News & Press releases

Number of entries: 113

August 2018

Finding the Happy Medium of Black Holes

Important evidence for populations of intermediate-mass black holes has been found.
Scientists have taken major steps in their hunt to find black holes that are neither very small nor extremely large. Finding these elusive intermediate-mass black holes could help astronomers better understand what the "seeds" for the largest black holes in the early Universe were.

The new research comes from two separate studies, each using data from NASA's Chandra X-ray Observatory and other telescopes. 

Black holes that contain between about one hundred and several hundred thousand times the mass of the Sun are called "intermediate mass" black holes, or IMBHs. This is because their mass places them in between the well-documented and frequently-studied "stellar mass" black holes on one end of the mass scale and the "supermassive black holes" found in the central regions of massive galaxies on the other.

While several tantalizing possible IMBHs have been reported in recent years, astronomers are still trying to determine how common they are and what their properties teach us about the formation of the first supermassive black holes.

One team of researchers used a large campaign called the Chandra COSMOS-Legacy survey to study dwarf galaxies, which contain less than one percent the amount of mass in stars as our Milky Way does. (COSMOS is an abbreviation of Cosmic Evolution Survey.) The characterization of these galaxies was enabled by the rich dataset available for the COSMOS field at different wavelengths, including data from NASA and ESA telescopes.

The Chandra data were crucial for this search because a bright, point-like source of X-ray emission near the center of a galaxy is a telltale sign of the presence of a black hole. The X-rays are produced by gas heated to millions of degrees by the enormous gravitational and magnetic forces near the black hole.

"We may have found that dwarf galaxies are a haven for these missing middleweight black holes," said Mar Mezcua of the Institute of Space Sciences in Spain who led one of the studies. "We didn't just find a handful of IMBHs — we may have found dozens."

Her team identified forty growing black holes in dwarf galaxies. Twelve of them are located at distances more than five billion light years from Earth and the most distant is 10.9 billion light years away, the most distant growing black hole in a dwarf galaxy ever seen. One of the dwarf galaxies is the least massive galaxy found to host a growing black hole in its center.

Most of these sources are likely IMBHs with masses that are about ten thousand to a hundred thousand times that of the Sun. One crucial result of this research is that the fraction of galaxies containing growing black holes is smaller for less massive galaxies than for their more massive counterparts.

A second team led by Igor Chilingarian of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., found a separate, important sample of possible IMBHs in galaxies that are closer to us. In their sample, the most distant IMBH candidate is about 2.8 billion light years from Earth and about 90% of the IMBH candidates they discovered are no more than 1.3 billion light years away.

With data from the Sloan Digital Sky Survey (SDSS), Chilingarian and his colleagues found galaxies with the optical light signature of growing black holes and then estimated their mass. They selected 305 galaxies with properties that suggested a black hole with a mass less than 300,000 times that of the Sun was lurking in the central regions of each of these galaxies.

Only 18 members of this list contained high quality X-ray observations that would allow confirmation that the sources are black holes. Detections with Chandra and with XMM-Newton were obtained for ten sources, showing that about half of the 305 IMBH candidates are likely to be valid IMBHs. The masses for the ten sources detected with X-ray observations were determined to be between 40,000 and 300,000 times the mass of the Sun.

"This is the largest sample of intermediate mass black holes ever found," said Chilingarian. "This black hole bounty can be used to address one of the biggest mysteries in astrophysics."

IMBHs may be able to explain how the very biggest black holes, the supermassive ones, were able to form so quickly after the Big Bang. One leading explanation is that supermassive black holes grow over time from smaller black holes "seeds" containing about a hundred times the Sun's mass. Some of these seeds should merge to form IMBHs. Another explanation is that they form very quickly from the collapse of a giant cloud of gas with a mass equal to hundreds of thousands of times that of the Sun.

Mezcua and her team may be seeing evidence in favor of the direct collapse idea, because this theory predicts that the less massive galaxies in their sample should be less likely to contain IMBHs.

"Our evidence is only circumstantial because it's possible that the IMBHs are just as common in the smaller galaxies but they're not consuming enough matter to be detected as X-ray sources", says Mezcua's co-author Francesca Civano of the CfA.

Chilingarian's team has a different conclusion.

"We're arguing that just the presence of intermediate mass black holes in the mass range we detected suggests that smaller black holes with masses of about a hundred Suns exist," says Chilingarian's co-author Ivan Yu. Katkov of Moscow State University in Russia. "These smaller black holes could be the seeds for the formation of supermassive black holes."

Another possibility is that both mechanisms actually occur. Both teams agree that to make firm conclusions much larger samples of black holes are needed using data from future satellites. The paper by Mar Mezcua and colleagues was published in the August issue of the Monthly Notices of the Royal Astronomical Society and is available online. The paper by Igor Chilingarian was recently accepted for publication in The Astrophysical Journal and is available online.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.
July 2018

Press-release of Kazan Federal University about recent PRD paper by S D Odintsov and V Oikonomou has attracted a lot of attention on main news portals in Russia /

Press-release on recent PRD paper by S D Odintsov and V Oikonomou
Recent PRD work by S D Odintsov and V K Oikonomou is devoted to study of possible future singularity of universe in frames of F(R) gravity. The method of dynamical system is used for that purpose. It is shown that for some DE models of F(R) gravity including quadratic on curvature terms the future singularity maybe avoided.
For detail:
Dynamical Systems Perspective of Cosmological Finite-time Singularities in f(R)f(R) Gravity and Interacting Multifluid Cosmology 
S. D. Odintsov (ICREA, Barcelona & ICE, Bellaterra & Barcelona, IEEC & Kazan State U.), V. K. Oikonomou (Aristotle U., Thessaloniki & TUSUR, Tomsk & Tomsk Pedagogical Inst.). Jun 19, 2018. 25 pp. 
Published in Phys.Rev. D98 (2018) no.2, 024013 
DOI: 10.1103/PhysRevD.98.024013

Link to the Russian news:
Other links in English:

June 2018

SEWM 2018 - Strong and ElectroWeak Matter Conference

Congreso SEWM 2018 - Strong and ElectroWeak Matter Conference en Barcelona
Del 25 al 29 de junio se celebrará en las instalaciones de CosmoCaixa en Barcelona la decimotercera edición del congreso Strong and ElectroWeak Matter Conference (SEWM 2018) organizado por el Instituto de Ciencias del Espacio (ICE-CSIC), Institut d’Estudis Espacials de Catalunya (IEEC), Institut de Física d’Altes Energies (IFAE) y Institut de Ciències del Cosmos Universitat de Barcelona (ICCUB), con la colaboración de la Fundación Bancaria “La Caixa”.
En este marco, el Nobel de Física de 2017 Barry Barish dará una conferencia pública el día 26 a las 19:00 en las instalaciones de CosmoCaixa con el título Ondas gravitacionales: de Einstein a una nueva ciencia.
June 2018

The observation of the nova ASASSN-18fv by the ESA's INTEGRAL gamma-ray satellite, triggered by our team, has been selected as the Picture of the Month

The nova ASASSN-18fv is being observed with INTEGRAL, searching for the7Be line at 478 keV; the OMC light curve is the picture of the month
INTEGRAL/OMC optical observations of the bright nova ASASSN-18fv
Background image: An artist's rendering of a classical nova; credit (c) David A. Hardy/
INTEGRAL/OMC optical observations of the bright nova ASASSN-18fv
  On 20 March a new bright transient optical source, near the Galactic plane, was found at V<10 mag by the All Sky Automated Survey for SuperNovae(ASAS-SN; ATel#11454). These observations showed an outburst amplitude of more than 7 mag. About a day later it was already brighter than 6 mag (see, e.g.,AAVSO Alert Notice 626). Early spectroscopy (ATels#11456,#11468) had not ruled out a Galactic classical nova, but the transient might also be a large outburst of a young stellar object or other peculiar explosion. Near-IR spectroscopy obtained about 10 days later (ATel#11506) showed that the spectrum is consistent with that observed for normal Fe II and transition (Fe II + He/N) class novae after peak. Two weeks later, Fermi/LAT and AGILE detected prolonged gamma-ray emission above ~100 MeV (ATels#11546,#11553), and a few days after that, NuSTAR in X-rays (3.5-78 keV; ATel#11608). If the source is indeed a classical nova, one may expect nucleosynthesis line emission from the decay of 7Be at 478 keV or of 22Na at 1275 keV, depending on the nova type. Since ASASSN-18fv looks like a CO nova, 7Be is favoured with respect to 22Na. This makes ASASSN-18fv a very interesting target for INTEGRAL.

On 23 April INTEGRAL started an out-of-TAC public observationof ASASSN-18fv. The source continued to be very bright (V~6.8 mag) one month after discovery, and it was, therefore, observed with the OMC in Fast monitoring mode. This is one of the few observations using this observing mode. With this mode, integrations of 3 seconds are performed at intervals of 4.5 seconds, and only the sections of the CCD containing the target of interest are read from the CCD and transmitted to ground. On 18 May, the OMC was configured back to Normal monitoring mode when the source brightness decayed to V~8.5 mag. Observations are currently still ongoing (see the INTEGRAL scheduling information).

The optical light curve as obtained by the AAVSO/visual estimates (open white circles) and with the INTEGRAL/OMC V-band (filled red circles) are shown in the main image. It can be seen that AAVSO data are important due to the large time span covered. The OMC data are of extremely good quality, although they are affected by time gaps produced when the source falls outside its FoV, because of the observing 5x5 dithering pattern. In the zoomed-in OMC light curve (inset figure top right) the excellent time resolution of the Fast monitoring mode and a photometric accuracy of about 0.02 mag can be seen (note that systematic effects not included). Short-scale time variability superposed on the general decline is clearly revealed. In some cases, the amplitude of the variability reaches 0.3 mag on timescales of several hours to one day. Due to the long INTEGRAL ToO program on ASASSN-18fv, the OMC is collecting a legacy optical data set on the source.

Oscillations were also reported in ATel#11508, around maximum light on timescales of days. According to the classification by Strope et al. (2010), ASASSN-18fv can be tentatively classified as a J-class nova. Light curves of this class are characterized by substantial jittering above the base level. The variations observed in the light curve could also resemble oscillations like those seen in the O-class. However, the observed variations by the OMC seem to be essentially random and start before the peak, while O-class novae are characterized by quasi-periodic oscillations which generally start around 3 mag below the peak. Future observations will help in the definitive classification of ASASSN-18fv.

Credits:Albert Domingo (INTEGRAL/OMC team, CAB/CSIC-INTA, Spain) and Margarita Hernanz (ICE-CSIC and IEEC, Spain).

We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this image.

Link to the original news in the ESA/INTEGRAL page.
May 2018

First polarimetric GNSS radio-occultation, obtained aboard the PAZ satellite

First polarimetric GNSS radio-occultation, obtained aboard the PAZ satellite
This last weekend polarimetric GNSS radio-occultation data were received for the first time ever! The ROHP-PAZ experiment will attempt simultaneous atmospheric thermodynamic and heavy rain retrievals. Over 200 profiles are acquired daily since then.
April 2018

El model d’espectre d’emissió de púlsars ens ajudarà a entendre millor la física relativista en entorns extrems

Descobertes fa escassament mig segle, les estrelles de neutrons o púlsars són encara un dels objectes més desconeguts de l’univers.
Diego F. Torres, professor ICREA i membre de l’Institut d’Estudis Espacials de Catalunya
La recent publicació a la revista Nature Astronomy d’un model teòric que n’explica la varietat dels espectres d’emissió de radiació aporta llum sobre la naturalesa d’aquests cossos. En parlem amb l’investigador a càrrec de la recerca, el director de l’Institut de Ciències de l’Espai, professor ICREA i membre de l’Institut d’Estudis Espacials de Catalunya, Diego F. Torres.

Les estrelles de neutrons van ser descobertes per la ciència fa relativament poc. ¿En què consisteixen?

Es tracta d’un dels finals possibles en l’evolució de les estrelles massives, que després d’esclatar com a supernoves conserven un romanent compacte que anomenem estrella de neutrons. Bàsicament és una estrella molt densa, de la mida d’una ciutat de 10 quilòmetres de radi, però amb una massa equivalent a la del sol. També presenten un camp magnètic molt intens i d’altres propietats físiques extremes. Aquests cossos roten molt ràpidament i en girar l’intens camp magnètic del seu voltant emet radiació en forma de feixos de llum. Quan aquests feixos, per les lleis de la geometria, es creuen amb la línia de la visual de la Terra es detecten en forma de pics de radiació anàlegs als dels fars, perquè ens en fem una idea gràfica. Com que això té una periodicitat d’acord amb la rotació de l’estrella, se’ls anomena púlsars. Es van descobrir fa només 50 anys, tot i que estaven previstes ja d’abans. Ara en coneixem moltes, en funció de la longitud d’ona de les seves emissions, que és precisament el focus d’atenció del nostre treball.

Una de les incògnites associades als púlsars era fins ara el seu divers rang d’espectre d’emissió de radiació.

Les estrelles de neutrons presenten espectres d’emissió molt variats, sobretot a altes energies de raigs X, però també a freqüències òptiques i cap a la part més energètica de l’espectre electromagnètic, que arriba als raigs gamma. Es dona el fet que fins i tot estrelles que semblen molt similars, amb propietats de període anàlogues, resulta que emeten radiació de forma molt diversa. D’aquí parteix la nostra pregunta: a què responen aquestes diferències i com es genera aquesta varietat? Hem volgut comprendre si a partir d’allò que mesurem en un rang de freqüències érem capaços de predir què veuríem en un altre. Això ens permetria augmentar la possibilitat de detectar nous púlsars, perquè si els detectem en les freqüències captades amb instruments més sensibles o amb capacitat de fer escombrades molt més àmplies, aleshores serà possible predir-los en d’altres rangs de freqüència.

¿El model que heu desenvolupat permet fer aquestes prediccions?

En efecte, en principi sembla que funciona correctament. El nostre model teòric parteix de la definició de només tres o quatre paràmetres físics, a partir dels quals podem explicar tots els espectres d’estrelles de neutrons que es coneixen. Emprant el model hem generat un sistema de predicció a partir de la detecció de púlsars en un determinat rang de freqüències, de forma que ens permeti calcular quines serien les emissions en la resta de rangs. El treball mostra que partint d’observacions en rajos gamma podem predir les estrelles que seran més brillants en raigs X, cosa que ens ha permès ja trobar noves estrelles de neutrons en aquesta banda de l’espectre electromagnètic.

¿Aquest model pot ser una eina per comprendre més profundament fenòmens cosmològics que fins ara se’ns escapaven?

Sobretot pel que fa a física d’altes energies, ja que en sistemes com els de les estrelles de neutrons totes les magnituds són bàsicament extremes. Cal tenir present que estem parlant de plasma en interacció amb camps magnètics que no és possible reproduir en laboratori, perquè les densitats, els valors magnètics i la pròpia acceleració de les partícules involucrades  corresponen a magnituds totalment relativistes. Esperem que el nostre treball ajudi a entendre una mica millor la física relativista en situacions d’entorn extrem.

¿També hi ha vinculació amb la branca quàntica de la física?

No directament, perquè malgrat que estem parlant de situacions que anomenem de camp fort, no hi ha un horitzó d'esdeveniments com en un forat negre.

¿Quines preveu que puguin ser les derivacions del model?

Hi ha dues línies principals de recerca. La primera parteix del fet que el nostre model es desenvolupa a partir de rajos X cap a freqüències més altes. Però també creiem que és possible fer-ho cap a longituds d’ona òptiques a l’abast dels telescopis normals a què estem habituats. La radiació òptica dels púlsars ha estat fins ara força misteriosa, no està ben entesa. Nosaltres creiem que l’extensió del model ens permetrà explicar-la i distingir quins púlsars es podrien veure en telescopis òptics. La segona derivació és un aspecte que el model encara no té incorporat: l’anàlisi de les corbes de llum. Els púlsars, a més de tenir un espectre, en tant que roten també presenten una variació repetitiva de llum. Per poder preveure corbes de llum hem d’estendre el model, una feina teòrica complicada que ara estem duent a terme per poder alhora predir i reproduir allò que veiem com a senyal temporal i com senyal en energia.


D.F. Torres. Order Parameters for the high-energy spectra of pulsars.
Nature Astronomy, 2, p. 247-256 (2018). DOI: 10.1038 / s41550-018-0384-5
April 2018

Today ICE-CSIC/IEEC receive the first data of the experiment aboard the PAZ satellite

Today ICE-CSIC/IEEC receive the first data of the experiment aboard the PAZ satellite
The experiment led by ICE-CSIC/IEEC aboard the PAZ satellite, called Radio-Occultation and Heavy Precipitation with PAZ (ROHP-PAZ) was activated on April 4, 2018. Today the ICE/CSIC-IEEC received the first data sets! 
March 2018

La missió ARIEL seleccionada com a propera missió científica mitjana de l'ESA

La missió ARIEL seleccionada com a propera missió científica mitjana de l'ESA
Impressió artística d’ARIEL fent camí cap al punt de Lagrange L2. Aquí, ARIEL estarà protegida del Sol i tindrà una visió sense obstacles de tot el cel.
ESA/STFC RAL Space/UCL/Europlanet-Science Office
ARIEL, una missió per respondre preguntes fonamentals sobre com es formen i evolucionen els sistemes planetaris, acaba de ser seleccionada per l'Agència Espacial Europea (ESA) com la seva propera missió científica de classe mitjana, amb data de llançament prevista pel 2028. Al llarg de 4 anys, ARIEL observarà 1000 planetes al voltant d’altres estrelles i farà el primer estudi a gran escala sobre la química de les atmosferes d’aquests exoplanetes.
La missió ARIEL ha estat desenvolupada per un consorci de més de 60 instituts de 15 països de l'ESA, inclosos el Regne Unit, França, Itàlia, Polònia, Espanya, Països Baixos, Bèlgica, Àustria, Dinamarca, Irlanda, Hongria, Suècia, Alemanya i Portugal, amb una col·laboració addicional de la NASA als EUA. L’IEEC lidera la participació espanyola i, a banda de la contribució científica, participa en la construcció de la missió, que inclou el sistema electrònic del satèl·lit que controla el telescopi i els moviments del mirall secundari, així com els programes informàtics que planifiquen les observacions dels planetes i que s’utilitzen a la base d’operacions de l’ESA. La resta d’institucions espanyoles involucrades són el Centro de Astrobiología, la Universidad Politécnica de Madrid i l’Instituto de Astrofísica de Canarias.
L'investigador principal d’ARIEL a Espanya, el Dr. Ignasi Ribas, de l'Institut de Ciències de l'Espai (ICE, CSIC) i director del Institut d'Estudis Espacials de Catalunya (IEEC) comenta: "Tot i que fins ara hem descobert al voltant de 3800 planetes que orbiten altres estrelles, la naturalesa d'aquests exoplanetes continua sent molt misteriosa. ARIEL n’estudiarà una mostra estadísticament gran per donar-nos una imatge veritablement representativa de què estan fets aquests planetes. Això ens permetrà respondre preguntes sobre com la química d'un planeta es vincula amb l'entorn en què es forma i com el seu naixement i evolució depenen de l’estrella a la qual orbiten". ARIEL estudiarà una població diversa d'exoplanetes que van des de la mida de Júpiter i Neptú fins a les anomenades súper-Terres, en una gran varietat d'ambients. El focus principal de la missió seran els planetes en òrbites properes a la seva estrella. Els exoplanetes calents, amb temperatures de fins a 2000ºC, representen un laboratori natural on estudiar la química i la formació planetàries, ja que les altes temperatures mantenen les diferents espècies moleculars en circulació per l'atmosfera i això les fa observables remotament.
ARIEL tindrà un telescopi amb un mirall primari d’un metre de diàmetre per recollir la llum visible i infraroja d’aquests sistemes planetaris que orbiten estrelles distants. Un espectròmetre descompondrà la llum en un "arc de Sant Martí" per extreure les empremtes químiques de les molècules atmosfèriques quan el planeta passa per davant o darrere de l'estrella. Un fotòmetre i un sistema de rastreig recolliran informació sobre la presència dels núvols a les atmosferes dels exoplanetes i permetran que el telescopi espacial apunti a cada estrella amb una gran estabilitat i precisió.
ARIEL serà llançat des de Kourou a la Guaiana Francesa i es posarà en òrbita al punt 2 de Lagrange (L2), que és un punt d'equilibri gravitacional a 1,5 milions de quilòmetres de l'òrbita de la Terra al voltant del Sol. Aquí, ARIEL estarà protegida del Sol i tindrà una visió sense obstacles de tot el cel per observar un gran nombre d’exoplanetes.
El gestor del projecte a nivell espanyol, el Dr. Josep Colomé, de l’ICE (ICE, CSIC) i del IEEC, declara: "La selecció d’ARIEL per part de l’ESA és una fantàstica notícia. És un reconeixement a la feina d’enginyeria feta durant els dos darrers anys i un impuls per la tecnologia espacial que desenvolupem per aquesta i altres missions i amb una estreta col·laboració amb la indústria del sector. ARIEL ens permet treballar amb centres de referència a nivell mundial i ens situa a la primera divisió de la tecnologia espacial."
ARIEL (Atmospheric Remote-Sensing Infrared Exoplanet Large-survey) en dades i xifres:

Mirall primari el·líptic: 1,1 x 0,7 metres
Instrumentació: 3 canals fotomètrics i 3 espectròmetres de baixa resolució que cobreixen de 0,5 a 7,8 micres en longitud d'ona.
Durada de la missió: 4 anys en òrbita
Data de llançament: 2028
Massa de càrrega útil: ~ 450 kg
Massa total: ~ 1300 kg
Destinació: Punt L2 de Lagrange Sol - Terra
Cost de la missió de l'ESA: ~ 450 milions d'euros, a més de les contribucions nacionals a la càrrega útil
Llançament del vehicle: Ariane 6-2 des de la Guaiana Francesa Imatges
March 2018

ESA's INTEGRAL satellite selects a figure in one of our papers as picture of the month

ESA's INTEGRAL satellite selects a figure in one of our papers as picture of the month (March 2018)
Which gamma-ray pulsars can be seen by INTEGRAL and why?
D.F. Torres, Nature Astronomy (2018), and Diego F. Torres/JWildfire for the artistic pulsar representation.
Which gamma-ray pulsars can be seen by INTEGRAL and why?

Neutron stars are a common compact endpoint of the life of stars. They have an extreme density (stars of about 10 km in size, with the mass of our Sun), and harbour the strongest magnetic fields (from 108 to 1014 times that of our Sun). Magnetized and rotating neutron stars emit beams of radiation, which can only be seen when the observer and the beam are aligned. Periodic recurrence of such alignment gives rise to pulsations, and to the name pulsar used for these objects.

Pulsars emit at all wavelengths, and their energy distribution (that is, how much power they yield at each wavelength band) varies strongly within the population. From the more than 2000 radio pulsars known, and the more than 200 gamma-ray pulsars known, we only know less than 20 of them which pulse in X-rays. The number of those detected in INTEGRAL's energy range is even lower.

What makes a pulsar shine in gamma-rays and/or X-ray energies? Ultimately, how can we predict, which pulsar will be visible to a particular X-ray instrument?

Despite the extreme precision of the observations, and the underlying complexity of the processes involved, recently a theoretical model for the spectra of pulsars has been proposed: just four physical parameters suffice to fit the emission spectrum of all gamma and/or X-ray pulsars known. When analyzing the properties for all pulsars, by grouping of these parameters, relevant correlations appear, explaining the different observational behaviours. As a bonus, the model acts as a tool to predict whether X-ray pulsars can be seen, starting from Fermi gamma-ray data. Detailed analysis of already detected and future gamma-ray pulsars can then lead to predicting whether or not INTEGRAL can detect them too.

Expectations for possible detections are shown in the image at the bottom, in the setting of the correlation of model parameters fitting the high-energy spectra of pulsars. Red and blue dots stand for gamma-ray-only, normal and millisecond pulsars, respectively. They show a strong correlation depicted with a dashed line (the shadowed region represents the 2 sigma uncertainty in this correlation). White (grey) crosses within a red/blue colored point denote a predicted flux of at least 10-13 erg cm-2 s-1 (10-14 erg cm-2 s-1) at 10 keV. The parameters of the X/gamma-ray and X-ray-only pulsars are shown with orange and black points, respectively, together with their 1 sigma uncertainty. The light cylinder Rlc of all pulsars is also noted (green pentagons). Uncertainties in the model parameters are larger (smaller) when only X-ray (both X- and GeV gamma-ray) data are available.

Reference: "Order Parameters for the high-energy spectra of pulsars"
D.F. Torres,
Nature Astronomy (2018), doi:10.1038/s41550-018-0384-5
February 2018

Stars in the Milky Way halo: Cosmic Space Invaders or Victims of Galactic Eviction?

Two stellar groups on opposite sides of the galactic plane originated by tidal interaction between the Milky Way and a satellite galaxy
The Milky Way perturbed by tidal interaction with a satellite galaxy
A group of astronomers led by the Max Planck Institute for Astronomy (MPIA) and with participation of the Institute of Space Sciences (ICE, CSIC) and the Institute of Space Studies of Catalonia (IEEC) have investigated a small population of stars in the halo of the Milky Way Galaxy, finding its chemical composition to closely match that of the Galactic disk. This similarity provides compelling evidence that these stars have originated from within the disc, rather than from merged dwarf galaxies. The reason for this stellar migration is thought to be the theoretically proposed oscillations of the Milky Way disc as a whole, induced by the tidal interaction of the Milky Way’s dark matter halo with a passing massive satellite galaxy.

The stars investigated belong to two different structures located in the Galactic halo, the Triangulum-Andromeda (TriAnd) and A13 overdensities. These two structures are located on opposite sides of the Milky Way, about 14.000 light years below and above the Galactic plane (see Figure 1), and were initially thought to be the debris left behind by smaller galaxies that invaded the Milky Way in the past.

But in the study published today in Nature, astronomers present compelling evidence showing that these structures actually originate from the Milky Way's disc itself, but were kicked out towards the halo.
The key to understanding the origin of these stars lies in their detailed abundance patterns, obtained using the high-resolution spectra taken with the Keck and the VLT (Very Large Telescope, ESO) telescopes. "The analysis of chemical abundances is a very powerful test which allows, similarly to the DNA matching, to identify the parent population of the star. Different parent populations, such as the Milky Way disc or halo, dwarf satellite galaxies or globular clusters, are known to have radically different chemical compositions. So once we know what the stars are made of, we can immediately link them to their parent populations.”, explains M. Bergemann, the leading author of the study.

The comparison of the chemical composition of these stars shows that they are almost identical, both within and between groups. Even more surprisingly, their composition closely matches the typical abundance pattern of Milky Way disk stars. This provides compelling evidence that these stars likely originate from the Galactic thin disk itself, rather than being debris from the disruption of one or many of the smaller galaxies that are thought to have been accreted by our Galaxy in the past.

But how did the stars then get to these extreme positions above and below the Galactic disk? Models of the evolution of the Milky Way predict this "Galactic eviction" to happen as the result of oscillations of the Galactic disk as a whole. The favoured explanation for these oscillations is the tidal interaction of the Milky Way with a massive satellite galaxy.
The results published in Nature now provide the clearest evidence for these oscillations of the Milky Way's disk so far. These results indicate that the dynamics of the Milky Way's disk is significantly more complex than previously thought and disk stars can be relocated to distant locations from their birthplace.

“Future work includes the improved determination of the distances and motions of the stars in these overdensities, especially critical will be the data from the Gaia space mission. This will test our interpretation that the overdensities are the crests of the large-scale Galactic wave. Moreover, future determination of ages of the stars with asteroseismology will allow to date when the interaction of the Milky Way and the satellite galaxy happened.” said M. Bergemann and A. Serenelli.

Background informationThe results described here were published in Bergemann et al., “Two chemically similar stellar overdensities on opposite sides of the Galactic disc plane" in the journal Nature ( using the identifier 10.1038/nature25490)

Aldo Serenelli, from the Institute of Space Sciences (ICE, CSIC) and Institute of Space Studies of Catalonia (IEEC) in collaboration with Maria Bergemann (Max Planck Institute for Astronomy, MPIA) , Branimir Sesar (MPIA), Judith G. Cohen (California Institute of Technology), Allyson Sheffield (City University of New York), Ting S. Li (Fermi National Accelerator Laboratory), Luca Casagrande (The Australian National University), Kathryn Johnston and Chervin F.P. Laporte (both Columbia University, New York), Adrian M. Price-Whelan (Princeton University) and Ralph Schönrich (University of Oxford, UK), Andrew Gould (MPIA)
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An institute of the Consejo Superior de Investigaciones Científicas
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