I. Shlosman - Academia.edu (original) (raw)

Papers by I. Shlosman

The Astrophysical Journal, 2012

Theoretical and numerical modeling of the assembly of dark-matter halos predicts that the most ma... more Theoretical and numerical modeling of the assembly of dark-matter halos predicts that the most massive and luminous galaxies at high redshift are surrounded by overdensities of fainter companions. We test this prediction with Hubble Space Telescope observations acquired by our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very bright z ∼ 8 candidates as Y 098 -dropout sources in four of the 23 non-contiguous WFC3 fields observed. We extend here the search for Y 098dropouts to fainter luminosities (M * galaxies with M AB ∼ −20), with detections at 5σ confidence (compared to the 8σ confidence threshold adopted earlier) identifying 17 new candidates. We demonstrate that there is a correlation between number counts of faint and bright Y 098 -dropouts at 99.84% confidence. Field BoRG58, which contains the best bright z ∼ 8 candidate (M AB = −21.3), has the most significant overdensity of faint Y 098 -dropouts. Four new sources are located within 70 ′′ (corresponding to 3.1 comoving Mpc at z = 8) from the previously known brighter z ∼ 8 candidate. The overdensity of Y 098 -dropouts in this field has a physical origin to very high confidence (p > 99.975%), independent of completeness and contamination rate of the Y 098 -dropout selection. We modeled the overdensity by means of cosmological simulations and estimate that the principal dark matter halo has mass M h ≈ (4 − 7) × 10 11 M ⊙ (∼ 5σ density peak) and is surrounded by several M h ≈ 10 11 M ⊙ halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M h > 2 × 10 14 M ⊙ galaxy cluster by z = 0. Follow-up observations with ground and space 2 Trenti et al.

The Astrophysical Journal, 2014

We have explored prevailing modes of galaxy growth for redshifts z ∼ 6 − 14, comparing substantia... more We have explored prevailing modes of galaxy growth for redshifts z ∼ 6 − 14, comparing substantially overdense and normal regions of the universe, using high-resolution zoom-in cosmological simulations. Such rare overdense regions have been projected to host high-z quasars. We demonstrate that galaxies in such environments grow predominantly by a smooth accretion from cosmological filaments which dominates the mass input from major, intermediate and minor mergers. We find that by z ∼ 6, the accumulated galaxy mass fraction from mergers falls short by a factor of 10 of the cumulative accretion mass for galaxies in the overdense regions, and by a factor of 5 in the normal environments. Moreover, the rate of the stellar mass input from mergers also lies below that of an in-situ star formation (SF) rate. The fraction of stellar masses in galaxies contributed by mergers in overdense regions is ∼ 12%, and ∼ 33% in the normal regions, at these redshifts. Our median SF rates for ∼ few × 10 9 M galaxies agrees well with the recently estimated rates for z ∼ 7 galaxies from Spitzer's SURF-UP survey. Finally, we find that the main difference between the normal and overdense regions lies in the amplified growth of massive galaxies in massive dark matter halos. This leads to the formation of > ∼ 10 10 M galaxies due to the ∼ 100-fold increase in mass during the above time period. Such galaxies are basically absent in the normal regions at these redshifts.

The Astrophysical Journal, 2002

We analyze the observed properties of nested and single stellar bar systems in disk galaxies. The... more We analyze the observed properties of nested and single stellar bar systems in disk galaxies. The 112 galaxies in our sample comprise the largest matched Seyfert vs. non-Seyfert galaxy sample of nearby galaxies with complete near-infrared or optical imaging sensitive to lengthscales ranging from tens of pc to tens of kpc. The presence of bars is deduced by fitting ellipses to isophotes in HST H-band images up to 10 ′′ radius, and in ground-based near-infrared and optical images outside the H-band images. This is a conservative approach that is likely to result in an underestimate of the true bar fraction. We find that a significant fraction of the sample galaxies, 17% ± 4%, has more than one bar, and that 28% ± 5% of barred galaxies have nested bars. The bar fractions appear to be stable according to reasonable changes in our adopted bar criteria. For the nested bars, we detect a clear division in length between the large-scale (primary) bars and small-scale (secondary) bars, both in absolute and normalized (to the size of the galaxy) length. We argue that this bimodal distribution can be understood within the framework of disk resonances, specifically the inner Lindblad resonances (ILRs), which are located where the gravitational potential of the innermost galaxy switches effectively from 3D to 2D. This conclusion is further strengthened by the observed distribution of the sizes of nuclear rings which are dynamically associated with the ILRs. While primary bars are found to correlate with the host galaxy sizes, no such correlation is observed for the secondary bars. Moreover, we find that secondary bars differ morphologically from single bars. Our matched Seyfert and non-Seyfert samples show a statistically significant excess of bars among the Seyfert galaxies at practically all lengthscales. We confirm our previous results that bars are more abundant in Seyfert hosts than in non-Seyferts, and that Seyfert galaxies always show a preponderance of "thick" bars compared to the bars in non-Seyfert galaxies. Finally, no correlation is observed between the presence of a bar and that of companion galaxies, even relatively bright ones. Overall, since star formation and dust extinction can be significant even in the H-band, the stellar dynamics of the central kiloparsec cannot always be revealed reliably by the use of near-infrared surface photometry alone.

The Astrophysical Journal, 1995

New optical and near-infrared (NIR) K-band images of the inner 3 kpc region of the nearby Virgo s... more New optical and near-infrared (NIR) K-band images of the inner 3 kpc region of the nearby Virgo spiral M100 (NGC 4321) display remarkable morphological changes with wavelength. While in the optical the light is dominated by a circumnuclear zone of enhanced star formation, the morphological features in the 2:2 m image correspond to a newly discovered kpc-size stellar bar, and a pair of leading arms situated inside an ovally-shaped region. Analysis of the K image con rms its symmetry: only a very small percentage of the ux, some 5%, is emitted in antisymmetric structures. This indicates that the overall morphology observed in the NIR is dominated by a global density wave. Making a rst-order correction for the presence of localized dust extinction in the K light using the I K image, we nd that the observed leading arm morphology is not caused or enhanced, but in fact slightly hidden by dust. Possible mechanisms responsible for the optical and NIR morphology are discussed, and tests are proposed to discriminate between them. Our dynamical conclusions are supported with an evolutionary stellar population model reproducing the observed optical and NIR colors in a number of star forming zones. We argue that the observed morphology is compatible with the presence of a pair of inner Lindblad resonances in the region, and show this explicitly in an accompanying paper by detailed numerical modeling. The phenomena observed in NGC 4321 may provide insight into physical processes leading to central activity in galaxies.

The Astrophysical Journal, 2002

Through observations and modeling, we demonstrate how the recently discovered large-scale bar in ... more Through observations and modeling, we demonstrate how the recently discovered large-scale bar in NGC 5248 generates spiral structure that extends from 10 kpc down to 100 pc, fuels star formation on progressively smaller scales, and drives disk evolution. Deep inside the bar, two massive molecular spirals cover nearly 180 in azimuth, show streaming motions of 20-40 km s À1 , and feed a starburst ring of super-star clusters at 375 pc. They also connect to two narrow K-band spirals that delineate the UV bright star clusters in the ring. The data suggest that the K-band spirals are young, and the starburst has been triggered by a bar-driven spiral density wave (SDW). The latter may even have propagated closer to the center where a second H ring and a dust spiral are found. The molecular and Hubble Space Telescope data support a scenario where stellar winds and supernovae efficiently clear out gas from dense star-forming regions on timescales less than a few Myr. We have investigated the properties of massive CO spirals within the framework of bar-driven SDWs, incorporating the effect of gas self-gravity. We find good agreement between the model predictions and the observed morphology, kinematics, and pitch angle of the spirals. This combination of observations and modeling provides the best evidence to date for a strong dynamical coupling between the nuclear region and the surrounding disk. It also confirms that a low central mass concentration, which may be common in late-type galaxies, is particularly favorable to the propagation of a bar-driven gaseous SDW deep into the central region of the galaxy, whereas a large central mass concentration favors other processes, such as the formation and decoupling of nuclear bars.

The Astrophysical Journal, 2003

The masses of supermassive black holes (SBHs) show correlations with bulge properties in disk and... more The masses of supermassive black holes (SBHs) show correlations with bulge properties in disk and elliptical galaxies. We study the formation of galactic structure within flat-core triaxial haloes and show that these correlations can be understood within the framework of a baryonic component modifying the orbital structure in the underlying potential. In particular, we find that terminal properties of bulges and their central SBHs are constrained by the destruction of box orbits in the harmonic cores of dark haloes and the emergence of progressively less eccentric loop orbits there. SBH masses, M • , should exhibit a tighter correlation with bulge velocity dispersions, σ B , than with bulge masses, M B , in accord with observations, if there is a significant scatter in the M H − σ H relation for the halo. In the context of this model the observed M • − σ B relation implies that haloes should exhibit a Faber-Jackson type relationship between their masses and velocity dispersions. The most important prediction of our model is that halo properties determine the bulge and SBH parameters. The model also has important implications for galactic morphology and the process of disk formation.

The Astrophysical Journal, 2009

We compare the substructure evolution in pure dark matter (DM) halos with those in the presence o... more We compare the substructure evolution in pure dark matter (DM) halos with those in the presence of baryons, hereafter PDM and BDM models. The prime halos have been analyzed in the previous work, Romano-Diaz et al. Models have been evolved from identical initial conditions which have been constructed by means of the Constrained Realization method. The BDM model includes star formation and feedback from stellar evolution onto the gas. A comprehensive catalog of subhalo populations has been compiled and individual and statistical properties of subhalos analyzed, including their orbital differences. We find that subhalo population mass functions in PDM and BDM are consistent with a single power law, M α sbh , for each of the models in the mass range of ∼ 2 × 10 8 M ⊙ − 2 × 10 11 M ⊙ . However, we detect a nonnegligible shift between these functions, the time-averaged α ∼ −0.86 for the PDM and −0.98 for the BDM models. Overall, α appears to be a nearly constant with variations of ±15%. Second, we find that the radial mass distribution of subhalo populations can be approximated by a power law, R γ sbh with a steepening that occurs at the radius of a maximal circular velocity, R vmax , in the prime halos. Here we find that the γ sbh ∼ −1.5 for the PDM and -1 for the BDM models, when averaged over time inside R vmax . The slope is steeper outside this region and approaches −3. We detect little spatial bias (less than 10%) between the subhalo populations and the DM distribution of the main halos. Also, the subhalo population exhibits much less triaxiality in the presence of baryons, in tandem with the shape of the prime halo. Finally, we find that, counter-intuitively, the BDM population is depleted at a faster rate than the PDM one within the central 30 kpc of the prime halo. The reason for this is that although the baryons provide a substantial glue to the subhalos, the main halo exhibits the same trend. This assures a more efficient tidal disruption of the BDM subhalo population. However, this effect can be reversed for a more efficient feedback from stellar evolution and the central supermassive black holes, which will expel baryons from the center and decrease the central concentration of the prime halo. We compare our results with via Lactea and Aquarius simulations and other published results.

The Astrophysical Journal, 2013

We use numerical simulations to explore whether direct collapse can lead to the formation of supe... more We use numerical simulations to explore whether direct collapse can lead to the formation of supermassive black hole (SMBH) seeds at high redshifts. Using the adaptive mesh refinement code ENZO, we follow the evolution of gas within slowly tumbling dark matter (DM) halos of M vir ∼ 2 × 10 8 M ⊙ and R vir ∼ 1 kpc. For our idealized simulations, we adopt cosmologically motivated DM and baryon density profiles and angular momentum distributions. Our principal goal is to understand how the collapsing flow overcomes the centrifugal barrier and whether it is subject to fragmentation which can potentially lead to star formation, decreasing the seed SMBH mass. We find that the collapse proceeds from inside out and leads either to a central runaway or to off-center fragmentation. A disk-like configuration is formed inside the centrifugal barrier, growing via accretion. For models with a more cuspy DM distribution, the gas collapses more and experiences a bar-like perturbation and a central runaway on scales of < ∼ 1 − 10 pc. We have followed this inflow down to ∼ 10 −4 pc (∼ 10 AU), where it is estimated to become optically thick. The flow remains isothermal and the specific angular momentum, j, is efficiently transferred by gravitational torques in a cascade of nested bars. This cascade is triggered by finite perturbations from the large-scale mass distribution and by gas self-gravity, and supports a self-similar, disk-like collapse where the axial ratios remain constant. The mass accretion rate shows a global minimum on scales of ∼ 1 − 10 pc at the time of the central runaway. In the collapsing phase, virial supersonic turbulence develops and fragmentation is damped. Models with progressively larger initial DM cores evolve similarly, but the timescales become longer. In models with more organized initial rotation -when the rotation of spherical shells is constrained to be coplanar -a torus forms on scales ∼ 20−50 pc outside the disk, and appears to be supported by turbulent motions driven by accretion from the outside. The overall evolution of the models depends on the competition between two timescales, corresponding to the onset of the central runaway and of off-center fragmentation. In models with less organized rotation -when the rotation of spherical shells is randomized (but the total angular momentum remains unchanged) -the torus is greatly weakened, the central accretion timescale is shortened, and off-center fragmentation is suppressedtriggering the central runaway even in previously 'stable' models. The resulting seed SMBH masses is found in the range M • ∼ 2 × 10 4 M ⊙ − 2 × 10 6 M ⊙ , substantially higher than the mass range of Population III remnants. We argue that the above upper limit on M • appears to be more realistic, and lies close to the cutoff mass of detected SMBHs. Corollaries of this model include a possible correlation between SMBH and DM halo masses, and similarity between the SMBH and halo mass functions, at time of formation.

The Astrophysical Journal, 2000

We present a deep ASCA observation of a Broad Absorption Line Quasar (BALQSO) PG0946+301. The sou... more We present a deep ASCA observation of a Broad Absorption Line Quasar (BALQSO) PG0946+301. The source was clearly detected in one of the gas imaging spectrometers, but not in any other detector. If BALQSOs have intrinsic X-ray spectra similar to normal radio-quiet quasars, our observations imply that there is Thomson thick X-ray absorption (N H > ∼ 10 24 cm −2 ) toward PG0946+301. This is the largest column density estimated so far toward a BALQSO. The absorber must be at least partially ionized and may be responsible for attenuation in the optical and UV. If the Thomson optical depth toward BALQSOs is close to one, as inferred here, then spectroscopy in hard X-rays with large telescopes like XMM would be feasible.

We initiated a deep campaign to probe one typical broad absorption line quasar in X-rays. We obse... more We initiated a deep campaign to probe one typical broad absorption line quasar in X-rays. We observed PG0946+301 with ASCA for 80 ksec. We will discuss the observational results and their implications. This work is supported in parts by NASA grant NAG5-3249 (LTSA).

The Astrophysical Journal, 2011

Observed high-redshift QSOs, at z ∼ 6, may reside in massive dark matter (DM) halos of more than ... more Observed high-redshift QSOs, at z ∼ 6, may reside in massive dark matter (DM) halos of more than 10 12 M ⊙ and are thus expected to be surrounded by overdense regions. In a series of 10 constrained simulations, we have tested the environment of such QSOs. Comparing the computed overdensities with respect to the unconstrained simulations of regions empty of QSOs, assuming there is no bias between the DM and baryon distributions, and invoking an observationally-constrained duty-cycle for Lyman Break Galaxies, we have obtained the galaxy count number for the QSO environment. We find that a clear discrepancy exists between the computed and observed galaxy counts in the Kim et al. samples. Our simulations predict that on average eight z ∼ 6 galaxies per QSO field should have been observed, while Kim et al. detect on average four galaxies per QSO field compared to an average of three galaxies in a control sample (GOODS fields). While we cannot rule out a small number statistics for the observed fields to high confidence, the discrepancy suggests that galaxy formation in the QSO neighborhood proceeds differently than in the field. We also find that QSO halos are the most massive of the simulated volume at z ∼ 6 but this is no longer true at z ∼ 3. This implies that QSO halos, even in the case they are the most massive ones at high redshifts, do not evolve into most massive galaxy clusters at z = 0.

The Astrophysical Journal, 2012

Theoretical and numerical modeling of the assembly of dark-matter halos predicts that the most ma... more Theoretical and numerical modeling of the assembly of dark-matter halos predicts that the most massive and luminous galaxies at high redshift are surrounded by overdensities of fainter companions. We test this prediction with Hubble Space Telescope observations acquired by our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very bright z ∼ 8 candidates as Y 098 -dropout sources in four of the 23 non-contiguous WFC3 fields observed. We extend here the search for Y 098dropouts to fainter luminosities (M * galaxies with M AB ∼ −20), with detections at 5σ confidence (compared to the 8σ confidence threshold adopted earlier) identifying 17 new candidates. We demonstrate that there is a correlation between number counts of faint and bright Y 098 -dropouts at 99.84% confidence. Field BoRG58, which contains the best bright z ∼ 8 candidate (M AB = −21.3), has the most significant overdensity of faint Y 098 -dropouts. Four new sources are located within 70 ′′ (corresponding to 3.1 comoving Mpc at z = 8) from the previously known brighter z ∼ 8 candidate. The overdensity of Y 098 -dropouts in this field has a physical origin to very high confidence (p > 99.975%), independent of completeness and contamination rate of the Y 098 -dropout selection. We modeled the overdensity by means of cosmological simulations and estimate that the principal dark matter halo has mass M h ≈ (4 − 7) × 10 11 M ⊙ (∼ 5σ density peak) and is surrounded by several M h ≈ 10 11 M ⊙ halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M h > 2 × 10 14 M ⊙ galaxy cluster by z = 0. Follow-up observations with ground and space 2 Trenti et al.

The Astrophysical Journal, 2014

We have explored prevailing modes of galaxy growth for redshifts z ∼ 6 − 14, comparing substantia... more We have explored prevailing modes of galaxy growth for redshifts z ∼ 6 − 14, comparing substantially overdense and normal regions of the universe, using high-resolution zoom-in cosmological simulations. Such rare overdense regions have been projected to host high-z quasars. We demonstrate that galaxies in such environments grow predominantly by a smooth accretion from cosmological filaments which dominates the mass input from major, intermediate and minor mergers. We find that by z ∼ 6, the accumulated galaxy mass fraction from mergers falls short by a factor of 10 of the cumulative accretion mass for galaxies in the overdense regions, and by a factor of 5 in the normal environments. Moreover, the rate of the stellar mass input from mergers also lies below that of an in-situ star formation (SF) rate. The fraction of stellar masses in galaxies contributed by mergers in overdense regions is ∼ 12%, and ∼ 33% in the normal regions, at these redshifts. Our median SF rates for ∼ few × 10 9 M galaxies agrees well with the recently estimated rates for z ∼ 7 galaxies from Spitzer's SURF-UP survey. Finally, we find that the main difference between the normal and overdense regions lies in the amplified growth of massive galaxies in massive dark matter halos. This leads to the formation of > ∼ 10 10 M galaxies due to the ∼ 100-fold increase in mass during the above time period. Such galaxies are basically absent in the normal regions at these redshifts.

The Astrophysical Journal, 2002

We analyze the observed properties of nested and single stellar bar systems in disk galaxies. The... more We analyze the observed properties of nested and single stellar bar systems in disk galaxies. The 112 galaxies in our sample comprise the largest matched Seyfert vs. non-Seyfert galaxy sample of nearby galaxies with complete near-infrared or optical imaging sensitive to lengthscales ranging from tens of pc to tens of kpc. The presence of bars is deduced by fitting ellipses to isophotes in HST H-band images up to 10 ′′ radius, and in ground-based near-infrared and optical images outside the H-band images. This is a conservative approach that is likely to result in an underestimate of the true bar fraction. We find that a significant fraction of the sample galaxies, 17% ± 4%, has more than one bar, and that 28% ± 5% of barred galaxies have nested bars. The bar fractions appear to be stable according to reasonable changes in our adopted bar criteria. For the nested bars, we detect a clear division in length between the large-scale (primary) bars and small-scale (secondary) bars, both in absolute and normalized (to the size of the galaxy) length. We argue that this bimodal distribution can be understood within the framework of disk resonances, specifically the inner Lindblad resonances (ILRs), which are located where the gravitational potential of the innermost galaxy switches effectively from 3D to 2D. This conclusion is further strengthened by the observed distribution of the sizes of nuclear rings which are dynamically associated with the ILRs. While primary bars are found to correlate with the host galaxy sizes, no such correlation is observed for the secondary bars. Moreover, we find that secondary bars differ morphologically from single bars. Our matched Seyfert and non-Seyfert samples show a statistically significant excess of bars among the Seyfert galaxies at practically all lengthscales. We confirm our previous results that bars are more abundant in Seyfert hosts than in non-Seyferts, and that Seyfert galaxies always show a preponderance of "thick" bars compared to the bars in non-Seyfert galaxies. Finally, no correlation is observed between the presence of a bar and that of companion galaxies, even relatively bright ones. Overall, since star formation and dust extinction can be significant even in the H-band, the stellar dynamics of the central kiloparsec cannot always be revealed reliably by the use of near-infrared surface photometry alone.

The Astrophysical Journal, 1995

New optical and near-infrared (NIR) K-band images of the inner 3 kpc region of the nearby Virgo s... more New optical and near-infrared (NIR) K-band images of the inner 3 kpc region of the nearby Virgo spiral M100 (NGC 4321) display remarkable morphological changes with wavelength. While in the optical the light is dominated by a circumnuclear zone of enhanced star formation, the morphological features in the 2:2 m image correspond to a newly discovered kpc-size stellar bar, and a pair of leading arms situated inside an ovally-shaped region. Analysis of the K image con rms its symmetry: only a very small percentage of the ux, some 5%, is emitted in antisymmetric structures. This indicates that the overall morphology observed in the NIR is dominated by a global density wave. Making a rst-order correction for the presence of localized dust extinction in the K light using the I K image, we nd that the observed leading arm morphology is not caused or enhanced, but in fact slightly hidden by dust. Possible mechanisms responsible for the optical and NIR morphology are discussed, and tests are proposed to discriminate between them. Our dynamical conclusions are supported with an evolutionary stellar population model reproducing the observed optical and NIR colors in a number of star forming zones. We argue that the observed morphology is compatible with the presence of a pair of inner Lindblad resonances in the region, and show this explicitly in an accompanying paper by detailed numerical modeling. The phenomena observed in NGC 4321 may provide insight into physical processes leading to central activity in galaxies.

The Astrophysical Journal, 2002

Through observations and modeling, we demonstrate how the recently discovered large-scale bar in ... more Through observations and modeling, we demonstrate how the recently discovered large-scale bar in NGC 5248 generates spiral structure that extends from 10 kpc down to 100 pc, fuels star formation on progressively smaller scales, and drives disk evolution. Deep inside the bar, two massive molecular spirals cover nearly 180 in azimuth, show streaming motions of 20-40 km s À1 , and feed a starburst ring of super-star clusters at 375 pc. They also connect to two narrow K-band spirals that delineate the UV bright star clusters in the ring. The data suggest that the K-band spirals are young, and the starburst has been triggered by a bar-driven spiral density wave (SDW). The latter may even have propagated closer to the center where a second H ring and a dust spiral are found. The molecular and Hubble Space Telescope data support a scenario where stellar winds and supernovae efficiently clear out gas from dense star-forming regions on timescales less than a few Myr. We have investigated the properties of massive CO spirals within the framework of bar-driven SDWs, incorporating the effect of gas self-gravity. We find good agreement between the model predictions and the observed morphology, kinematics, and pitch angle of the spirals. This combination of observations and modeling provides the best evidence to date for a strong dynamical coupling between the nuclear region and the surrounding disk. It also confirms that a low central mass concentration, which may be common in late-type galaxies, is particularly favorable to the propagation of a bar-driven gaseous SDW deep into the central region of the galaxy, whereas a large central mass concentration favors other processes, such as the formation and decoupling of nuclear bars.

The Astrophysical Journal, 2003

The masses of supermassive black holes (SBHs) show correlations with bulge properties in disk and... more The masses of supermassive black holes (SBHs) show correlations with bulge properties in disk and elliptical galaxies. We study the formation of galactic structure within flat-core triaxial haloes and show that these correlations can be understood within the framework of a baryonic component modifying the orbital structure in the underlying potential. In particular, we find that terminal properties of bulges and their central SBHs are constrained by the destruction of box orbits in the harmonic cores of dark haloes and the emergence of progressively less eccentric loop orbits there. SBH masses, M • , should exhibit a tighter correlation with bulge velocity dispersions, σ B , than with bulge masses, M B , in accord with observations, if there is a significant scatter in the M H − σ H relation for the halo. In the context of this model the observed M • − σ B relation implies that haloes should exhibit a Faber-Jackson type relationship between their masses and velocity dispersions. The most important prediction of our model is that halo properties determine the bulge and SBH parameters. The model also has important implications for galactic morphology and the process of disk formation.

The Astrophysical Journal, 2009

We compare the substructure evolution in pure dark matter (DM) halos with those in the presence o... more We compare the substructure evolution in pure dark matter (DM) halos with those in the presence of baryons, hereafter PDM and BDM models. The prime halos have been analyzed in the previous work, Romano-Diaz et al. Models have been evolved from identical initial conditions which have been constructed by means of the Constrained Realization method. The BDM model includes star formation and feedback from stellar evolution onto the gas. A comprehensive catalog of subhalo populations has been compiled and individual and statistical properties of subhalos analyzed, including their orbital differences. We find that subhalo population mass functions in PDM and BDM are consistent with a single power law, M α sbh , for each of the models in the mass range of ∼ 2 × 10 8 M ⊙ − 2 × 10 11 M ⊙ . However, we detect a nonnegligible shift between these functions, the time-averaged α ∼ −0.86 for the PDM and −0.98 for the BDM models. Overall, α appears to be a nearly constant with variations of ±15%. Second, we find that the radial mass distribution of subhalo populations can be approximated by a power law, R γ sbh with a steepening that occurs at the radius of a maximal circular velocity, R vmax , in the prime halos. Here we find that the γ sbh ∼ −1.5 for the PDM and -1 for the BDM models, when averaged over time inside R vmax . The slope is steeper outside this region and approaches −3. We detect little spatial bias (less than 10%) between the subhalo populations and the DM distribution of the main halos. Also, the subhalo population exhibits much less triaxiality in the presence of baryons, in tandem with the shape of the prime halo. Finally, we find that, counter-intuitively, the BDM population is depleted at a faster rate than the PDM one within the central 30 kpc of the prime halo. The reason for this is that although the baryons provide a substantial glue to the subhalos, the main halo exhibits the same trend. This assures a more efficient tidal disruption of the BDM subhalo population. However, this effect can be reversed for a more efficient feedback from stellar evolution and the central supermassive black holes, which will expel baryons from the center and decrease the central concentration of the prime halo. We compare our results with via Lactea and Aquarius simulations and other published results.

The Astrophysical Journal, 2013

We use numerical simulations to explore whether direct collapse can lead to the formation of supe... more We use numerical simulations to explore whether direct collapse can lead to the formation of supermassive black hole (SMBH) seeds at high redshifts. Using the adaptive mesh refinement code ENZO, we follow the evolution of gas within slowly tumbling dark matter (DM) halos of M vir ∼ 2 × 10 8 M ⊙ and R vir ∼ 1 kpc. For our idealized simulations, we adopt cosmologically motivated DM and baryon density profiles and angular momentum distributions. Our principal goal is to understand how the collapsing flow overcomes the centrifugal barrier and whether it is subject to fragmentation which can potentially lead to star formation, decreasing the seed SMBH mass. We find that the collapse proceeds from inside out and leads either to a central runaway or to off-center fragmentation. A disk-like configuration is formed inside the centrifugal barrier, growing via accretion. For models with a more cuspy DM distribution, the gas collapses more and experiences a bar-like perturbation and a central runaway on scales of < ∼ 1 − 10 pc. We have followed this inflow down to ∼ 10 −4 pc (∼ 10 AU), where it is estimated to become optically thick. The flow remains isothermal and the specific angular momentum, j, is efficiently transferred by gravitational torques in a cascade of nested bars. This cascade is triggered by finite perturbations from the large-scale mass distribution and by gas self-gravity, and supports a self-similar, disk-like collapse where the axial ratios remain constant. The mass accretion rate shows a global minimum on scales of ∼ 1 − 10 pc at the time of the central runaway. In the collapsing phase, virial supersonic turbulence develops and fragmentation is damped. Models with progressively larger initial DM cores evolve similarly, but the timescales become longer. In models with more organized initial rotation -when the rotation of spherical shells is constrained to be coplanar -a torus forms on scales ∼ 20−50 pc outside the disk, and appears to be supported by turbulent motions driven by accretion from the outside. The overall evolution of the models depends on the competition between two timescales, corresponding to the onset of the central runaway and of off-center fragmentation. In models with less organized rotation -when the rotation of spherical shells is randomized (but the total angular momentum remains unchanged) -the torus is greatly weakened, the central accretion timescale is shortened, and off-center fragmentation is suppressedtriggering the central runaway even in previously 'stable' models. The resulting seed SMBH masses is found in the range M • ∼ 2 × 10 4 M ⊙ − 2 × 10 6 M ⊙ , substantially higher than the mass range of Population III remnants. We argue that the above upper limit on M • appears to be more realistic, and lies close to the cutoff mass of detected SMBHs. Corollaries of this model include a possible correlation between SMBH and DM halo masses, and similarity between the SMBH and halo mass functions, at time of formation.

The Astrophysical Journal, 2000

We present a deep ASCA observation of a Broad Absorption Line Quasar (BALQSO) PG0946+301. The sou... more We present a deep ASCA observation of a Broad Absorption Line Quasar (BALQSO) PG0946+301. The source was clearly detected in one of the gas imaging spectrometers, but not in any other detector. If BALQSOs have intrinsic X-ray spectra similar to normal radio-quiet quasars, our observations imply that there is Thomson thick X-ray absorption (N H > ∼ 10 24 cm −2 ) toward PG0946+301. This is the largest column density estimated so far toward a BALQSO. The absorber must be at least partially ionized and may be responsible for attenuation in the optical and UV. If the Thomson optical depth toward BALQSOs is close to one, as inferred here, then spectroscopy in hard X-rays with large telescopes like XMM would be feasible.

We initiated a deep campaign to probe one typical broad absorption line quasar in X-rays. We obse... more We initiated a deep campaign to probe one typical broad absorption line quasar in X-rays. We observed PG0946+301 with ASCA for 80 ksec. We will discuss the observational results and their implications. This work is supported in parts by NASA grant NAG5-3249 (LTSA).

The Astrophysical Journal, 2011

Observed high-redshift QSOs, at z ∼ 6, may reside in massive dark matter (DM) halos of more than ... more Observed high-redshift QSOs, at z ∼ 6, may reside in massive dark matter (DM) halos of more than 10 12 M ⊙ and are thus expected to be surrounded by overdense regions. In a series of 10 constrained simulations, we have tested the environment of such QSOs. Comparing the computed overdensities with respect to the unconstrained simulations of regions empty of QSOs, assuming there is no bias between the DM and baryon distributions, and invoking an observationally-constrained duty-cycle for Lyman Break Galaxies, we have obtained the galaxy count number for the QSO environment. We find that a clear discrepancy exists between the computed and observed galaxy counts in the Kim et al. samples. Our simulations predict that on average eight z ∼ 6 galaxies per QSO field should have been observed, while Kim et al. detect on average four galaxies per QSO field compared to an average of three galaxies in a control sample (GOODS fields). While we cannot rule out a small number statistics for the observed fields to high confidence, the discrepancy suggests that galaxy formation in the QSO neighborhood proceeds differently than in the field. We also find that QSO halos are the most massive of the simulated volume at z ∼ 6 but this is no longer true at z ∼ 3. This implies that QSO halos, even in the case they are the most massive ones at high redshifts, do not evolve into most massive galaxy clusters at z = 0.