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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, 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
34 % in H, 43 % in FUV, 37 % in 24 m, and
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 1.4
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)/([N II]/H)]) 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+[S II]/H)) and N2S2H ( log([N
II]/[S II]) + 0.264log([N II]/H)) 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
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