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Hyper-massive Black Holes have Faint Broad and Narrow Emission Lines

Bhat Harshitha K., Chakravorty Susmita, Sengupta Dhrubojyoti, Elvis Martin, Datta Sudeb Ranjan, Roy Nirupam, Bertemes Caroline, Ferland Gary, Ezhikode Savithri H.
02 Jul 2020 astro-ph.GA arxiv.org/abs/2007.01438

The EUV provides most of the ionization that creates the high equivalent width (EW) broad and narrow emission lines (BELs, NELs) of quasars. Spectra of Hypermassive Schwarzschild black holes (HMBHs, $M_{BH} \geq 10^{10} M_{\odot}$) with $\alpha$-discs, decline rapidly in the EUV suggesting much lower EWs. Model spectra for black holes of mass $10^{6}-10^{12} M_{\odot}$ and accretion rates $0.03 \leq L_{bol}/L_{edd} \leq 1.0$ were input to the CLOUDY photoionization code. BELs become $\sim$100 times weaker in EW from $M_{BH} \sim 10^8 M_{\odot}$ to $M_{BH} \sim 10^{10} M_{\odot}$. The high ionization BELs (O VI 1034 $\overset{\circ}{\mathrm {A}}$, C IV 1549 $\overset{\circ}{\mathrm {A}}$, He II 1640 $\overset{\circ}{\mathrm {A}}$) decline in EW from ($M_{BH} \geq 10^6 M_{\odot}$, reproducing the Baldwin effect, but regain EW for $M_{BH} \geq 10^{10} M_{\odot}$). The low ionization lines (MgII 2798 $\overset{\circ}{\mathrm {A}}$, H$\beta$ 4861 $\overset{\circ}{\mathrm {A}}$ and H$\alpha$ 6563 $\overset{\circ}{\mathrm {A}}$) remain weak. Lines for maximally spinning HMBHs behave similarly. Line ratio diagrams for the BELs show that high OVI/H$\beta$ and low CIV/H$\alpha$ may pick out HMBH, although OVI is often hard to observe. In NEL BPT diagrams HMBHs lie among star-forming regions, except for highly spinning, high accretion rate HMBHs. In summary, the BELs expected from HMBHs would be hard to detect using the current optical facilities. From 100 to $10^{12} M_{\odot}$, the emission lines used to detect AGN only have high EW in the $10^6 - 10^9 M_{\odot}$ window, where most AGN are found. This selection effect may be distorting reported distributions of $M_{BH}$.

Calibration Requirements for Epoch of Reionization 21-cm signal observations -- I. Effect of time-correlated gains

Kumar Jais, Dutta Prasun, Roy Nirupam
14 May 2020 astro-ph.IM astro-ph.CO arxiv.org/abs/2005.07126

The residual gain errors add to the systematics of the radio interferometric observations. In case of the high dynamic range observations, these systematic effects dominates over the thermal noise of the observation. In this work, we investigate the effect of time-correlated residual gain errors in the estimation of the power spectrum of the sky brightness distribution in high dynamic range observations. Particularly, we discuss a methodology to estimate the bias in the power spectrum estimator of the redshifted 21-cm signal from neutral hydrogen in the presence of bright extragalactic compact sources. We find, that for the visibility based power spectrum estimators, particularly those use nearby baseline correlations to avoid noise bias, the bias in the power spectrum arises mainly from the time correlation in the residual gain error. The bias also depends on the baseline distribution for a particular observation. Analytical calculations show that the bias is dominant for certain types of baseline pairs used for the visibility correlation. We perform simulated observation of extragalactic compact sources in the presence of residual gain errors with the GMRT like array and estimate the bias in the power spectrum. Our results indicate that in order to estimate the redshifted 21-cm power spectrum, better calibration techniques and estimator development are required.

Characterizing EoR foregrounds: A study of the Lockman Hole Region at 325 MHz

Mazumder Aishrila, Chakraborty Arnab, Datta Abhirup, Choudhuri Samir, Roy Nirupam, Wadadekar Yogesh, Ishwara-Chandra C. H.
11 May 2020 astro-ph.CO astro-ph.GA arxiv.org/abs/2005.05205

One of the key science goals for the most sensitive telescopes, both current and upcoming, is the detection of the redshifted 21-cm signal from the Cosmic Dawn and Epoch of Reionization. The success of detection relies on accurate foreground modeling for their removal from data sets. This paper presents the characterization of astrophysical sources in the Lockman Hole region. Using 325 MHz data obtained from the GMRT, a $6^\circ \times 6^\circ$ mosaiced map is produced with an RMS reaching 50 $\mu$Jy $\mathrm{beam}^{-1}$. A source catalog containing 6186 sources is created, and the Euclidean normalized differential source counts have been derived from it, consistent with previous observations as well as simulations. A detailed comparison of the source catalog is also made with previous findings - at both lower and higher frequencies. The angular power spectrum (APS) of the diffuse Galactic synchrotron emission is determined for three different galactic latitudes using the Tapered Gridded Estimator. The values of the APS lie between $\sim$1 mK$^2$ to $\sim$100 mK$^2$. Fitting a power law of the form $A\ell^{-\beta}$ gives values of $A$ and $\beta$ varying across the latitudes considered. This paper demonstrates, for the first time, the variation of the power-law index for diffuse emission at very high galactic locations. It follows the same trend that is seen at locations near the galactic plane, thus emphasizing the need for low-frequency observations for developing better models of the diffuse emission.

Milky Way globular cluster dynamics: are they preferentially co-rotating?

Das Saikat, Roy Nirupam
28 Mar 2020 astro-ph.GA arxiv.org/abs/2003.12763

The motion of the baryonic components of the Milky Way is governed by both luminous and dark matter content of the Galaxy. Thus, the dynamics of the Milky Way globular clusters can be used as tracers to infer the mass model of the Galaxy up to a large radius. In this work, we use the directly observable line-of-sight velocities to test if the dynamics of the globular cluster population is consistent with an assumed axisymmetric gravitational potential of the Milky Way. For this, we numerically compute the phase space distribution of the globular cluster population where the orbits are either oriented randomly or co-/counter- rotating with respect to the stellar disk. Then we compare the observed position and line-of-sight velocity distribution of $\sim$ 150 globular clusters with that of the models. We found that, for the adopted mass model, the co-rotating scenario is the favored model based on various statistical tests. We do the analysis with and without the GCs associated to the progenitors of early merger events. This analysis can be extended in the near future to include precise and copious data to better constrain the Galactic potential up to a large radius.

All sky angular power spectrum: I. Estimating brightness temperature fluctuations using TGSS 150 MHz survey

Choudhuri Samir, Ghosh Abhik, Roy Nirupam, Bharadwaj Somnath, Intema Huib. T., Ali Sk. Saiyad
17 Mar 2020 astro-ph.CO astro-ph.GA arxiv.org/abs/2003.07869

Measurements of the Galactic synchrotron emission is relevant for the 21-cm studies from the Epoch of Reionization. The study of the synchrotron emission is also useful to quantify the fluctuations in the magnetic field and the cosmic ray electron density of the turbulent interstellar medium (ISM) of our Galaxy. Here, we present the all-sky angular power spectrum $(C_{\ell})$ measurements of the diffuse synchrotron emission using the TIFR GMRT Sky Survey (TGSS) at 150 {\rm MHz}. We estimate $C_{\ell}$ using visibility data both before and after subtracting the modelled point sources. The amplitude of the measured $C_{\ell}$ falls significantly after subtracting the point sources, and it is also slightly higher in the Galactic plane for the residual data. The residual $C_{\ell}$ is most likely to be dominated by the Galactic synchrotron emission. The amplitude of the residual $C_{\ell}$ falls significantly away from the Galactic plane. We find the measurements are quite symmetric in the Northern and Southern hemispheres except in the latitude range $15-30^{\circ}$ which is the transition region from the disk dominated to diffuse halo dominated region. The comparison between this interferometric measurement with the scaled version of the Haslam rms map at 150 {\rm MHz} shows that the correlation coefficient $(r)$ is more than 0.5 for most of the latitude ranges considered here. This signifies the TGSS survey is quite sensitive to the diffuse Galactic synchrotron radiation.

The study of the angular and spatial distribution of radio selected AGNs and star-forming galaxies in the ELAIS N1 field

Chakraborty Arnab, Dutta Prasun, Datta Abhirup, Roy Nirupam
27 Feb 2020 astro-ph.GA astro-ph.CO arxiv.org/abs/2002.12383

The cosmic evolution of bias of different source populations with underlying dark matter density field in post reionization era can shed light on large scale structures. Studying the angular and spatial distribution of different compact sources using deep radio catalogue at low-frequency is essential to understand the matter distribution of the present Universe. Here, we investigate the relationship of luminous matter with their host dark matter haloes by measuring the angular and spatial clustering of sources (two-point statistics), using deep radio observation of ELAIS N1 (EN1) field with upgraded Giant Metrewave Radio Telescope (uGMRT) at 300-500 MHz. We also analyze the 612 MHz GMRT archival data of the same field to understand the cosmic evolution of clustering of different source populations. We classify the sources as star-forming galaxies (SFGs) and active galactic nuclei (AGN) based on their radio luminosity. We find that the spatial clustering length and bias to the dark matter density field of SFGs are smaller than AGNs at both frequencies. This proves that AGNs are mainly hosted by massive haloes and hence strongly clustered. However, a small decrease in the bias for both kind of sources at higher frequency indicates that we are most likely tracing the faint objects residing in less massive haloes at higher frequencies. Our results are in excellent agreement with previous findings at radio and multi-frequency surveys. However, comparison with SKADS simulation suggests that the halo mass for different populations used in the simulation is systematically lower. This work quantifies the spatial distribution of extragalactic compact objects in EN1 field and bridges the gap between shallow and deep surveys.

Characterization of unresolved and unclassified sources detected in radio continuum surveys of the Galactic plane

Chakraborty Arnab, Roy Nirupam, Wang Y., Datta Abhirup, Beuther H., Medina S. -N. X., Menten K. M., Urquhart J. S., Brunthaler A., Dzib S. A.
07 Jan 2020 astro-ph.GA astro-ph.CO arxiv.org/abs/2001.02358

The continuum emission from 1 to 2 GHz of The HI/OH/Recombination line survey of the inner Milky Way (THOR) at $\lesssim$18" resolution covers $\sim 132$ square degrees of the Galactic plane and detects 10387 sources. Similarly, the first data release of the Global View of Star Formation in the Milky Way (GLOSTAR) surveys covers $\sim 16$ square degrees of the Galactic plane from 4-8 GHz at 18" resolution and detects 1575 sources. However, a large fraction of the unresolved discrete sources detected in these radio continuum surveys of the Galactic plane remain unclassified. Here, we study the Euclidean-normalized differential source counts of unclassified and unresolved sources detected in these surveys and compare them with simulated extragalactic radio source populations as well as previously established source counts. We find that the differential source counts for THOR and GLOSTAR surveys are in excellent agreement with both simulation and previous observations. We also estimate the angular two-point correlation function of unclassified and unresolved sources detected in THOR survey. We find a higher clustering amplitude in comparison with the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey up to the angular separation of $5^{\circ}$. The decrease in angular correlation with increasing flux cut and the excellent agreement of clustering pattern of sources above 1 mJy with high $z$ samples ($z >0.5$) of the FIRST survey indicates that these sources might be high $z$ extragalactic compact objects. The similar pattern of one-point and two-point statistics of unclassified and compact sources with extragalactic surveys and simulations confirms the extragalactic origin of these sources.

Gas-dust correlations in nearby galaxies: a case study of NGC 3184 and NGC 7793

Saikia Gautam, Patra Narendra Nath, Roy Nirupam, Jog Chanda J.
01 Jan 2020 astro-ph.GA arxiv.org/abs/2001.00168

The study of gas-dust interactions occurring in the interstellar medium of a galaxy is essential for understanding various physical processes taking place within it. A comparison of such events at different locations corresponding to diverse astrophysical environments provides more insight into the star-formation as well as dust destruction conditions and time-scales. We present a case study for two galaxies: NGC 3184 and NGC 7793, which are typical examples of a grand design spiral' and aflocculent spiral', respectively. We investigate the gas-dust correlations at various spatially resolved locations within each galaxy, including spiral arms, using archival data. Moreover, we have segregated the neutral gas into wide (warm) and narrow (cold) velocity components to check the correlations with individual dust emission bands. We find a positive correlation between the gas and the dust, with the total atomic gas emission mainly dominated by its warm component in both the galaxies. We also find the dust population in NGC 7793 to have a greater fraction of emission coming from cold and diffuse, larger-sized dust particles as compared to NGC 3184. This nearby galaxy pilot study could serve as a template for similar studies of larger galaxy samples with analogous morphologies.

A study of Kepler supernova remnant: angular power spectrum estimation from radio frequency data

Saha Preetha, Bharadwaj Somnath, Roy Nirupam, Choudhuri Samir, Chattopadhyay Debatri
07 Sep 2019 astro-ph.GA astro-ph.HE arxiv.org/abs/1909.03195

Supernova remnants (SNRs) have a variety of overall morphology as well as rich structures over a wide range of scales. Quantitative study of these structures can potentially reveal fluctuations of density and magnetic field originating from the interaction with ambient medium and turbulence in the expanding ejecta. We have used $1.5$GHz (L band) and $5$GHz (C band) VLA data to estimate the angular power spectrum $C_{\ell}$ of the synchrotron emission fluctuations of the Kepler SNR. This is done using the novel, visibility based, Tapered Gridded Estimator of $C_{\ell}$. We have found that, for $\ell = (1.9 - 6.9) \times 10^{4}$, the power spectrum is a broken power law with a break at $\ell = 3.3 \times 10^{4}$, and power law index of $-2.84\pm 0.07$ and $-4.39\pm 0.04$ before and after the break respectively. The slope $-2.84$ is consistent with 2D Kolmogorov turbulence and earlier measurements for the Tycho SNR. We interpret the break to be related to the shell thickness of the SNR ($0.35 $ pc) which approximately matches $\ell = 3.3 \times 10^{4}$ (i.e., $0.48$ pc). However, for $\ell > 6.9 \times 10^{4}$, the estimated $C_{\ell}$ of L band is likely to have dominant contribution from the foregrounds while for C band the power law slope $-3.07\pm 0.02$ is roughly consistent with $3$D Kolmogorov turbulence like that observed at large $\ell$ for Cas A and Crab SNRs.

Detailed study of ELAIS N1 field with the uGMRT -- II. Source Properties and Spectral Variation Of Foreground Power Spectrum from 300-500 MHz Observations

Chakraborty Arnab, Roy Nirupam, Datta Abhirup, Choudhuri Samir, Datta Kanan K., Dutta Prasun, Bharadwaj Somnath, Intema Huib, Choudhury Madhurima, Pal Srijita
27 Aug 2019 astro-ph.CO arxiv.org/abs/1908.10380

Understanding the low-frequency radio sky in depth is necessary to subtract foregrounds in order to detect the redshifted 21 cm signal of neutral hydrogen from the Cosmic Dawn, Epoch of Reionization (EoR) and post-reionization era. In this second paper of the series, we present the upgraded Giant Metrewave Radio Telescope (uGMRT) observation of the ELAIS N1 field made at 300-500 MHz. The image covers an area of $\sim 1.8$ $\mathrm{deg}^{2}$ and has a central background rms noise of $\sim$ 15 $\mu \mathrm{Jy}$ $\mathrm{beam}^{-1}$. We present a radio source catalog containing 2528 sources (with flux densities > 100 $\mu$Jy) and normalized source counts derived from that. The detailed comparison of detected sources with previous radio observations is shown. We discuss flux scale accuracy, positional offsets, spectral index distribution and correction factors in source counts. The normalized source counts are in agreement with previous observations of the same field, as well as model source counts from the Square Kilometre Array Design Study (SKADS) simulation. It shows a flattening below $\sim$1 mJy which corresponds to rise in population of star forming galaxies and radio-quiet AGN. For the first time, we estimated the spectral characteristics of the angular power spectrum or Multi-Frequency Angular Power Spectrum (MFAPS) of diffuse Galactic synchrotron emission (DGSE) over the wide frequency bandwidth of $300-500$~MHz from radio interferometric observations. This work demonstrates the improved capabilities of the uGMRT.