Astrophysics of Galaxies (astro-ph.GA)

Abstract: Machine learning feature importance calculations are used to determine the molecular substructures that are responsible for mid and far-infrared (IR) emission features of neutral polycyclic aromatic hydrocarbons (PAHs). Using the extended-connectivity fingerprint as a descriptor of chemical structure, a random forest model is trained on the spectra of 14,124 PAHs to evaluate the importance of 10,632 molecular fragments for each band within the range of 2.761 to 1172.745 microns. The accuracy of the results is confirmed by comparing them with previously studied unidentified infrared emission (UIE) bands. The results are summarized in two tables available as Supplementary Data, which can be used as a reference for assessing possible UIE carriers. We demonstrate that the tables can be used to explore the relation between the PAH structure and the spectra by discussing about the IR features of nitrogen-containing PAHs and super-hydrogenated PAHs.

Abstract: Changing-look Active Galactic Nuclei (CL AGN) can be generally confirmed by the emergence (turn-on) or disappearance (turn-off) of broad emission lines, associated with a transient timescale (about $100\sim5000$ days) that is much shorter than predicted by traditional accretion disk models. We carry out a systematic CL AGN search by cross-matching the spectra coming from the Dark Energy Spectroscopic Instrument and the Sloan Digital Sky Survey. Following previous studies, we identify CL AGN based on $\rm{H}\alpha $, $\rm{H}\beta$, and Mg\,{\sc ii} at $z\leq0.75$ and Mg\,{\sc ii}, C\,{\sc iii}], and C\,{\sc iv} at $z>0.75$. We present 130 CL AGN based on visual inspection and three selection criteria, including 2 $\rm{H}\alpha$, 45 $\rm{H}\beta$, 38 Mg\,{\sc ii}, 61 C\,{\sc iii}], and 10 C\,{\sc iv} CL AGN. Twenty cases show simultaneous appearances/disappearances of two broad emission lines while three AGN exhibit the concurrent appearance of three broad emission lines. We also present 91 CL AGN candidates with significant flux variation of broad emission lines but remaining strong broad components. In the confirmed CL AGN, 42 cases show additional CL candidate features for different lines. In this paper, we find 1) a 95:35 ratio of a turn-on to turn-off CL AGN; 2) the highest redshift CL AGN ($z=3.56$) ever discovered; 3) an upper limit transition timescale ranging from 244 to 5762 days in the rest-frame; 4) the majority of CL AGN follow the bluer-when-brighter trend. Our results greatly increase the current CL census ($30\sim50\%$) and would be conducive to explore the underlying physical mechanism.

Abstract: Analytical models describing the dynamics of lobed radio sources are essential for interpretation of the tens of millions of radio sources that will be observed by the Square Kilometre Array and pathfinder instruments. We propose that historical models can be grouped into two classes in which the forward expansion of the radio source is driven by either the jet momentum flux or lobe internal pressure. The most recent generation of analytical models combines these limiting cases for a more comprehensive description. We extend the mathematical formalism of historical models to describe source expansion in non-uniform environments, and directly compare different model classes with each other, and with hydrodynamic numerical simulations. We quantify differences in predicted observable characteristics for lobed radio sources due to the different model assumptions for their dynamics. We make our code for the historical models analysed in this review openly available to the community.

Abstract: Radio-loud compact radio sources (CRSs) are characterised by morphological compactness of the jet structure centred on the active nucleus of the galaxy. Most of the local elliptical galaxies are found to host a CRS with nuclear luminosities lower than those of typical quasars, $\lesssim$10$^{42}\, {\rm erg\, s}^{-1}$. Recently, low-luminosity CRSs with a LINER-like optical spectrum have been named Fanaroff-Riley (FR) type 0 to highlight their lack of substantially extended radio emission at kpc scales, in contrast with the other Fanaroff-Riley classes, full-fledged FRIs and FRII radio galaxies. FR0s are the most abundant class of radio galaxies in the local Universe, and characterised by a higher core dominance, poorer Mpc-scale environment and smaller (sub-kpc scale, if resolved) jets than FRIs. However, FR0s share similar host and nuclear properties with FRIs. A different accretion-ejection paradigm from that in place in FRIs is invoked to account for the parsec-scale FR0 jets. This review revises the state-of-the-art knowledge about FR0s, their nature, and which open issues the next generation of radio telescopes can solve in this context.

Abstract: Atomic hydrogen (HI) is a critical stepping stone in the gas evolution cycle of the interstellar medium (ISM) of the Milky Way. Hi traces both the cold, premolecular state before star formation and the warm, diffuse ISM before and after star formation. This review describes new, sensitive HI absorption and emission surveys, which, together with high angular and spectral resolution Hi emission data, have revealed the physical properties of HI, its structure, and its association with magnetic fields. We give an overview of the HI phases and discuss how Hi properties depend on the environment and what its structure can tell us about feedback in the ISM. Key findings include the following: - The mass fraction of the cold neutral medium is $\lesssim 40$\% on average, increasing with $A_V$ due to the increase of mean gas density. - The cold disk extends to at least $R\sim 25$ kpc. - Approximately 40% of the HI is warm, with structural characteristics that derive from feedback events. - Cold HI is highly filamentary, whereas warm HI is more smoothly distributed. We summarize future observational and simulation opportunities that can be used to unravel the 3D structure of the atomic ISM and the effects of heating and cooling on HI properties.

Abstract: The age distribution of the open cluster system is a key piece of information to decipher the star formation history of the Milky Way disk. Recently, a remarkable earlier drop of its older end was found, which caught our attention. Precisely, we analyzed in detail the population of open clusters older than 1 Gyr located inside a circle of 2.0 kpc from the Sun contained in the Milky Way Star Cluster catalog, using the Data Release 3.0 of the Gaia survey, and found that it contains a slightly larger old open cluster population with respect to that witnessing the earlier drop age distribution. However, there are still some aspects that deserve further attention in order to undoubtedly handle a statistically complete cluster sample, that allows us to comprehensively know the older end of the open cluster age distribution function. We discuss some reasons that affect such a completeness, among them, the photometric depth of the database employed, the performance of machine learning techniques used to recognize open clusters, the cleaning of cluster color-magnitude diagrams from field star contamination, etc.

Abstract: This work presents the results of a kinematic analysis of the Galaxy that uses a new model as applied to the newest available Gaia data. We carry out the Taylor decomposition of the velocity field up to second order for 18 million high luminosity stars (i.e., young OB, giants and subgiants) from the Gaia DR3 data. We determine the components of mean stellar velocities and their first and second partial derivatives (relative to cylindrical coordinates) for more than 28 thousand points in the plane of our Galaxy. We estimate Oort's constants A, B, C, and K and other kinematics parameters and map them as a function of Galactocentric coordinates. The values found confirm the results of our previous works and are in excellent agreement with those obtained by other authors. In addition, the introduction of second order partial derivatives of the stellar velocity field allows us to determine the values of the vertical gradient of the Galaxy azimuthal, radial and vertical velocities. Also, we determine the mean of the Galaxy rotation curve for Galactocentric distances from 4 kpc to 18 kpc by averaging Galactic azimuths in the range -30$^\circ$<$\theta$<+30$^\circ$ about the direction Galactic Centre -- Sun --Galactic anticentre. Maps of the velocity components and of their partial derivatives with respect to coordinates within 10 kpc of the Sun reveal complex substructures, which provide clear evidence of non-axisymmetric features of the Galaxy. Finally, we show evidence of differences in the Northern and Southern hemispheres stellar velocity fields.

Abstract: The excellent sensitivity and optimum resolution of LoTSS DR2 at 144 MHz has enabled us to discover a giant radio galaxy (J1225+4011) with three distinct episodes of jet activity, making it a member of a class of objects called triple-double radio galaxies (TDRGs). The source extends overall up to 1.35 Mpc in projected size, with the second episode extending to 572 kpc, and the inner episode to 118 kpc. J1225+4011 is only the fourth radio source showing a triple-double radio structure. All four sources have overall sizes greater than 700 kpc, making them giants. We also present the LoTSS 144 MHz map of the TDRG J0929+4146 and report its updated size. Lastly, we have summarised and discussed the radio properties of all TDRGs for the first time to understand their growth and evolution. Our observations suggest that the power of their jets may decrease with time.

Abstract: We report the discovery of a quiescent galaxy at $z=2.34$ with a stellar mass of only $M_\star = 9.5^{+1.8}_{-1.2} \times 10^{8} \mathrm{M}_\odot$, based on deep JWST/NIRSpec spectroscopy. This is the least massive quiescent galaxy found so far at high redshift. We use a Bayesian approach to model the spectrum and photometry, and find the target to have been quiescent for 0.6 Gyr with a mass-weighted average stellar age of 0.8-1.7 Gyr (dominated by systematics). The galaxy displays an inverse colour gradient with radius, consistent with environment-driven quenching. Based on a combination of spectroscopic and robust (medium- and broad-band) photometric redshifts, we identify a galaxy overdensity near the location of the target (5-$\sigma$ above the background level at this redshift). We stress that had we been specifically targetting galaxies within overdensities, the main target would not have been selected on photometry alone; therefore, environment studies based on photometric redshifts are biased against low-mass quiescent galaxies. The overdensity contains three spectroscopically confirmed, massive, old galaxies ($M_\star = 8-17 \times 10^{10} \mathrm{M}_\odot$). The presence of these evolved systems points to accelerated galaxy evolution in overdensities at redshifts z > 2, in agreement with previous works. In projection, our target lies only 35 pkpc away from the most massive galaxy in this overdensity (spectroscopic redshift z = 2.349) which is located close to overdensity's centre. This suggests the low-mass galaxy was quenched by environment, making it possibly the earliest evidence for environment-driven quenching to date.

Abstract: After a quarter century since the detection of the last interstellar carboxylic acid, acetic acid (CH$_3$COOH), we report the discovery of a new one, the \textit{cis-trans} form of carbonic acid (HOCOOH), toward the Galactic Center molecular cloud G+0.693-0.027. HOCOOH stands as the first interstellar molecule containing three oxygen atoms and also the third carboxylic acid detected so far in the interstellar medium. Albeit the limited available laboratory measurements (up to 65 GHz), we have also identified several pairs of unblended lines directly in the astronomical data (between 75-120 GHz), which allowed us to slightly improve the set of spectroscopic constants. We derive a column density for \textit{cis-trans} HOCOOH of $N$ = (6.4 $\pm$ 0.4) $\times$ 10$^{12}$ cm$^{-2}$, which yields an abundance with respect to molecular H$_2$ of 4.7 $\times$ 10$^{-11}$. Meanwhile, the extremely low dipole moment (about fifteen times lower) of the lower-energy conformer, \textit{cis-cis} HOCOOH, precludes its detection. We obtain an upper limit to its abundance with respect to H$_2$ of $\leq$ 1.2 $\times$10$^{-9}$, which suggests that \textit{cis-cis} HOCOOH might be fairly abundant in interstellar space, although it is nearly undetectable by radio astronomical observations. We derive a \textit{cis-cis}/\textit{cis-trans} ratio $\leq$ 25, consistent with the smaller energy difference between both conformers compared with the relative stability of \textit{trans-} and \textit{cis}-formic acid (HCOOH). Finally, we compare the abundance of these acids in different astronomical environments, further suggesting a relationship between the chemical content found in the interstellar medium and the chemical composition of the minor bodies of the Solar System, which could be inherited during the star formation process.