Browsing by Author "Samui, Saumyadip"

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Constrained semi-analytical models of Galactic outflows
(2008-01) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
We present semi-analytic models of galactic outﬂows that are constrained by available ob- servations 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 . 109 M ) for a wide range of model parameters while this is not the case in high mass halos (M & 1011 M ). The ﬂow generically accelerates within the halo virial radius, then starts to decelerate, and traverses well into the intergalactic medium (IGM), before freezing to the Hubble ﬂow. The acceleration phase can result in shell fragmentation due to the Rayleigh-Taylor instability, although the ﬁnal outﬂow radius is not signiﬁcantly altered. The gas phase metallicity of the outﬂow and within the galaxy are computed assuming uniform instantaneous mixing. 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 using porosity weighted averages and probability density functions of various physical quantities. Models with only atomic cooled halos signiﬁcantly ﬁll the IGM at z 3 with metals (with 2:5 & [Z=Z ] & 3:7), the actual extent depending on the efﬁciency of winds, the initial mass function (IMF) and the fractional mass that goes through star formation. The reionization history has a signiﬁcant effect on the volume ﬁlling factor, due to radiative feedback. In these models, a large fraction of outﬂows at z 3 are supersonic, hot (T 105K) 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- 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 ] 3:6). The order unity ﬂuctuations at z 8 that becomes the mildly non-linear ﬂuctuations traced by Lyman- forest at z < 4 will then have this metallicity. 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.
Cosmic ray driven outflows from high redshift galaxies
(2010-01-10) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
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(SNe)shocks inside galaxies. We obtain solutions of such CR driven free winds in a gravitational potential of the Navarro-Frenk-White (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 = 5=3.We show that cosmic rays 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 ( w) is predicted to be ~ 0:2 0:5 for massive galaxies, with masses M ~ 1011 1012M .We show w to be inversely proportional to the square of the circular velocity. Magnetic ﬁelds at the G levels are also required in these galaxies to have a signiﬁcant mass loss. A large w for small mass galaxies implies that cosmic ray 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 intergalactic medium. These conclusions are applicable to the class of free wind models where the source region is conﬁned to be within the sonic point.
Models of high redshift luminosity functions and galactic outflows: The dependence on halo mass function
(2009-02-01) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
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 historically used 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 di erent mass functions. We show that the reionization history and UV luminosity function of Lyman break galaxies (LBGs) predicted by the PS mass function di ers from those using any other existing mass function, like Sheth-Tormen (ST) mass func- tion. In particular the reionization e ciency 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 2 - minimization, we nd that the observed UV luminosity functions of LBGs at 3:0 z 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 lling factor of the metals expelled from the galaxies through super- novae driven out ows di ers very little between models with di erent mass functions. It depends on the way we treat merging out ows. We also show that the porosity weighted average quantities related to the out ow are not very sensitive to the di erences in the halo mass function
Probing the star formation history using the redshift evolution of luminosity fuctions
(2007-03-30) Samui, Saumyadip; Srianand, R.; Subramanian, Kandaswamy
We present a self-consistent, semi-analytical ΛCDM model of star formation and reionization. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the electron scattering optical depth to reionization, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3 ≤ z ≤ 6 for a reasonable range of model parameters. While simple photoionization feedback produces the correct shape of LF at z = 6, for z = 3 we need additional feedback that suppresses star formation activities in halos with 1010 . (M/M⊙) . 1011. Models with prolonged continuous star formation activities are preferred over those with short bursts as they are consistent with the existence of a Balmer break in considerable fraction of observed galaxies even at z ∼ 6. The halo number density evolution from the standard ΛCDMstructure formation model that ﬁts LF up to z = 6 is consistent with the upper limits on z ≃ 7 LF and source counts at 8 ≤ z ≤ 12 obtained fromthe Hubble Ultra Deep Field (HUDF) observations without requiring any dramatic change in the nature of star formation. However, to reproduce the observed LF at 6 ≤ z ≤ 10, obtained from the near-IR observations around strong lensing clusters, we need a strong evolution in the initial mass function, reddening correction and the mode of star formation at z & 8. We show that low mass molecular cooled halos, which may be important for reionizing the universe, are not detectable in the present deep ﬁeld observations even if a considerable fraction of its baryonic mass goes through a star burst phase. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the luminosity function in the redshift range 6 ≤ z ≤ 12. In our model calculations, the contribution of low mass halos to global SFR density prior to reionization reveals itself in the form of second peak at z ≥ 6. However this peak will not be visible in the observed SFR density as a function of z as most of these galaxies have luminosity below the detection threshold of various ongoing deep ﬁeld surveys. Accurately measuring the LF at high redshifts can be used to understand the nature of star formation in the dark ages and probe the history of reionization.
Understanding the redshift evolution of the luminosity functions of Lyman-alpha emitters
(2009-06-01) Samui, Saumyadip; Srianand, R.; Subramanian, Kandaswamy
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-α 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-α luminosity functions at z . 4 are well reproduced with only . 10% of the LBGs emitting Lyman-α 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-α 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-α 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-α photons. At z > 5 the observed Lyman-α 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-α escape from these galaxies