arXiv subjects

Saumyadip Samui

Publications and source records attributed to Saumyadip Samui.

At least 19 recordsLinked to original sources

arXiv2026

Signature of Bursty Star Formation in the High-Redshift Galaxies Detected with JWST

Recent JWST observations reveal an unexpectedly slow evolution in ultraviolet luminosity functions (UV LFs) at redshifts $z > 10$. To investigate this phenomenon, we develop a semi-analytical model of the UV LF, calibrated against well-constrained measurements at $z \sim 2-10$. Our analysis identifies a transition in star formation modes across cosmic epochs: at $z \lesssim 5$, a longer characteristic star formation timescale with nearly constant star formation efficiency ($f_\star$) dominates, whereas at $6 \lesssim z \lesssim 10$, shorter timescales prevail without requiring an increase in $f_\star$. For $z > 10$, the slow UV LF evolution is best explained by a shift toward even shorter star formation timescales without changing the star formation efficiency. Dust-free conditions or a top-heavy initial mass function (IMF) alone cannot reproduce the observations at $z\sim 14$. By combining UV LF with stellar mass estimates from Prospector-based SED fitting, we try to break degeneracies between IMF variations and star formation histories. Our results indicate that evolving star formation timescales rather than IMF or dust changes are the primary drivers of the observed high-redshift UV LF evolution, reflecting changing physical conditions during the earliest phases of galaxy assembly. Additionally, we show that moderate AGN activity could further boost UV luminosities at $z \sim 14$, potentially explaining the observed UV LF without changes in stellar parameters.

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arXiv2022

Impact of cosmic rays on the global 21-cm signal during cosmic dawn

It is extremely important to understand the processes through which the thermal state of the inter-galactic medium (IGM) evolved in the early universe in order to study the evolution of HI 21-cm signal during cosmic dawn. Here, we consider the heating of the IGM due to cosmic ray protons generated by the supernovae from both early Pop III and Pop II stars. The low energy cosmic ray protons from Pop III supernovae can escape from minihalos and heat the IGM via collision and ionization of hydrogen. Furthermore, high energy protons generated in Pop II supernovae can escape the hosting halos and heat the IGM via magnetosonic Alfvén waves. We show that the heating due to these cosmic ray particles can significantly impact the IGM temperature and hence the global 21-cm signal at $z\sim 14-18$. The depth, location, and duration of the 21-cm absorption profile are highly dependent on the efficiencies of cosmic ray heating. In particular, the EDGES signal can be well fitted by the cosmic ray heating along with the Lyman-$α$ coupling, and the dark matter-baryon interaction that we consider to achieve a `colder IGM background'. Further, we argue that the properties of cosmic rays and the nature of first generation of stars could be constrained by accurately measuring the global 21-cm absorption signal during the cosmic dawn.

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arXiv2022

Studying Cosmic Dawn using redshifted HI 21-cm signal: A brief review

In this review article, we briefly outline our current understanding of the physics associated with the HI 21-cm signal from cosmic dawn. We discuss different phases of cosmic dawn as the ambient gas and the background radiations evolve with the redshift. We address the consequences of several possible heating sources and radiation background on the global 21-cm signal. We further review our present perspective of other important aspects of the HI 21-cm signal such as the power spectrum and imaging. Finally, we highlight the future key measurements of the Square Kilometre Array and other ongoing/upcoming experiments that will enlighten our understanding of the early Universe.

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arXiv2021

Exploring the Voids: Luminosity Functions and Magnetic Field

We present semi-analytical models for magnetisation of the void inter-galactic medium (IGM) by outflows from void galaxies. The number density of dark matter haloes in an under-dense region (i.e., void) is obtained using the excursion set method extended for such low density environment. The star formation in such haloes has been estimated, taking account of the negative feedback by supernovae. The galaxy formation/evolution model is tuned to provide the $r$-band luminosity function, the stellar mass function and also the color of void galaxies as obtained from recent observations. This star formation model is used to study possible outflows from void galaxies driven by the hot thermal gas and cosmic ray pressures. These outflows drag the magnetic fields present in those galaxies to the void IGM. We show that such a model can magnetise $\sim 30\%$ of the void IGM with the magnetic field strength of $10^{-12}-10^{-10}$ G while considering only magnetic flux freezing condition. Along with this, the megapersec size of individual outflows can explain the non-detection of GeV photons in TeV blazars that put a lower limit of $10^{-16}$ G for void IGM magnetic field with a Mpc coherent length scale.

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arXiv2020

Primordial magnetic fields during the cosmic dawn in light of EDGES 21-cm signal

We study prospects of constraining the primordial magnetic field (PMF) and its evolution during the dark ages and cosmic dawn in light of EDGES 21-cm signal. Our analysis has been carried out on a `colder IGM' background which is one of the promising avenues to interpret the EDGES signal. We consider the dark matter-baryon interactions for the excess cooling. We find that the colder IGM suppresses both the residual free electron fraction and the coupling coefficient between the ionised and neutral components. The Compton heating also gets affected in colder IGM background. Consequently, the IGM heating rate due to the PMF enhances compared to the standard scenario. Thus, a significant fraction of the magnetic energy, for $B_0 \lesssim 0.5 \, {\rm nG}$, gets transferred to the IGM and the magnetic field decays at a much faster rate compared to the simple $(1+z)^2$ scaling during the dark ages and cosmic dawn. This low PMF is an unlikely candidate for explaining the rise of the EDGES absorption signal at lower redshift. We also see that the PMF and DM-baryon interaction together introduces a plateau-like feature in the redshift evolution of the IGM temperature. We find that the upper limit on the PMF depends on the underlying DM-baryon interaction. Higher PMF can be allowed when the interaction cross-section is higher and/or the DM particle mass is lower. Our study shows that the PMF with $B_0$ up to $\sim 0.4 \, {\rm nG}$, which is ruled out in the standard model, can be allowed if DM-baryon interaction with suitable cross-section and DM mass is considered.

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arXiv2019

On the star formation efficiency in high redshift Lyman-α emitters

We present semi-analytical models of high redshift Lyman-α emitters (LAEs) in order to constrain the star formation efficiency in those galaxies. Our supernova feedback induced star formation model along with Sheth-Tormman halo mass function correctly reproduces the shape, amplitude and the redshift evolution of UV and Lyman-α luminosity functions of LAEs in the redshift range z = 2 to 7.3. We show that the fraction of Lyman-α emitting galaxies increases with increasing redshifts reaching to unity just after the reionisation. However, we show that star formation efficiency in those LAEs does not show any redshift evolution within the uncertainty in available observations. This would have significant repercussion on the reionisation of the intergalactic medium.

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arXiv2018

Probing feedback in high-$z$ galaxies using extended UV luminosity functions

We fit the recently updated UV luminosity functions (LF) of high-$z$ ($1.5\le z\le8.0$) galaxies using our semi-analytical models of galaxy formation that take into account various feedback processes. In order to reproduce the overall redshift evolution we require the efficiency of converting gas into stars to decrease with decreasing redshift. Even for $z\ge 6$, our models require supernovae (SNe) feedback to reproduce the observed LF suggesting the prevalence of galactic winds that could have polluted the inter-galactic medium even at very high redshifts. The observed LF in the low luminosity end for $z<2.5$ shows an upward turn. In our models we reproduce this trend using passively evolving population of galaxies. Measuring stellar mass, age and metallicity of these galaxies using multi-band spectral energy distribution (SED) fitting will place strong constraints on the existence of such galaxies. While Active galactic Nuclei (AGN) feedback is essential to reproduce the LF at high luminosity end for $z<4$, it may not needed in the case of $z\ge 6$. We show that the expected turn around in the LF due to cooling criteria occurs at luminosity much lower than what has been probed with present day observations. With future deep observations, that can measure the LF more accurately, we will be able to distinguish between different modes of SNe feedback and get insights into the physical processes that drive the galaxy evolution.

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arXiv2018

Efficient cold outflows driven by cosmic rays in high redshift galaxies and their global effects on the IGM

We present semi-analytical models of galactic outflows in high redshift galaxies driven by both hot thermal gas and non-thermal cosmic rays. Thermal pressure alone may not sustain a large scale outflow in low mass galaxies (i.e $M\sim 10^8$~M$_\odot$), in the presence of supernovae (SNe) feedback with large mass loading. We show that inclusion of cosmic ray pressure allows outflow solutions even in these galaxies. In massive galaxies for the same energy efficiency, cosmic ray driven winds can propagate to larger distances compared to pure thermally driven winds. On an average gas in the cosmic ray driven winds has a lower temperature which could aid detecting it through absorption lines in the spectra of background sources. Using our constrained semi-analytical models of galaxy formation (that explains the observed UV luminosity functions of galaxies) we study the influence of cosmic ray driven winds on the properties of the intergalactic medium (IGM) at different redshifts. In particular, we study the volume filling factor, average metallicity, cosmic ray and magnetic field energy densities for models invoking atomic cooled and molecular cooled halos. We show that the cosmic rays in the IGM could have enough energy that can be transferred to the thermal gas in presence of magnetic fields to influence the thermal history of the intergalactic medium. The significant volume filling and resulting strength of IGM magnetic fields can also account for recent $γ$-ray observations of blazars.

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arXiv2016

Modelling the 21 cm Signal From the Epoch of Reionization and Cosmic Dawn

Studying the cosmic dawn and the epoch of reionization through the redshifted 21 cm line are among the major science goals of the SKA1. Their significance lies in the fact that they are closely related to the very first stars in the universe. Interpreting the upcoming data would require detailed modelling of the relevant physical processes. In this article, we focus on the theoretical models of reionization that have been worked out by various groups working in India with the upcoming SKA in mind. These models include purely analytical and semi-numerical calculations as well as fully numerical radiative transfer simulations. The predictions of the 21 cm signal from these models would be useful in constraining the properties of the early galaxies using the SKA data.

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arXiv2016

Photo-$z$ with CuBAN$z$: An improved photometric redshift estimator using Clustering aided Back Propagation Neural network

We present an improved photometric redshift estimator code, CuBAN$z$, that is publicly available at https://goo.gl/fpk90V}{https://goo.gl/fpk90V. It uses the back propagation neural network along with clustering of the training set, which makes it more efficient than existing neural network codes. In CuBAN$z$, the training set is divided into several self learning clusters with galaxies having similar photometric properties and spectroscopic redshifts within a given span. The clustering algorithm uses the color information (i.e. $u-g$, $g-r$ etc.) rather than the apparent magnitudes at various photometric bands as the photometric redshift is more sensitive to the flux differences between different bands rather than the actual values. Separate neural networks are trained for each cluster using all possible colors, magnitudes and uncertainties in the measurements. For a galaxy with unknown redshift, we identify the closest possible clusters having similar photometric properties and use those clusters to get the photometric redshifts using the particular networks that were trained using those cluster members. For galaxies that do not match with any training cluster, the photometric redshifts are obtained from a separate network that uses entire training set. This clustering method enables us to determine the redshifts more accurately. SDSS Stripe 82 catalog has been used here for the demonstration of the code. For the clustered sources with redshift range $z_{\rm spec}<0.7$, the residual error ($\langle (z_{\rm spec}-z_{\rm phot})^2\rangle^{1/2} $) in the training/testing phase is as low as 0.03 compared to the existing ANNz code that provides residual error on the same test data set of 0.05. Further, we provide a much better estimate of the uncertainty of the derived photometric redshift.

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arXiv2014

Star formation in high redshift galaxies including Supernova feedback: effect on stellar mass and luminosity functions

We present a semi-analytical model of high redshift galaxy formation. In our model the star formation inside a galaxy is regulated by the feedback from supernova (SNe) driven outflows. We derive a closed analytical form for star formation rate in a single galaxy taking account of the SNe feedback in a self-consistent manner. We show that our model can explain the observed correlation between the stellar mass and the circular velocity of galaxies from dwarf galaxies to massive galaxies of $10^{12} M_\odot$. For small mass dwarf galaxies additional feedback other than supernova feedback is needed to explain the spread in the observational data. Our models reproduce the observed 3-D fundamental correlation between the stellar mass, gas phase metallicity and star formation rate in galaxies establishing that the SNe feedback plays a major role in building this relation. Further, the observed UV luminosity functions of Lyman-Break galaxies (LBGs) are well explained by our feedback induced star formation model for a vast redshift range of $1.5 \le z \le 8$. In particular, the flattening of the luminosity functions at the low luminosity end naturally arises due to our explicit SNe feedback treatment.

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arXiv2012

Spatial Clustering of High Redshift Lyman Break Galaxies

We present a physically motivated semi-analytic model to understand the clustering of high redshift LBGs. We show that the model parameters constrained by the observed luminosity function, can be used to predict large scale (θ> 80 arcsec) bias and angular correlation function of galaxies. These predictions are shown to reproduce the observations remarkably well. We then adopt these model parameters to calculate the halo occupation distribution (HOD) using the conditional mass function. The halo model using this HOD is shown to provide a reasonably good fit to the observed clustering of LBGs at both large (θ>80") and small (θ< 10") angular scales for z=3-5 and several limiting magnitudes. However, our models underpredict the clustering amplitude at intermediate angular scales, where quasi-linear effects are important. The average mass of halos contributing to the observed clustering is found to be 6.2 x 10^{11} M_\odot and the characteristic mass of a parent halo hosting satellite galaxies is 1.2 \times 10^{12} M_\odot for a limiting absolute magnitude of -20.5 at z=4. For a given threshold luminosity these masses decrease with increasing z and at any given z these are found to increase with increasing value of threshold luminosity. We find that approximately 40 % of the halos above a minimum mass M_{min}, can host detectable central galaxies and about 5-10 % of these halos are likely to also host a detectable satellite. The satellites form typically a dynamical timescale prior to the formation of the parent halo. The small angular scale clustering is due to central-satellite pairs and is quite sensitive to changes in the duration of star formation in a halo. The present data favor star formation in a halo lasting typically for a few dynamical time-scales. Our models also reproduce different known trends between parameters related to star formation.

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arXiv2011

Weighing neutrinos using high redshift galaxy luminosity functions

Laboratory experiments measuring neutrino oscillations, indicate small mass differences between different mass eigenstates of neutrinos. The absolute mass scale is however not determined, with at present the strongest upper limits coming from astronomical observations rather than terrestrial experiments. The presence of massive neutrinos suppresses the growth of perturbations below a characteristic mass scale, thereby leading to a decreased abundance of collapsed dark matter halos. Here we show that this effect can significantly alter the predicted luminosity function (LF) of high redshift galaxies. In particular we demonstrate that a stringent constraint on the neutrino mass can be obtained using the well measured galaxy LF and our semi-analytic structure formation models. Combining the constraints from the Wilkinson Microwave Anisotropy Probe 7 year (WMAP7) data with the LF data at z = 4, we get a limit on the sum of the masses of 3 degenerate neutrinos Σm_ν< 0.52 eV at the 95 % CL. The additional constraints using the prior on Hubble constant strengthens this limit to Σm_ν< 0.29 eV at the 95 % CL. This neutrino mass limit is a factor of order 4 improvement compared to the constraint based on the WMAP7 data alone, and as stringent as known limits based on other astronomical observations. As different astronomical measurements may suffer from different set of biases, the method presented here provides a complementary probe of Σm_ν. We suggest that repeating this exercise on well measured luminosity functions over different redshift ranges can provide independent and tighter constraints on Σm_ν.

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arXiv2009

Cosmic ray driven outflows from high redshift galaxies

We study winds in high redshift galaxies driven by a relativistic cosmic ray (proton) component in addition to the hot thermal gas component. Cosmic rays (CRs) are likely to be efficiently generated in supernova shocks inside galaxies. We obtain solutions of such CR driven free winds in a gravitational potential of the NFW form, relevant to galaxies. Cosmic rays naturally provide the extra energy and/or momentum input to the system, needed for a transonic wind solution in a gas with adiabatic index $γ=5/3$. We show that CRs can effectively drive winds even when the thermal energy of the gas is lost due to radiative cooling. These wind solutions predict an asymptotic wind speed closely related to the circular velocity of the galaxy. Furthermore, the mass outflow rate per unit star formation rate (eta_w) is predicted to be ~ 0.2-0.5 for massive galaxies, with masses $M \sim 10^{11}-10^{12} M_\odot$. We show eta_w to be inversely proportional to the square of the circular velocity. Magnetic fields at the $μ$G levels are also required in these galaxies to have a significant mass loss. A large eta_w for small mass galaxies implies that CR driven outflows could provide a strong negative feedback to the star formation in dwarf galaxies. Further, our results will also have important implications to the metal enrichment of the IGM. These conclusions are applicable to the class of free wind models where the source region is confined to be within the sonic point.

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arXiv2009

Understanding the redshift evolution of the luminosity functions of Lyman-alpha emitters

We present a semi-analytical model of star formation which explains simultaneously the observed UV luminosity function of high redshift Lyman break galaxies (LBGs) and luminosity functions of Lyman-alpha emitters. We consider both models that use the Press-Schechter (PS) and Sheth-Tormen (ST) halo mass functions to calculate the abundances of dark matter halos. The Lyman-alpha luminosity functions at z < 4 are well reproduced with only <10% of the LBGs emitting Lyman-alpha lines with rest equivalent width greater than the limiting equivalent width of the narrow band surveys. However, the observed luminosity function at z > 5 can be reproduced only when we assume that nearly all LBGs are Lyman-alpha emitters. Thus it appears that 4 < z < 5 marks the epoch when a clear change occurs in the physical properties of the high redshift galaxies. As Lyman-alpha escape depends on dust and gas kinematics of the inter stellar medium (ISM), this could mean that on an average the ISM at z > 5 could be less dusty, more clumpy and having more complex velocity field. All of these will enable easier escape of the Lyman-alpha photons. At z > 5 the observed Lyman-alpha luminosity function are well reproduced with the evolution in the halo mass function along with very minor evolution in the physical properties of high redshift galaxies. In particular, upto z = 6.5, we do not see the effect of evolving inter galactic medium (IGM) opacity on the Lyman-alpha escape from these galaxies.

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arXiv2009

Models of high redshift luminosity functions and galactic outflows: The dependence on halo mass function

The form of the halo mass function is a basic ingredient in any semi-analytical galaxy formation model. We study the existing forms of the mass functions in the literature and compare their predictions for semi-analytical galaxy formation models. Two methods are used in the literature to compute the net formation rate of halos, one by simply taking the derivative of the halo mass function and the other using the prescription due to Sasaki (1994). For the Press-Schechter (PS) mass function, we compare various model predictions, using these two methods. However, as the Sasaki formalism cannot be easily generalized for other mass functions, we use the derivative while comparing model predictions of different mass functions. We show that the reionization history and UV luminosity function of Lyman break galaxies (LBGs) predicted by the PS mass function differs from those using any other existing mass function, like Sheth-Tormen (ST) mass function.In particular the reionization efficiency of molecular cooled halos has to be substantially reduced when one uses the ST and other mass functions obtained from the simulation instead of the PS mass function. Using $χ^2$-minimization, we find that the observed UV luminosity functions of LBGs at $3.0\le z\le 7.4$ are better reproduced by models using the ST mass function compared to models that use the PS mass function. On the other hand, the volume filling factor of the metals expelled from the galaxies through supernovae driven outflows differs very little between models with different mass functions. It depends on the way we treat merging outflows. We also show that the porosity weighted average quantities related to the outflow are not very sensitive to the differences in the halo mass function.

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arXiv2008

Constrained semi-analytical models of Galactic outflows

We present semi-analytic models of galactic outflows, constrained by available observations on high redshift star formation and reionization. Galactic outflows are modeled in a manner akin to models of stellar wind blown bubbles. Large scale outflows can generically escape from low mass halos (M<10^9 M_sun) for a wide range of model parameters but not from high mass halos (M> 10^{11} M_sun). The gas phase metallicity of the outflow and within the galaxy are computed. Ionization states of different metal species are calculated and used to examine the detectability of metal lines from the outflows. The global influence of galactic outflows is also investigated. Models with only atomic cooled halos significantly fill the IGM at z~3 with metals (with -2.5>[Z/Z_sun]>-3.7), the actual extent depending on the efficiency of winds, the IMF, the fractional mass that goes through star formation and the reionization history of the universe. In these models, a large fraction of outflows at z~3 are supersonic, hot (T> 10^5 K) and have low density, making metal lines difficult to detect. They may also result in significant perturbations in the IGM gas on scales probed by the Lyman-alpha forest. On the contrary, models including molecular cooled halos with a normal mode of star formation can potentially volume fill the universe at z> 8 without drastic dynamic effects on the IGM, thereby setting up a possible metallicity floor (-4.0<[Z/Z_sun]<-3.6). Interestingly, molecular cooled halos with a ``top-heavy'' mode of star formation are not very successful in establishing the metallicity floor because of the additional radiative feedback, that they induce. (Abridged)

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arXiv2007

Probing the star formation history using the redshift evolution of luminosity functions

We present a self-consistent, semi-analytical LCDM model of star formation and reionization. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the optical depth to reionization, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3 8. Molecular cooled halos, which may be important for reionizing the universe, are not detectable in the present deep field observations. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the LF at 6<z< 12.Accurately measuring the LF at high z can be used to understand the nature of star formation in the dark ages and probe the history of reionization. (Abridged)

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