arXiv subjects

A. Escala

Publications and source records attributed to A. Escala.

14 recordsLinked to original sources

arXiv2026

Constraints on dynamically-formed massive black holes in Little Red Dots from X-ray non-detections

The existence of massive, compact galaxies (Little Red Dots, LRDs) at $z \sim 2$ challenges early structure formation models, suggesting rapid stellar and black hole (BH) assembly. While LRDs are efficient environments for BH growth, many show no X-ray evidence of strong AGN emission. We utilize a subsample of X-ray non-detected LRDs to test the compatibility of collision-based BH formation scenarios and constrain physical parameters like metallicity and column density. Our results indicate LRDs are ideal birthplaces for massive BHs, particularly given a mass-radius relation $R_{gal} \propto M_{gal}^{0.6}$. Collision-based models suggest seed masses larger than those in the local Universe, consistent with high-redshift BH mass-radius relations. We modeled BH seed formation and X-ray emission (0.3-7 keV) against observed upper limits. We find that mass-radius exponents $> 0.55$ favor the collision-based scenario; however, consistency with stacked X-ray analysis requires specific accretion and obscuration parameters. Constant or increasing SFR scenarios with high Eddington ratios are feasible but necessitate larger column densities or higher metal enrichment. Alternatively, moderate sub-Eddington accretion reconciles massive seeds with observed masses and X-ray weakness. We conclude that even if LRDs began as starbursts, they should eventually evolve into AGNs.

astro-ph.GA↗

arXiv2025

Efficient black hole seed formation in low metallicity and dense stellar clusters with implications for JWST sources

Recent observations with the James Webb Space Telescope (JWST) reveal young massive clusters (YMCs) as key building blocks of early galaxies. They are not only important constituents of galaxies, but also potential birthplaces of very massive stars (VMSs) and black hole (BH) seeds. We explore stellar dynamics in extremely dense clusters with initial half-mass densities of $\rho_h \gtrsim 10^8M_\odot{\rm pc}^{-3}$ at very low metallicity, comparable to some of the densest clusters seen by JWST. Using direct N-body and Monte Carlo simulations with stellar evolution, we show that VMS formation through collisions is unavoidable, with final masses reaching $5\times10^3$ to $4\times10^4M_\odot$. These results support the existence of a critical mass scale above which collisions become highly efficient, enabling the formation of VMSs and intermediate-mass BHs (IMBHs). Our models, using nbody6++gpu and MOCCA with updated SSE/BSE routines, show that dense clusters rapidly form VMSs via stellar bombardment. The VMSs then collapse into BH seeds of a few $10^3$ to $10^4M_\odot$ in less than 4 Myr. We identify a critical mass-density threshold beyond which clusters undergo runaway collisions that yield massive BH seeds. For typical YMCs detected by JWST, efficiencies up to 10% are expected, implying BH masses up to $10^5M_\odot$ if formed via collisions. We predict a scaling relation for BH mass, $\log(M_{\rm BH}/M_\odot)=-0.76+0.76\log(M/M_\odot)$. Frequent VMS formation may also explain the high nitrogen abundance observed in galaxies at high redshift.

astro-ph.GA↗

arXiv2025

Insights into Supermassive Black Hole Mergers from the Gravitational Wave Background

At the Kavli Institute for Theoretical Physics, participants of the rapid response workshop on the gravitational wave background explored discrepancies between experimental results and theoretical models for a background originating from supermassive black hole binary mergers. Underestimated theoretical and/or experimental uncertainties are likely to be the explanation. Another key focus was the wide variety of search methods for supermassive black hole binaries, with the conclusion that the most compelling detections would involve systems exhibiting both electromagnetic and gravitational wave signatures

astro-ph.HE↗

arXiv2024

The supermassive black hole population from seeding via collisions in Nuclear Star Clusters

The coexistence of nuclear star clusters (NSCs) and supermassive black holes (SMBHs) in galaxies with stellar masses $\sim 10^{10}~$M$_\odot$, the scaling relations between their properties and properties of the host galaxy (e.g., $M_{NSC}^{stellar}-M_{galaxy}^{stellar}$, $M_{BH}-M_{galaxy}^{stellar}$), and the fact that NSCs seem to take on the role of SMBHs in less massive galaxies and vice versa in the more massive ones, suggest that the origin of NSCs and SMBHs is related. In this study, we implement an 'in-situ' NSC formation scenario, where NSCs are formed in the center of galaxies due to star formation in the accumulated gas. We explore the impact of the free parameter $A_{res}$ which regulates the amount of gas transferred to the NSC reservoir, playing a crucial role in shaping the cluster's growth. Simultaneously, we include a BH seed formation recipe based on stellar collisions within NSCs in the Semi-Analytical Model (SAM) Galacticus to explore the resulting population of SMBHs. We determine the parameter space of the NSCs that form a BH seed and find that in initially more compact NSCs the formation of these BH seeds is more favorable, leading to the formation of light, medium and heavy BH seeds which finally reach masses up to $\sim 10^9$~M$_\odot$ and is comparable with the observed SMBH mass function at masses above $10^8$~M$_\odot$. Additionally, we compare the resulting population of NSCs with a derived NSC mass function from the stellar mass function of galaxies from the GAMA survey at $z<0.06$ finding a well agreement in shape terms. We also find a considerable overlap in the observed scaling relations between the NSC mass, and the host galaxy stellar mass and velocity dispersion which is independent of the value of $A_{res}$. However, the chi-square analysis suggests that the model requires further refinement to achieve better quantitative agreement.

astro-ph.GA↗

arXiv2024

Efficiency of black hole formation via collisions in stellar systems: An analysis of data from simulations and observations

This paper explores the theoretical relation between star clusters and black holes within, focusing on the potential role of nuclear star clusters (NSCs), globular clusters (GCs), and ultra compact dwarf galaxies (UCDs) as environments that allow for black hole formation via stellar collisions. This study aims to identify the optimal conditions for stellar collisions across a range of stellar systems leading to the formation of very massive stars that subsequently collapse into black holes. We analyze data from numerical simulations and observations of diverse stellar systems, encompassing various initial conditions, initial mass functions, and evolution scenarios. We computed a critical mass, determined by the interplay of collision time, system age, and initial properties of the star cluster. The efficiency of black hole formation ($\epsilon_{\mathrm{BH}}$) is defined as the ratio of initial stellar mass divided by critical mass. We find that stellar systems with a ratio of initial stellar mass over critical mass above 1 exhibit a high efficiencies in terms of black hole formation, ranging from $30-100\%$. While there is some scatter, potentially attributed to complex system histories and the presence of gas, the results highlight the potential for achieving high efficiencies via a purely collisional channel in black hole formation. In conclusion, this theoretical exploration elucidates the connection between star clusters and black hole formation. The study underscores the significance of UCDs, GCs, and NSCs as environments conducive to the black hole formation scenario via stellar collisions. The defined black hole formation efficiency ($\epsilon_{\mathrm{BH}}$) is shown to be influenced by the ratio of the initial stellar mass to the critical mass.

astro-ph.GA↗

arXiv2023

Origin of supermassive black holes: predictions for the black hole population

The presence of supermassive black holes at redshift z > 6 raises some questions about their formation and growth in the early universe. Due to the construction of new telescopes like the ELT to observe and detect SMBHs, it will be useful to derive theoretical estimates for the population and to compare observations and model predictions in the future. In consequence our main goal is to estimate the population of SMBHs using a semi-analytic code known as Galacticus which is a code for the formation and evolution of galaxies where we are about to include different scenarios for SMBHs formation indicating the initial mass of the black hole seed, its formation conditions and recipes for the evolution of the components of the galaxies. We found that the principal mechanism of growing SMBHs is is via galaxy mergers and accretion of matter. For the comparison of our results with observations, we calculate the radius of influence of the black hole to estimate which part of the population could be detected, leading to relations similar to the observed ones.

astro-ph.CO↗

arXiv2023

Global instability by runaway collisions in nuclear stellar clusters: Numerical tests of a route for massive black hole formation

The centres of galaxies host nuclear stellar clusters, supermassive black holes, or both. The origin of this dichotomy is still a mystery. Nuclear stellar clusters are the densest stellar system in the Universe, so they are ideal places for runaway collisions to occur. Previous studies have proposed the possible existence of a critical mass scale in such clusters, for which the occurrence of collisions becomes very frequent and leads to the formation of a very massive object. While it is difficult to directly probe this scenario with simulations, we here aim for a proof of concept using toy models where the occurrence of such a transition is shown based on simplified compact systems, where the typical evolution time-scales will be faster compared to the real Universe. Indeed our simulations confirm that such a transition takes place and that up to 50 per cent of the cluster mass can go into the formation of a central massive object for clusters that are above the critical mass scale. Our results thus support the proposed new scenario on the basis of idealized simulations. A preliminary analysis of observed nuclear star clusters shows similar trends related to the critical mass as in our simulations. We further discuss the caveats for the application of the proposed scenario in real nuclear star clusters.

astro-ph.GA↗

arXiv2019

The kiloparsec-scale gas kinematics in two star-forming galaxies at $z \sim 1.47$ seen with ALMA and VLT-SINFONI

We present Atacama Large Millimeter/submillimeter Array (ALMA) CO($J=2-1$) observations of two main-sequence star-forming galaxies at $z\sim1.47$ taken from the High-Z Emission Line Survey (HiZELS). These two systems have been previously reported to be molecular gas rich $f_{\rm H_2} \equiv M_{\rm H_2}/(M_{\rm H_2} + M_\star) \sim 0.8$. We carried out a follow-up study to resolve, at $\sim$kpc-scales, the CO emission. These new observations are combined with our earlier ALMA observations (sensitive to diffuse CO emission) and compared with our previous H$α$-based study at matched spatial resolution. One target is marginally resolved in CO(2-1), showing complex dynamics with respect to the ionised gas traced by H$α$. While the other source is spatially resolved, enabling a detailed exploration of its internal dynamical properties. In this system, both gaseous phases show similar spatial extension, rotation velocities and velocity dispersions ($V_{\rm rot} \sim σ_v \sim 100$\,km\,s$^{-1}$) suggesting a rotational velocity to velocity dispersion ratio consistent with unity. By comparing the ionized and molecular gas tracers through the use of a two-dimensional kinematic model, we estimate a median depletion time $τ_{\rm dep}=2.3 \pm 1.2$\,Gyr for the galaxy as a whole. This value is in agreement with the average $τ_{\rm dep}$ value observed in local star-forming galaxies at similar spatial scales. Using a thick-disk dynamical modelling, we derive a dynamical mass $M_{\rm dyn} = (1.59\pm0.19) \times 10^{11}$\,$M_\odot$ within $\approx 6$\,kpc. This suggests a dark matter fraction ($f_{\rm DM} \equiv M_{\rm DM}/M_{\rm dyn}$) of $0.59\pm0.10$, in agreement with the average $f_{\rm DM}$ value derived from stacked rotation curve analysis of galaxies at similar redshift range.

astro-ph.GA↗

arXiv2018

VALES V: A kinematic analysis of the molecular gas content in $H$-ATLAS galaxies at $z\sim0.03-0.35$ using ALMA

We present Atacama Large Millimeter/submillimeter Array (ALMA) resolved observations of molecular gas in galaxies up to $z=0.35$ to characterise the role of global galactic dynamics on the global interstellar medium (ISM) properties. These observations consist of a sub-sample of 39 galaxies taken from the Valparaíso ALMA Line Emission Survey (VALES). From the CO($J=1-0)$ emission line, we quantify the kinematic parameters by modelling the velocity fields. We find that the IR luminosity increases with the rotational to dispersion velocity ratio ($V_{\rm rot}/σ_v$, corrected for inclination). We find a dependence between $V_{\rm rot}/σ_v$ and the [CII]/IR ratio, suggesting that the so-called `[CII] deficit' is related to the dynamical state of the galaxies. We find that global pressure support is needed to reconcile the dynamical mass estimates with the stellar masses in our systems with low $V_{\rm rot}/σ_v$ values. The star formation rate (SFR) is weakly correlated with the molecular gas fraction ($f_{\rm H_2}$) in our sample, suggesting that the release of gravitational energy from cold gas may not be the main energy source of the turbulent motions seen in the VALES galaxies. By defining a proxy of the `star formation efficiency' parameter as the SFR divided by the CO luminosity (SFE$'\equiv$ SFR/L$'_{\rm CO}$), we find a constant SFE$'$ per crossing time ($t_{\rm cross}$). We suggest that $t_{\rm cross}$ may be the controlling timescale in which the star formation occurs in dusty $z\sim0.03-0.35$ galaxies.

astro-ph.GA↗

arXiv2017

Cusps in the center of galaxies: a real conflict with observations or a numerical artefact of cosmological simulations?

Galaxy observations and N-body cosmological simulations produce conflicting dark matter halo density profiles for galaxy central regions. While simulations suggest a cuspy and universal density profile (UDP) of this region, the majority of observations favor variable profiles with a core in the center. In this paper, we investigate the convergency of standard N-body simulations, especially in the cusp region, following the approach proposed by \protect{\citep{13}}. We simulate the well known Hernquist model using the SPH code Gadget-3 and consider the full array of dynamical parameters of the particles. We find that, although the cuspy profile is stable, all integrals of motion characterizing individual particles suffer strong unphysical variations along the whole halo, revealing an effective interaction between the test bodies. This result casts doubts on the reliability of the velocity distribution function obtained in the simulations. Moreover, we find unphysical Fokker-Planck streams of particles in the cusp region. The same streams should appear in cosmological N-body simulations, being strong enough to change the shape of the cusp or even to create it. Our analysis, based on the Hernquist model and the standard SPH code, strongly suggests that the UDPs generally found by the cosmological N-body simulations may be a consequence of numerical effects. A much better understanding of the N-body simulation convergency is necessary before a 'core-cusp problem' can properly be used to question the validity of the CDM model.

astro-ph.GA↗

arXiv2016

SINFONI-HiZELS: The dynamics, merger rates & metallicity gradients of `typical' star-forming galaxies at $z$ = 0.8-2.2

We present adaptive optics (AO) assisted SINFONI integral field unit (IFU) spectroscopy of eleven H$α$ emitting galaxies selected from the High-Z Emission Line Survey (HiZELS). We obtain spatially resolved dynamics on ~kpc-scales of star-forming galaxies (stellar mass M$_\star$ = 10$^{9.5-10.5}$ M$_\odot$ and star formation rate SFR = 2-30 M$_\odot$ yr$^{-1}$) near the peak of the cosmic star-formation rate history. Combining these observations with our previous SINFONI-HiZELS campaign, we construct a sample of twenty homogeneously selected galaxies with IFU AO-aided observations -- the `SHiZELS' survey, with roughly equal number of galaxies per redshift slice, at $z$ = 0.8, 1.47, and 2.23. We measure the dynamics and identify the major kinematic axis by modelling their velocity fields to extract rotational curves and infer their inclination-corrected rotational velocities. We explore the stellar mass Tully-Fisher relationship, finding that galaxies with higher velocity dispersions tend to deviate from this relation. Using kinemetry analyses we find that galaxy interactions might be the dominant mechanism controlling the star-formation activity at $z$ = 2.23 but they become gradually less important down to $z$ = 0.8. Metallicity gradients derived from the [NII]/H$α$ emission line ratio show a median negative gradient for the SHiZELS survey of $Δ$log(O/H)/$Δ$R = -0.026$\pm$0.008 dex kpc$^{-1}$. We find that metal-rich galaxies tend to show negative gradients, whereas metal-poor galaxies tend to exhibit positive metallicity gradients. This result suggests that the accretion of pristine gas in the periphery of galaxies plays an important role in replenishing the gas in `typical' star-forming galaxies.

astro-ph.GA↗

arXiv2014

Super Star Clusters in Luminous Infrared Galaxies: the SUNBIRD Survey

We present recent results from an adaptive optics imaging survey of 40 Luminous IR Galaxies (LIRGs) searching for obscured core collapse supernovae and studying the galaxies themselves. Here, in particular, we discuss the Super Star Clusters (SSC) populations in the LIRGs. We have constructed the first statistically significant samples of Luminosity Functions (LF) of SSCs in the near-IR, and find evidence that the LF slopes in LIRGs are shallower than in more quiescent spiral galaxies. Distance and blending effects were investigated in detail paving the way for SSC studies further out than done previously. We have also correlated the luminosities of the brightest clusters with the star formation rates (SFR) of the hosts. The relation is similar, though somewhat steeper than that found in the optical and at lower SFR levels, suggesting systematic extinction and/or age effects. We find that the characteristics of the relation suggest an underlying physical driver rather than solely a size-of-sample effect. In particular, a truncated luminosity/mass function would naturally explain the small scatter we find. Finally, we are modelling the ages and masses of our near-IR detected clusters in conjunction with HST optical data and present early results of using SSC properties to trace the histories of the target LIRG systems.

astro-ph.GA↗

arXiv2009

Four Highly Luminous Massive Star Forming Regions in the Norma Spiral Arm II. Deep NIR imaging

We present sensitive NIR (J, H and K) imaging observations toward four luminous massive star forming regions in the Norma Spiral Arm: G324.201+0.119, G328.307+0.432, G329.337+0.147 and G330.949-0.174. We identify three clusters of young stellar objects (YSO) based on surface density diagnostics. We also find that sources detected only in the H and K-bands and with colors corresponding to spectral types earlier than B2, are likely YSOs. We analyze the spatial distribution of stars of different masses and find signatures in two clusters of primordial mass segregation which can't be explained as due to incompleteness effects. We show that dynamic interactions of cluster members with the dense gas from the parent core can explain the observed mass segregation, indicating that the gas plays an important role in the dynamics of young clusters.

astro-ph.SR↗

arXiv2009

The Evolution of Swift/BAT blazars and the origin of the MeV background

We use 3 years of data from the Swift/BAT survey to select a complete sample of X-ray blazars above 15 keV. This sample comprises 26 Flat-Spectrum Radio Quasars (FSRQs) and 12 BL Lac objects detected over a redshift range of 0.03<z<4.0. We use this sample to determine, for the first time in the 15--55 keV band, the evolution of blazars. We find that, contrary to the Seyfert-like AGNs detected by BAT, the population of blazars shows strong positive evolution. This evolution is comparable to the evolution of luminous optical QSOs and luminous X-ray selected AGNs. We also find evidence for an epoch-dependence of the evolution as determined previously for radio-quiet AGNs. We interpret both these findings as a strong link between accretion and jet activity. In our sample, the FSRQs evolve strongly, while our best-fit shows that BL Lacs might not evolve at all. The blazar population accounts for 10-20 % (depending on the evolution of the BL Lacs) of the Cosmic X-ray background (CXB) in the 15--55 keV band. We find that FSRQs can explain the entire CXB emission for energies above 500 keV solving the mystery of the generation of the MeV background. The evolution of luminous FSRQs shows a peak in redshift ($z_c$=4.3$\pm0.5$) which is larger than the one observed in QSOs and X-ray selected AGNs. We argue that FSRQs can be used as tracers of massive elliptical galaxies in the early Universe.

astro-ph.CO↗