Spatially-resolved studies of nearby star-forming galaxies

Spatially-resolved studies of nearby star-forming galaxies are essential to understand various physical and chemical phenomena at play in the interstellar medium in the galaxies, and consequently to obtain a comprehensive picture of galaxy formation and evolution. In this thesis, I perform spatially-resolved analyses of chemical abundances and star-formation in nearby star-forming galaxies – blue compact dwarf galaxies (BCDs) and spiral galaxies. I map various properties of H II regions and the surrounding gas within three BCDs, using integral field spectroscopic (IFS) data from the Gemini Multi-Object Spectrograph-North. While answering questions related to chemical homogeneity, ionisation mechanisms and stellar populations within BCDs, I address more profound issues, which go beyond the characterisation of studied BCDs and aim to explain global phenomena with broader implications. The BCD NGC 4449 hosts a metal-poor central star-forming region, which I explain by various scenarios related to the interplay between star-formation, metal-distribution and gas dynamics within galaxies. The BCD NGC 4670 shows an unusual negative relationship between the nitrogen-to-oxygen ratio and oxygen abundance at spatially-resolved scales. I explore this relation with chemical evolution models and by comparison to other star-forming galaxies and suggest that nitrogen enrichment, variations in star-formation efficiency or hydrodynamical effects may be responsible for the observed relation. For another BCD, SBS 1415+437, the spatially-resolved abundances on average agree with the integrated abundance, implying that low-redshift spatially-resolved results may be directly compared with unresolved high-redshift results. I study spiral galaxies to address long-standing issues related to the reliability of metallicity calibrators and the Schmidt Law of star-formation. Using IFS data of twenty-four spiral galaxies taken with the Multi-Unit Spectroscopic Explorer, I find that the current strong-line metallicity calibrators for H II regions are unsuitable for regions dominated by diffuse ionised gas (DIG). I devise new recipes for estimating the metal-content of the DIG. For another set of nine spiral galaxies, I use multi-wavelength data to show that the spatially-resolved Schmidt relation is very sensitive to the consideration of diffuse background, which is a component unrelated to the current star-formation. Removal of this component from the SFR tracers and the atomic gas results in similar local and global Schmidt relation. To conclude, the spatially-resolved analyses presented in this thesis have led to discoveries and further questions, which I will address in my ongoing and future works

The effect of diffuse background on the spatially-resolved Schmidt relation in nearby spiral galaxies

The global Schmidt law of star formation provides a power-law relation between the surface densities of star-formation rate (SFR) and gas, and successfully explains plausible scenarios of galaxy formation and evolution. However, star formation being a multi-scale process, requires spatially-resolved analysis for a better understanding of the physics of star formation. It has been shown that the removal of a diffuse background from SFR tracers, such as H$\alpha$, far-ultraviolet (FUV), infrared, leads to an increase in the slope of the sub-galactic Schmidt relation. We reinvestigate the local Schmidt relations in nine nearby spiral galaxies taking into account the effect of inclusion and removal of diffuse background in SFR tracers as well as in the atomic gas.We used multiwavelength data obtained as part of the surveys such as SINGS, KINGFISH, THINGS, and HERACLES. Making use of a novel split of the overall light distribution as a function of spatial scale, we subtracted the diffuse background in the SFR tracers as well as the atomic gas. Using aperture photometry, we study the Schmidt relations on background subtracted and unsubtracted data at physical scales varying between 0.5--2 kpc. The fraction of diffuse background varies from galaxy to galaxy and accounts to $\sim$34 % in H$\alpha$, $\sim$43 % in FUV, $\sim$37 % in 24 $\mu$m, and $\sim$75\% in H I on average. We find that the inclusion of diffuse background in SFR tracers leads to a linear molecular gas Schmidt relation and a bimodal total gas Schmidt relation. However, the removal of diffuse background in SFR tracers leads to a super-linear molecular gas Schmidt relation. A further removal of the diffuse background from atomic gas results in a slope $\sim$1.4 $\pm$ 0.1, which agrees with dynamical models of star formation accounting for flaring effects in the outer regions of galaxies.Comment: 33 pages, 25 figures, 6 tables, accepted for publication in A&

Metallicity calibrations for diffuse ionised gas and low ionisation emission regions

Using integral field spectroscopic data of 24 nearby spiral galaxies obtained with the Multi-Unit Spectroscopic Explorer (MUSE), we derive empirical calibrations to determine the metallicity of the diffuse ionized gas (DIG) and/or of the low-ionisation emission region (LI(N)ER) in passive regions of galaxies. To do so, we identify a large number of HII--DIG/LIER pairs that are close enough to be chemically homogeneous and we measure the metallicity difference of each DIG/LIER region relative to its HII region companion when applying the same strong line calibrations. The O3N2 diagnostic ($=$log [([O III]/H$\beta$)/([N II]/H$\alpha$)]) shows a minimal offset (0.01--0.04 dex) between DIG/LIER and HII regions and little dispersion of the metallicity differences (0.05 dex), suggesting that the O3N2 metallicity calibration for HII regions can be applied to DIG/LIER regions and that, when used on poorly resolved galaxies, this diagnostic provides reliable results by suffering little from DIG contamination. We also derive second-order corrections which further reduce the scatter (0.03--0.04 dex) in the differential metallicity of HII-DIG/LIER pairs. Similarly, we explore other metallicity diagnostics such as O3S2 ($=$log([O III]/H$\beta$+[S II]/H$\alpha$)) and N2S2H$\alpha$ ($=$ log([N II]/[S II]) + 0.264log([N II]/H$\alpha$)) and provide corrections for O3S2 to measure the metallicity of DIG/LIER regions. We propose that the corrected O3N2 and O3S2 diagnostics are used to measure the gas-phase metallicity in quiescent galaxies or in quiescent regions of star-forming galaxies.Comment: Accpeted for publication in MNRAS, 40 pages, 1 Table, 33 figures (including appendix and figures' resolution reduced

Maternal and perinatal outcomes of pregnancies complicated by cardiac disease at tertiary level hospital of Uttarakhand

Background: Cardiac disease is the most common cause of indirect maternal deaths and most common cause of death overall. It complicates 1% of all maternal deaths. The maternal and neonatal risks associated with pregnancy in women with cardiac disease receiving comprehensive prenatal care have not been well defined. Objective of present study was to evaluate the burden of cardiac diseases in pregnancy in our hospital, their clinical presentation, type of cardiac lesion, associated complications, maternal and fetal outcome.Methods: A retrospective study was conducted in Obstetrics and gynecology department at Shri Guru Ram Rai Institute of medical and health sciences, Dehradun from January- 2013 to December-2015. All patients with cardiac diseases during pregnancy or developed during postpartum period were included in the study. Detailed Demographic profile of patients, mode of delivery, perinatal outcome, associated antenatal, intranatal and postnatal complications were analyzed in detail.Results: There were 37 patients with cardiac disease during pregnancy in our study period. Out of these 32.4% were diagnosed during current pregnancy. Rheumatic heart diseases were seen in 54.05% patients and congenital heart disease seen in 35.1%. out of these 81.1% patients belonged to NYHA class I and II and 18.9% patients in class III & IV. Majority of patients (78.3%) had vaginal delivery. We observed 5.4% maternal mortality and 2.7% of neonatal mortality rate.Conclusions: Pregnancy in women with heart disease is associated with significant cardiac and neonatal complications. Multidisciplinary approach involving cardiologist, obstetrician and neonatologist improves fetomaternal outcome

Evaluation of Demirjian stage of the third molar as a reliable predictor of 18 years age

Background: In accordance with section 3(1) of the 1875 Indian majority act, any individual residing in India must reach the majority age when they reach the age of 18 and not before. Third molar formation begins approximately the age of 18. This study aims to distinguish the third molar Demirjian stage(s) indicate that the individual is under the age of 18, and which do not, we will be able to determine the person's age. Methods: In this research, 202 patients' OPG radiographs aged 14-25 years were obtained. Their radiographs revealed 742 third molars in total. Third molar development was examined using Demirjian's eight-stage method, and average age was then calculated for each step of third molar growth. Stages under the age of eighteen and stages over the age of eighteen were analyzed. Results: Data revealed that a person was in stage C most likely under the age of 18 (minor) and at stage H, a person was most likely over the age of 18 (major). Stages A and B were not evident in the age range studied. The age associated with stage C was less than 18 years, while the age associated with stage H was greater than 18 years. Conclusions: Assessing third molar growth stages is a reliable non-invasive method for estimating an individual's age. Stage H indicates a likely age over 18 with completed root development, while stage C suggests an age under 18 with ongoing root development, making it a quick and useful approach

Pre-supernova stellar feedback in nearby starburst dwarf galaxies

Context. Stellar feedback in dwarf galaxies remains, to date, poorly explored, yet is crucial to understanding galaxy evolution in the early Universe. In particular, pre-supernova feedback has recently been found to play a significant role in regulating and disrupting star formation in larger spiral galaxies, but it remains uncertain if it also plays this role in dwarfs.Aims. We study the ionised gas properties and stellar content of individual star-forming regions across three nearby, low-metallicity (12 + log(O/H)∼7.5), dwarf (M* ∼ 40 × 106 M⊙), starburst (log(SFR)∼ − 2.8) galaxies (J0921, KKH046, and Leo P) to investigate how massive stars influence their surroundings and how this influence changes as a function of environment.Methods. We extracted integrated spectra of 30 HII regions from archival VLT/MUSE integral field spectroscopic observations of these three dwarf starburst galaxies. We fitted the HII regions’ main emission lines with Gaussian profiles to derive their oxygen abundances, electron densities, and luminosities, and we used the Stochastically Lighting Up Galaxies (SLUG) code to derive the stellar mass, age, and bolometric luminosity of the stellar populations driving the HII regions. We quantified two pre-supernova stellar feedback mechanisms, namely the direct radiation pressure and photoionisation feedback, and explored how feedback strength varies with HII region properties.Results. Our findings suggest that stellar feedback has less of an impact on evolved regions, with both the pressure of the ionised gas and the direct radiation pressure decreasing as a function of HII region size (i.e. the evolutionary stage). We find that these stellar feedback mechanisms are also dependent on the metallicity of the HII regions. These findings extend results from stellar feedback studies of more massive star-forming galaxies to the low-mass, low-metallicity regime. In addition, we conclude that the use of stochastic stellar population models significantly affects the relationships found between feedback-related pressure terms and HII region properties, and in particular that non-stochastic models can severely underestimate the bolometric luminosity of low-mass stellar populations