arXiv subjects

Huanian Zhang

Publications and source records attributed to Huanian Zhang.

45 records · Page 3Linked to original sources

arXiv2018

Systematically Measuring Ultra Diffuse Galaxies (SMUDGes). I. Survey Description and First Results in the Coma Galaxy Cluster and Environs

We present a homogeneous catalog of 275 large (effective radius $\gtrsim$ 5.3 arcsec) ultra-diffuse galaxy (UDG) candidates lying within an $\approx$ 290 square degree region surrounding the Coma cluster. The catalog results from our automated postprocessing of data from the Legacy Surveys, a three-band imaging survey covering 14,000 square degrees of the extragalactic sky. We describe a pipeline that identifies UDGs and provides their basic parameters. The survey is as complete in these large UDGs as previously published UDG surveys of the central region of the Coma cluster. We conclude that the majority of our detections are at roughly the distance of the Coma cluster, implying effective radii $\ge 2.5$ kpc, and that our sample contains a significant number of analogs of DF 44, where the effective radius exceeds 4 kpc, both within the cluster and in the surrounding field. The $g-z$ color of our UDGs spans a large range, suggesting that even large UDGs may reflect a range of formation histories. A majority of the UDGs are consistent with being lower stellar mass analogs of red sequence galaxies, but we find both red and blue UDG candidates in the vicinity of the Coma cluster and a relative overabundance of blue UDG candidates in the lower density environments and the field. Our eventual processing of the full Legacy Surveys data will produce the largest, most homogeneous sample of large UDGs.

astro-ph.GA↗

arXiv2018

Emission line ratios for the Circumgalactic Medium and the "Bimodal" Nature of Galaxies

We find significantly different diagnostic emission line ratios for the circumgalactic gas associated with galaxies of stellar masses above and below $10^{10.4}$ M$_\odot$ using SDSS spectroscopy. Specifically, in a sample of 17,393 galaxies, intersected by 18,535 lines of sight at projected radii between 10 and 50 kpc, we stack measured fluxes for nebular strong emission lines, [O {\small III}] $λ$5007, H$α$ and [N {\small II}] $\lambda6583$, and find that the gas surrounding the lower mass galaxies exhibits similar line ratios to those of gas ionized by star formation and that surrounding the higher mass galaxies similar to those of gas ionized by AGN or shocks. This finding highlights yet another characteristic of galaxies that is distinctly different above and below this stellar mass threshold, but one that is more closely connected to the gas accretion behavior hypothesized to be responsible for this dichotomy.

astro-ph.GA↗

arXiv2018

Emission from the Ionized Gaseous Halos of Low Redshift Galaxies and Their Neighbors

Using a sample of nearly half a million galaxies, intersected by over 8 million lines of sight from the Sloan Digital Sky Survey Data Release 12, we extend our previous study of the recombination radiation emitted by the gaseous halos of nearby galaxies. We identify an inflection in the radial profile of the H$α$+N[{\small II}] radial emission profile at a projected radius of $\sim 50$ kpc and suggest that beyond this radius the emission from ionized gas in spatially correlated halos dominates the profile. We confirm that this is a viable hypothesis using results from a highly simplified theoretical treatment in which the dark matter halo distribution from cosmological simulations is straightforwardly populated with gas. Whether we fit the fraction of halo gas in a cooler (T $= 12,000$ K), smooth ($c = 1$) component (0.26 for galaxies with M$_* = 10^{10.88}$ M$_\odot$ and 0.34 for those with M$_* = 10^{10.18}$ M$_\odot$) or take independent values of this fraction from published hydrodynamical simulations (0.19 and 0.38, respectively), this model successfully reproduces the radial location and amplitude of the observed inflection. We also observe that the physical nature of the gaseous halo connects to primary galaxy morphology beyond any relationship to the galaxy's stellar mass and star formation rate. We explore whether the model reproduces behavior related to the central galaxy's stellar mass, star formation rate, and morphology. We find that it is unsuccessful in reproducing the observations at this level of detail and discuss various shortcomings of our simple model that may be responsible.

astro-ph.GA↗

arXiv2018

Higgs and $Z$ Assisted Stop Searches at Hadron Colliders

Current searches for the light top squark (stop) mostly focus on the decay channels of $\tilde{t} \rightarrow t χ_1^0$ or $\tilde{t} \rightarrow b χ_1^\pm \rightarrow bW χ_1^0$, leading to $t\bar{t}/bbWW+\met$ final states for stop pair productions at the LHC. However, in supersymmetric scenarios with light neutralinos and charginos other than the neutralino lightest supersymmetric particle (LSP), more than one decay mode of the stop could be dominant. While those new decay modes could significantly weaken the current stop search limits at the LHC, they also offer alternative discovery channels for stop searches. In this paper, we studied the scenario with light Higgsino next-to-LSPs (NLSPs) and Bino LSP. The light stop decays primarily via $\tilde t_1 \to t χ_2^0/χ_3^0$, with the neutralinos subsequent decaying to a $Z$ boson or a Higgs boson: $χ_2^0/χ^0_3 \to χ_1^0 h/Z$. Pair production of light stops at the LHC leads to final states of $t \bar t hh\met$, $t \bar t hZ\met$ or $t \bar t ZZ\met$. We consider three signal regions: one charged lepton (1$\ell$), two opposite sign charged leptons (2 OS $\ell$) and at least three charged leptons ($ \ge 3 \ell$). We found that the 1$\ell$ signal region of channel $t \bar t hZ\met$ has the best reach sensitivity for light stop searches. For 14 TeV LHC with 300 ${\rm fb}^{-1}$ integrated luminosity, a stop mass up to 900 GeV can be discovered at 5$σ$ significance, or up to 1050 GeV can be excluded at 95\% C.L. Combining all three decay channels for $1 \ell$ signal region extends the reach for about 100$-$150 GeV. We also studied the stop reach at the 100 TeV $pp$ collider with 3 ${\rm ab}^{-1}$ luminosity, with discovery and exclusion reach being 6 TeV and 7 TeV, respectively.

hep-ph↗

arXiv2017

The Galaxy's Veil of Excited Hydrogen

Many of the baryons in our Galaxy probably lie outside the well known disk and bulge components. Despite a wealth of evidence for the presence of some gas in galactic halos, including absorption line systems in the spectra of quasars, high velocity neutral hydrogen clouds in our Galaxy halo, line emitting ionised hydrogen originating from galactic winds in nearby starburst galaxies, and the X-ray coronas surrounding the most massive galaxies, accounting for the gas in the halo of any galaxy has been observationally challenging primarily because of its low density in the expansive halo. The most sensitive measurements come from detecting absorption by the intervening gas in the spectra of distant objects such as quasars or distant halo stars, but these have typically been limited to a few lines of sight to sufficiently bright objects. Massive spectroscopic surveys of millions of objects provide an alternative approach to the problem. Here, we present the first evidence for a widely distributed, neutral, excited hydrogen component of the Galaxy's halo. It is observed as the slight, (0.779 $\pm$ 0.006)\%, absorption of flux near the rest wavelength of H$α$ in the combined spectra of hundreds of thousands of galaxy spectra and is ubiquitous in high latitude lines of sight. This observation provides an avenue to tracing, both spatially and kinematically, the majority of the gas in the halo of our Galaxy.

astro-ph.GA↗

arXiv2016

Sbottom discovery via mixed decays at the LHC

In the search for bottom squark (sbottom) in SUSY at the LHC, the common practice has been to assume a $100\%$ decay branching fraction for a given search channel. In realistic MSSM scenarios, there are often more than one significant decay modes to be present, which significantly weaken the current sbottom search limits at the LHC. On the other hand, the combination of the multiple decay modes offers alternative discovery channels for sbottom searches. In this paper, we present the sbottom decays in a few representative mass parameter scenarios. We then analyze the sbottom signal for the pair production in QCD with one sbottom decaying via $\tilde{b}\rightarrow b χ_1^0,\ b χ_2^0$, and the other one decaying via $\tilde{b} \rightarrow t χ_1^\pm$. With the gaugino subsequent decaying to gauge bosons or a Higgs boson $χ_2^0 \rightarrow Z χ_1^0,\ h χ_1^0$ and $χ_1^\pm \rightarrow W^\pm χ_1^0$, we study the reach of those signals at the 14 TeV LHC with 300 ${\rm fb^{-1}}$ integrated luminosity. For a left-handed bottom squark, we find that a mass up to 920 GeV can be discovered at 5$σ$ significance for 250 GeV $< m_{χ_1^0} <$ 350 GeV, or excluded up to 1050 GeV at the 95\% confidence level for the $h$ channel ($μ>0$); similarly, it can be discovered up to 840 GeV, or excluded up to 900 GeV at the 95\% confidence level for the $Z$ channel ($μ<0$). The sbottom and stop signals in the same SUSY parameter scenario are combined to obtain the optimal sensitivity, which is about 150 GeV better than the individual reach of the sbottom or stop. For a right-handed bottom squark with $\tilde{b} \tilde{b}^* \rightarrow b χ_1^0,\ t χ_1^\pm$ channel, we find that the sbottom mass up to 880 GeV can be discovered at 5$σ$ significance, or excluded up to 1060 GeV at the 95\% confidence level.

hep-ph↗

arXiv2016

Hydrogen Emission from the Ionized Gaseous Halos of Low Redshift Galaxies

Using a sample of nearly half million galaxies, intersected by over 7 million lines of sight from the Sloan Digital Sky Survey Data Release 12, we trace H$α$ + [N{\small II}] emission from a galactocentric projected radius, $r_p$, of 5 kpc to more than 100 kpc. The emission flux surface brightness is $\propto r_p^{-1.9 \pm 0.4}$. We obtain consistent results using only the H$α$ or [N{\small II}] flux. We measure a stronger signal for the bluer half of the target sample than for the redder half on small scales, $r_p <$ 20 kpc. We obtain a $3σ$ detection of H$α$ + [N{\small II}] emission in the 50 to 100 kpc $r_p$ bin. The mean emission flux within this bin is $(1.10 \pm 0.35) \times 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ Å$^{-1}$, which corresponds to $1.87 \times 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ arcsec$^{-2}$ or 0.0033 Rayleigh. This detection is 34 times fainter than a previous strict limit obtained using deep narrow-band imaging. The faintness of the signal demonstrates why it has been so difficult to trace recombination radiation out to large radii around galaxies. This signal, combined with published estimates of n$_{\rm H}$, lead us to estimate the temperature of the gas to be 12,000 K, consistent with independent empirical estimates based on metal ion absorption lines and expectations from numerical simulations.

astro-ph.GA↗

arXiv2015

Examining early-type galaxy scaling relations using simple dynamical models

We use dynamical models that include bulk rotation, velocity dispersion anisotropy and both stars and dark matter to explore the conditions that give rise to the early-type galaxy scaling relations referred to as the Fundamental Plane (FP) and Manifold (FM). The modelled scaling relations generally match the observed relations and are remarkably robust to all changes allowed within these models. The empirical relationships can fail beyond the parameter ranges where they were calibrated and we discuss the nature of those failures. Because the location of individual models relative to the FP and FM is sensitive to the adopted physical scaling of the models, unconstrained rescaling produces a much larger scatter about the scaling relations than that observed. We conclude that only certain combinations of scaling values, which define the physical radial and kinematic scale of the model, produce low scatter versions of the FP and FM. These combinations further result in reproducing a condition observed previously for galaxies, $r_c ρ_0 = $ constant, where $r_c$ is the scaling radius and $ρ_0$ is the central density. As such, we conclude that this empirical finding and global galaxy scaling relations are not independent and that finding the physical cause of one should lead to the solution to the other. Although our models are strictly for pressure supported galaxies, these results may well hold generally because the central density constraint was first identified in dwarf spheroidals but later extended to rotating giant galaxies and the FM applies to galaxies of any morphological type and luminosity class.

astro-ph.GA↗

arXiv2015

Complex decay chains of top and bottom squarks

Current searches for the top squark mostly focus on the decay channels of $\tilde{t}_1 \rightarrow t χ_1^0$ or $\tilde{t}_1 \rightarrow b χ_1^\pm \rightarrow bW χ_1^0$, leading to $tt/bbWW+\not\mathrel{E}_T$ final states for top squark pair production at the LHC. In supersymmetric scenarios with light gauginos other than the neutralino lightest supersymmetric particle (LSP), different decay modes of the top squark could be dominant, which significantly weaken the current top squark search limits at the LHC. Additionally, new decay modes offer alternative discovery channels for top squark searches. In this paper, we study the top squark and bottom squark decay in the Bino-like LSP case with light Wino or Higgsino next-to-LSPs (NLSPs), and identify cases in which additional decay modes become dominant. We also perform a collider analysis for top squark pair production with mixed top squark decay final states of $\tilde{t}_1 \to t χ_2^0 \to th χ_1^0$, $\tilde{t}_1 \to b χ_1^\pm \to bW χ_1^0 $, leading to the $bbbbjj\ell+\not\mathrel{E}_T$ collider signature. The branching fraction for such decay varies between 25\% and 50\% for a top squark mass larger than 500 GeV with $M_2=M_1+150$ GeV. At the 14 TeV LHC with 300 ${\rm fb}^{-1}$ integrated luminosity, the top squark can be excluded up to about 1040 GeV at the 95\% C.L., or be discovered up to 940 GeV at 5$σ$ significance.

hep-ph↗