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From First Light to Newborn Stars

From First Light to Newborn Stars

March 14-17, 2010
Tucson, AZ

The Spitzer Legacy Survey of the Cygnus-X Region

Lori Allen, Joe Hora, and the Cygnus-X Legacy team

We describe the first data delivery from the Spitzer Legacy Survey of the Cygnus-X region, which is a massive star formation complex containing the richest known concentration of massive protostars and the largest OB association within 2 kpc. This unbiased survey of 24 square degrees with the Spitzer IRAC and MIPS instruments has the sensitivity to detect young stars to a limit of 0.5 Mo. The data release consists of a source catalog and mosaics of the images in each band that combine all of the frames obtained in the various observation epochs. The source catalog contains the band-merged Spitzer 3.6, 4.5, 5.8, 8.0, and 24 mm photometry as well as the 2MASS J, H, and K bands. A preliminary source classification using this data set has yielded over 2000 deeply embedded and Class I (protostar) candidates, and over 12000 Class II (star+disk) candidate objects. The Cygnus-X survey is an important part of the Spitzer legacy in the study of star formation, and has provided a data set that will be key in future investigations carried out with the warm Spitzer, Herschel, and JWST missions, as well as ground-based observations.

Surveying the Andromeda Stellar Halo

Rachael L. Beaton, Steven R. Majewski, Richard J. Patterson and XXX

We are currently in the process of surveying the Andromeda stellar halo with the Kitt Peak 4-meter telescope using the Washington+DDO51 photometry system on the MOSAIC Imager. As of Fall 2009, we have imaged nearly twenty-five square degrees including seventeen of the twenty known dwarf spheroidals in the Andromeda satellite population. Additionally, the SPLASH collaboration has obtained spectroscopic follow up with the DEIMOS spectrograph on the Keck telescopes, covering roughly 10% of our sampled area, including eleven dwarf spheroidals. Studies of the low surface brightness, old stellar populations in the Andromeda stellar halo suffer from contamination from red dwarf stars in the Milky Way foreground. The Washington+DDO51 photometry system utilizes the surface gravity sensitivity of the MgH absorption feature at 5051 angstroms to separate target red giants from nearby dwarfs on a star by star basis. This detailed preselection has allowed for highly efficient follow up stellar spectroscopy and the development of a detailed five component maximum likelihood analysis to isolate our target populations that utilizes all available data products from our photometric and spectroscopic observations on a star by star basis. Using these methods, we have been able to trace the structure of the Andromeda stellar halo to a projected distance of 165 kpc and to accurately derive detailed mass distributions and highly robust structural parameters for the Andromeda dwarf spheroidals. In this poster, I will explain our methodology using completed observations of several dwarf spheroidal galaxies, a marriage of the Kitt Peak 4-meter +MOSAIC and Keck+DEIMOS data. I will also preview ongoing work to use our existing spectroscopic and photometric data to develop photometry only statistical methods to isolating old stellar populations in the Andromeda stellar halo at surface brightness too low for efficient spectroscopic follow up.

Star Formation in a Spiral Arm: W51 molecular clouds and YSO mass function

J.H. Bieging, M. Kang, M. Povich, & Y. Lee

We have mapped the molecular clouds in the W51 region, a GMC complex in the Sagittarius spiral arm at a distance of about 6 kpc, in 12CO and 13CO emission lines using the Steward Observatory Heinrich Hertz Telescope. In the mapped 1.25 square degree field we identified some 700 YSO candidates from the GLIMPSE point source catalog and determined their properties by SED fitting to the models of Whitney et al. We find that the YSO mass function for stars >5 M_sun is significantly shallower (log slope = -1.3) in the denser, most active star-forming parts of the clouds than in the outer regions (slope = -2.4). Comparison of the positions of YSOs with the spatial distribution of the molecular gas reveals evidence for triggered star formation as well as clustered formation of massive stars.

The Star Formation Rate Density at z~4-10: Deep Lymanbreak galaxy searches and the UV perspective

Rychard J. Bouwens

One of the simplest and most direct ways of estimating the starformation rate (SFR) density at high redshift is from the UV light of star-forming galaxies. Young stars emit a large fraction of their energy at these wavelengths, and it is very easy using current instrumentation to measure this energy to very low SFRs (~1 solar mass / year) and to high redshifts, e.g., z~8 with the new WFC3/IR instrument on HST. Of course, dust extinction presents us with a significant challenge in estimating these SFRs. Fortunately, dust (and its signatures) is now increasingly well characterized at z<1 and z~2, and therefore quite plausible corrections can be made for its effects. In my presentation, I will give an overview of the latest SFR density estimates at z~4-8 derived from Lyman-break galaxy (LBG) searches and the corrections for dust. I will also compare the SFR density estimates derived from LBGs with that seen in the submm and as derived from other approaches (like those that use GRBs). New WFC3/IR results will also be featured throughout.

Star Formation in Clusters

Rupali Chandar

There is now strong evidence that many, and possibly most stars form in groups and clusters. This means that understanding the life cycles of star clusters, from their birth in molecular clouds to their dissolution into the field star population, also tell us about the build up of galaxies in general. The mass and age distributions of clusters in particular, provide critical insight into their formation and evolution. These distributions have now been studied for a number of nearby galaxies of different types and environments (e.g., dwarf, giant, spiral, irregular, interacting, and quiescent galaxies). One key question currently being debated is how “universal” is the process of star cluster formation (and destruction) in these different environments. I will review recent observational results for clusters, both young and old, near and far, and discuss what these results tell us about the process of star formation, and about the build up of the stellar populations of entire galaxies.

The Star Formation History of the Universe

Ranga Chary

I will summarize the results on the dust obscured and unobscured star-formation history of the Universe from recent optical and IR extragalactic surveys. I will highlight the evolution of dust obscuration with redshift, constraints that the extragalactic background light place on galaxy evolution and the nature of galaxies that dominate the SFR density with redshift. Finally, I will try to assess the different physical mechanisms (i.e. mergers, cooling flows etc) which are driving star-formation at different redshifts.

Massive Star Formation in the Large Magellanic Cloud

You-Hua Chu

The Large Magellanic Cloud (LMC), with its 50 kpc distance and nearly face-on viewing angle, provides an excellent laboratory to study the formation of massive stars and its interplay with feedback both locally at parsec scale and globally across the entire galaxy. Surveys of both stars and interstellar medium (ISM) of the LMC are available. Recent Spitzer survey of the LMC, combined with CTIO Blanco 4m images in H-alpha, R, J, and K, allow us to identify massive young stellar objects (YSOs). I will use multi-wavelength observations to illustrate the relationship among molecular clouds, star formation, and feedback over a time-span of ~15 Myr and a spatial scale ranging from pc to kpc.

Star Formation at High Redshift

Romeel Dave

I will discuss recent progress in measuring star formation rates in galaxies from today back to the reionization epoch. Multiwavelength data sets have opened up new views on the nature and evolution of star forming galaxies across cosmic time. Fundamental insights have come from this, such as the emergence of a “main sequence” of galaxy formation, which has profound implications for how galaxies grow and evolve. Finally, I will discuss recent developments in understanding galaxy growth from numerical simulations, including the importance of feedback in regulating star formation, and some quandaries in reconciling various indirect measures of cosmic star formation.

The Dirt on Dry Mergers

Vandana Desai, Arjun Dey, Emma Cohen, Tom Soifer, Emeric XXX

Dry merging (i.e., merging without gas) is invoked in models of hiearchical galaxy formation as an important mode of galaxy assembly, necessary to reproduce the observed fractions and luminosities of galaxies in the red sequence. In one prominent study, van Dokkum (2005), hereafter vD05, found that 70% of nearby (z ~ 0.1) optically red early-type galaxies show signs of tidal interaction, and concluded that the majority of luminous field ellipticals were formed via dry mergers. I will present the long wavelength Spitzer/MIPS (3.6-70 micron) SEDs of the vD05 sample. We find that a significant fraction of the dry mergers identified by vD05 are found to have mid-IR emission in excess of what would be expected from a passively evolving galaxy. Based on mid-IR colors, dusty star formation is the likely source of this mid-IR excess. The derived SFRs are large for passive galaxies, with approximately 25% of the dry merger candidates exhibiting SFRs > 1 Msun/yr. I will discuss the implications of these results for the relevance of dry merging in the formation of early-type galaxies.

Star formation in high redshift galaxies: insights from resolved observations of the ionized and molecular gas

N. M. Förster Schreiber

Recent observational and theoretical results indicate that smooth yet efficient modes of gas accretion and internal secular processes may be important in driving the star formation and evolution of massive z ~ 1 - 3 galaxies. Studies of the spatially-resolved dynamics, star formation, and molecular gas content have provided some of the most convincing evidence in support of this scenario. I will highlight recent results, notably from Ha and CO observations at z ~ 1 - 3 with SINFONI at the VLT and the IRAM Plateau de Bure Interferometer, and will discuss the implications on the star formation activity and scaling laws of massive star-forming galaxies at these early epochs.

A Star and Gas Surface Density Correlation within Nearby Molecular Clouds

R. Gutermuth, J. Pipher, T. Megeath, T. Allen, P. Myers, L. Allen

Large scale surveys with Spitzer have provided a near complete census of young stellar objects within many nearby molecular clouds. In addition, the column density structure of the dense, star-forming portions of these clouds are reliably probed by near-IR extinction mapping using 2MASS photometry. I will describe the recent discovery and characterization of a correlation between the surface densities of young stellar objects and their local natal gas within highly resolved molecular clouds.

Star formation from (local) molecular clouds to spiral arms

Lee Hartmann

I will discuss the observations we have of nearby star-forming regions, emphasizing that molecular clouds are highly structured, with the dense regions accounting for only a small fraction of the cloud mass, the importance of stellar energy input (feedback) for limiting cloud lifetimes, and that star formation must proceed rapidly upon cloud formation, which in turn is a result of molecular cloud formation by large-scale flows and gravitational collapse. I will then briefly consider the implications of what we find locally for more distant, unresolved objects, and make some suggestions concerning the parameters which set star formation rates and the high-mass stellar IMF.

Comparing the Star Formation Rate and Gas Surface Density Relations in the Milky Way to Extragalactic Relations

Amanda Heiderman, Neal J. Evans II, Lori Allen, and Tracy Huard

We investigate the relation between star formation rate (SFR) and gas surface densities in young stellar objects (YSOs) and massive dense clumps. Our YSO sample consists of Flat SED and Class I, II, and III objects located in 19 molecular clouds from the Spitzer c2d and Gould’s Belt surveys. These data allow us to probe the low mass star formation regime that is essentially invisible to tracers (such as H-alpha emission) used to establish extragalactic relations (eg., Schmidt-Kennicutt Law). We estimate gas surface densities from extinction maps and CO maps for two of the c2d clouds and measure YSO SFR surface densities from YSO counts, assuming a mean mass and star formation timescale. We also separate clouds by contours of extinction and count only the youngest YSOs: Flat SED and Class I objects. For our sample of massive star forming clumps, we derive SFRs from the infrared and use dense gas maps estimate gas surface densities. We find that Flat SED and Class I YSOs at higher gas and SFR surface densities overlap with the massive clumps. We also find that on average, all YSOs and massive clumps lie above the extragalactic SFR-gas relations. These two unique data sets allow us to test the established extragalactic SFR-gas surface density relations by comparing them to both regions of low-mass and high-mass star formation for the first time.

Quark-Novae, cosmic reionization, and early r-process element production

Rachid Ouyed, Ralph Pudritz, Prashanth Jaikumar

We examine the case for Quark-Novae as possible sources for the reionization of the universe. We find that dualshock Quark-Nova events can produce enough photons to reionize Hydrogen in the entire Inter-Galactic medium (IGM) by z~6. Such events can explain the large optical depth tau_e ~ 0.1 as measured by WMAP, if the clumping factor of the material being ionized is small. We suggest a way in which a normal initial mass function (IMF) for the oldest stars can be reconciled with a large optical depth as well as the mean metallicity of the early IGM post reionization. We point out the main cosmological signatures and means of detection for Quark-Novae, and their interpretation as high redshift (z~7-8) gamma-ray bursts.

MAGES: The MIPS AGN and Galaxy Evolution Surve

B. Jannuzi, B. Weiner, M. Block, C. Borys, D. Eisenstein, C. Ko

MAGES, a Spitzer Legacy program, consists of MIPS farinfrared imaging of the 9 square degree Bootes field of the NOAO Deep Wide-Field Survey. The observations were executed in January of 2009. Complemented by a wealth of multi-wavelength data available for the Bootes field, the new MIPS observations enable the study of the properties of z<1 LIRGS and ULIRGS, the far-IR emission of obscured and unobscured QSOs and AGN, the spectral energy distributions of IR-luminous galaxies, and the clustering of IR-selected source populations. Our new MIPS observations yielded significantly improved long-wavelength (70 and 160 micron bands) observations compared to previous observations obtained early in the lifetime of the cryogenic mission of the Spitzer Space Telescope. Our catalogs can be used to study the history of star formation rate in galaxies over more than half of the age of the Universe.

Probing the Birth of Super Star Clusters

Kelsey Johnson

Super star clusters are among the most extreme star formation environments known; they have incredible stellar densities, and each can harbor thousands of massive stars within radii of only a few parsecs. The most robust of these clusters may even be precursors to the ancient globular clusters ubiquitous around massive galaxies in the local universe today. Understanding the formation and feedback of super star clusters has the potential to provide us with insight into the evolution of starburst episodes throughout the universe. At present the relationship between the local physical conditions and the voracity of star formation is not well-constrained. Some progress has been made: over the last decade, a number of natal super star clusters have been discovered, providing us with a glimpse into their early evolution. However, the set of existing observations is anemic, and our current physical model for these natal clusters in simplistic. I will overview what we think we know about these objects based on existing observations and outline some of the most significant gaps in our current understanding.

Star Formation in Extreme Environments: II Zw 40

Amanda Kepley, Amy Reines, Kelsey Johnson

Dwarf starburst galaxies like II Zw 40 provide a window into the formation of massive clusters in conditions like those in the early universe, i.e., low mass and low metallicity. We present new very sensitive, very high resolution (~6 pc) VLA+Pie Town radio continuum observations of the central star forming region of II Zw 40. We are able to distinguish at least four super star clusters and derive their properties (electron density, size, number of ionizing photons). We also present a comparison of the radio continuum emission with high resolution optical images.

Star Formation in W5

Xavier Koenig and Lori Allen

We present images and results from our extensive Spitzer Space Telescope imaging survey of the W5 Hii region with the Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS). We detect dense clusters of stars, centered on the O stars: HD 18326, BD +60 586, HD 17505 and HD 17520. At 24μm, substantial extended emission is visible, presumably from heated large dust grains that survive in the strongly ionizing environment of the Hii region. With photometry of more than 18000 point sources, we analyze the clustering properties of objects classified as young stars by their IR spectral energy distributions (a total of 1882 sources) across the region using a minimal-spanning-tree algorithm. We find 58% of infrared excess sources belong to clusters with _30 members. We find that within the evacuated cavity of the HII regions that make up W5, the ratio of class II to class I sources is 7 times higher than for objects detected coincident with molecular gas as traced by12CO emission and near-IR extinction maps. We attribute this contrast to an age difference between the two locations, and postulate that at least two distinct generations of star formation are visible across W5. Our preliminary analysis shows that triggering is a plausible mechanism to explain the multiple generations of star formation in W5, and merits further investigation.

Star-Formation at z<1: Galaxy Mergers v. Gas Accretion

Jennifer Lotz

Galaxies grow with time through both discrete galaxy mergers and smooth gas accretion. When and how this growth occurs, and the role of mergers in the star-formation histories of galaxies, remain outstanding observational questions. I will review recent observations of star-forming galaxies and galaxy mergers at z<1, as well as theoretical predictions for merger driven star-formation. Although galaxy mergers are common, the evolution in the gas properties of galaxies during this epoch is likely to be more important for the evolution in the global star-formation rate density since z~1.

The Spitzer Survey of the Orion Molecular Cloud Complex

Tom Megeath

The Orion Molecular Cloud complex is our nearest example of low and high mass star formation in an extended OB association. Over the duration of the cyro-mission, the Spitzer space telescope surveyed 10 sq degrees of the Orion complex, the most extensive survey of young stars and protostars in Orion to date. Using this survey, I will address three questions: what fractions of stars form in clusters, what are the properties of the clusters, and how does Orion compare to other nearby molecular clouds and to distant super star clusters?

The Herschel Orion Protostar Survey (HOPS)

Tom Megeath and the HOPS Team

The Orion molecular cloud complex is the most active region of star formation within 500 pc of the Sun, with over 400 protostars identified by Spitzer in a diverse range of environments. We are now collecting extensive observations on 280 Orion protostars, including 5-40 um Spitzer spectroscopy, near-IR imaging and spectroscopy with Hubble and ground-based telescopes, and a Herschel open time key project of PACS far-IR imaging and spectroscopy. With these data we can determine the fundamental properties (multiplicity, gas infall rate, bolometric luminosity, outflow cavity geometry) of a large sample of protostars in a single cloud complex. We are additionally obtaining data on the temperature, turbulence and column density of the surrounding molecular gas from a number of ground-based telescopes. This program will provide new insights into protostellar evolution and how protostars are influenced by their surrounding environment (molecular gas properties, density of stars, radiation fields). To date, we have acquired Herschel 70 and 160 um images of a group of four protostars associated with the Herbig-Haro complex HH1-2. With these and other data, we constructed 1-160 micron SEDs and fit them to model SEDs generated with radiative transfer codes. The inferred mass infall rates vary by 3 orders of magnitude between the protostars, hinting at a sharp decrease in the infall rate over time. This field also includes the NGC 1999 nebula; we show with our Herschel data and data from the Kitt Peak 4-meter, Magellan and APEX that the well known dark feature in the nebula is not due to a foreground dark globule, but is a cavity in the nebula.

Exploring Star Formation Intensity, Gas Pressures, and Feedback

J. Monkiewicz and S. Malhotra

We study the effects of feedback on star formation by examining the relation between ISM pressures and global star formation rate intensity for ~40 nearby star-forming galaxies observed with ISO and GALEX. Using FIR and UV surface brightness as our measures of global star formation intensity, we compare to warm neutral gas pressures, as derived from far-infrared [CII] and [OI] line widths. We find a strong correlation between pressure and surface brightness up to pressures of 6 K cm-3. Above this values, surface brightness appears to plateau, consistent with results over a wide range of redshifts. We also examine the relationship of pressure to total UV and IR luminosity, and find that this correlation much weaker, suggesting that star formation intensity is a much more fundamental indicator of the global state of a galaxy’s ISM than is the total amount of star formation.

The Kennicutt-Schmidt Relation at z~2

Desika Narayanan

Understanding the potential evolution of the Kennicutt-Schmidt (KS) relation with redshift is a formidable challenge. Here, we describe a novel methodology for deriving the KS law at z~2, the epoch when the bulk of the Universe’s stars formed. The method has three main components. (1) First, by tying together high resolution SPH simulations of isolated galaxies and galaxy mergers with dust radiative transfer modeling, we provide a physical model for the evolution of various high redshift galaxy populations. In particular, we focus on reproducing observed BzK and Submillimeter galaxies, and verify our model with comparisons to a number of observations. (2) Second, we describe how the KS law relates to an observationally tractable relationship, the LIR-molecular line relation at z~2. We utilize the theoretical framework developed by Krumholz & Thompson (2007) and Narayanan et al. (2008) to show that the observed LIR-Lmol relations (at all redshifts) are fundamentally anchored by the underlying KS law, and quantify how a given observed (e.g.) SFR-HCN relation in z~2 galaxies will betray the underlying KS law at this epoch. (3) Finally, we utilize our models to derive robust observational SFR tracers in BzK and Submillimeter Galaxies (to avoid contamination by e.g. AGN or older stellar populations in the SFR determination). Utilizing these three components, we derive an imminently observable experiment for constraining the KS law in z~2 galaxies. If time permits, we will apply this methodology to the recent observational surveys of Tacconi and coworkers.

Effect of reionization redshift and virialization temperature of CDM halos on the cosmic star formation rate.

Ramona Núñez-López

In present work we calculate the Pop III stars formation in the cold dark matter halo. Comoving number density of collapsed is calculated in the frame of the Press-Schechter (PS) and the Seth-Tormen (ST) formalism. By integration of the calculated star formation rate (SFR), the cosmic SFR is obtained. The dependence of the cosmic SFR on defferent important parameters is discussed.

Correlations of Quasars and Luminous Red Galaxies

Dara Norman (NOAO), Roberto DePropris (CTIO), Nicholas Ro

Public data from the 2dF quasar survey and 2dF/Sloan Digital Sky Survey LRG and QSO, with their vast reservoirs of spectroscopically located and identified sources, afford us the chance to more accurately study their real-space correlations in the hopes of identifying the physical processes that trigger quasar activity. We have used these two public databases to measure the projected cross-correlation, Omega_p , between quasars and luminous red galaxies. We find the projected twopoint correlation to have a fitted clustering radius of r_0 = 5.3 ű 0.6 and a slope gamma = 1.83 ű 0.42 on scales from 0.7 to 27 /h Mpc. We attempt to understand this strong correlation by separating the LRG sample into two populations of blue and red galaxies. We measure at the cross-correlation with each population. We find that these quasars have a stronger correlation amplitude with the bluer, more recently star-forming population in our sample than the redder passively evolving population, which has a correlation that is much more noisy and seems to flatten on scales < 5 /h Mpc. We compare this result to published work on hierarchical models. The stronger correlation of bright quasars with LRGs that have undergone a recent burst of star formation suggests that the physical mechanisms that produce both activities are related and that minor mergers or tidal effects may be important triggers of bright quasar activity and/or that bright quasars are less highly biased than faint quasars.

The Star Formation History of M31’s Bulge and Disk

K.A.G. Olsen, A.W. Stephens, and R.D. Blum

We will discuss near-infrared observations of ten fields in the bulge and disk of M31, obtained with the NIRI instrument and ALTAIR adaptive optics system on Gemini North. These are the some of the highest resolution near-infrared observations obtained to date of the extremely crowded high surface brightness regions of M31. From fits to the K-band luminosity functions of the bulge and innermost disk fields, we have found the stellar population mix to be dominated by old, nearly solar metallicity stars independent of radius. Analyses of disk fields located in the vicinity of M31’s 10 kpc ring of star formation reveals very different star formation histories. While an old, moderately metal-poor population contains the majority of the stellar mass, up to 30% of the mass is found in a solar metallicity star formation burst of intermediate age. The burst age varies between ~200 Myr and 1 Gyr, depending on location. We attribute the burst to the event that formed the ring itself, and will discuss its implications for the formation of M31’s disk.

Alpha-enhancement and Evolution of Very Strong MgII Absorbers

Paola Rodriguez Hidalgo, Kaylan Wessels, Jane Charlton, Ther XXX

The ratio of the equivalent widths (Wr) of FeII to MgII provide information about gas properties, such as ionization parameter and density, as well as galactic properties, such as star formation history. We present a study of the evolution of this ratio over a large fraction of cosmic history: 2.3-8.7 Gyrs of the age of the Universe. Our sample consists of 88 strong (Wr(2796)>0.3 Ã...) and 100 weak (Wr(2796)<0.3 Ã...) MgII absorbers, with redshifts 0.3<z<2.5, measured in 81 quasar spectra obtained from the Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES) archives of high-resolution spectra (R~45,000). We find an evolution in the distribution of the ratio of Wr(FeII)/Wr(MgII) for very strong MgII absorbers (those with Wr(2796)>1 Ã...). Statistical tests confirm that the population of absorbers with low redshift (0.3<z<1.2) is not drawn from the same population as the high redshift absorbers (1.2<z<2.5). At low redshift, we find an absence of very strong MgII absorbers with small Wr(FeII)/Wr(MgII) while at higher redshifts absorbers with smaller Wr(FeII)/Wr(MgII) are present. In contrast, previous studies have shown that weak absorbers show an absence of large ratios of Wr(FeII)/Wr(MgII) at high redshifts. After ruling out high ionization conditions, we conclude that this trend may be caused by (1) alpha-enhancement on very strong MgII absorbers at high redshift, and/or (2) lack of FeII production in the high redshift. The alpha-enhancement may be due to superwinds, as they make a large contribution to strong MgII absorption at high redshift, particularly for the strongest absorbers. Since Fe is mostly produced in Type Ia supernovae, which occur approximately 1 Gyr after the massive core supernovae start polluting the galactic gas, the lack of FeII at high redshifts indicates early stages in the star formation of these galaxies. This research was funded by NASA under grant 428-09-57DN.

How clusters supress star formation over the last 7 billion years

Gregory Rudnick, Rose Finn, Vandana Desai, Bianca Poggianti, XXX

It is a well known that the global star formation rate (SFR) has declined precipitously since z>1. Despite the accurate characterization of this decline in the field it is not yet clear what role environment plays in the shutoff of star formation in individual galaxies and in their subsequent migration to the red sequence. To address this we have completed the largest multi-wavelength survey of clusters at 0.4<z<0.8, when the Universe was 50% of its current age. Our 24-micron data from Spitzer combined with extensive optical spectroscopy give us a robust view of star formation without the complications of uncertain extinction. I will present results from the ESO Distant Cluster Survey (EDisCS) that highlight the role of environment in governing star formation in galaxies. A main result is that there is a great degree of cluster-to-cluster variation in the star forming properties, necessitating a large sample to draw robust conclusions. The 24-micron imaging and optical spectroscopy of our 16 clusters demonstrates that the star forming fraction and mean SFR of cluster galaxies is suppressed with respect to the field, and that the lower fraction exists at all radii less than 1.5 times the virial radius, with significant suppression existing in the cluster cores. Despite this, ~1/4 of the clusters have a fraction of vigorously star forming galaxies above the field value, indicating that star formation may be enhanced in some cluster environments. When studying the redshift evolution down to z=0, we see that the fraction of vigorously star forming galaxies in clusters decreases towards the present day with a similar rate to that in the field. I will discuss the implications that our results have for the possible mechanisms that suppress star formation in galaxies. I will also address how this suppression is linked to the rapid build-up of the faint end of the cluster red sequence that has been observed in these same clusters.

Star Formation in Dense & Extreme Environments

Nick Scoville

I will review the standard properties of star forming molecular clouds and then focus on the physics of accelerated star formation occurring in galactic nuclei and the massive OB star forming regions where the star formation timescales are 10-30 times shorter. In the latter instances, star are forming at maximal self-limited rates, regulated by feedback. In general, the molecules last much longer than a dynamical timescale (~million yrs) although this gas may be shuffled between different size clouds over their lifetime. In galactic nuclei, only very dense and massive clouds can collapse due to the higher turbulent pressures and tidal effects and much of the ISM will be distributed in disk-like structures not self-gravitating clouds. The gas in these regions is most appropriately traced via higher excitation transitions (e.g. HCN) than CO. Given the very high column densities of gas and dust, the star formation rates can only be reliably probed in the far infrared/submm or in the free-free continuum and ALMA holds tremendous promise for opening up these regions. These extreme conditions are likely to be much more dominant for star formation in the early universe.

Star Formation and Dust Extinction of z~4 Galaxies in GOODS

Hyunjin Shim, Ranga-Ram Chary, Mark Dickinson and the GOODS team

We present the rest-frame optical star formation rates of z~4 galaxies with spectroscopic redshifts selected over the Great Observatories Origins Deep Survey (GOODS) fields. Among 74 galaxies detected in Spitzer IRAC 3.6um and 4.5um, more than 70% of the galaxies show clear excess in 3.6μm compared to the expected flux using stellar continuum only. We suggest that this 3.6um excess is due to Halpha emission line redshifted into 3.6um band at 3.8<z<5.0, and consider these 3.6um excess galaxies to be Halpha emitter (HAE) candidates at z~4. The selection for z~4 HAE candidates using broadband flux excess is sensitive to galaxies with strong Halpha equivalent widths, i.e., rest-frame equivalent width larger than 350A. The Halpha inferred star formation rates of z~4 HAE candidates range 20-1000 Msun/yr, ~20 times larger than UVderived star formation rates when no extinction correction is applied. The ratio between the Halpha-derived star formation rates and the UV-derived star formation rates is an independent measure of dust extinction for high-redshift starforming galaxies besides the UV spectral slope beta. The correlation between beta and the star formation ratio at different wavelengths raise the possibility that at least ~30% of z~4 HAE candidates follow the extinction law other than local starburst extinction law. The correlation between large Halpha equivalent widths and the stellar population ages also show that half of z~4 HAE candidates have continuous star formation over a relatively long time scale (tau>100Myr). The lower limit of stellar mass density at z~2 produced by these z~4 HAE candidates with continuous star formation is about 10% of the observed value, which show z~4 HAE emitters are possible to be generators of red, massive galaxies at z~2. The non-merger morphology and the large current star formation rates for z~4 HAE candidates with continuous star formation could be an evidence of “cold-flow accretion” in high-redshift star formation.

History of Star Formation in Local Galaxies

Tammy Smecker-Hane

Galaxies in the Local Group show a surprisingly wide array of star formation histories, even among the dwarf spheroidal galaxies (dSphs) that are the lowest mass galaxies. I will review what we know about the star formation histories of Local Group galaxies, specifically highlighting how we are making great strides in quantifying galaxy evolution by studying individual stars in these galaxies. By combining imaging of star fields obtained with NOAO mosaic cameras and the Hubble Space telescope, which yields highly precise color-magnitude diagrams of the main-sequence turnoff region, with multi-object spectroscopy obtained at NOAO telescopes, which constrains the chemical abundances of the stars in the galaxies, we can unambiguously derive the chemical evolution and star formation histories of these galaxies. Many interesting puzzles remain to be solved and in this coming decade we will see these results tested against increasingly sophisticated theoretical simulations of galaxy evolution that will quantify the physical processes (delayed infall of gas, energetic feedback from supernovae and massive stars, outflows of metal-enriched winds, etc.) that regulate galaxy evolution.

The Steady March of Feedback-Driven Star Formation in the Carina Nebula

Nathan Smith

The Carina Nebula is an ideal laboratory in which to study feedback from young massive stars. Its extreme stellar content samples a scale similar to regions like 30 Doradus and more distant starbursts, but it is close enough to study the details of how low-mass star formation proceeds in such an extreme environment. I will describe some recent results from large surveys with HST and Spitzer. In particular, I will focus on the spatial relationships between O-type stars, low-mass protostars, outflows, warm dust, and ionized photoevaporative flows compared to dense clumps and dust pillars. The star formation rate and initial mass function imply that this mode of feedback-driven star formation in pillars plays a key role in the gradual construction of a large unbound OB association.

The Mass Function of the First Stars

Aparna Venkatesan

The mass function of the first stars is an important input in cosmological calculations to estimate the feedback from these objects in the early universe. I present the current constraints on the first stars’ mass function from reionization and nucleosynthetic observations, as well as from the most recent cosmic microwave background data. I comment particularly on the effects of X-rays and helium-ionizing radiation from high redshift galaxies on the ionization and thermal balance in the intergalactic medium, and show how these separate effects can be disentangled and used to place limits on first-light sources.

Spatial clustering of infrared-luminous galaxies, a clue to their fates

Benjamin Weiner, Alison Coil

Infrared-luminous galaxies are powered by star formation or active galactic nuclei, but emit much of their light as radiation reprocessed by dust into the far infrared. The most massive starbursts in both the local and high redshift universe manifest themselves as ultraluminous infrared galaxies. However, it remains controversial what IR-luminous galaxies at z=1 are, and what they will evolve into. Are IR-luminous galaxies at high redshift mostly galaxy mergers, as they are at low redshift? Are ultraluminous IR galaxies strongly clustered, and can we infer whether they must evolve into cluster galaxies today? We measure the spatial clustering of LIRGs and ULIRGs at z=1 using Spitzer/MIPS sources cross-correlated with galaxies from the DEEP2 redshift survey. Because the evolution of clustering strength is well understood, the correlation lengths constrain the galaxy populations that LIRGs and ULIRGs will evolve into, and the classes of AGN to which they may be linked.

Modeling Emission from Natal Super Star Clusters: Some Problems, and Potential Solutions

D. G. Whelan, K. E. Johnson

Three-dimensional models with clumpy dust distributions suffer from degeneracies with sightline: the same physical parameters can look vastly different in the infrared, depending on viewing angle. I am looking for simple infrared diagnostics that can be used by observers to help determine physical parameters like size, density, stellar luminosity, the amount of dust that is distributed in clumps as opposed to smoothly, and where the small grain emission comes from, so that those variables may be constrained in future models. The advent of Herschel for providing relatively high spatial resolution farinfrared observations provides a powerful tool for testing the usefulness of these models and constraining super star cluster physical parameters.

Local Galaxies in Global Contexts

Dennis Zaritsky & Michael Eskew

I will present results from our work on reconstructed star formation histories of nearby galaxies in the context of diagrams used to study more distant galaxies (Tully-Fisher, color-magnitude diagram, the build-up of stellar mass). By using the histories of individual galaxies one begins to explore the excursions that are possible in such diagnostic plots over the course a galaxy’s lifetime. In particular, I will focus on how some of these galaxies move in the color magnitude diagram to explore the nature of the blue cloud, green valley, and red sequence.


NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation.

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