The Blazing Sky: LAMOST Observations Reveal Nature of Unknown Gamma-ray Sources

Fig. 1 Artistic representation of an active galaxy jet
Image by M. Kornmesser/ESO

Fig. 2 Example of the completely featureless optical spectrum of the BL Lac known as J065046.49+250259.6
Image by Harold A. Peña Herazo

An international team of astronomers has unveiled the nature of hundreds of gamma-ray emitting sources, discovering that most of them belong to the class of active galaxies known as blazars. 

Their recent study was published in The Astronomical Journal.

One of the most intriguing challenges in modern gamma-ray astronomy is searching for low-energy counterparts of unidentified gamma-ray sources. Unidentified sources constitute about 1/3 of all celestial objects detected by the Fermi satellite to date, the most recent gamma-ray mission with unprecedented capabilities for observing the high energy sky.

Since the largest population of known gamma-ray sources are blazars, astronomers believe they can also classify most unidentified gamma-ray sources as blazars. However, they can completely understand their nature only by observing blazar candidates at visible frequencies.

Blazars are extremely rare, black hole-powered galaxies. They host a supermassive black hole in their central regions that sweep out matter at almost the speed of light in the form of a powerful jet pointing towards the Earth. Particles accelerated in these jets can emit light up to the most energetic gamma-rays, thus being visible by instruments onboard the Fermi satellite.

The team, led by Dr. Harold Peña Herazo from Mexico's National Institute of Astrophysics, Optics, and Electronics (INAOE), analyzed hundreds of optical spectra collected by the Large Sky Area Multi-Object Fabre Spectroscopic Telescope (LAMOST) at the Xinglong Station in China.

LAMOST is hosted by National Astronomical Observatories of Chinese Academy of Sciences. It provided a unique opportunity to unveil the nature of blazar-like sources that can potentially be counterparts of unidentified gamma-ray sources.

From the list of sources discovered by the Fermi satellite, the researchers selected a sample of Blazar Candidates of Uncertain type (BCUs), which share several properties in common with blazars. However, optical spectroscopic observations are necessary to determine their proper classification and confirm their nature.

Using spectroscopic data available in the LAMOST archive, the researchers were able to classify tens of BCUs as blazars. "LAMOST data also permitted verifying the nature of hundreds of additional blazars by searching for emission or absorption lines used to determine their cosmological distances," said Prof. GU Minfeng from Shanghai Astronomical Observatory of Chinese Academy of Sciences.

The vast majority of sources belong to the blazar class known as BL Lac objects and have a completely featureless optical spectrum. This makes measuring their cosmological distances an extremely challenging task. However, thanks to the LAMOST observations, a few more of them have luckily revealed visible signatures in their optical spectra.

"Our analysis showed great potential for the LAMOST survey and allowed us to discover a few changing-look blazars," said Dr. Peña Herazo, currently a postdoctoral fellow at the East Asian Observatory.

"It is worth noting that the possibility of using LAMOST observations to estimate blazar cosmological distances is critical to studying this population, its cosmological evolution, the imprint in the extragalactic gamma-ray background light in the gamma-ray spectra, and the blazar contribution to the extragalactic gamma-ray background," said Prof. Francesco Massaro from the University of Turin.

"I started working on this optical campaign and analyzing spectroscopic data in 2015, and nowadays, thanks to the observations available in LAMOST archive, we certainly made a significant step toward the identification of gamma-ray sources with blazars. Future perspectives achievable thanks to LAMOST datasets will definitively reveal the nature of hundreds of new blazars in the years to come," commented Dr. Federica Ricci at Bologna University and INAF-OAS.

The group’s previous study was also published in The Astronomical Journal. 

 

Contact:

XU Ang 

National Astronomical Observatories

E-mail: annxu@nao.cas.cn

 References:

• An Optical Overview of Blazars with LAMOST. II. Gamma-Ray Blazar Candidates and Updated Classifications

• An Optical Overview of Blazars with LAMOST. I. Hunting Changing-look Blazars and New Redshift Estimates

 

Related Articles

• Novel Method for Locating High-energy Dissipation Region in Blazar Proposed

• Is Blazar Flux Variation in Complete Electromagnetic Spectrum Intrinsic?

 

 Source: Chinese Academy of Sciences/News

---

Discovery of a Young Blazar Produced by the Merger of Two Galaxies

The galaxy TXS 2116-077 (seen on the right) collides with another spiral-shaped galaxy of similar mass, creating a relativistic jet in TXS 2116-077's centre. Both galaxies have AGNs. Shown here is an image of the emission in the H-alpha line with superposed isophotes of the infrared emission in the J band. Also shown in black lines is the region where a spectrum was obtained with ISIS on the WHT and OSIRIS on the Gran Telescopio Canarias (GTC). Figure extracted from Paliya et al., 2020, ApJ, 892, 133. Large format: PNG. 

A blazar is a particular type of active galactic nucleus (AGN) with a central supermassive black hole which emits a jet, a flux of highly energetic particles and radiation moving almost at the velocity of light, and which is aligned along the observer's line-of-sight. An international team of researchers has observed the birth of one of these objects for the first time by combining observations from several telescopes, among them the William Herschel Telescope (WHT). 

As a point of reference, scientists believe all large galaxies have centrally located massive black holes, but only about one per cent of these have active nuclei. For example, our Milky Way's massive black hole is dormant. The emission from an AGN can often exceed that of the host galaxy, and originates from the central black hole accreting circumnuclear gas. But not all this gas is accreted onto the black hole; some gets accelerated and spewed out in the form of narrow, bi-polar jets.

"Active galaxies which have jets are usually big, old elliptical galaxies which, according to the models, are formed by the merger of two or more smaller galaxies, so that we think that these mergers are the cause of the activation of the jets" says Rubén García-Benito, a researcher at the Instituto de Astrofísica de Andalucía (IAA-CSIC), who has participated in the discovery. "A galactic collision is a very efficient way to make large masses of gas fall to the centre of a galaxy, which feeds the supermassive black hole and can produce the emergence of the jet". 

Now, astronomers have imaged the formation of a jet from two younger, spiral-shaped galaxies, in the process of merging. In scientific terminology these young spiral galaxies containing jets are called Narrow Line Seyfert 1 gamma ray emitter galaxies (γ-NLSy1). 

Each merging galaxy shows a supermassive black hole at its centre. The more massive of the two shows a very young jet, with an estimated age less than 15,000 years, whose existence can be attributed to the interacton between the galaxies, which started at least 500 million years ago.

"We are seeing the jet face-on" explains Enrique Pérez Jiménez, a researcher at the IAA-CSIC and a co-author of the study, "so that we have found the precursor of a blazar. As an analogy we could say that if a blazar is an adult, a γ-NLSy1 is a child".

Jets are the most powerful astrophysical phenomena in the universe. They can emit more energy into the universe in one second than our sun will produce in its entire lifetime. That energy is in the form of radiation, such as intense radio waves, X-rays, and gamma-rays.

In general blazars are so bright that they occult the galaxies which host them, so that studying their environment is difficult. However the detected jet in this young galaxy is less energetic, which has permitted the study of the gas and the stars of the host galaxy, providing very valuable information to trace the origin of the jet.

The team obtained the image and the spectra using several of the largest ground-based telescopes in the world, such as the GTC and the WHT on La Palma, as well as the optical/infrared Subaru telescope on Hawaii, and NASA's Chandra X-ray satellite observatory. The WHT ISIS observations were obtained as part of a service programme.

---

More information:

Vaidehi S. Paliya, Enrique Pérez, Rubén García-Benito, Marco Ajello, Francisco Prada, Antxon Alberdi, Hyewon Suh, C. H. Ishwara Chandra, Alberto Domínguez, Stefano Marchesi, Tiziana Di Matteo, Dieter Hartmann, and Marco Chiaberge, 2020, "TXS 2116-077: A Gamma-Ray Emitting Relativistic Jet Hosted in a Galaxy Merger", ApJ, 892, 133. [ ADS ]

Clemson researchers capture first-ever photographic proof of power-packed jet emerging from colliding galaxies, Clemson University press release, 7th April 2020.

La Palma telescopes participate in the discovery of a young blazar produced by the merger of two galaxies, IAC press release, 7 April 2020.

La fusión de dos galaxias genera la versión juvenil de un blázar, uno de los objetos más energéticos conocidos, IAA press release, 7 April 2020.

Jets from a Galaxy Collision, AAS NOVA research highlights, 8 April 2020.


Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.

---

Contact:

Javier Méndez  (Public Relations Officer)

 Source: Isaac Newton Group of Telescopes (ING)

---

Powering the Extreme Jets of Active Galaxies

Black-hole-powered galaxies called blazars have powerful jets that are thought to be fortuitously aimed directly toward Earth. Astronomers have used multi-band observations, from the gamma-ray to the radio, to study the powerful jets and their driving sources. Credit: NASA; M. Weiss/CfA

An active galaxy nucleus (AGN) contains a supermassive black hole that is vigorously accreting material. It typically ejects jets of particles that move at close to the speed of light, radiating across many wavelengths, in particular the X-ray, in processes are among the most energetic phenomena in the universe. The jets are often also highly collimated and extend far beyond their host galaxy, and if they happen to be pointed along our line of sight they are the most spectacular class of this phenomenon: blazars.

A few years ago astronomers noticed that some types of blazars have jet powers that appear to exceed the power provided by the accretion. Two ideas were put forward to explain the difference: the jets are also extracting power from the spin of the black hole or from the magnetic flux around the object. How either process happens – if indeed they do happen - is hotly debated, but one popular line of argument asserts that the processes are somehow related to the mass of the supermassive black hole, with the most massive cases (more than a hundred million solar-masses) being the most anomalous. Recently the Fermi Gamma-Ray Space Telescope detected gamma-rays (even more energetic photons than X-rays) coming from jets in a class of galaxies called Seyferts, spiral galaxies with relatively small supermassive black hole masses, typically about ten million solar-masses. Astronomers speculated that these relatively low-mass yet powerful emission engines might provide keys to sorting out the various sources of jet power.

CfA astronomer Mislav Balokovic and his colleagues completed a multi-wavelength study of the bright blazar-like Seyfert galaxy PKSJ1222+0413 and included data from the gamma-ray to the radio, both archival and new observations, including new results from the NuSTAR space observatory They then undertook a complete modeling of this source, the most distant one of its type known - its light has been traveling towards us for about eight billion years. They detected the pronounced signature of an accretion disk, and estimated the mass of the supermassive black hole from the widths and strengths of the emission lines to be about two hundred million solar-masses, about ten times higher than most other Seyferts of its type. The jet luminosity is only about half the accretion luminosity, unlike cases like galaxies whose jet power exceeds the accretion. But the object nonetheless clearly falls into a transition regime for jet strengths, enabling future studies to study in more detail the origins of jet power both Seyfert galaxies and in blazars.

Reference(s):

"The Relativistic Jet of the γ-ray Emitting Narrow-Line Seyfert 1 Galaxy PKS J1222+0413," Daniel Kynoch, Hermine Landt, Martin J. Ward, Chris Done, Catherine Boisson, Mislav Balokovic, Emmanouil Angelakis, and Ioannis Myserlis, MNRAS 487, 181, 2019.

Source: Smithsonian Astrophysical Observatory (SAO)

---

Radio Weak Blazars

Black-hole-powered galaxies called blazars have powerful jets that are thought to be fortuitously aimed directly toward Earth. Blazars emit at wavelengths from the radio to the gamma-rays, but astronomers have now found two objects that are blazar like in many ways but which are radio-quiet. Credit: NASA; M. Weiss/CfA

A blazar is a galaxy whose central nucleus is bright at wavelengths from the low energy radio band to high energy gamma rays (each gamma ray photon is over a hundred million times more energetic than the X-rays seen by the Chandra X-ray Observatory). Astronomers think that the blazar nucleus contains a supermassive black hole, similar to a quasar nucleus. The emission results when matter falls onto the vicinity of the black hole and erupts into powerful, narrow jets of radiating charged particles moving close to the speed of light. Two defining characteristics of blazars, strong radio emission and high variability, are results of the accretion and jets.

Although the nuclei of other galaxies also eject jets of particles, the class of blazars is thought to result from our unique viewing angle: staring directly down the throats of these jets. The orientation makes these objects unique probes of exotic physical activity, with the relative intensities of the radiation providing key diagnostics. In most other galaxies, for example, infrared radiation comes from heated dust, but in blazars the infrared colors indicate that it comes from jet emission. Because the jet emission is so bright the underlying galaxy light can be masked, with the result that in the class of BL Lac blazars emission and absorption lines are not detected, making their distances difficult to determine.

CfA astronomers Raffaele D'Abrusco and Howard Smith and their four colleagues report discovering blazars that challenge this general paradigm. They found two BL Lac blazars with no apparent radio emission: "radio weak" BL Lacs. The astronomers discovered them by using the Fermi catalog of very high energy sources to identify a set of possible new blazars, and the WISE infrared sky catalog to reinforce the categorization and to pinpoint the locations of the sources in the sky. After searching radio catalogs for counterparts to the sources, they discovered two that had no detected radio emission.

Since blazars are by definition highly variable, and since not all of the wavelengths were measured at the same time, the scientists review the possibility that the emission at one or more wavelengths varied enough to account for the peculiar observations; they also examine some other possibilities. In the end, they conclude that although variability might be a possible explanation, if these candidates behaved like other blazars, variability alone could not resolve the mystery of the radio silence. If confirmed, these new Radio Weak BL Lac objects challenge the basic explanation of blazars. How many radio weak BL Lacs exist, how far away they are, and how they are formed and evolve - indeed why they exist at all - are now pressing questions in extragalactic astronomy.

Reference(s): 

"Radio-weak BL Lac Objects in the Fermi Era," F. Massaro, E. J. Marchesini, R. D'Abrusco, N. Masetti, I. Andruchow & Howard A. Smith, ApJ 834, 113, 2017.

Source:  Smithsonian Astrophysical Observatory (SAO)

NASA's WISE, Fermi Missions Reveal a Surprising Blazar Connection

An analysis of blazar properties observed by the Wide-field Infrared Survey Explorer (WISE) and Fermi's Large Area Telescope (LAT) reveal a correlation in emissions from the mid-infrared to gamma rays, an energy range spanning a factor of 10 billion. When plotted by gamma-ray and mid-infrared colors, confirmed Fermi blazars (gold dots) form a unique band not shared by other sources beyond our galaxy. A blue line marks the best fit of these values. The relationship allows astronomers to identify potential new gamma-ray blazars by studying WISE infrared data.

Credits: NASA's Goddard Space Flight Center/Francesco Massaro, University of Turin

Black-hole-powered galaxies called blazars are the most common sources detected by NASA's Fermi Gamma-ray Space Telescope. As matter falls toward the supermassive black hole at the galaxy's center, some of it is accelerated outward at nearly the speed of light along jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, as illustrated here, the galaxy appears especially bright and is classified as a blazar. Credits: M. Weiss/CfA. Hi-res image

Astronomers studying distant galaxies powered by monster black holes have uncovered an unexpected link between two very different wavelengths of the light they emit, the mid-infrared and gamma rays. The discovery, which was accomplished by comparing data from NASA’s Wide-field Infrared Survey Explorer (WISE) and Fermi Gamma-ray Space Telescope, has enabled the researchers to uncover dozens of new blazar candidates.

Francesco Massaro at the University of Turin in Italy and Raffaele D’Abrusco at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, show for the first time that the mid-infrared colors of blazars in WISE data correlate to an equivalent measurement of their gamma-ray output.

"This connection links two vastly different forms of light over an energy range spanning a factor of 10 billion," said Massaro. "Ultimately, it will help us decipher how supermassive black holes in these galaxies manage to convert the matter around them into vast amounts of energy."

Blazars constitute more than half of the discrete gamma-ray sources seen by Fermi's Large Area Telescope (LAT). At the heart of a blazar lies a supersized black hole with millions of times the sun's mass surrounded by a disk of hot gas and dust. As material in the disk falls toward the black hole, some of it forms dual jets that blast subatomic particles straight out of the disk in opposite directions at nearly the speed of light. A blazar appears bright to Fermi for two reasons. Its jets produce many gamma rays, the highest-energy form of light, and we happen to be viewing the galaxy face on, which means one of its jets is pointing in our direction.

From January to August 2010, NASA's WISE mapped the entire sky in four infrared wavelengths, cataloging more than half a billion sources. In 2011, Massaro, D’Abrusco and their colleagues began using WISE data to investigate Fermi blazars.

"WISE made it possible to explore the mid-infrared colors of known gamma-ray blazars," said D’Abrusco. "We found that when we plotted Fermi blazars by their WISE colors in a particular way, they occupied a distinctly different part of the plot than other extragalactic gamma-ray sources."

The scientists detail new aspects of the infrared/gamma-ray connection in a paper published in The Astrophysical Journal on Aug. 9. They say the electrons, protons and other particles accelerated in blazar jets leave a specific "fingerprint" in the infrared light they emit. This same pattern is also clearly evident in their gamma rays. The relationship effectively connects the dots for blazars across an enormous swath of the electromagnetic spectrum.

About a thousand Fermi sources remain unassociated with known objects at any other wavelength. Astronomers suspect many of these are blazars, but there isn't enough information to classify them. The infrared/gamma-ray connection led the authors to search for new blazar candidates among WISE infrared sources located within the positional uncertainties of Fermi's unidentified gamma-ray objects. When the researchers applied this relationship to Fermi's unknown sources, they quickly found 130 potential blazars. Efforts are now under way to confirm the nature of these objects through follow-up studies and to search for additional candidates using the WISE connection.

"About a third of the gamma-ray objects seen by Fermi remained unknown in the most recent catalog, and this result represents an important advance in understanding their natures," said David Thompson, a Fermi deputy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

NASA's Jet Propulsion Laboratory in Pasadena, California, manages and operates WISE for NASA's Science Mission Directorate in Washington. The spacecraft was put into hibernation mode in 2011 after twice scanning the entire sky, thereby completing its main objectives. In September 2013, WISE was reactivated, renamed NEOWISE and assigned a new mission to assist NASA's efforts to identify potentially hazardous near-Earth objects.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

For more information on Fermi, visit:  www.nasa.gov/fermi

For more information on WISE, visit:  www.nasa.gov/wise

---

For additional information, please contact:

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov

---

By Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland

X-ray Echoes of a Shredded Star Provide Close-up of 'Killer' Black Hole

NASA Goddard astronomer Erin Kara discusses the discovery of X-ray echoes from Swift J1644+57, a black hole that shattered a passing star. X-rays produced by flares near this million-solar-mass black hole bounced off the nascent accretion disk and revealed its structure.Credits: NASA's Goddard Space Flight Center. Download high-resolution video at NASA’s Scientific Visualization Studio

Now astronomers using archival observations from Swift, the European Space Agency's (ESA) XMM-Newton observatory and the Japan-led Suzaku satellite have identified the reflections of X-ray flares erupting during the event. Led by Erin Kara, a postdoctoral researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland, College Park (UMCP), the team has used these light echoes, or reverberations, to map the flow of gas near a newly awakened black hole for the first time.

"While we don't yet understand what causes X-ray flares near the black hole, we know that when one occurs we can detect its echo a couple of minutes later, once the light  has reached and illuminated parts of the flow," Kara explained. "This technique, called X-ray reverberation mapping, has been previously used to explore stable disks around black holes, but this is the first time we've applied it to a newly formed disk produced by a tidal disruption."

In this artist's rendering, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close. Stellar debris has fallen toward the black hole and collected into a thick chaotic disk of hot gas. Flashes of X-ray light near the center of the disk result in light echoes that allow astronomers to map the structure of the funnel-like flow, revealing for the first time strong gravity effects around a normally quiescent black hole. Credits: NASA/Swift/Aurore Simonnet, Sonoma State University

Stellar debris falling toward a black hole collects into a rotating structure called an accretion disk. There the gas is compressed and heated to millions of degrees before it eventually spills over the black hole's event horizon, the point beyond which nothing can escape and astronomers cannot observe. The Swift J1644+57 accretion disk was thicker, more turbulent and more chaotic than stable disks, which have had time to settle down into an orderly routine. The researchers present the findings in a paper published online in the journal Nature on Wed., June 22.

One surprise from the study is that high-energy X-rays arise from the inner part of the disk. Astronomers had thought most of this emission originated from a narrow jet of particles accelerated to near the speed of light.

In blazars, the most luminous galaxy class powered by supermassive black holes, jets produce most of the highest-energy emission.

"We do see a jet from Swift J1644, but the X-rays are coming from a compact region near the black hole at the base of a steep funnel of inflowing gas we're looking down into," said co-author Lixin Dai, a postdoctoral researcher at UMCP. "The gas producing the echoes is itself flowing outward along the surface of the funnel at speeds up to half the speed of light."

X-rays originating near the black hole excite iron ions in the whirling gas, causing them to fluoresce with a distinctive high-energy glow called iron K-line emission. As an X-ray flare brightens and fades, the gas follows in turn after a brief delay depending on its distance from the source.

"Direct light from the flare has different properties than its echo, and we can detect reverberations by monitoring how the brightness changes across different X-ray energies," said co-author Jon Miller, a professor of astronomy at the University of Michigan in Ann Arbor.

Swift J1644+57 is one of only three tidal disruptions that have produced high-energy X-rays, and to date it remains the only event caught at the peak of this emission. These star shredding episodes briefly activate black holes astronomers wouldn't otherwise know about. For every black hole now actively accreting gas and producing light, astronomers think nine others are dormant and dark. These quiescent black holes were active when the universe was younger, and they played an important role in how galaxies evolved. Tidal disruptions therefore offer a glimpse of the silent majority of supersized black holes.

Images from Swift's Ultraviolet/Optical (white, purple) and X-Ray telescopes (yellow and red) were combined in this composite of Swift J1644+57, an X-ray outburst astronomers classify as a tidal disruption event. The event is seen only in the X-ray image, which is a 3.4-hour exposure taken on March 28, 2011. The outburst was triggered when a passing star came too close to a supermassive black hole. The star was torn apart, and much of the gas fell toward the black hole. To date, this is the only tidal disruption event emitting high-energy X-rays that astronomers have caught at peak luminosity. Credits: NASA/Swift/Stefan Immler.  Click here for an unlabeled version of this image.

"If we only look at active black holes, we might be getting a strongly biased sample," said team member Chris Reynolds, a professor of astronomy at UMCP. "It could be that these black holes all fit within some narrow range of spins and masses. So it’s important to study the entire population to make sure we’re not biased."

The researchers estimate the mass of the Swift J1644+57 black hole at about a million times that of the sun but did not measure its spin. With future improvements in understanding and modeling accretion flows, the team thinks it may be possible to do so.     

ESA's XMM-Newton satellite was launched in December 1999 from Kourou, French Guiana. NASA funded elements of the XMM-Newton instrument package and provides the NASA Guest Observer Facility at Goddard, which supports use of the observatory by U.S. astronomers. Suzaku operated from July 2005 to August 2015 and was developed at the Japanese Institute of Space and Astronautical Science, which is part of the Japan Aerospace Exploration Agency, in collaboration with NASA and other Japanese and U.S. institutions.

NASA's Swift satellite was launched in November 2004 and is managed by Goddard. It is operated in collaboration with Penn State University in University Park, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corp. in Dulles, Virginia, with international collaborators in the U.K., Italy, Germany and Japan.

---

By Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland

Editor: Ashley Morrow

Source: NASA/Black Holes