Galaxies

10 pages, 1034 KiB

Open AccessReview

X-ray Polarization of Blazars and Radio Galaxies Measured by the Imaging X-ray Polarimetry Explorer

by Alan P. Marscher, Laura Di Gesu, Svetlana G. Jorstad, Dawoon E. Kim, Ioannis Liodakis, Riccardo Middei and Fabrizio Tavecchio

Galaxies 2024, 12(4), 50; https://doi.org/10.3390/galaxies12040050 - 22 Aug 2024

Viewed by 518

Abstract

X-ray polarization, which now can be measured by the Imaging X-ray Polarimetry Explorer (IXPE), is a new probe of jets in the supermassive black hole systems of active galactic nuclei (AGNs). Here, we summarize IXPE observations of radio-loud AGNs that have been published [...] Read more.

X-ray polarization, which now can be measured by the Imaging X-ray Polarimetry Explorer (IXPE), is a new probe of jets in the supermassive black hole systems of active galactic nuclei (AGNs). Here, we summarize IXPE observations of radio-loud AGNs that have been published thus far. Blazars with synchrotron spectral energy distributions (SEDs) that peak at X-ray energies are routinely detected. The degree of X-ray polarization is considerably higher than at longer wavelengths. This is readily explained by energy stratification of the emission regions when electrons lose energy via radiation as they propagate away from the sites of particle acceleration as predicted in shock models. However, the 2–8 keV polarization electric vector is not always aligned with the jet direction as one would expect unless the shock is oblique. Magnetic reconnection may provide an alternative explanation. The rotation of the polarization vector in Mrk421 suggests the presence of a helical magnetic field in the jet. In blazars with lower-frequency peaks and the radio galaxy Centaurus A, the non-detection of X-ray polarization by IXPE constrains the X-ray emission mechanism. Full article

(This article belongs to the Special Issue X-ray Polarization: A New Era Begins)

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18 pages, 4170 KiB

Open AccessArticle

Semi-Empirical Estimates of the Cosmic Planet Formation Rate

by Andrea Lapi, Lumen Boco, Francesca Perrotta and Marcella Massardi

Galaxies 2024, 12(4), 49; https://doi.org/10.3390/galaxies12040049 - 18 Aug 2024

Viewed by 379

Abstract

We devise and exploit a data-driven, semi-empirical framework of galaxy formation and evolution, coupling it to recipes for planet formation from stellar and planetary science, to compute the cosmic planet formation rate, and the properties of the planets’ preferred host stellar and galactic [...] Read more.

We devise and exploit a data-driven, semi-empirical framework of galaxy formation and evolution, coupling it to recipes for planet formation from stellar and planetary science, to compute the cosmic planet formation rate, and the properties of the planets’ preferred host stellar and galactic environments. We also discuss how the rates and formation sites of planets are affected when considering their habitability, and when including possible threatening sources related to star formation and nuclear activity. Overall, we conservatively estimate a cumulative number of some

10^{20}

Earth-like planets and around

10^{18}

habitable Earths in our past lightcone. Finally, we find that a few

10^{17}

are older than our own Earth, an occurrence which places a loose lower limit a few

10^{- 18}

to the odds for a habitable world to ever host a civilization in the observable Universe. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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13 pages, 933 KiB

Open AccessArticle

Determination of the Hubble Constant and Sound Horizon from Dark Energy Spectroscopic Instrument Year 1 and Dark Energy Survey Year 6 Baryon Acoustic Oscillation

by Jose Agustin Lozano Torres

Galaxies 2024, 12(4), 48; https://doi.org/10.3390/galaxies12040048 - 13 Aug 2024

Viewed by 560

Abstract

We perform new measurements of the expansion rate and the sound horizon at the end of the baryon decoupling, and derive constraints on cosmic key parameters in the framework of the

Λ

CDM model, wCDM model, non-flat

Λ

CDM model and the [...] Read more.

We perform new measurements of the expansion rate and the sound horizon at the end of the baryon decoupling, and derive constraints on cosmic key parameters in the framework of the

Λ

CDM model, wCDM model, non-flat

Λ

CDM model and the phenomenological emergent dark energy (PEDE) model. We keep

r_{d}

and

H_{0}

completely free, and use the recent Dark Energy Spectroscopic Instrument (DESI) Year 1 and Dark Energy Survey (DES) Year 6 BAO measurements in the effective redshift range

0.3 < z < 2.33

, combined with the compressed form of the Pantheon sample of Type Ia supernovae, the latest 34 observational

H (z)

measurements based on the differential age method, and the recent

H_{0}

measurement from SH0ES 2022 as an additional Gaussian prior. Combining BAO data with the observational

H (z)

measurements, and the Pantheon SNe Ia data, we obtain

H_{0} = 69.70 \pm 1.11

km

s^{- 1}

{Mpc}^{- 1}

,

r_{d} = 147.14 \pm 2.56

Mpc in flat

Λ

CDM model,

H_{0} = 70.01 \pm 1.14

km

s^{- 1}

{Mpc}^{- 1}

,

r_{d} = 146.97 \pm 2.45

Mpc in PEDE model. The spatial curvature is

Ω_{k} = 0.023 \pm 0.025

, and the dark energy equation of state is

w = - 1.029 \pm 0.051

, consistent with a cosmological constant. We apply the Akaike information and the Bayesian information criterion test to compare the four models, and see that the PEDE model performs better. Full article

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15 pages, 1308 KiB

Open AccessFeature PaperReview

Considerations on Possible Directions for a Wide Band Polarimetry X-ray Mission

by Paolo Soffitta, Enrico Costa, Nicolas De Angelis, Ettore Del Monte, Klaus Desch, Alessandro Di Marco, Giuseppe Di Persio, Sergio Fabiani, Riccardo Ferrazzoli, Markus Gruber, Takahashi Hiromitsu, Saba Imtiaz, Philip Kaaret, Jochen Kaminski, Dawoon E. Kim, Fabian Kislat, Henric Krawczynski, Fabio La Monaca, Carlo Lefevre, Hemanth Manikantan, Herman L. Marshall, Romana Mikusincova, Alfredo Morbidini, Fabio Muleri, Stephen L. O’Dell, Takashi Okajima, Mark Pearce, Vladislavs Plesanovs, Brian D. Ramsey, Ajay Ratheesh, Alda Rubini, Shravan Vengalil Menon and Martin C. Weisskopf Show full author list Hide full author list

Galaxies 2024, 12(4), 47; https://doi.org/10.3390/galaxies12040047 - 8 Aug 2024

Viewed by 509

Abstract

The Imaging X-ray Polarimetry Explorer (IXPE) has confirmed that X-ray polarimetry is a valuable tool in astronomy, providing critical insights into the emission processes and the geometry of compact objects. IXPE was designed to be sensitive in the 2–8 keV energy range for [...] Read more.

The Imaging X-ray Polarimetry Explorer (IXPE) has confirmed that X-ray polarimetry is a valuable tool in astronomy, providing critical insights into the emission processes and the geometry of compact objects. IXPE was designed to be sensitive in the 2–8 keV energy range for three primary reasons: (1) celestial X-ray sources are bright within this range, (2) the optics are effective, and (3) most sources across various classes were expected to exhibit some level of polarization. Indeed, IXPE is a great success, and its discoveries are necessitating the revision of many theoretical models for numerous sources. However, one of IXPE’s main limitations is its relatively narrow energy band, coupled with rapidly declining efficiency. In this paper, we will demonstrate the benefits of devising a mission focused on a broader energy band (0.1–79 keV). This approach leverages current technologies that align well with theoretical expectations and builds on the successes of IXPE. Full article

(This article belongs to the Special Issue X-ray Polarization: A New Era Begins)

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14 pages, 1795 KiB

Open AccessReview

X-ray Polarimetry of X-ray Pulsars

by Juri Poutanen, Sergey S. Tsygankov and Sofia V. Forsblom

Galaxies 2024, 12(4), 46; https://doi.org/10.3390/galaxies12040046 - 7 Aug 2024

Viewed by 461

Abstract

Radiation from X-ray pulsars (XRPs) was expected to be strongly linearly polarized owing to a large difference in their ordinary and extraordinary mode opacities. The launch of IXPE allowed us to check this prediction. IXPE observed a dozen X-ray pulsars, discovering pulse-phase dependent [...] Read more.

Radiation from X-ray pulsars (XRPs) was expected to be strongly linearly polarized owing to a large difference in their ordinary and extraordinary mode opacities. The launch of IXPE allowed us to check this prediction. IXPE observed a dozen X-ray pulsars, discovering pulse-phase dependent variation of the polarization degree (PD) and polarization angle (PA). Although the PD showed rather erratic profiles resembling flux pulse dependence, the PA in most cases showed smooth variations consistent with the rotating vector model (RVM), which can be interpreted as a combined effect of vacuum birefringence and dipole magnetic field structure at a polarization-limiting (adiabatic) radius. Application of the RVM allowed us to determine XRP geometry and to confirm the free precession of the NS in Her X-1. Deviations from RVM in two bright transients led to the discovery of an unpulsed polarized emission likely produced by scattering off the accretion disk wind. Full article

(This article belongs to the Special Issue X-ray Polarization: A New Era Begins)

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8 pages, 2851 KiB

Open AccessReview

The IXPE Science of Pulsars and Their Nebulae

by Niccolò Bucciantini, Roger W. Romani, Fei Xie and Josephine Wong

Galaxies 2024, 12(4), 45; https://doi.org/10.3390/galaxies12040045 - 7 Aug 2024

Viewed by 442

Abstract

Pulsars (PSRs) and Pulsar Wind Nebulae (PWNe) form some of the most interesting high-energy astrophysical systems. Their prominent synchrotron emission makes them ideal candidates for polarimetry. Here, after briefly summarizing the scientific rationale underpinning the importance of their polarimetric studies, we review the [...] Read more.

Pulsars (PSRs) and Pulsar Wind Nebulae (PWNe) form some of the most interesting high-energy astrophysical systems. Their prominent synchrotron emission makes them ideal candidates for polarimetry. Here, after briefly summarizing the scientific rationale underpinning the importance of their polarimetric studies, we review the current status and achievements obtained by the IXPE mission. For the first time, we have space- and phase-resolved X-ray data that allow us to set constraints on the geometry and level of turbulence of the magnetic field which, in turn, can help us to better understand which acceleration and radiation model(s) might be at work in these systems. Interestingly, PWNe show a large variety in terms of polarization degree that might indicate key physical differences, still to be further investigated. Full article

(This article belongs to the Special Issue X-ray Polarization: A New Era Begins)

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21 pages, 2057 KiB

Open AccessArticle

A Dust-Scattering Model for M1-92: A Revised Estimate of the Mass Distribution and Inclination

by Yun Qi Li, Mark R. Morris and Raghvendra Sahai

Galaxies 2024, 12(4), 44; https://doi.org/10.3390/galaxies12040044 - 5 Aug 2024

Viewed by 549

Abstract

Preplanetary nebulae (PPNe) are formed from mass-ejecting late-stage AGB stars. Much of the light from the star gets scattered or absorbed by dust particles, giving rise to the observed reflection nebula seen at visible and near-IR wavelengths. Precursors to planetary nebulae (PNe), PPNe [...] Read more.

Preplanetary nebulae (PPNe) are formed from mass-ejecting late-stage AGB stars. Much of the light from the star gets scattered or absorbed by dust particles, giving rise to the observed reflection nebula seen at visible and near-IR wavelengths. Precursors to planetary nebulae (PNe), PPNe generally have not yet undergone any ionization by UV radiation from the still-buried stellar core. Bipolar PPNe are a common form of observed PPNe. This study lays the groundwork for future dynamical studies by reconstructing the dust density distribution of a particularly symmetric bipolar PPN, M1-92 (Minkowski’s Footprint, IRAS 19343+2926). For this purpose, we develop an efficient single-scattering radiative transfer model with corrections for double-scattering. Using a V-band image from the Hubble Space Telescope (HST), we infer the dust density profile and orientation of M1-92. These results indicate that M1-92’s slowly expanding equatorial torus exhibits an outer radial cutoff in its density, which implicates the influence of a binary companion during the formation of the nebula. Full article

(This article belongs to the Special Issue Origins and Models of Planetary Nebulae)

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8 pages, 1762 KiB

Open AccessFeature PaperReview

The IXPE View of Neutron Star Low-Mass X-ray Binaries

by Francesco Ursini, Andrea Gnarini, Fiamma Capitanio, Anna Bobrikova, Massimo Cocchi, Alessandro Di Marco, Sergio Fabiani, Ruben Farinelli, Fabio La Monaca, John Rankin, Mary Lynne Saade and Juri Poutanen

Galaxies 2024, 12(4), 43; https://doi.org/10.3390/galaxies12040043 - 4 Aug 2024

Viewed by 522

Abstract

Low-mass X-ray binaries hosting weakly magnetized neutron stars (NS-LMXBs) are among the brightest sources in the X-ray sky. Since 2021, the Imaging X-ray Polarimetry Explorer (IXPE) has provided new measurements of the X-ray polarization of these sources. IXPE observations have revealed that most [...] Read more.

Low-mass X-ray binaries hosting weakly magnetized neutron stars (NS-LMXBs) are among the brightest sources in the X-ray sky. Since 2021, the Imaging X-ray Polarimetry Explorer (IXPE) has provided new measurements of the X-ray polarization of these sources. IXPE observations have revealed that most NS-LMXBs are significantly polarized in the X-rays, providing unprecedented insight into the geometry of their accretion flow. In this review paper, we summarize the first results obtained by IXPE on NS-LMXBs, the emerging trends within each class of sources (atoll/Z), and possible physical interpretations. Full article

(This article belongs to the Special Issue X-ray Polarization: A New Era Begins)

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18 pages, 661 KiB

Open AccessArticle

Steps toward Unraveling the Structure and Formation of Five Polar Ring Galaxies

by Kyle E. Lackey, Varsha P. Kulkarni and Monique C. Aller

Galaxies 2024, 12(4), 42; https://doi.org/10.3390/galaxies12040042 - 31 Jul 2024

Viewed by 564

Abstract

Polar ring galaxies (PRGs) are unusual relative to common galaxies in that they consist of a central host galaxy—usually a gas-poor, early-type S0 or elliptical galaxy—surrounded by a ring of gas, dust and stars that orbit perpendicular to the major axis of the [...] Read more.

Polar ring galaxies (PRGs) are unusual relative to common galaxies in that they consist of a central host galaxy—usually a gas-poor, early-type S0 or elliptical galaxy—surrounded by a ring of gas, dust and stars that orbit perpendicular to the major axis of the host. Despite the general quiescence of early-type galaxies (ETGs) and the rings’ lack of spiral density waves, PRGs are the sites of significant star formation relative to typical ETGs. To study these structures and improve PRG statistics, we obtained and analyzed infrared (IR) images from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope, and combined these IR data with archival optical data from both the Sloan Digital Sky Survey and the Hubble Space Telescope, and with optical imaging data we obtained with the Gemini South Observatory. We performed structural decomposition and photometry for five PRGs, and fit the spectral energy distributions (SEDs) of each PRG component to estimate the stellar masses, ages, and other physical properties of the PRG components. We show that PRC B-12 and PRC B-22, both lacking previous analysis, obey trends commonly observed among PRGs. We find that the stellar masses of polar rings can be a significant fraction of the host galaxy’s stellar masses (∼10–30%). We note, however, that our estimates of stellar mass and other physical properties are the results of SED fitting and not direct measurements. Our findings corroborate both previous theoretical expectations and measurements of existing samples of PRGs and indicate the utility of SED fitting in the context of these unusual galaxies, which historically have lacked multi-wavelength photometry of their stellar components. Finally, we outline future improvements needed for more definitive studies of PRGs and their formation scenarios. Full article

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15 pages, 4117 KiB

Open AccessArticle

A Study of Interstellar Medium in the Line of Sight of Transient Neutron Star Low-Mass X-ray Binary, MXB 1659-298, by Timing and Spectral Analysis

by Rabindra Mahato, Parag Bhattacharya and Monmoyuri Baruah

Galaxies 2024, 12(4), 41; https://doi.org/10.3390/galaxies12040041 - 31 Jul 2024

Viewed by 471

Abstract

This work is dedicated to the study of interstellar medium (ISM) along the line of sight (LOS) of the transient low-mass X-ray binary, MXB 1659-298, capitalizing the high resolving power of XMM-Newton in the soft energy range. We emphasized the analysis of reflection [...] Read more.

This work is dedicated to the study of interstellar medium (ISM) along the line of sight (LOS) of the transient low-mass X-ray binary, MXB 1659-298, capitalizing the high resolving power of XMM-Newton in the soft energy range. We emphasized the analysis of reflection grating spectrometer (RGS) data in the energy range 0.5–2.15 keV, suitable for the study of ISM. The paper includes an explanation of why, in the soft X-ray energy range, only two observations (out of seven) were deemed eligible for analysis. Three absorption lines associated with highly ionized Fe XX (1s²2p²-2p² (³p) 4d), Si XIV (1s²-1s2p), and Mg XI (1s²-1s6p) were identified in the observations, with IDs of 8620701(2001) and 748391601(2015). These new absorption lines and the absorption edge due to the neutral oxygen K edge seen in the spectra validate the multiphase structure of ISM. The predominance of interstellar medium over the ionized absorber is established along the direction of the source. The equivalent hydrogen column density measured is nearly equal to the galactic HI value derived previously. The small value of the ionic column density of Fe, Si, and Mg in the site of the high-temperature region resembles previous findings. Full article

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5 pages, 298 KiB

Open AccessArticle

SS 433: Flares and L₂ Overflow Spirals

by Michael Bowler

Galaxies 2024, 12(4), 40; https://doi.org/10.3390/galaxies12040040 - 18 Jul 2024

Viewed by 485

Abstract

Flaring in the SS 433 microquasar is dominated by outbursts from material at distances from the centre of mass of the binary system comparable to the separation of the two components. This note completes a demonstration that ejected plasma leaves the system in [...] Read more.

Flaring in the SS 433 microquasar is dominated by outbursts from material at distances from the centre of mass of the binary system comparable to the separation of the two components. This note completes a demonstration that ejected plasma leaves the system in the region of the L₂ point, there overflowing the outer Roche lobe and giving rise to a spiral structure as it leaves the system as part of the local environment. It also provides a new measure of the mass ratio of the binary. Full article

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19 pages, 5710 KiB

Open AccessFeature PaperReview

Planetary Nebulae Research: Past, Present, and Future

by Sun Kwok

Galaxies 2024, 12(4), 39; https://doi.org/10.3390/galaxies12040039 - 17 Jul 2024

Viewed by 936

Abstract

We review the evolution of our understanding of the planetary nebulae phenomenon and their place in the scheme of stellar evolution. The historical steps leading to our current understanding of central star evolution and nebular formation are discussed. Recent optical imaging, X-ray, ultraviolet, [...] Read more.

We review the evolution of our understanding of the planetary nebulae phenomenon and their place in the scheme of stellar evolution. The historical steps leading to our current understanding of central star evolution and nebular formation are discussed. Recent optical imaging, X-ray, ultraviolet, infrared, millimeter wave, and radio observations have led to a much more complex picture of the structure of planetary nebulae. The optically bright regions have multiple shell structures (rims, shells, crowns, and haloes), which can be understood within the interacting winds framework. However, the physical mechanism responsible for bipolar and multipolar structures that emerged during the proto-planetary nebulae phase is yet to be identified. Our morphological classifications of planetary nebulae are hampered by the effects of sensitivity, orientation, and field-of-view coverage, and the fraction of bipolar or multipolar nebulae may be much higher than commonly assumed. The optically bright bipolar lobes may represent low-density, ionization-bounded cavities carved out of a neutral envelope by collimated fast winds. Planetary nebulae are sites of active synthesis of complex organic compounds, suggesting that planetary nebulae play a major role in the chemical enrichment of the Galaxy. Possible avenues of future advancement are discussed. Full article

(This article belongs to the Special Issue Origins and Models of Planetary Nebulae)

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9 pages, 2148 KiB

Open AccessReview

On the Making of IXPE

by Martin C. Weisskopf and Brian Ramsey

Galaxies 2024, 12(4), 38; https://doi.org/10.3390/galaxies12040038 - 16 Jul 2024

Viewed by 501

Abstract

Drs. Weisskopf and Ramsey were the original Principal and Deputy Principal Investigators of the Imaging X-ray Polarimetry Explorer (IXPE). They outline the path to the development of IXPE and discuss the technical and programmatic history that led up to the mission, a partnership [...] Read more.

Drs. Weisskopf and Ramsey were the original Principal and Deputy Principal Investigators of the Imaging X-ray Polarimetry Explorer (IXPE). They outline the path to the development of IXPE and discuss the technical and programmatic history that led up to the mission, a partnership between the Italian Space Agency and NASA, and the first fully dedicated to imaging X-ray polarimetry in the 2–8 keV band. An admittedly biased, as seen through the eyes of the original and Deputy Principal Investigators, technical overview of the development of the historical and critical scientific instrumentation is provided. The outstanding, and often paradigm-shifting results are presented in the papers following this one. Full article

(This article belongs to the Special Issue X-ray Polarization: A New Era Begins)

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9 pages, 1343 KiB

Open AccessArticle

Correlations between IR Luminosity, Star Formation Rate, and CO Luminosity in the Local Universe

by Matteo Bonato, Ivano Baronchelli, Viviana Casasola, Gianfranco De Zotti, Leonardo Trobbiani, Erlis Ruli, Vidhi Tailor and Simone Bianchi

Galaxies 2024, 12(4), 37; https://doi.org/10.3390/galaxies12040037 - 8 Jul 2024

Viewed by 607

Abstract

We exploit the DustPedia sample of galaxies within approximately 40 Mpc, selecting 388 sources, to investigate the correlations between IR luminosity (L_IR), the star formation rate (SFR), and the CO(1-0) luminosity (L_CO) down to much lower luminosities than reached [...] Read more.

We exploit the DustPedia sample of galaxies within approximately 40 Mpc, selecting 388 sources, to investigate the correlations between IR luminosity (L_IR), the star formation rate (SFR), and the CO(1-0) luminosity (L_CO) down to much lower luminosities than reached by previous analyses. We find a sub-linear dependence of the SFR on L_IR. Below

log (L_{IR} / L_{⊙}) ≃ 10

or

SFR ≃ 1 M_{⊙} {yr}^{- 1}

, the SFR/L_IR ratio substantially exceeds the standard ratio for dust-enshrouded star formation, and the difference increases with decreasing L_IR values. This implies that the effect of unobscured star formation overcomes that of dust heating by old stars, at variance with results based on the Planck ERCSC galaxy sample. We also find that the relations between the L_CO and L_IR or the SFR are consistent with those obtained at much higher luminosities. Full article

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23 pages, 34923 KiB

Open AccessFeature PaperReview

Molecular Gas Kinematics in Local Early-Type Galaxies with ALMA

by Ilaria Ruffa and Timothy A. Davis

Galaxies 2024, 12(4), 36; https://doi.org/10.3390/galaxies12040036 - 2 Jul 2024

Viewed by 877

Abstract

Local early-type galaxies (ETGs) are mostly populated by old stars, with little or no recent star formation activity. For this reason, they have historically been believed to be essentially devoid of cold gas, which is the fuel for the formation of new stars. [...] Read more.

Local early-type galaxies (ETGs) are mostly populated by old stars, with little or no recent star formation activity. For this reason, they have historically been believed to be essentially devoid of cold gas, which is the fuel for the formation of new stars. Over the past two decades, however, increasingly-sensitive instrumentation observing the sky at (sub-)millimetre wavelengths has revealed the presence of significant amounts of cold molecular gas in the hearts of nearby ETGs. The unprecedented capabilities offered by the Atacama Large Millimeter/submillimeter Array (ALMA), in particular, have allowed us to obtain snapshots of the central regions of these ETGs with unprecedented detail, mapping this gas with higher sensitivity and resolution than ever before possible. Studies of the kinematics of the observed cold gas reservoirs are crucial for galaxy formation and evolution theories, providing, e.g., constraints on the fundamental properties and fuelling/feedback processes of super-massive black holes (SMBHs) at the centre of these galaxies. In this brief review, we summarise what the first 10 years of ALMA observations have taught us about the distribution and kinematics of the cold molecular gas component in nearby ellipticals and lenticulars. Full article

(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)

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11 pages, 548 KiB

Open AccessReview

An Examination of the Very First Polarimetric X-ray Observations of Radio-Quiet Active Galactic Nuclei

by Frédéric Marin, Vittoria E. Gianolli, Adam Ingram, Dawoon E. Kim, Andrea Marinucci, Daniele Tagliacozzo and Francesco Ursini

Galaxies 2024, 12(4), 35; https://doi.org/10.3390/galaxies12040035 - 2 Jul 2024

Viewed by 672

Abstract

Active galactic nuclei (AGNs), either radio-quiet or radio-loud, had never been observed in X-ray polarized light until the advent of the Imaging X-ray Polarimetry Explorer (IXPE) in the end of 2021. This satellite opened a new observational window for studying supermassive black holes [...] Read more.

Active galactic nuclei (AGNs), either radio-quiet or radio-loud, had never been observed in X-ray polarized light until the advent of the Imaging X-ray Polarimetry Explorer (IXPE) in the end of 2021. This satellite opened a new observational window for studying supermassive black holes and their complex environment. In this regard, radio-quiet AGNs are probably better targets than radio-loud objects to probe accretion processes due to the lack of synchrotron emission from jets that can dilute the polarized signal from the central engine. Their relatively clean environment not only allows to detect and measure the X-ray polarization originating from the hot corona responsible for X-ray emission, but also to assess the geometry of the media immediately surrounding the supermassive black hole. Such geometrical measurements work just as well for characterizing the corona morphology in pole-on AGNs as it does for determining the three-dimensional shape of the circumnuclear cold obscurer (the so-called torus) in edge-on AGNs. In this review paper, we will return to each of the observations made by IXPE so far in the field of radio-quiet AGNs and highlight the fundamental contribution of X-ray polarimetry to our understanding of how light is emitted and how matter is shaped around supermassive black holes. Full article

(This article belongs to the Special Issue X-ray Polarization: A New Era Begins)

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15 pages, 537 KiB

Open AccessArticle

The First VHE Activity of OJ 287 and the Extragalactic Background Light

by Sameer Yadav and Pankaj Kushwaha

Galaxies 2024, 12(4), 34; https://doi.org/10.3390/galaxies12040034 - 30 Jun 2024

Viewed by 660

Abstract

The BL Lacertae (BL Lac) object OJ 287 underwent an intense X-ray activity phase, exhibiting its brightest recorded X-ray flare in 2016–2017, characterized by much softer X-ray spectra and, concurrently, its first-ever recorded very-high-energy (VHE) emission (100–560 GeV), reported by the VERITAS observatory. [...] Read more.

The BL Lacertae (BL Lac) object OJ 287 underwent an intense X-ray activity phase, exhibiting its brightest recorded X-ray flare in 2016–2017, characterized by much softer X-ray spectra and, concurrently, its first-ever recorded very-high-energy (VHE) emission (100–560 GeV), reported by the VERITAS observatory. Broadband spectral energy distribution reveals a new jet emission component similar to high-synchrotron-peaked BL Lac objects, thereby implying the soft X-ray spectrum for the synchrotron emission. Using the advantage of simultaneous X-ray and VHE spectral information, as well as the source being a low-synchrotron-peaked BL Lac object, we systematically explored the extragalactic background light (EBL) spectrum by demanding that the VHE spectrum cannot be harder than the X-ray spectrum. We used three different phenomenological forms of the EBL spectral shape (power-law, parabola, and polynomial) motivated by current constraints on the EBL with the Bayesian Monte Carlo approach to infer the credible EBL range. Our study favors an almost flat power-law spectral shape and is consistent with previous studies. The other spectral forms capable of capturing curvature though result in a better statistics value; the improvement is statistically insignificant given the additional parameters. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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14 pages, 919 KiB

Open AccessArticle

More Luminous Red Novae That Require Jets

by Noam Soker

Galaxies 2024, 12(4), 33; https://doi.org/10.3390/galaxies12040033 - 26 Jun 2024

Cited by 1 | Viewed by 933

Abstract

In this paper, I study two intermediate luminosity optical transients (ILOTs), classified as luminous red novae (LRNe), and argue that their modeling with a common envelope evolution (CEE) without jets encounters challenges. LRNe are ILOTs powered by violent binary interaction. Although in the [...] Read more.

In this paper, I study two intermediate luminosity optical transients (ILOTs), classified as luminous red novae (LRNe), and argue that their modeling with a common envelope evolution (CEE) without jets encounters challenges. LRNe are ILOTs powered by violent binary interaction. Although in the literature it is popular to assume a CEE is the cause of LRNe, I here repeat an old claim that many LRNe are powered by grazing envelope evolution (GEE) events; the GEE might end in a CEE or a detached binary system. I find that the LRN AT 2021biy might have continued to experience mass ejection episodes after its eruption and, therefore, might not have suffered a full CEE during the outburst. This adds to an earlier finding that a jetless model does not account for some of its properties. I find that a suggested jetless CEE model for the LRN AT 2019zhd does not reproduce its photosphere radius evolution. These results that challenge jetless models of two LRNe strengthen a previous claim that jets play major roles in powering ILOTs and shaping their ejecta and that, in many LRNe, the more compact companion launches the jets during a GEE. Full article

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16 pages, 608 KiB

Open AccessArticle

A 3 mm Spectral Line Study of the Central Molecular Zone Infrared Dark Cloud G1.75-0.08

by Oskari Miettinen and Miguel Santander-García

Galaxies 2024, 12(4), 32; https://doi.org/10.3390/galaxies12040032 - 25 Jun 2024

Viewed by 914

Abstract

Infrared dark clouds (IRDCs) are fruitful objects to study the fragmentation of interstellar filaments and initial conditions and early stages of high-mass (

M > 8

M_⊙) star formation. We used the Yebes 40 m and Institut de Radioastronomie Millimétrique (IRAM) [...] Read more.

Infrared dark clouds (IRDCs) are fruitful objects to study the fragmentation of interstellar filaments and initial conditions and early stages of high-mass (

M > 8

M_⊙) star formation. We used the Yebes 40 m and Institut de Radioastronomie Millimétrique (IRAM) 30 m radio telescopes to carry out the first single-pointing spectral line observations towards the IRDC G1.75-0.08, which is a filamentary Central Molecular Zone (CMZ) cloud. Our aim is to reach an improved understanding of the gas kinematics and dynamical state of the cloud and its two clumps that we call clumps A and B. We also aim to determine the fractional abundances of the molecules detected at 3 mm towards G1.75-0.08. We detected HNCO

(J_{K_{a}, K_{c}} = 4_{0, 4} - 3_{0, 3})

, HCN

(J = 1 - 0)

, and HCO⁺

(J = 1 - 0)

towards both clumps. The N₂H⁺

(J = 1 - 0)

line was detected only in clump B, while N₂D⁺

(J = 1 - 0)

was not detected at all. The HCN and HNCO spectra exhibit two velocity components. The abundances of the detected species are comparable to those in other IRDCs. An upper limit to the [N₂D⁺]/[N₂H⁺] deuterium fraction of <0.05 derived towards clump B is consistent with values observed in many high-mass clumps. The line mass analysis suggests that the G1.75-0.08 filament is subcritical by a factor of

11 \pm 6

, and the clumps were found to be gravitationally unbound (

α_{vir} > 2

). Our finding that G1.75-0.08 is strongly subcritical is atypical compared to the general population of Galactic filamentary clouds. The cloud’s location in the CMZ might affect the cloud kinematics similar to what has been found for the Brick IRDC, and the cloud’s dynamical state might also be the result of the turbulent motions or shear and tidal forces in the CMZ. Because the target clumps are dark at 70

μ

m and massive (several

10^{3}

M_⊙), they can be considered candidates for being high-mass starless clumps but not prestellar because they are not gravitationally bound. Full article

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18 pages, 2170 KiB

Open AccessArticle

Constraints on Prospective Deviations from the Cold Dark Matter Model Using a Gaussian Process

by Martiros Khurshudyan and Emilio Elizalde

Galaxies 2024, 12(4), 31; https://doi.org/10.3390/galaxies12040031 - 24 Jun 2024

Cited by 1 | Viewed by 559

Abstract

Recently, using Bayesian Machine Learning, a deviation from the cold dark matter model on cosmological scales has been put forward. Such a model might replace the proposed non-gravitational interaction between dark energy and dark matter, and help solve the

H_{0}

tension problem. [...] Read more.

Recently, using Bayesian Machine Learning, a deviation from the cold dark matter model on cosmological scales has been put forward. Such a model might replace the proposed non-gravitational interaction between dark energy and dark matter, and help solve the

H_{0}

tension problem. The idea behind the learning procedure relies on a generated expansion rate, while the real expansion rate is just used to validate the learned results. In the present work, however, the emphasis is put on a Gaussian Process (GP), with the available

H (z)

data confirming the possible existence of the already learned deviation. Three cosmological scenarios are considered: a simple one, with an equation-of-state parameter for dark matter

ω_{d m} = ω_{0} \neq 0

, and two other models, with corresponding parameters

ω_{d m} = ω_{0} + ω_{1} z

and

ω_{d m} = ω_{0} + ω_{1} z / (1 + z)

. The constraints obtained on the free parameters

ω_{0}

and

ω_{1}

hint towards a dynamical nature of the deviation. The dark energy dynamics is also reconstructed, revealing interesting aspects connected with the

H_{0}

tension problem. It is concluded, however, that improved tools and more data are needed, to reach a better understanding of the reported deviation. Full article

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Galaxies, Volume 12, Issue 4 (August 2024) – 20 articles

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