52 research outputs found
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How Do Galaxies Form Their Stars Over Cosmic Time?
Galaxies in the past were forming more stars than those today, but the driving force behind this increase in activity remains uncertain. In this thesis I explore the origin of high star-formation rates today and in the past by studying the properties of gas and dust in the cold interstellar medium (ISM) of dusty galaxies over cosmic time. Critically, we do not yet understand how these galaxies could form so many stars. This work began with my discovery of unusual infrared (IR) emission line ratios in the class of dusty galaxies where most of the Universe’s stars were formed. To fully understand the source of these unusual emission line ratios, I turn to local analogs of the distant galaxies I study at high-redshift to investigate in detail the ratio of far-IR fine-structure line emission to mid-IR Polycylic Aromatic Hydrocarbons (PAHs). I find that gas within young star-forming regions heats and cools differently when it is compressed to high star-formation rate surface densities. I use radio spectroscopy of CO(1-0) and sub-millimeter dust continuum measurements to test how changes in heating and cooling impact the total gas reservoir, and find more efficient star- formation in compact galaxies at all redshifts. The high star-formation rates of distant galaxies may be sustained by this more efficient mode. With spatially resolved studies, I find the IR properties of star-forming, dusty galaxies to be comparable for fixed IR surface density at low- and high-redshift. Finally, to match the formation of stars with the synchronous growth of supermassive black holes I analyze numerical simulations of galaxy formation to study the radiative feedback of active galactic nuclei on dust. Rapidly accreting supermassive black holes can heat the dust in their host galaxies, powering a significant fraction of the cold dust luminosity and biasing IR-derived star-formation rates if left unaccounted for. With the recent launch of the James Webb Space Telescope, the future of extragalactic infrared observations is wide open and this research provides motivation for the continued study of the cold gas and dust conditions from which new stars form
Neutral Gas Properties and Ly Escape in Extreme Green Pea Galaxies
Mechanisms regulating the escape of Ly photons and ionizing radiation
remain poorly understood. To study these processes we analyze VLA 21cm
observations of one Green Pea (GP), J160810+352809 (hereafter J1608), and HST
COS spectra of 17 GP galaxies at . All are highly ionized: J1608 has the
highest [O III] /[O II] for star-forming galaxies in
SDSS, and the 17 GPs have [O III]/[O II] . We set an upper limit on
J1608's HI mass of , near or below average compared to
similar mass dwarf galaxies. In the COS sample, eight GPs show Ly
absorption components, six of which also have Ly emission. The HI
column densities derived from Ly absorption are high, cm, well above the LyC optically thick limit. Using
low-ionization absorption lines, we measure covering fractions
(f_{\mbox{cov}}) of , and find that f_{\mbox{cov}} strongly
anti-correlates with Ly escape fraction. Low covering fractions may
facilitate Ly and LyC escape through dense neutral regions. GPs with
f_{\mbox{cov}}\sim1 all have low neutral gas velocities, while GPs with lower
f_{\mbox{cov}}=0.2-0.6 have a larger range of velocities. Conventional
mechanical feedback may help establish low f_{\mbox{cov}} in some cases,
whereas other processes may be important for GPs with low velocities. Finally,
we compare f_{\mbox{cov}} with proposed indicators of LyC escape. Ionizing
photon escape likely depends on a combination of neutral gas geometry and
kinematics, complicating the use of emission-line diagnostics for identifying
LyC emitters.Comment: 21 pages, 11 figures, accepted for publication in Ap
Measuring the Total Ultraviolet Light from Galaxy Clusters at z=0.5-1.6: The Balance of Obscured and Unobscured Star-Formation
Combined observations from UV to IR wavelengths are necessary to fully
account for the star-formation in galaxy clusters. Low mass (log M/Msun<10)
galaxies are typically not individualy detected, particularly at higher
redshifts (z~1-2) where galaxy clusters are undergoing rapid transitions from
hosting mostly active, dust-obscured star-forming galaxies to quiescent,
passive galaxies. To account for these undetected galaxies, we measure the
total light emerging from GALEX/NUV stacks of galaxy clusters between
z=0.5-1.6. Combined with existing measurements from Spitzer, WISE, and
Herschel, we study the average UV through far-infrared (IR) spectral energy
distribution (SED) of clusters. From the SEDs, we measure the total stellar
mass and amount of dust-obscured and unobscured star-formation arising from all
cluster-member galaxies, including the low mass population. The relative
fraction of unobscured star-formation we observe in the UV is consistent with
what is observed in field galaxies. There is tentative evidence for lower than
expected unobscured star-formation at z~0.5, which may arise from rapid
redshift evolution in the low mass quenching efficiency in clusters reported by
other studies. Finally, the GALEX data places strong constraints on derived
stellar-to-halo mass ratios at z<1 which anti-correlate with the total halo
mass, consistent with trends found from local X-ray observations of clusters.
The data exhibit steeper slopes than implementations of the cluster
star-formation efficiency in semi-analytical models.Comment: 13 pages, 5 figures, accepted to Ap
Neutral Gas Properties and Lyα Escape in Extreme Green Pea Galaxies
Mechanisms regulating the escape of Lyα photons and ionizing radiation remain poorly understood. To study these processes, we analyze Very Large Array 21 cm observations of one Green Pea (GP), J160810+352809 (hereafter J1608), and Hubble Space Telescope Cosmic Origins Spectrograph (COS) spectra of 17 GP galaxies at . All are highly ionized: J1608 has the highest [O iii] λ5007/[O ii] λ3727 for star-forming galaxies in Sloan Digital Sky Survey, and the 17 GPs have [O iii]/[O ii] ≥ 6.6. We set an upper limit on J1608\u27s H i mass of , near or below average compared to similar-mass dwarf galaxies. In the COS sample, eight GPs show Lyα absorption components, six of which also have Lyα emission. The H i column densities derived from Lyα absorption are high, cm−2 = 19–21, well above the LyC optically thick limit. Using low-ionization absorption lines, we measure covering fractions () of 0.1–1 and find that strongly anticorrelates with Lyα escape fraction. Low covering fractions may facilitate Lyα and LyC escape through dense neutral regions. GPs with all have low neutral gas velocities, while GPs with lower have a larger range of velocities. Conventional mechanical feedback may help establish low in some cases, whereas other processes may be important for GPs with low velocities. Finally, we compare with proposed indicators of LyC escape. Ionizing photon escape likely depends on a combination of neutral gas geometry and kinematics, complicating the use of emission-line diagnostics for identifying LyC emitters
Measuring the Heating and Cooling of the Interstellar Medium at High Redshift: PAH and [C II] Observations of the Same Star-forming Galaxies at z ∼ 2
Star formation depends critically on cooling mechanisms in the interstellar medium (ISM); however, thermal properties of gas in galaxies at the peak epoch of star formation (z ~ 2) remain poorly understood. A limiting factor in understanding the multiphase ISM is the lack of multiple tracers detected in the same galaxies, such as Polycyclic Aromatic Hydrocarbon (PAH) emission, a tracer of a critical photoelectric heating mechanism in interstellar gas, and [C ii] 158 μm fine-structure emission, a principal coolant. We present ALMA Band 9 observations targeting [C ii] in six z ~ 2 star-forming galaxies with strong Spitzer IRS detections of PAH emission. All six galaxies are detected in dust continuum and marginally resolved. We compare the properties of PAH and [C ii] emission, and constrain their relationship as a function of total infrared luminosity (L_(IR)) and IR surface density. [C ii] emission is detected in one galaxy at high signal-to-noise (34σ), and we place a secure upper limit on a second source. The rest of our sample are not detected in [C ii] likely due to redshift uncertainties and narrow ALMA bandpass windows. Our results are consistent with the deficit in [C ii]/L_(IR) and PAH/L_(IR) observed in the literature. However, the ratio of [C ii] to PAH emission at z ~ 2 is possibly much lower than what is observed in nearby dusty star-forming galaxies. This could be the result of enhanced cooling via [O i] at high-z, hotter gas and dust temperatures, and/or a reduction in the photoelectric efficiency, in which the coupling between interstellar radiation and gas heating is diminished
Measuring the Heating and Cooling of the Interstellar Medium at High Redshift: PAH and [C II] Observations of the Same Star-forming Galaxies at z ∼ 2
Star formation depends critically on cooling mechanisms in the interstellar medium (ISM); however, thermal properties of gas in galaxies at the peak epoch of star formation (z ~ 2) remain poorly understood. A limiting factor in understanding the multiphase ISM is the lack of multiple tracers detected in the same galaxies, such as Polycyclic Aromatic Hydrocarbon (PAH) emission, a tracer of a critical photoelectric heating mechanism in interstellar gas, and [C ii] 158 μm fine-structure emission, a principal coolant. We present ALMA Band 9 observations targeting [C ii] in six z ~ 2 star-forming galaxies with strong Spitzer IRS detections of PAH emission. All six galaxies are detected in dust continuum and marginally resolved. We compare the properties of PAH and [C ii] emission, and constrain their relationship as a function of total infrared luminosity (L_(IR)) and IR surface density. [C ii] emission is detected in one galaxy at high signal-to-noise (34σ), and we place a secure upper limit on a second source. The rest of our sample are not detected in [C ii] likely due to redshift uncertainties and narrow ALMA bandpass windows. Our results are consistent with the deficit in [C ii]/L_(IR) and PAH/L_(IR) observed in the literature. However, the ratio of [C ii] to PAH emission at z ~ 2 is possibly much lower than what is observed in nearby dusty star-forming galaxies. This could be the result of enhanced cooling via [O i] at high-z, hotter gas and dust temperatures, and/or a reduction in the photoelectric efficiency, in which the coupling between interstellar radiation and gas heating is diminished
Tracing the Total Stellar Mass and Star Formation of High-Redshift Protoclusters
As the progenitors of present-day galaxy clusters, protoclusters are
excellent laboratories to study galaxy evolution. Since existing observations
of protoclusters are limited to the detected constituent galaxies at UV and/or
infrared wavelengths, the details of how typical galaxies grow in these young,
pre-virialized structures remain uncertain. We measure the total stellar mass
and star formation within protoclusters, including the contribution from faint
undetected members by performing a stacking analysis of 211
protoclusters selected as Planck cold sources. We stack WISE and Herschel/SPIRE
images to measure the angular size and the spectral energy distribution of the
integrated light from the protoclusters. The fluxes of protoclusters selected
as Planck cold sources can be contaminated by line of sight interlopers. Using
the WebSky simulation, we estimate that a single protocluster contributes
% of the flux of a Planck cold source on average. After this
correction, we obtain a total star formation rate of $7.3\pm3.2 \times 10^3\
M_{\odot} {\rm yr}^{-1}4.9\pm 2.2\times 10^{12}\
M_{\odot}z=2-4L_{IR} < 3 \times 10^{12} L_{\odot}$). Lastly, we find that protoclusters
subtend a half-light radius of 2.8' (4.2-5.8 cMpc) which is consistent with
simulations.Comment: Accepted for publication in Ap
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