Exploring the limits of star formation from the extreme environment of galaxy interactions to the Milky Way

textIn this thesis, I explore the rate at which molecular gas is converted to stars through detailed studies of a sample of molecular clouds in the Milky Way, IFU spatially resolved observations of gas-rich nearby interacting galaxies, as well as the environmental dependence of star formation and galaxy morphology in a galaxy supercluster. This thesis is composed of three individual projects that investigate nearby star formation within the local 500 pc of our Sun, to neighboring extreme star forming environments of interacting starburst galaxies, and finally studying how star formation varies with galaxy morphology in a galaxy supercluster a z~0.165. I discuss the relation between the star formation rate (SFR) and molecular gas surface densities (e.g., Schmidt-Kennicutt relation) in Galactic star forming regions and find there is a discrepancy between my study and extragalactic relations. The discrepancy is attributed to extragalactic measurements that are averaged over large >kpc scales and include star forming molecular gas (above some threshold) and molecular gas the is not dense enough to form stars. I find a steep increase in the Galactic SFR-gas surface density relation indicative of a threshold for efficient star formation that is best fit to a broken power law with a linear slope above 129 Msun pc⁻². I introduce the VIRUS-P Investigation of the eXtreme ENviroments of Starbursts (VIXENS) project which is a survey of interacting is a large integral field unit survey of nearby infrared bright (L_IR>3x10¹⁰ Lsun) interacting/starburst galaxies. The main goal of VIXENS is to investigate the relation between star formation and gas content on spatially resolved scales of ~0.1-1 kpc in the extreme star forming environments of interacting/starburst galaxies. The VIXENS sample is composed of systems in a range interaction stages with morphological signatures from early phase (close pairs) to late stage mergers (single system with multiple nuclei), SFRs, and gas surface densities. I highlight the first results from the VIXENS survey in the late interaction phase galaxy merger Arp 299. I find 1.3 kpc regions in Arp 299 to lie along the SFR-gas surface density relation found for mergers at high redshift, but this relation is highly dependent on the CO to molecular hydrogen (H₂) conversion factor. I find evidence for a Galactic CO-to-H₂ conversion factor using metallicity and dust temperature measurements, which would place 1.3 kpc regions in the Arp 299 merger in between the high redshift and Kennicutt-Schmidt relations. Comparing the SFR to dense gas surface densities as traced by HCN and HCO⁺, I find an agreement between the spatially resolved measurements and that found on global scales in spirals and (ultra)luminous infrared galaxies. Finally, I present an investigation of the influence of environment on frequency, distribution, color, and star formation properties of galaxy mergers and non-interacting galaxies in the Abell 901/902 supercluster at z~0.165. I find galaxy mergers be preferentially blue in color and have an enhanced SFR by a factor of ~2 compared to non-interacting galaxies. This result may be due to a decrease in galaxy velocity dispersion in the cluster outskirt, favoring galaxy-galaxy interactions, or to interacting galaxies that are part of groups or field galaxies being accreted along cosmological filaments by the clusters. I compare to N-body simulations of groups and field galaxies accreting onto the clusters and find the fraction of mergers are similar to that predicated at group overdensities. I find the SFR of galaxies in the supercluster to be depressed compared to field galaxies in both the core and cluster outskirts, suggesting that an environmental process such as ram pressure stripping is effective throughout the cluster. The results of a modest SFR enhancement and a low merger fraction culminate in my finding that mergers contribute only a small fraction (between 10% and 15%) of the total SFR density of the Abell 901/902 clusters.Astronom

The Gould Belt 'MISFITS' Survey: The Real Solar Neighborhood Protostars

We present an HCO$^{+}$ J=3-2 survey of Class 0+I and Flat SED young stellar objects (YSOs) found in the Gould Belt clouds by surveys with Spitzer. Our goal is to provide a uniform Stage 0+I source indicator for these embedded protostar candidates. We made single point HCO$^{+}$ J = 3-2 measurements toward the source positions at the CSO and APEX of 546 YSOs (89% of the Class 0+I + Flat SED sample). Using the criteria from van Kempen et al. (2009), we classify sources as Stage 0+I or bona fide protostars and find that 84% of detected sources meet the criteria. We recommend a timescale for the evolution of Stage 0+I (embedded protostars) of 0.54 Myr. We find significant correlations of HCO$^{+}$ integrated intensity with ${\alpha}$ and $T_{bol}$ but not with $L_{bol}$. The detection fraction increases smoothly as a function of ${\alpha}$ and $L_{bol}$, while decreasing smoothly with $T_{bol}$. Using the Stage 0+I sources tightens the relation between protostars and high extinction regions of the cloud; 89% of Stage I sources lie in regions with $A_{V}$ >8 mag. Class 0+I and Flat SED YSOs that are not detected in HCO$^{+}$ have, on average, a factor of ~2 higher $T_{bol}$ and a factor of ~5 lower $L_{bol}$ than YSOs with HCO$^{+}$ detections. We find less YSO contamination, defined as the number of undetected YSOs divided by the total number surveyed, for sources with $T_{bol}\lesssim$ 600 K and $L_{bol} \gtrsim$ 1 $L_{\odot}$. The contamination percentage is >90% at $A_{V}$< 4 mag and decreases as $A_{V}$ increases.Comment: 28 pages, 11 figures, 5 tables. Accepted for publication to the Astrophysical Journa

The VIRUS-P Exploration of Nearby Galaxies (VENGA): Survey Design, Data Processing, and Spectral Analysis Methods

We present the survey design, data reduction, and spectral fitting pipeline for the VIRUS-P Exploration of Nearby Galaxies (VENGA). VENGA is an integral field spectroscopic survey, which maps the disks of 30 nearby spiral galaxies. Targets span a wide range in Hubble type, star formation activity, morphology, and inclination. The VENGA data-cubes have 5.6'' FWHM spatial resolution, ~5A FWHM spectral resolution, sample the 3600A-6800A range, and cover large areas typically sampling galaxies out to ~0.7 R_25. These data-cubes can be used to produce 2D maps of the star formation rate, dust extinction, electron density, stellar population parameters, the kinematics and chemical abundances of both stars and ionized gas, and other physical quantities derived from the fitting of the stellar spectrum and the measurement of nebular emission lines. To exemplify our methods and the quality of the data, we present the VENGA data-cube on the face-on Sc galaxy NGC 628 (a.k.a. M 74). The VENGA observations of NGC 628 are described, as well as the construction of the data-cube, our spectral fitting method, and the fitting of the stellar and ionized gas velocity fields. We also propose a new method to measure the inclination of nearly face-on systems based on the matching of the stellar and gas rotation curves using asymmetric drift corrections. VENGA will measure relevant physical parameters across different environments within these galaxies, allowing a series of studies on star formation, structure assembly, stellar populations, chemical evolution, galactic feedback, nuclear activity, and the properties of the interstellar medium in massive disk galaxies.Comment: Accepted for publication in AJ, 25 pages, 18 figures, 6 table

The VIRUS-P Exploration of Nearby Galaxies (VENGA): Survey Design and First Results

VENGA is a large-scale extragalactic IFU survey, which maps the bulges, bars and large parts of the outer disks of 32 nearby normal spiral galaxies. The targets are chosen to span a wide range in Hubble types, star formation activities, morphologies, and inclinations, at the same time of having vast available multi-wavelength coverage from the far-UV to the mid-IR, and available CO and 21cm mapping. The VENGA dataset will provide 2D maps of the SFR, stellar and gas kinematics, chemical abundances, ISM density and ionization states, dust extinction and stellar populations for these 32 galaxies. The uniqueness of the VIRUS-P large field of view permits these large-scale mappings to be performed. VENGA will allow us to correlate all these important quantities throughout the different environments present in galactic disks, allowing the conduction of a large number of studies in star formation, structure assembly, galactic feedback and ISM in galaxies.Comment: 7 pages, 3 figures, proceedings of the "Third Biennial Frank N. Bash Symposium, New Horizons in Astronomy" held in Austin, TX, Oct. 2009. To be published in the Astronomical Society of the Pacific Conference Series, eds. L. Stanford, L. Hao, Y. Mao, J. Gree

Young Stellar Objects in the Gould Belt

We present the full catalog of Young Stellar Objects (YSOs) identified in the 18 molecular clouds surveyed by the Spitzer Space Telescope "cores to disks" (c2d) and "Gould Belt" (GB) Legacy surveys. Using standard techniques developed by the c2d project, we identify 3239 candidate YSOs in the 18 clouds, 2966 of which survive visual inspection and form our final catalog of YSOs in the Gould Belt. We compile extinction corrected SEDs for all 2966 YSOs and calculate and tabulate the infrared spectral index, bolometric luminosity, and bolometric temperature for each object. We find that 326 (11%), 210 (7%), 1248 (42%), and 1182 (40%) are classified as Class 0+I, Flat-spectrum, Class II, and Class III, respectively, and show that the Class III sample suffers from an overall contamination rate by background AGB stars between 25% and 90%. Adopting standard assumptions, we derive durations of 0.40-0.78 Myr for Class 0+I YSOs and 0.26-0.50 Myr for Flat-spectrum YSOs, where the ranges encompass uncertainties in the adopted assumptions. Including information from (sub)millimeter wavelengths, one-third of the Class 0+I sample is classified as Class 0, leading to durations of 0.13-0.26 Myr (Class 0) and 0.27-0.52 Myr (Class I). We revisit infrared color-color diagrams used in the literature to classify YSOs and propose minor revisions to classification boundaries in these diagrams. Finally, we show that the bolometric temperature is a poor discriminator between Class II and Class III YSOs.Comment: Accepted for publication in ApJS. 29 pages, 11 figures, 14 tables, 4 appendices. Full versions of data tables (to be published in machine-readable format by ApJS) available at the end of the latex source cod

The Spatially Resolved Star Formation Law from Integral Field Spectroscopy: VIRUS-P Observations of NGC 5194

We investigate the relation between the star formation rate surface density (Sigma_SFR) and the mass surface density of gas (Sigma_gas) in NGC 5194. VIRUS-P integral field spectroscopy is used to measure H-alpha, H-beta [NII]6548,6584, and [SII]6717,6731 emission line fluxes for 735 regions ~170 pc in diameter, and derive extinction corrected Sigma_SFRs. HI 21cm and CO maps are used to measure the HI and H_2 gas surface density for each region. We present a new method for fitting the Star Formation Law (SFL), which includes the intrinsic scatter in the relation as a free parameter, allows the inclusion of non-detections, and is free of the systematics involved in performing linear correlations over incomplete data in logarithmic space. We use the [SII]/H-alpha ratio to separate the H-alpha flux contribution from the diffuse ionized gas (DIG). After removing the DIG, we measure a slope N=0.82+/-0.05, and an intrinsic scatter epsilon=0.43+/-0.02 dex for the molecular gas SFL. We also measure a typical depletion timescale tau~2 Gyr, in good agreement with Bigiel et al. (2008). The HI density shows no correlation with the SFR, and the total gas SFL closely follows the molecular gas SFL. We assess the validity of corrections applied in narrow-band H-alpha measurements to overcome issues related to continuum subtraction, underlying photospheric absorption, and contamination by the [NII] doublet. The disagreement with the super-linear molecular SFL measured by Kennicutt et al. (2007) is due to differences in the fitting method. Our results support a low and close to constant star formation efficiency (SFE = 1/tau) in the molecular ISM. The data shows excellent agreement with the model of Krumholz et al (2009). The large intrinsic scatter may imply the existence of other important parameters setting the SFR.Comment: 23 pages, 19 figures, 14 pages of tables. Accepted for publication in ApJ. (Figure 16 has been corrected from the first submitted version.

Barred Galaxies in the Abell 901/2 Supercluster with STAGES

We present a study of bar and host disk evolution in a dense cluster environment, based on a sample of ~800 bright (MV <= -18) galaxies in the Abell 901/2 supercluster at z~0.165. We use HST ACS F606W imaging from the STAGES survey, and data from Spitzer, XMM-Newton, and COMBO-17. We identify and characterize bars through ellipse-fitting, and other morphological features through visual classification. (1) We explore three commonly used methods for selecting disk galaxies. We find 625, 485, and 353 disk galaxies, respectively, via visual classification, a single component S'ersic cut (n <= 2.5), and a blue-cloud cut. In cluster environments, the latter two methods miss 31% and 51%, respectively, of visually-identified disks. (2) For moderately inclined disks, the three methods of disk selection yield a similar global optical bar fraction (f_bar-opt) of 34% +10%/-3%, 31% +10%/-3%, and 30% +10%/-3%, respectively. (3) f_bar-opt rises in brighter galaxies and those which appear to have no significant bulge component. Within a given absolute magnitude bin, f_bar-opt is higher in visually-selected disk galaxies that have no bulge as opposed to those with bulges. For a given morphological class, f_bar-opt rises at higher luminosities. (4) For bright early-types, as well as faint late-type systems with no evident bulge, the optical bar fraction in the Abell 901/2 clusters is comparable within a factor of 1.1 to 1.4 to that of field galaxies at lower redshifts (5) Between the core and the virial radius of the cluster at intermediate environmental densities, the optical bar fraction does not appear to depend strongly on the local environment density and varies at most by a factor of ~1.3. We discuss the implications of our results for the evolution of bars and disks in dense environments.Comment: accepted for publication in ApJ, abstract abridged, for high resolution figures see http://www.as.utexas.edu/~marinova/STAGES/STAGES_bars.pd

The VIRUS-P Exploration of Nearby Galaxies (VENGA): The X CO Gradient in NGC 628

We measure the radial profile of the ^(12)CO(1-0) to H_2 conversion factor (X_(CO)) in NGC 628. The Hα emission from the VENGA integral field spectroscopy is used to map the star formation rate (SFR) surface density (Σ_(SFR)). We estimate the molecular gas surface density (Σ_(H2)) from Σ_(SFR) by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure X_(CO). We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (Hα versus far-UV plus 24 μm), and CO maps from different telescopes (single-dish and interferometers). The observed X_(CO) profile is robust against these systematics, drops by a factor of two from R ~ 7 kpc to the center of the galaxy, and is well fit by a gradient Δlog(X_(CO)) = 0.06 ± 0.02 dex kpc^(–1). We study how changes in X_(CO) follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in X_(CO) can be explained by a combination of decreasing metallicity, and decreasing Σ_(H2) with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of X_(CO) in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star-forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single X_(CO) value when large changes in gas surface density or metallicity are present