the foremost intergovernmental astronomy organization in Europe and the world’s most productive ground-based astronomical observatory by far. It has 16 Member States: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and with Australia as a Strategic Partner. ESO carries out an ambitious program focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organizing cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-meter Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky.”

## Dwarf Star –“Hosts First Nearby Super-Earth that Could Harbor Life”

“This is exciting, as this is humanity’s first nearby super-Earth that could harbor life – uncovered with help from TESS, our small, mighty mission with a huge reach.” said Lisa Kaltenegger, director of Cornell’s Carl Sagan Institute.

NASA’s Transiting Exoplanet Survey Satellite (TESS), a mission designed to comb the heavens for exoplanets, has discovered its first potentially habitable world outside of our own solar system – and an international team of astronomers has characterized the super-Earth, about 31 light-years away.

In a new paper in the The Astrophysical Journal Letters, a team led by Kaltenegger, associate professor of astronomy, models the conditions under which the planet — discovered in early 2019 — could sustain life.

As this super-Earth exoplanet is more massive than our own blue planet, Kaltenegger said this discovery will provide insight into Earth’s heavyweight planetary cousins. “With a thick atmosphere, the planet GJ 357 d could maintain liquid water on its surface like Earth and we could pick out signs of life with upcoming telescopes soon to be online,” she said.

Astronomers from the Institute of Astrophysics of the Canary Islands and the University of La Laguna, both of Spain, announced the discovery of the system July 31 in the journal Astronomy & Astrophysics. They showed that the distant solar system – with a diminutive M-type dwarf sun, about one-third the size of our own sun – harbors three planets, with one of those in that system’s habitable zone: GJ 357 d.

Last February, the TESS satellite observed that the dwarf sun GJ 357 dimmed very slightly every 3.9 days, evidence of a transiting planet moving across the star’s face. That planet was GJ 357 b, a so-called “hot Earth” about 22 percent larger than Earth, according to the NASA Goddard Space Flight Center, which guides TESS.

Follow up observations from the ground led to the discovery of two more exoplanetary siblings: GJ 357 c and GJ 357 d. The international team of scientists collected Earth-based telescopic data going back two decades – to reveal the newly found exoplanets’ tiny gravitational tugs on its host star, according to NASA.

Exoplanet GJ 357 c sizzles at 260 degrees Fahrenheit, and has at least 3.4 times Earth’s mass. However, the system’s outermost known sibling planet – GJ 357 d, a super-Earth – could provide conditions just like on Earth and orbits the dwarf star every 55.7 days at a distance about 20 percent of Earth’s distance from the Sun. It is not yet known if this planet transits its sun.

Kaltenegger, doctoral candidate Jack Madden and undergraduate student Zifan Lin simulated light fingerprints, climates and remotely detectable spectra for the planet, which could range from a rocky composition to a water world.

Madden explained that investigating new discoveries provides an opportunity to test theories and models. “We built the first models of what this new world could be like,” he said. “Just knowing that liquid water can exist on the surface of this planet motivates scientists to find ways of detecting signs of life.”

## Star Clusters Could Be Galaxy Remnants

Globular star clusters are like spherical cathedrals of light – collections of millions of stars lumped into a space only a few dozen light-years across. If the Earth resided within a globular cluster, our night sky would be alight with thousands of stars more brilliant than Sirius.

Our own Milky Way Galaxy currently holds about 200 globular clusters, but once possessed many more. According to the hierarchical theory of galaxy formation, galaxies have grown larger over time by consuming smaller dwarf galaxies and star clusters. And sometimes, it seems that the unfortunate prey is not swallowed whole but instead is munched like a peach, stripped of its outer layers to leave behind only the pit. New research by Paul Martini (Harvard-Smithsonian Center for Astrophysics) and Luis Ho (Observatories of the Carnegie Institution of Washington) shows that some globular clusters may be remnants of dwarf galaxies that were stripped of their outer stars, leaving only the galaxy’s nucleus behind.

Martini and Ho reported their results in the July 20, 2004, issue of The Astrophysical Journal.

Their findings hint at an important yet puzzling connection between the largest globular clusters and the smallest dwarf galaxies. “Star clusters and galaxies are quite different from a structural standpoint – star clusters are much more centrally concentrated, for example – and so the mechanisms that form them must be quite different. Identification of star clusters in the same mass range as galaxies is a very important step toward understanding how both types of objects form,” says Martini.

For their investigation, the team studied 14 globular clusters in the large elliptical galaxy Centaurus A (NGC 5128) using the 6.5-meter-diameter Magellan Clay telescope at Carnegie’s Las Campanas Observatory, Chile. The clusters were selected for their brightness, and since brighter clusters tend to contain more stars and more mass, were expected to be massive. Yet their results surprised even Martini and Ho, showing that the globular clusters of Centaurus A are much more massive than most globulars in the Local Group of galaxies (which includes the Milky Way and the Andromeda Galaxy).

“The essence of our findings is that these 14 globulars are 10 times more massive than the smaller globulars in our neighborhood, and whatever process makes them can produce some really huge objects – they begin to overlap with the smallest galaxies,” says Martini.

Martini also points out the recent discovery of a suspected intermediate-mass black hole in the Andromeda Galaxy globular cluster known as G1, which offers further evidence linking globular clusters to dwarf galaxies. The presence of a moderate-sized black hole is more understandable if it once occupied the center of a dwarf galaxy – a galaxy that lost its outer stars to the pull of Andromeda, leaving it only a shadow of its former self.

Ho, a co-discoverer of the intermediate-mass black hole in G1, adds, “One of the most surprising findings is that the black hole in G1 obeys the same tight correlation between black hole mass and host galaxy mass that has been well established for supermassive black holes in the centers of big galaxies. This puzzling result is more understandable if G1 was once the nucleus of a larger galaxy. A very interesting question is whether some of the massive clusters in Centaurus A also contain central black holes.”

Although most of our Galaxy’s globular clusters are much smaller than those of Centaurus A, the largest Milky Way globulars (such as the omega Centauri star cluster) rival those of the elliptical galaxy. The similarities between massive globulars in both galaxies may point to similar formation mechanisms. Future studies of these most massive globular clusters will explore connections between the formation processes for star clusters and galaxies.

Centaurus A is located approximately 12.5 million light-years away. It is about 65,000 light-years across and is more massive than the Milky Way and Andromeda galaxies put together. Centaurus A possesses a total of about 2000 globular clusters – more than all of the galaxies in the Local Group combined. Recent Spitzer Space Telescope observations of Centaurus A uncovered evidence that it merged with a spiral galaxy about 200 million years ago.

## “An Incredible Mystery” –Colossal Supernova Obscured the Glow of Entire Galaxies

“A billion years ago, something in the whirling darkness of space erupted with a fury that obscured the glow of entire galaxies,” reports Robin Andrews in Quanta after scientists at the Harvard-Smithsonian Center for Astrophysics announced the discovery of the most massive star ever known to be destroyed by a supernova explosion that could rewrite the physics of stars, The event challenges existing models of how massive stars die, providing insight into the death of the first stars in the universe

“Eventually”, continued Andrews , “the light from that cataclysm reached Earth, and in November 2016, it was captured by a group of intrepid humans at the European Space Agency’s Gaia Satellite . They found that the conflagration wasn’t just unfathomably energetic, but, like a lonely bonfire, it kept on burning, dimming so slowly that its glow can still be seen years after it began.”

A Unique Beast –In a Previously Uncharted Galaxy

Three years of intensive follow up observations of the supernova SN2016iet revealed characteristics—incredibly long duration and large energy, unusual chemical fingerprints, and an environment poor in metals—for which there are no analogues in the existing astronomical literature.

“When we first realized how thoroughly unusual SN2016iet is, my reaction was ‘whoa – did something go horribly wrong with our data?'” said Sebastian Gomez , Harvard University graduate student and lead author of the paper. “After a while we determined that SN2016iet is an incredible mystery, located in a previously uncatalogued galaxy one billion light years from Earth.”

The team used a variety of telescopes, including the CfA | Harvard & Smithsonian’s MMT Observatory located at the Fred Lawrence Whipple Observatory in Amado, AZ, and the Magellan Telescopes at the Las Campanas Observatory in Chile to show that SN2016iet is different than the thousands of supernovas observed by scientists for decades.

“Everything about this supernova looks different—its change in brightness with time, its spectrum, the galaxy it is located in, and even where it’s located within its galaxy, said Dr. Edo Berger, Professor of Astronomy at Harvard University and an author on the paper. “We sometimes see supernovas that are unusual in one respect, but otherwise are normal this one is unique in every possible way.”

Death of a Colossal Star 200 Xs Mass of Our Sun

The observations and analysis show that SN2016iet began as an incredibly massive star 200 times the mass of Earth’s Sun that mysteriously formed in isolation roughly 54,000 light years from the center of its host dwarf galaxy. The star lost about 85 percent of its mass during a short life of only a few million years, all the way up to its final violent collapse when runaway thermonuclear reactions occur and the star explodes. The collision of the explosion-debris with the material shed in the final decade before explosion led to SN2016iet’s unusual appearance, providing scientists with the first strong case of a pair-instability supernova — a supernova where pairs of matter and antimatter particles (namely electrons and positrons) are produced.

A Pair-instability Supernova

“The idea of pair-instability supernovas has been around for decades,” said Berger. “But finally having the first observational example that puts a dying star in the right regime of mass, with the right behavior, and in a metal-poor dwarf galaxy is an incredible step forward. SN2016iet represents the way in which the most massive stars in the universe, including the first stars, die.”

The team will continue to observe and study SN2016iet for years, watching for additional clues as to how it formed, and how it will evolve. “Most supernovas fade away and become invisible against the glare of their host galaxies within a few months. But because SN2016iet is so bright and so isolated we can study its evolution for years to come,” said Gomez. “These observations are already in progress and we can’t wait to see what other surprises this supernova has in store for us.”

The Daily Galaxy, Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via Harvard-Smithsonian CfA , Quanta , Astrophysical Journal . Avi was formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL).

image at top of page: shows the supermassive star Eta Carinae, destined to explode as a massive supernova. Before its demise, it is kicking off material, much like SN2016iet did previous to its eruption. The “bells” recorded in this image from the Hubble Space Telescope, were first seen in 1840, and may be the result of a collision with another star. This body sits 7,500 light years from Earth. Image credit: NASA, ESA, N. Smith (University of Arizona) and J. Morse (BoldlyGo Institute).

Your free twice-weekly fix of stories of space and science –a random journey from Planet Earth through the Cosmos– that has the capacity to provide clues to our existence and add a much needed cosmic perspective in our Anthropocene epoch.

## Astronomers Discover How Lowly Dwarf Galaxy Gets Concentrated Star Power

An international team of astronomers [1] using the Atacama Large Millimeter/submillimeter Array (ALMA) has discovered an unexpected population of compact interstellar clouds hidden within the nearby dwarf irregular galaxy [2] Wolf—Lundmark—Melotte, more commonly known as WLM.

These clouds, which are nestled within a heavy blanket of interstellar material, help explain how dense star clusters [3] are able to form in the tenuous environs of a galaxy thousands of times smaller and far more diffuse than our own Milky Way.

“For many reasons, dwarf irregular galaxies like WLM are poorly equipped to form star clusters,” noted Monica Rubio, an astronomer with the University of Chile and lead author on a paper to appear in the scientific journal Nature. “These galaxies are fluffy with very low densities. They also lack the heavy elements that contribute to star formation. Such galaxies should only form dispersed stars rather than concentrated clusters, but that is clearly not the case.”

By studying this galaxy with ALMA, the astronomers were able to locate, for the first time, compact regions that appear able to emulate the nurturing environments found in larger galaxies.

These regions were discovered by pinpointing the almost imperceptible and highly localized millimeter wavelength light emitted by carbon monoxide (CO) molecules, which are typically associated with star-forming interstellar clouds.

Earlier, an affiliated team of astronomers led by Deidre Hunter at the Lowell Observatory in Flagstaff, Ariz., first detected CO in the WLM galaxy with the single-dish Atacama Pathfinder Experiment (APEX) telescope [4]. These initial, low-resolution observations could not resolve where the molecules reside, but they did confirm that WLM contains the lowest abundance of CO ever detected in any galaxy. This lack of CO and other heavy elements should put a serious damper on star formation, the astronomers note.

“Molecules, and carbon monoxide in particular, play an important role in star formation,” said Rubio. “As gas clouds begin to collapse, temperatures and densities rise, pushing back against gravity. That’s where these molecules and dust particles come to the rescue by absorbing some of the heat through collisions and radiating it into space at infrared and submillimeter wavelengths.” This cooling effect enables gravity to continue the collapse until a star forms.

The problem previously was that in WLM and similar galaxies with very low abundances of heavy elements, astronomers simply didn’t see enough of this material to account for the new star clusters they observed.

The reason the CO was initially so difficult to see, the researchers discovered, is that unlike in normal galaxies, the WLM clouds are very tiny compared to their overlying envelopes of molecular and atomic gas.

To become viable star factories, the concentrated CO clouds need these enormous envelopes of transitional gas to bear down on them, giving the cores of CO a high enough density to allow them to form a normal cluster of stars.

“Like a diver being squeezed at the bottom of a deep abyss, these bundles of star-forming gas are under tremendous pressure, even though the surrounding ocean of interstellar gas is much more shallow,” said Bruce Elmegreen, a co-author on the paper and researcher at the IBM T.J. Watson Research Center in Yorktown Heights, N.Y. “By discovering that the carbon monoxide is confined to highly concentrated regions within a vast expanse of transitional gas, we could finally understand the mechanisms that led to the impressive stellar neighborhoods we see in the galaxy today.”

Further studies with ALMA will also help determine the conditions that formed the globular clusters found in the halo of the Milky Way. Astronomers believe these much larger clusters may have originally formed in dwarf galaxies and later migrated to the halo after their host dwarf galaxies dispersed.

WLM is a relatively isolated dwarf galaxy located approximately 3 million light-years away on the outer edges of the Local Group: the collection of galaxies that includes the Milky Way, the Magellanic Clouds, Andromeda, M33, and dozens of smaller galaxies.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

[1] Collaborators in the present study include Monica Rubio, Universidad de Chile, Santiago Bruce G. Elmegreen, IBM T.J. Watson Research Center, Yorktown Heights, N.Y. Deidre A. Hunter, Lowell Observatory, Flagstaff, Ariz Elias Brinks, University of Hertfordshire, UK Juan R. Cortes, Joint ALMA Observatory and National Radio Astronomy Observatory, Santiago, Chile and Phil Cigan, New Mexico Institute of Mining and Technology, Socorro.

[2] Irregular galaxies lack the distinctive shapes of spiral and elliptical galaxies. Dwarf irregulars, like WLM, are hundreds of times smaller than the larger variety and contain only a few hundred million stars instead of tens of billions. Though small, some are now known to harbor massive black holes at their centers.

[3] Star clusters, like the Pleiades found in our own Milky Way galaxy, are made up of hundreds of stars. Others, like globular clusters, can contain hundreds of thousands to a few million stars. Though many stars in the Milky Way originally form in clusters, some – like the Sun – drift away from their stellar nurseries and move freely throughout their home galaxy. Stars in the largest and densest clusters, like those observed in WLM, remain relatively close together.

[4] The APEX team was led by Deidre Hunter at the Lowell Observatory in Flagstaff, Ariz., and Elias Brinks at the University of Hertfordshire, U.K. It also included Monica Rubio Bruce Elmegreen Andreas Schruba, California Institute of Technology, Pasadena, Calif. and Celia Verdugo, University of Chile.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

## Rare Black Hole Survives Galaxy's Destruction

Like a fossil hinting at a long-gone animal, a black hole is offering clues about a now-destroyed galaxy that may once have existed around it.

The Hubble Space Telescope recently spied a cluster of young blue stars surrounding a rare mid-weight black hole that suggests the black hole was once at the center of a dwarf galaxy. Astronomers think this galaxy was torn apart by the gravity of a larger host galaxy that it orbited.

The violent encounter would have stripped away most of the dwarf galaxy's stars, but it also could have compressed the gas around its central black hole, triggering a new wave of star formation. It is these new stars that Hubble recently saw signs of.

The observations suggest that the young stars must be less than 200 million years old, meaning the collision between the parent galaxy and its dwarf likely occurred around that time.

"The fact that there's a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very low-mass dwarf galaxy,"Sean Farrell, of the Sydney Institute for Astronomy in Australia, said in a statement. "The dwarf galaxy was then swallowed by the more massive galaxy." [When Galaxies Collide: Photos of Great Galactic Crashes]

Besides revealing clues about the lost galaxy, the black hole itself, called HLX-1 (Hyper-Luminous X-ray source 1), is scientifically interesting, researchers said.

When Farrell and his colleagues discovered HLX-1in 2009, it was the first intermediate-mass black hole known. Scientists think it may represent a class of middleweight black holes that are the building blocks for the supermassive black holes lurking at the center of most large galaxies, including our own Milky Way.

"This black hole is unique in that it's the only intermediate-mass black hole we've found so far. Its rarity suggests that these black holes are only visible for a short time," said Mathieu Servillat, a member of the research team who conducted his work at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

This specimen contains the mass of about 20,000 suns, and is located roughly 290 million light-years from Earth. In comparison, the supermassive black hole at the center of the Milky Way is as massive as 4 million suns.

By studying this rare middleweight black hole, scientists hope to learn more about how they, and their larger supermassive brethren, form.

"For the first time, we have evidence on the environment, and thus the origin, of this middle-weight black hole," Servillat said.

The researchers report their findings in the Feb. 15 issue of the Astrophysical Journal Letters.

## 50% calcium, 100% unique

Many images of supernovae show face-on galaxies, with the explosion a bright point of light sprinkled in somewhere. We can’t get photographs of SN 2005E that look anything like this, however, because NGC 1032 appears edge-on to us, presenting only a side view. In the initial Lick Observatory images — taken using the Katzman Automatic Imaging Telescope (KAIT) — the supernova appears only as a dot relatively far from the host galaxy, 22.9 kpc radially from the center and a surprising 11.3 kpc above the disk. If we didn’t have earlier images of NGC 1032 from the Sloan Digital Sky Survey (SDSS), showing only a blank space where the supernova appeared, it might be mistaken for a field star, or a separate background galaxy.

Nonetheless, follow-up observations and spectroscopy confirmed that SN 2005E was a supernova. The spectra showed no hydrogen lines, ruling out a Type II supernova, but also lacked the silicon features of a Type Ia supernova, initially leading the team to classify it as a Type Ib supernova, with a massive star that had been stripped of its hydrogen envelope as a progenitor. However, the spectra also indicated that only about 0.3 solar masses had been ejected, which seemed to imply that a massive star couldn’t have been responsible. Additionally, there was no star formation anywhere nearby.

This presented a puzzle. The mass of the ejecta was also too low for a normal Type Ia supernova. Further study of it showed remarkably strong calcium lines, meaning that 40–50% of the ejecta, by mass, consisted of calcium. This had never been observed before, although some models of white dwarf-white dwarf systems predicted that it could happen through a detonation of donated helium on the surface of one of the components, with double-white dwarf progenitors required to explain the extreme low luminosity of SN 2005E, dim even compared to Type Ia supernovae.

One question remained: How did SN 2005E stray so far from NGC 1032? The most likely explanation was that the progenitor had been a hypervelocity star, originally ejected from the inner region of the galaxy after an encounter with a supermassive black hole (SMBH) or SMBH binary. The speeds needed to propel a massive star this far from the center before it exploded ranged from 300 km/s to 1600 km/s — not unreasonable. However, the rate at which such encounters happen for high-mass stars make this event unlikely to be seen by the telescope, and coupled with the observations, it remained unlikely that the progenitor was a single massive star, but instead a white dwarf binary system, similarly ejected from the galaxy’s center.

## Has a new dwarf galaxy been found hiding behind Andromeda?

One of the greatest challenges of astronomy is locating objects in space that are obscured by the light of nearby, brighter objects. In addition to making extra-solar planets very difficult to directly image, this problem also intrudes on surveys of the local Universe, where astronomers are unable to detect dwarf and isolated galaxies because of all the brighter ones surrounding them.

Because of this, astronomers are unable to do a full inventory of small galaxies in the volume of space surrounding the Milky Way (aka the local volume). However, thanks to the efforts of an amateur astronomer and an international team of scientists, a dwarf spheroidal galaxy was recently discovered lurking behind the Andromeda Galaxy. The discovery of this object, named Donatiello I, could help astronomers learn more about the process of galaxy formation.

The study which describes these findings recently appeared in the journal Astronomy & Astrophysics. The research team was led by David Martínez-Delgado of the Center for Astronomy of Heidelberg University, and included members from the Institute for Research in Fundamental Sciences (IPM) in Tehran, the National Institute for Astrophysics (INAF), the Instituto de Astrofísica de Canarias (IAC), the Special Astrophysical Observatory in Russia, Nuovo Orione, and multiple observatories and universities.

At present, the most widely accepted cosmological model (the Lambda-CDM model) predicts that there are a large number of small dark matter halos in the Local Volume, but it is unclear how many of them are associated with baryonic matter (i.e. star clusters and dwarf galaxies). As such, the ability to do a full inventory of dwarf and isolated galaxies would help resolve this question and allow astronomers to learn more about the history of galaxy formation.

As Dr. Martínez-Delgado told Universe Today via email:

Diagram showing the Lambda-CBR universe, from the Big Bang to the the current era. Credit: Alex Mittelmann/Coldcreation
“There is a discrepancy between the observed number of low mass systems in the Local Group and its surroundings and the predicted one in cosmological simulations. A complete census of dwarf galaxies is necessary to understand the actual origin of this problem, as for example, the ingredients and assumptions in the computation of cosmological simulations or the lack of deep observations needed to trace the lowest surface brightness system in the local universe. The census of dwarf galaxies can shed light on key questions on galaxy formation and evolution.”

Unfortunately, creating such an inventory presents many problems, not the least of which is the fact that no large scale, deep imaging surveys have been possible so far. Whereas surveys like the Sloan Digital Sky Survey (SDSS) or the PANoramic Survey Telescope And Rapid Response System (PanSTARRs) survey have been comprehensive, they have surface brightness limits (

24.5-25 magn/parcsec²) that are too high to detect fainter objects.

Another problem is the limits of radio surveys to detect these galaxies, like those being performed by the the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFALFA) survey. Using this method, astronomers look for hydrogen gas in the spectra lines of distant objects. But since low surface brightness galaxies have a negligible gas content, they do not appear in such surveys.

As Martínez-Delgado explained, this is why the only means to detect them at present is through ultra-deep imaging, which is well-suited to amateur astronomers:

“That is, the only way to find them is by means of ultra-deep imaging in wide areas of the sky. Amateur telescopes can obtain ultra-deep imaging of nearby galaxies or wide sky fields with surface brightness reaching surface brightness limit of 29 magn/arcsec2 or deeper. This provides new chance of discovery for amateurs in the field of galaxy evolution, a topic that was not feasible for amateurs one decade ago. The new generation of large scale surveys (e.g. LSST) will have a tremendous impact on this research topic too.

The Sloan Digital Sky Survey telescope stands out against the breathtaking backdrop of the Sacramento Mountains. Credit : SDSS, Fermilab Visual Media Services
It was one such amateur astronomer who was responsible for the discovery of the dwarf spheroidal galaxy – an Italian amateur astronomer named Giuseppe Donatiello. Using a 12.7-cm telescope, Donatiello captured a mosaic of deep images of the Andromeda galaxy in 2016, the purpose of which was to detect the stellar streams, satellites galaxies and diffuse streams that had been recently discovered around Andromeda.

It was as he was examining one of these images (on Sept. 3rd, 2016), Donatiello noted the presence of an object located one degree from the star Mirach (beta Andromedae). This object was then confirmed by Martínez-Delgado and his colleagues using archival images from the SDSS, and by follow-up observations made on Nov. 27th, 2016.

These were performing using the 3.58 meter Telescopio Nazionale Galileo (TNG) and the 10.4 meter Gran Telescopio Canarias (GTC), both of which are located at the IAC’s Roque de Los Muchachos Observatory on the island of La Palma, Spain.

This allowed the team to determine the dwarf galaxies distance from Earth, to resolve and study its stars, and confirm that it was indeed a dwarf galaxy in the proximity of the Local Group. This discovery not only demonstrated the effectiveness of ultra-deep surveys conducted by amateur astronomers, it also has immense significance when it comes to the study of low-luminosity dwarf galaxies.

“Donatiello I can be an isolated galaxy that stopped its star formation long time ago,” said Martínez-Delgado. “It is difficult to understand what mechanism was the responsible of this quenched star formation when there is not interaction with any massive host galaxy, like in the case of Donatiello I.”

Local Group of galaxies, including the massive members M31 (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies. Credit: Wikipedia Commons/Antonio Ciccolella
Because of this discovery, isolated dwarf galaxies could serve as laboratories to test theories of star formation in low-mass systems. This would be of immense use to cosmologists conducting simulations to better understand the history of star formation in the Local Group galaxies, which could help resolve the aforementioned discrepancies between observational astronomy and cosmology.

And as Martínez-Delgado concluded, the discovery of this dwarf galaxy also opens opportunities for further surveys in the region:

“Donatiello I could be one of the brightest members of a large population of isolated, unbound dwarf galaxies that remains undiscovered around the Local Group. A statistical comparison of these observations is extremely important to probe the predictions of state-of-the-art cosmological simulations. I think its discovery must encourage a more systematic survey of this kind of low surface brightness systems with modest instruments.”

This discovery is one of many that demonstrates how improvements in technology and data-sharing are allowing for new opportunities for amateur astronomers. Today, individuals with their own equipment, knowledge and access to scientific databases are able to contribute to the discovery process.

And with next-generation instruments looking farther into the cosmos and in greater detail, we are likely to see more objects that were previously undetectable. These and other discoveries will teach us much about how our Universe came to be

## Dwarf galaxy WLM becomes star-forming powerhouse

ALMA discovers an unexpected population of compact interstellar clouds inside the dwarf irregular galaxy WLM. These star-forming clouds provide the necessary nurturing environment to form star clusters. As seen in relation to an optical image of the galaxy taken with the Blanco 4-metre telescope, (box upper left) an overlaying blanket of hydrogen gas (red) imaged with NRAO’s VLA telescope provides the pressure necessary to concentrate molecules of carbon monoxide (yellow) as seen with ALMA. These regions correspond to dense cores capable of forming clusters like those found in the Milky Way and other large galaxies. Image credit: B. Saxton (NRAO/AUI/NSF) M. Rubio et al., Universidad de Chile, ALMA (NRAO/ESO/NAOJ) D. Hunter and A. Schruba, VLA (NRAO/AUI/NSF) P. Massey/Lowell Observatory and K. Olsen (NOAO/AURA/NSF). An international team of astronomers using the Atacama Large Millimeter/submillimetre Array (ALMA) has discovered an unexpected population of compact interstellar clouds hidden within the nearby dwarf irregular galaxy Wolf—Lundmark—Melotte, more commonly known as WLM.

These clouds, which are nestled within a heavy blanket of interstellar material, help explain how dense star clusters are able to form in the tenuous environs of a galaxy thousands of times smaller and far more diffuse than our own Milky Way.

“For many reasons, dwarf irregular galaxies like WLM are poorly equipped to form star clusters,” noted Monica Rubio, an astronomer with the University of Chile and lead author on a paper to appear in the scientific journal Nature. “These galaxies are fluffy with very low densities. They also lack the heavy elements that contribute to star formation. Such galaxies should only form dispersed stars rather than concentrated clusters, but that is clearly not the case.”

By studying this galaxy with ALMA, the astronomers were able to locate, for the first time, compact regions that appear able to emulate the nurturing environments found in larger galaxies.

These regions were discovered by pinpointing the almost imperceptible and highly localized millimeter wavelength light emitted by carbon monoxide (CO) molecules, which are typically associated with star-forming interstellar clouds.

Earlier, an affiliated team of astronomers led by Deidre Hunter at the Lowell Observatory in Flagstaff, Arizona, first detected CO in the WLM galaxy with the single-dish Atacama Pathfinder Experiment (APEX) telescope. These initial, low-resolution observations could not resolve where the molecules reside, but they did confirm that WLM contains the lowest abundance of CO ever detected in any galaxy. This lack of CO and other heavy elements should put a serious damper on star formation, the astronomers note.

“Molecules, and carbon monoxide in particular, play an important role in star formation,” said Rubio. “As gas clouds begin to collapse, temperatures and densities rise, pushing back against gravity. That’s where these molecules and dust particles come to the rescue by absorbing some of the heat through collisions and radiating it into space at infrared and submillimetre wavelengths.” This cooling effect enables gravity to continue the collapse until a star forms.

The problem previously was that in WLM and similar galaxies with very low abundances of heavy elements, astronomers simply didn’t see enough of this material to account for the new star clusters they observed.

The reason the CO was initially so difficult to see, the researchers discovered, is that unlike in normal galaxies, the WLM clouds are very tiny compared to their overlying envelopes of molecular and atomic gas.

To become viable star factories, the concentrated CO clouds need these enormous envelopes of transitional gas to bear down on them, giving the cores of CO a high enough density to allow them to form a normal cluster of stars.

“Like a diver being squeezed at the bottom of a deep abyss, these bundles of star-forming gas are under tremendous pressure, even though the surrounding ocean of interstellar gas is much more shallow,” said Bruce Elmegreen, a co-author on the paper and researcher at the IBM T.J. Watson Research Center in Yorktown Heights, N.Y. “By discovering that the carbon monoxide is confined to highly concentrated regions within a vast expanse of transitional gas, we could finally understand the mechanisms that led to the impressive stellar neighbourhoods we see in the galaxy today.”

Further studies with ALMA will also help determine the conditions that formed the globular clusters found in the halo of the Milky Way. Astronomers believe these much larger clusters may have originally formed in dwarf galaxies and later migrated to the halo after their host dwarf galaxies dispersed.

WLM is a relatively isolated dwarf galaxy located approximately 3 million light-years away on the outer edges of the Local Group: the collection of galaxies that includes the Milky Way, the Magellanic Clouds, Andromeda, M33, and dozens of smaller galaxies.