The Study Of Neutral Hydrogen In Dwarf Galaxies In The Nearby Universe

Dwarf galaxies,as the weakest type of galaxies,are characterized by small-scale,low-luminosity,usually with thousands to billions of stars.The vast majority of galaxies in the universe are dwarf galaxies and often exist as satellite galaxies,surrounding large galaxies.According to the hierarchical structure formation theory,larger galaxies are assembled of smaller galaxies during their formation,so understanding dwarf galaxies is an essential part to unravel the mystery of galactic formation and evolution.The HI content of galaxies is an important parameter for studying galaxy evolution.For example,gas-rich late-type galaxies in high-density regions such as galaxy clusters can transform into gas-starved galaxies over time under the influence of multiple gas removal mechanisms.HI gas is detected in most satellite galaxies more than 270 kpc from the Milky Way or M31,while it’s not detected within this distance.These gas removal mechanisms include:merger and tidal interactions,ram-pressure stripping,etc.Therefore,the gas content of dwarf galaxies will be affected to varying degrees in different environments,especially the distance gradient of the host galaxy,which will show obvious environmental dependence.The gas content of dwarf galaxies in the universe provides a good research direction for studying the formation and evolution of galaxies in the universe.At the same time,these dwarf galaxies may also be inspiring for some basic cosmological problems.These factors include the nature of dark matter,the difference between the observed number of dwarf galaxies in the local group of galaxies and the prediction ofΛCDM,and the difference between the observed baryonic matter in the Milky Way and the theoretical prediction.This thesis explores these questions by studying the neutral hydrogen（HI）component of dwarf galaxies.We selected a group of spectral line observations of neutral hydrogen absorbers and FRB,with a total of 58 observations.All data come from the newly built FAST telescope in China,which is the most sensitive single-dish telescope in the world,providing us with the possibility to detect high-redshift neutral hydrogen signals.We hope to use the high sensitivity and high resolution of FAST to find some dwarf galaxies,especially high redshift galaxies and dark galaxies,by detecting the HI emission line.The method utilized in this paper is to fit and deduct the baseline of the spectral lines by second-order polynomial and FFT,and then build a model of HI emission lines to search the spectral lines with 6σcriterion,filtering out all the emission line bands that exceed the 6σcriterion as HI signal candidates.By crossing them with the optical catalog GSWLC,we can authenticate whether the signal source is a galaxy by their coordinate and redshift.Through the above method,we found 62 HI signal candidates in total.Seven neutral hydrogen signals were finally identified as galactic,five of which were cross-certified with ALFALFA survey,and the flux errors were largely within 5%,establishing the reliability of the data processing method.The fluxes and HI masses for the other two dwarf galaxies were also calculated.For the remaining 2 HI signal candidates,no optical counterparts have been found,and further confirmation is needed as to whether these emission line signals are from dwarf or dark galaxies or from some RFI.In addition,the newly discovered dwarf galaxies COSMOS-dw1 and Pisces VII were also observed,and the former measured the HI signal(Flux=0.0445 Jy km/s,=5.031*10⁶_⊙).No signal was observed for the latter,and only an upper HI mass limit was given:₇4)8))=1.67×10⁶ M_⊙.Pisces VII is only 72 kpc from its host galaxy,M33,which also verifies the view that dwarf galaxies lack HI gas as they approach the host galaxy.There are some high-redshift,low peak flux candidates in the selected samples,and if they can be certified as coming from galaxies,then it is feasible to find dwarf galaxies by HI signals.In the future work,further authentication of candidates that have not found optical counterparts is needed,such as mapping or deep imaging observation,so that we can eliminate RFI,and hope to find some dwarf galaxies that are very weak in the optical band through the HI emission line,and continuously expand the dwarf galaxy sample to provide some help for understanding galaxy evolution and cold dark matter models.