Researchers Identify Massive Black Holes that Seemed “hidden” in Dwarf Galaxies

black holes
  • An investigation conducted by two researchers from the Institute of Space Sciences (ICE-CSIC), a research unit of the Institute of Space Studies of Catalonia (IEEC), has been published today in the Astrophysical Journal Letters
  • The researchers have found massive black holes in 37 dwarf galaxies and have identified active galactic nuclei not seen until now. These nuclei are similar to the seed black holes that produced the massive black holes
  • The study constitutes the widest work so far done in dwarf galaxies using the integral field spectroscopy technique

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Generic picture of a dwarf galaxy. Image credit: NASA’s Goddard Space Flight Center/Jenny Hottle

A project conducted by the Institute of Space Science (ICE – CSIC), a research unit of the Institute of Space Studies of Catalonia (IEEC), has used the integral field spectroscopy technique (also known as IFU – integral field unit) to identify massive black holes in dwarf galaxies. The researchers have found 37 of these phenomena, 23 of which are new as evidence of their presence were not found in previous works of the same galaxies. This is the widest study ever done with this technique in dwarf galaxies.

The analysis, published today 28 July in the Astrophysical Journal Letters, is the widest study of active galactic nuclei (AGN) in dwarf galaxies ever done using the almost 5,000 observations of galaxies measured by MaNGA (Mapping Nearby Galaxies at Apache Point Observatory) with the IFU technique.

An AGN is a compact area in the centre of a galaxy that emits energy in its central region, usually generated by a massive black hole, among other elements.

“Thanks to the observations with IFU we have been able to find AGNs that seemed to be hidden in previous works”, emphasises Mar Mezcua, an IEEC researcher at ICE-CSIC.

The other co-author of the study and ICE-CSIC researcher, Helena Domínguez Sánchez, says: “The advantage of the IFU technique with respect to the classic observations with long-slit spectroscopy, that gives one spectrum per object, is that it allows us to obtain multiple spectra, sometimes more than one thousand per galaxy, in different regions”. “This way”, she adds, “we can study with much more detail the stellar populations, their gas and the kinematics of both”.

The “light echo” from the black hole has been captured by the spectroscopy

From the almost 1,609 dwarf galaxies that have been studied, the researchers have found AGNs in 37 of them, 23 of which are new cases that had not been previously identified.

“The classic spectroscopy has the limitation that it detects only the dominant energy source”, clarifies Domínguez, “meaning that in galaxies where the energy emerging from the stellar formation dominates its total emission, the AGN would be left «hidden»”.

The factors that explain the difficulty of observing these nuclei could be related to their activity or their location. On one hand, it can be that the AGN is no longer active and the IFU technique detects its last emission, the “light echo”, generally very weak. On the other hand, the AGN can be active but outside the centre of the galaxy.  At the same time, it could be that the nucleus is active and in the centre of the galaxy, but the stellar emission from the central region is brighter than the active nucleus, which makes its observation more difficult.

“With this investigacion we conclude that the IFU technique allows us to identify the last emission from nuclei that are no longer active, something that can not be done with other techniques”, says Mezcua. “Moreover, the active nuclei found are much weaker than those known until now”.

Searching for active nuclei in dwarf galaxies to understand the beginning of the Universe

These active nuclei could contain the vestiges of the first black holes formed in the early Universe, those which did not grow until becoming supermassive. The search for AGNs or massive black holes in dwarf galaxies allows us to increase our knowledge about the origin of the Universe, since they are considered to be the type of galaxies most similar to the first ones.

“It is believed that the black holes powering the AGNs are very similar to the seed black holes, the ones that were first formed”, notes Mezcua. The researcher adds that the scientific community considers that the supermassive black holes, those with one-million-times larger mass than that of the Sun, could have grown from these seed black holes.





More information

This research is presented in a paper entitled “Hidden AGN in dwarf galaxies revealed by MaNGA: light echoes, off-nuclear wanderers, and a new broad-line AGN”, by Mezcua, M. & Domínguez Sánchez, H., and it has appeared in the journal Astrophysical Journal Letters,  2020, ApJL, 898, L30, on 28 July 2020.

The Institute of Space Studies of Catalonia (IEEC  — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of over 20 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centers, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.

IEEC is a private non-profit foundation, governed by a Board of Trustees composed of Generalitat de Catalunya and four other institutions that each have a research unit, which together constitute the core of IEEC R&D activity: the University of Barcelona (UB) with the research unit ICCUB — Institute of Cosmos Sciences; the Autonomous University of Barcelona (UAB) with the research unit CERES — Center of Space Studies and Research; the Polytechnic University of Catalonia (UPC) with the research unit CTE — Research Group in Space Sciences and Technologies; the Spanish Research Council (CSIC) with the research unit ICE — Institute of Space Sciences. IEEC is integrated in the CERCA network (Centres de Recerca de Catalunya).

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