Galaxy Groups: laboratories to study galaxy evolution

The main motivation of this mini-workshop is to discuss ideas/results/projects in the field of galaxy groups (and all the physical processes related with this topic). This mini-workshop will be held in La Serena, in order to motivate the participation of researchers and students working in different chilean institutions.

Pubblicità

IAU 312: Star Clusters and Black Holes in Galaxies Across Cosmic Time

Black Holes, dense star clusters, and galactic nuclei move into the focus of cosmological galaxy formation and evolution. High-resolution observations as well as simulations, in our local environment as well as in the distant universe approach the central engine and its co-evolution with stars and gas around. Black Holes formed in astrophysical environments are expected gravitational wave sources.

Satellite galaxies and dwarfs in the Local Group

The local neighborhood surrounding the Milky Way and M31 is teeming with small satellite galaxies whose properties are of immense interest. In the past few years the number of dwarfs observed in the local group has nearly doubled, mostly due to increased sensitivity in observations. These observations have posed a number of challenges to the theoretical modeling of dwarf galaxies.

The dance of stars

The study of dense stellar systems has constantly been an active area of research since half a century. Such systems, which mainly refer to young massive star clusters, globular clusters and dwarf galaxies, play the role of building blocks of the structures in the Universe. This is why it is crucial to understand them. Dense systems of stars present enough challenges to basically all schools in astronomy and in all wavelengths. Young systems challenge an optical/infrared observer to extract the stellar ingredient, their motions and distributions by overcoming the dense crowding of stars. Older systems puzzle one with large over-populations of exotic objects of a wide variety – millisecond radio pulsars, “blue-straggler” stars and X-ray binaries.

Dwarf Galaxies as Cosmological Probes

Recent wide-field surveys have revolutionized our understanding of the Local Group of Galaxies. The Sloan Digital Sky Survey, in particular, has more than doubled the number of known dwarf galaxies orbiting the Milky Way and revealed a new population of ultrafaint dwarf satellites. At the same time, advances in computational cosmology have led to improved predictions for the properties of the smallest dark matter halos that host dwarf galaxies in the current paradigm of structure formation, the Lambda Cold Dark Matter model.

A failed and faintest galaxy ever detected provides new clues about the primordial Universe

Segue 1 is 50 times dimmer than the star cluster pictured above but is 1000 times more massive, meaning most of its mass must be made up of dark matter. (Credit: Sloan Digital Sky Survey)

Located some 75.000 light years from us, a galaxy known as Segue 1 has some unusual properties: it is the faintest galaxy ever detected. It is very small, containing only about 1.000 stars. And it has a rare chemical composition, with vanishingly small amounts of metallic elements present.

More at MIT: A “wimpy” dwarf fossil galaxy reveals new facts about early universe

arXiv: Segue 1: An Unevolved Fossil Galaxy from the Early Universe

Seeds of supermassive black holes are quite massive themselves

The galaxy NGC 4395 is shown here in infrared light, captured by NASA’s Spitzer Space Telescope. This dwarf galaxy is relatively small in comparison with our Milky Way galaxy, which is nearly 1,000 times more massive. Image Credit: NASA/JPL-Caltech

Come fa un buco nero supermassiccio ad acquisire una massa che va tipicamente da qualche decina/centinaia di milioni fino a qualche miliardo di masse solari? Ad oggi, non c’è una risposta ben precisa ma alcuni dati ottenuti di recente dal telescopio spaziale Wide-field Infrared Survey Explorer (WISE) stanno facendo luce sulla natura di quei “siti cosmici” dove hanno origine i buchi neri. Inoltre, questi dati forniscono nuovi indizi che hanno lo scopo di comporre insieme il puzzle che descrive l’evoluzione di questi enigmatici oggetti che risiedono nei nuclei delle galassie attive.

Growing a black hole is not as easy as planting a seed in soil and adding water. The massive objects are dense collections of matter that are literally bottomless pits; anything that falls in will never come out. They come in a range of sizes. The smallest, only a few times greater in mass than our Sun, form from exploding stars. The biggest of these dark beasts, billions of times the mass of our Sun, grow together with their host galaxies over time, deep in the interiors. But how this process works is an ongoing mystery. Researchers using WISE addressed this question by looking for black holes in smaller, “dwarf” galaxies. These galaxies have not undergone much change, so they are more pristine than their heavier counterparts. In some ways, they resemble the types of galaxies that might have existed when the Universe was young, and thus they offer a glimpse into the nurseries of supermassive black holes. In this new study, using data of the entire sky taken by WISE in infrared light, up to hundreds of dwarf galaxies have been discovered in which buried black holes may be lurking. Infrared light, the kind that WISE collects, can see through dust, unlike visible light, so it’s better able to find the dusty, hidden black holes. The researchers found that the dwarf galaxies’ black holes may be about 1,000 to 10,000 times the mass of our Sun, larger than expected for these small galaxies. “Our findings suggest the original seeds of supermassive black holes are quite massive themselves“, said Shobita Satyapal of George Mason University, Fairfax, Va.

Daniel Stern, an astronomer specializing in black holes at NASA’s Jet Propulsion Laboratory, Pasadena, California, who was not a part of the new study, says: “The research demonstrates the power of an all-sky survey like WISE to find the rarest black holes. Though it will take more research to confirm whether the dwarf galaxies are indeed dominated by actively feeding black holes, this is exactly what WISE was designed to do: find interesting objects that stand out from the pack“.

The new observations argue against one popular theory of black hole growth, which holds that the objects bulk up in size through galaxy collisions.

When our Universe was young, galaxies were more likely to crash into others and merge. It is possible the galaxies’ black holes merged too, accumulating more mass. In this scenario, supermassive black holes grow in size through a series of galaxy mergers. The discovery of dwarf galaxy black holes that are bigger than expected suggests that galaxy mergers are not necessary to create big black holes. Dwarf galaxies don’t have a history of galactic smash-ups, and yet their black holes are already relatively big. Instead, supermassive black holes might form very early in the history of the Universe. Or, they might grow harmoniously with their host galaxies, feeding off surrounding gas.”We still don’t know how the monstrous black holes that reside in galaxy centers formed“, said Satyapal. “But finding big black holes in tiny galaxies shows us that big black holes must somehow have been created in the early Universe, before galaxies collided with other galaxies“.

NASA: The Search for Seeds of Black Holes

arXiv: Discovery of a Population of Bulgeless Galaxies with Extremely Red Mid-IR Colors: Obscured AGN Activity in the Low Mass Regime?

 

 

The Near-Field / Deep-Field Connection

The Southern California Center for Galaxy Evolution (CGE) and the University of California High-Performance AstroComputing Center (HiPACC) are bringing together theorists and observers for a three-day conference on the Near-Field Deep-Field Connection.

Topics of the workshop to be covered will include:

• local relics of reionization,
• connections between first stars and local metallicity,
• the evidence for and impact of IMF variation,
• the CGM of the Milky Way and beyond,
• dwarf galaxies at high and low z, and
• star-formation histories near and far.

 

La distribuzione della materia scura nelle galassie nane

Alcuni astronomi della Università del Texas a Austin ritengono di aver trovato la risposta ad un mistero che dura da circa 20 anni relativo alla distribuzione della materia scura nelle galassie nane. Gli scienziati hanno osservato che la distribuzione, in media, segue una legge molto semplice in cui la densità diminuisce dal centro della galassia, anche se la distribuzione può variare da galassia a galassia.

Dark matter is matter that gives off no light, but that astronomers detect by seeing its gravitational tug on other objects (like stars). Theories abound on what dark matter might be made of, unseen particles, dead stars, and more, but nobody knows for sure. Though mysterious, understanding the nature of dark matter is important, because it makes up most of the matter in the Universe. The only way to understand how the cosmos evolved to its present state is to decode dark matter’s role. For that reason, astronomers study the distribution of dark matter within galaxies and on even larger scales. Dwarf galaxies, in particular, make great laboratories to study dark matter, because they contain up to 1,000 times more dark matter than normal matter. Normal galaxies like the Milky Way, on the other hand, contain only 10 times more dark matter than normal matter. For the past 20 years, observational astronomers and theorists have debated how dark matter is distributed in galaxies. Observational astronomers, through their studies of telescope data, have argued that galaxies have a fairly uniform distribution of dark matter throughout. Theorists, backed by computer simulations from the 1990s, have argued that dark matter density decreases steadily from a galaxy’s core to its hinterlands. The disagreement is known as the “core/cusp debate.” This work set out to study the question using both data from telescopes and newly developed computer modeling. The project started out not assuming core or cusp theory is right but just asking ‘what is it?.’ These new models allowed to take this approach. Astronomers used telescope observations of several of the satellite galaxies orbiting the Milky Way, including the Carina, Draco, Fornax, Sculptor, and Sextans dwarf galaxies. The work involved running many supercomputer models for each galaxy to determine the distribution of dark matter within it, using the University’s Texas Advanced Computing Center (TACC).

They found that in some of the galaxies, the dark matter density decreased steadily from the center. In others, the density held constant. And some galaxies fell in between.

However, when all the galaxies’ distributions were analyzed together, the results showed that on average, the theorists were right. This seems to suggest that the theory behind dark matter in galaxies is correct on the whole, but that “each galaxy develops slightly differently.” The results do “pose more questions, questions about dark matter itself, and how normal matter interacted with dark matter to form the types of galaxies seen today. Possible next steps in this research include getting more telescope observations of these galaxies, both their centers and their extreme outlying regions, to understand the distribution of dark matter within them even better. More theory is also needed to explain the details of why certain galaxies’ dark matter halos deviate from the norm.

University of Texas: Texas Astronomers Unravel 20-Year Dark Matter Mystery with New Computer Models

arXiv: Variations in a Universal Density Profile

Did the Milky Way and Andromeda already collide?

Gli scienziati ritengono che tra circa 3 miliardi di anni la Via Lattea si scontrerà con Andromeda e che tale evento sarà il primo di una serie di collisioni galattiche. Oggi, però, un gruppo di astronomi guidati da Hongsheng Zhao della University of St Andrews propone un nuovo scenario in cui viene ipotizzato che le due galassie si sono già scontrate una volta, circa 10 miliardi di anni fa, e che la nostra conoscenze sulla gravità sono fondamentalmente errate. In realtà, questa idea potrebbe spiegare non solo la struttura della nostra galassia e quella di Andromeda ma anche la presenza delle galassie satelliti.

The Milky Way, made up of about 200 billion stars, is part of a group of galaxies called the Local Group. Astrophysicists often theorise that most of the mass of the Local Group is invisible, made of so-called dark matter. Most cosmologists believe that across the whole Universe, this matter outweighs ‘normal’ matter by a factor of five.

The dark matter in both Andromeda and the Milky Way then makes the gravitational pull between the two galaxies strong enough to overcome the expansion of the cosmos, so that they are now moving towards each other at around 100 km per second, heading for a collision 3 billion years in the future.

But this model is based on the conventional model of gravity devised by Newton and modified by Einstein a century ago, and it struggles to explain some properties of the galaxies we see around us. Zhao and his team argue that at present the only way to successfully predict the total gravitational pull of any galaxy or small galaxy group, before measuring the motion of stars and gas in it, is to make use of a model first proposed by Prof. Mordehai Milgrom of the Weizmann Institute in Israel in 1983. This modified gravity theory (Modified Newtonian Dynamics or MOND) describes how gravity behaves differently on the largest scales, diverging from the predictions made by Newton and Einstein.

Zhao and his colleagues have for the first time used this theory to calculate the motion of Local Group galaxies. Their work suggests that the Milky Way and Andromeda galaxies had a close encounter about 10 billion years ago.

If gravity conforms to the conventional model on the largest scales then taking into account the supposed additional pull of dark matter, the two galaxies would have merged. “Dark matter would work like honey: in a close encounter, the Milky Way and Andromeda would get stuck together, figuratively speaking“, says team member Prof. Pavel Kroupa from Bonn University. “But if Milgrom’s theory is right“, says his colleague Benoit Famaey (Observatoire Astronomique de Strasbourg), “then there are no dark particles and the two large galaxies could have simply passed each other thereby drawing matter from each other into long thin tidal arms“. New little galaxies would then form in these arms, a process often observed in the present-day Universe. Zhao explains: “The only way to explain how the two galaxies could come close to each other without merging is if dark matter isn’t there. Observational evidence for a past close encounter would then strongly support the Milgromian theory of gravity”. Just such a signature might already have been found. Astronomers struggle to account for the distribution of dwarf galaxies in orbit around both the Milky Way and Andromeda.

The dwarf galaxies could be explained if they were born from gas and stars ripped out of the two parent galaxies during their close encounter. Pavel Kroupa sees this as the ‘smoking gun’ for the collision.

“Given the arrangement and motion of the dwarf galaxies, I can’t see how any other explanation works”, he comments. The team now plan to model the encounter using Milgromian dynamics and are developing a computer code at Bonn University for this purpose. In the new model, the Milky Way and Andromeda are still going to crash into each other again in the next few billion years, but it will feel like ‘deja vu’. And the team believes that their discovery has profound consequences for our current understanding of the Universe. Pavel Kroupa concludes, “If we are right, the history of the cosmos will have to be rewritten from scratch”.

RAS: Did Andromeda crash into the Milky Way 10 billion years ago?

arXiv: Local Group timing in Milgromian dynamics. A past Milky Way-Andromeda encounter at z>0.8