Observing Supermassive Black Holes across cosmic time: from phenomenology to physics

In the last decade, a combination of high sensitivity, high spatial resolution observations and of coordinated multi-wavelength surveys has revolutionized our view of extra-galactic black hole (BH) astrophysics. We now know that supermassive black holes reside in the nuclei of almost every galaxy, grow over cosmological times by accreting matter, interact and merge with each other, and in the process liberate enormous amounts of energy that influence dramatically the evolution of the surrounding gas and stars, providing a powerful self-regulatory mechanism for galaxy formation. The different energetic phenomena associated to growing black holes and Active Galactic Nuclei (AGN), their cosmological evolution and the observational techniques used to unveil them, are the subject of this chapter. In particular, I will focus my attention on the connection between the theory of high-energy astrophysical processes giving rise to the observed emission in AGN, the observable imprints they leave at different wavelengths, and the methods used to uncover them in a statistically robust way. I will show how such a combined effort of theorists and observers have led us to unveil most of the SMBH growth over a large fraction of the age of the Universe, but that nagging uncertainties remain, preventing us from fully understating the exact role of black holes in the complex process of galaxy and large-scale structure formation, assembly and evolution.Comment: 46 pages, 21 figures. This review article appears as a chapter in the book: "Astrophysical Black Holes", Haardt, F., Gorini, V., Moschella, U and Treves A. (Eds), 2015, Springer International Publishing AG, Cha

Formation and Evolution of Supermassive Black Holes

The correlation between the mass of supermassive black holes in galaxy nuclei and the mass of the galaxy spheroids or bulges (or more precisely their central velocity dispersion), suggests a common formation scenario for galaxies and their central black holes. The growth of bulges and black holes can commonly proceed through external gas accretion or hierarchical mergers, and are both related to starbursts. Internal dynamical processes control and regulate the rate of mass accretion. Self-regulation and feedback are the key of the correlation. It is possible that the growth of one component, either BH or bulge, takes over, breaking the correlation, as in Narrow Line Seyfert 1 objects. The formation of supermassive black holes can begin early in the universe, from the collapse of Population III, and then through gas accretion. The active black holes can then play a significant role in the re-ionization of the universe. The nuclear activity is now frequently invoked as a feedback to star formation in galaxies, and even more spectacularly in cooling flows. The growth of SMBH is certainly there self-regulated. SMBHs perturb their local environment, and the mergers of binary SMBHs help to heat and destroy central stellar cusps. The interpretation of the X-ray background yields important constraints on the history of AGN activity and obscuration, and the census of AGN at low and at high redshifts reveals the downsizing effect, already observed for star formation. History appears quite different for bright QSO and low-luminosity AGN: the first grow rapidly at high z, and their number density decreases then sharply, while the density of low-luminosity objects peaks more recently, and then decreases smoothly.Comment: 31 pages, 13 figures, review paper for Astrophysics Update

X-ray Survey Results on Active Galaxy Physics and Evolution

This "pedagogical" review describes the key Chandra and XMM-Newton extragalactic surveys to date and details some of their implications for AGN physics and evolution. We additionally highlight two topics of current widespread interest: (1) X-ray constraints on the AGN content of luminous submillimeter galaxies, and (2) the demography and physics of high-redshift (z > 4) AGN as revealed by X-ray observations. Finally, we discuss prospects for future X-ray surveys with Chandra, XMM-Newton, and upcoming missions.Comment: 26 pages, in Physics of Active Galactic Nuclei at All Scales, eds. Alloin D., Johnson R., Lira P. (Springer-Verlag, Berlin), version with all figures at http://www.astro.psu.edu/users/niel/papers/papers.htm

Astronomical Distance Determination in the Space Age: Secondary Distance Indicators

The formal division of the distance indicators into primary and secondary leads to difficulties in description of methods which can actually be used in two ways: with, and without the support of the other methods for scaling. Thus instead of concentrating on the scaling requirement we concentrate on all methods of distance determination to extragalactic sources which are designated, at least formally, to use for individual sources. Among those, the Supernovae Ia is clearly the leader due to its enormous success in determination of the expansion rate of the Universe. However, new methods are rapidly developing, and there is also a progress in more traditional methods. We give a general overview of the methods but we mostly concentrate on the most recent developments in each field, and future expectations. © 2018, The Author(s)

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

Chandra Stacking Constraints on the Contribution of 24 μ\mum Spitzer Sources to the Unresolved Cosmic X-Ray Background

We employ X-ray stacking techniques to examine the contribution from X-ray undetected, mid-infrared-selected sources to the unresolved, hard (6-8 keV) cosmic X-ray background (CXB). We use the publicly available, 24 micron Spitzer Space Telescope MIPS catalogs from the Great Observatories Origins Deep Survey (GOODS) - North and South fields, which are centered on the 2 Ms Chandra Deep Field-North and the 1 Ms Chandra Deep Field-South, to identify bright (S_24 > 80 microJy) mid-infrared sources that may be powered by heavily obscured AGNs. We measure a significant stacked X-ray signal in all of the X-ray bands examined, including, for the first time, a significant (3.2 sigma) 6-8 keV stacked X-ray signal from an X-ray undetected source population. We find that the X-ray-undetected MIPS sources make up about 2% (or less) of the total CXB below 6 keV, but about 6% in the 6-8 keV band. The 0.5-8 keV stacked X-ray spectrum is consistent with a hard power-law (Gamma = 1.44 +/- 0.07), with the spectrum hardening at higher X-ray energies. Our findings show that these bright MIPS sources do contain obscured AGNs, but are not the primary source of the unresolved 50% of 6-8 keV CXB. Our study rules out obscured, luminous QSOs as a significant source of the remaining unresolved CXB and suggests that it most likely arises from a large population of obscured, high-redshift (z > 1), Seyfert-luminosity AGNs.Comment: 5 pages (emulateapj format), 4 figures; submitted to ApJ Letter

The Properties and Redshift Evolution of Intermediate-Luminosity Off-Nuclear X-Ray Sources in the Chandra Deep Fields

We analyze a population of intermediate-redshift (z~0.05-0.3) off-nuclear X-ray sources located within optically bright galaxies in the Great Observatories Origins Deep Survey and Galaxy Evolution from Morphology and SEDs fields. A total of 24 off-nuclear source candidates are classified using deep Chandra X-Ray Observatory exposures from the Chandra Deep Field-North, Chandra Deep Field-South, and Extended Chandra Deep Field-South 15 of these are newly identified. These sources have average X-ray spectral shapes and optical environments similar to those observed for off-nuclear intermediate-luminosity (LX>=1039 ergs s-1 in the 0.5-2.0 keV band) X-ray objects (IXOs; sometimes referred to as ultraluminous X-ray sources) in the local universe. This sample improves the available source statistics for intermediate-redshift off-nuclear sources with LX>~1039.5 ergs s-1, and it places significant new constraints on the redshift evolution of the off-nuclear source frequency in field galaxies. The fraction of intermediate-redshift field galaxies containing an off-nuclear source with LX>~1039 ergs s-1 is suggestively elevated (~80% confidence level) with respect to that observed for IXOs in the local universe; we calculate this elevation to be a factor of ~1.9+1.4-1.3. A rise in this fraction is plausibly expected as a consequence of the observed increase in global star formation density with redshift, and our results are consistent with the expected magnitude of the rise in this fraction

The X-Ray Evolution of Early-Type Galaxies in the Extended Chandra Deep Field-South

We investigate the evolution over the last 6.3 Gyr of cosmic time (i.e., since ) of the average X‐ray properties of early‐type galaxies within the Extended Chandra Deep Field–South (E‐CDF‐S). Our early‐type galaxy sample includes 539 objects with red sequence colors and Sérsic indices larger than , which were selected jointly from the COMBO‐17 (Classifying Objects by Medium‐Band Observations in 17 Filters) and GEMS (Galaxy Evolution from Morphologies and SEDs) surveys. We utilize the deep Chandra observations over the E‐CDF‐S and X‐ray stacking analyses to constrain primarily the average X‐ray emission from ``normal'' early‐type galaxies (i.e., those that are not dominated by luminous AGNs). We study separately optically luminous () and faint () galaxy samples, which we expect to have soft (0.5–2.0 keV) X‐ray emission dominated by hot (1 keV) interstellar gas and low‐mass X‐ray binary (LMXB) populations, respectively. The AGN fraction of our optically luminous sample evolves with redshift in a manner consistent with the evolution observed in other investigations of X‐ray–selected AGNs. We find that the X‐ray–to–B‐band mean luminosity ratio () for normal optically luminous galaxies does not evolve significantly over . This lack of X‐ray evolution implies a general balance between the heating and cooling of the hot gas. If transient AGN activity is largely responsible for maintaining this balance, then we infer that mechanical power must be dominating the feedback out to . Furthermore, in this scenario the average mechanical AGN power must remain roughly constant over the last half of cosmic time. For our optically faint early‐type galaxies, we find suggestive evidence that increases with redshift over