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At least 1,477 records · Page 82
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DOE OSTI2015

STRONG C{sup +} EMISSION IN GALAXIES AT z ∼ 1-2: EVIDENCE FOR COLD FLOW ACCRETION POWERED STAR FORMATION IN THE EARLY UNIVERSE

We have recently detected the [C II] 157.7 μm line in eight star-forming galaxies at redshifts 1 to 2 using the redshift (z) Early Universe Spectrometer (ZEUS). Our sample targets star formation dominant sources detected in PAH emission. This represents a significant addition to [C II] observations during the epoch of peak star formation. We have augmented this survey with observations of the [O I] 63 μm line and far infrared photometry from the PACS and SPIRE Herschel instruments as well as Spitzer IRS spectra from the literature showing PAH features. Our sources exhibit above average gas heating efficiency, many with both [O I]/FIR and [C II]/FIR of ∼1% or more. The relatively strong [C II] emission is consistent with our sources being dominated by star formation powered photo-dissociation regions, extending to kiloparsec scales. We suggest that the star formation mode in these systems follows a Schmidt-Kennicutt law similar to local systems, but at a much higher rate due to molecular gas surface densities 10-100 times that of local star-forming systems. The source of the high molecular gas surface densities may be the infall of neutral gas from the cosmic web. In addition to the high [C II]/FIR values, we also find high [C II]/PAH ratios and, in at least one source, a cool dust temperature. This source, SWIRE 4-5, bears a resemblance in these diagnostics to shocked regions of Stephan's Quintet, suggesting that another mode of [C II] excitation in addition to normal photoelectric heating may be contributing to the observed [C II] line.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

BUILDING LATE-TYPE SPIRAL GALAXIES BY IN-SITU AND EX-SITU STAR FORMATION

We analyze the formation and evolution of the stellar components in ''Eris'', a 120 pc resolution cosmological hydrodynamic simulation of a late-type spiral galaxy. The simulation includes the effects of a uniform UV background, a delayed-radiative-cooling scheme for supernova feedback, and a star formation recipe based on a high gas density threshold. It allows a detailed study of the relative contributions of ''in-situ'' (within the main host) and ''ex-situ'' (within satellite galaxies) star formation to each major Galactic component in a close Milky Way analog. We investigate these two star-formation channels as a function of galactocentric distance, along different lines of sight above and along the disk plane, and as a function of cosmic time. We find that: (1) approximately 70% of today's stars formed in-situ; (2) more than two thirds of the ex-situ stars formed within satellites after infall; (3) the majority of ex-situ stars are found today in the disk and in the bulge; (4) the stellar halo is dominated by ex-situ stars, whereas in-situ stars dominate the mass profile at distances ≲ 5 kpc from the center at high latitudes; and (5) approximately 25% of the inner, r ≲ 20 kpc, halo is composed of in-situ stars that have been displaced from their original birth sites during Eris' early assembly history.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE BLACK HOLE FORMATION PROBABILITY

A longstanding question in stellar evolution is which massive stars produce black holes (BHs) rather than neutron stars (NSs) upon death. It has been common practice to assume that a given zero-age main sequence (ZAMS) mass star (and perhaps a given metallicity) simply produces either an NS or a BH, but this fails to account for a myriad of other variables that may effect this outcome, such as spin, binarity, or even stochastic differences in the stellar structure near core collapse. We argue that instead a probabilistic description of NS versus BH formation may be better suited to account for the current uncertainties in understanding how massive stars die. We present an initial exploration of the probability that a star will make a BH as a function of its ZAMS mass, P {sub BH}(M {sub ZAMS}). Although we find that it is difficult to derive a unique P {sub BH}(M {sub ZAMS}) using current measurements of both the BH mass distribution and the degree of chemical enrichment by massive stars, we demonstrate how P {sub BH}(M {sub ZAMS}) changes with these various observational and theoretical uncertainties. We anticipate that future studies of Galactic BHs and theoretical studies of core collapse will refine P {sub BH}(M {sub ZAMS}) and argue that this framework is an important new step toward better understanding BH formation. A probabilistic description of BH formation will be useful as input for future population synthesis studies that are interested in the formation of X-ray binaries, the nature and event rate of gravitational wave sources, and answering questions about chemical enrichment.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

TIME-VARYING DYNAMICAL STAR FORMATION RATE

We present numerical evidence of dynamic star formation in which the accreted stellar mass grows superlinearly with time, roughly as t {sup 2}. We perform simulations of star formation in self-gravitating hydrodynamic and magnetohydrodynamic turbulence that is continuously driven. By turning the self-gravity of the gas in the simulations on or off, we demonstrate that self-gravity is the dominant physical effect setting the mass accretion rate at early times before feedback effects take over, contrary to theories of turbulence-regulated star formation. We find that gravitational collapse steepens the density profile around stars, generating the power-law tail on what is otherwise a lognormal density probability distribution function. Furthermore, we find turbulent velocity profiles to flatten inside collapsing regions, altering the size-line width relation. This local flattening reflects enhancements of turbulent velocity on small scales, as verified by changes to the velocity power spectra. Our results indicate that gas self-gravity dynamically alters both density and velocity structures in clouds, giving rise to a time-varying star formation rate. We find that a substantial fraction of the gas that forms stars arrives via low-density flows, as opposed to accreting through high-density filaments.

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DOE OSTI2015

CHARACTERIZING THE BROWN DWARF FORMATION CHANNELS FROM THE INITIAL MASS FUNCTION AND BINARY-STAR DYNAMICS

The stellar initial mass function (IMF) is a key property of stellar populations. There is growing evidence that the classical star-formation mechanism by the direct cloud fragmentation process has difficulties reproducing the observed abundance and binary properties of brown dwarfs and very-low-mass stars. In particular, recent analytical derivations of the stellar IMF exhibit a deficit of brown dwarfs compared to observational data. Here we derive the residual mass function of brown dwarfs as an empirical measure of the brown dwarf deficiency in recent star-formation models with respect to observations and show that it is compatible with the substellar part of the Thies-Kroupa IMF and the mass function obtained by numerical simulations. We conclude that the existing models may be further improved by including a substellar correction term that accounts for additional formation channels like disk or filament fragmentation. The term ''peripheral fragmentation'' is introduced here for such additional formation channels. In addition, we present an updated analytical model of stellar and substellar binarity. The resulting binary fraction and the dynamically evolved companion mass-ratio distribution are in good agreement with observational data on stellar and very-low-mass binaries in the Galactic field, in clusters, and in dynamically unprocessed groups of stars if all stars form as binaries with stellar companions. Cautionary notes are given on the proper analysis of mass functions and the companion mass-ratio distribution and the interpretation of the results. The existence of accretion disks around young brown dwarfs does not imply that these form just like stars in direct fragmentation.

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DOE OSTI2015

ORIGIN OF THE GALAXY MASS-METALLICITY-STAR FORMATION RELATION

We describe an equilibrium model that links the metallicity of low-redshift galaxies to stellar evolution models. It enables the testing of different stellar initial mass functions and metal yields against observed galaxy metallicities. We show that the metallicities of more than 80,000 Sloan Digital Sky Survey galaxies in the low-redshift range 0.07 ≤ z ≤ 0.3 considerably constrain stellar evolution models that simultaneously relate galaxy stellar mass, metallicity, and star formation rates to the infall rate of low-metallicity extragalactic gas and outflow of enriched matter. A feature of our model is that it encompasses both the active star forming phases of a galaxy and epochs during which the same galaxy may lie fallow. We show that the galaxy mass-metallicity-star formation relation can be traced to infall of extragalactic gas mixing with native gas from host galaxies to form stars of observed metallicities, the most massive of which eject oxygen into extragalactic space. Most consequential among our findings is that, on average, extragalactic infall accounts for one half of the gas required for star formation, a ratio that is remarkably constant across galaxies with stellar masses ranging at least from M* = 2 × 10{sup 9} to 6 × 10{sup 10} M {sub ☉}. This leads us to propose that star formation is initiated when extragalactic infall roughly doubles the mass of marginally stable interstellar clouds. The processes described may also account quantitatively for the metallicity of extragalactic space, though to check this the fraction of extragalactic baryons will need to be more firmly established.

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DOE OSTI2015

THE FORMATION AND ERUPTION OF A SMALL CIRCULAR FILAMENT DRIVEN BY ROTATING MAGNETIC STRUCTURES IN THE QUIET SUN

We present the first observation of the formation and eruption of a small circular filament driven by a rotating network magnetic field (RNF) in the quiet Sun. In the negative footpoint region of an inverse J-shaped dextral filament, the RNF was formed by the convergence to supergranular junctions of several magnetic flux patches of the same polarity, and it then rotated counterclockwise (CCW) for approximately 11 hr and showed up as a CCW rotating EUV cyclone, during which time the filament gradually evolved into a circular filament that surrounded the cyclone. When the calculated convergence and vortex flows appeared around the RNF during its formation and rotation phases, the injected magnetic helicity calculation also showed negative helicity accumulation during the RNF rotation that was consistent with the dextral chirality of the filament. Finally, the RNF rotation stopped and the cyclone disappeared, and, probably due to an emerging bipole and its forced cancellation with the RNF, the closure filament underwent an eruption along its axis in the (clockwise) direction opposite to the rotation directions of the RNF and cyclone. These observations suggest that the RNFs might play an important role in the formation of nearby small-scale circular filaments as they transport and inject magnetic energy and helicity, and the formation of the EUV cyclones may be a further manifestation of the helicity injected into the corona by the rotation of the RNFs in the photosphere. In addition, the new emerging bipole observed before the filament eruption might be responsible for destabilizing the system and triggering the magnetic reconnection which proves useful for the filament eruption.

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DOE OSTI2015

Metal-poor, strongly star-forming galaxies in the DEEP2 Survey: The relationship between stellar mass, temperature-based metallicity, and star formation rate

We report on the discovery of 28 z≈0.8 metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [O iii] λ4363 emission line, which provides a “direct” measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFR with stellar mass estimates from modeling the optical photometry. We find that these galaxies are 1.05 ± 0.61 dex above the z∼1 stellar mass–SFR relation and 0.23 ± 0.23 dex below the local mass–metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 dex, but significant dispersion remains (0.29 dex with 0.16 dex due to measurement uncertainties). This dispersion suggests that gas accretion, star formation, and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of ≈100{sub −75}{sup +310} Myr, which suggests stochastic star formation. Combining our sample with other z∼1 metal-poor galaxies, we find a weak positive SFR–metallicity dependence (at fixed stellar mass) that is significant at 94.4% confidence. We interpret this positive correlation as recent star formation that has enriched the gas but has not had time to drive the metal-enriched gas out with feedback mechanisms.

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DOE OSTI2015

LoCuSS: THE SLOW QUENCHING OF STAR FORMATION IN CLUSTER GALAXIES AND THE NEED FOR PRE-PROCESSING

We present a study of the spatial distribution and kinematics of star-forming galaxies in 30 massive clusters at 0.15 < z < 0.30, combining wide-field Spitzer 24 μm and GALEX near-ultraviolet imaging with highly complete spectroscopy of cluster members. The fraction (f{sub SF}) of star-forming cluster galaxies rises steadily with cluster-centric radius, increasing fivefold by 2r{sub 200}, but remains well below field values even at 3r{sub 200}. This suppression of star formation at large radii cannot be reproduced by models in which star formation is quenched in infalling field galaxies only once they pass within r{sub 200} of the cluster, but is consistent with some of them being first pre-processed within galaxy groups. Despite the increasing f{sub SF}-radius trend, the surface density of star-forming galaxies actually declines steadily with radius, falling ∼15× from the core to 2r{sub 200}. This requires star formation to survive within recently accreted spirals for 2–3 Gyr to build up the apparent over-density of star-forming galaxies within clusters. The velocity dispersion profile of the star-forming galaxy population shows a sharp peak of 1.44 σ{sub ν} at 0.3r{sub 500}, and is 10%–35% higher than that of the inactive cluster members at all cluster-centric radii, while their velocity distribution shows a flat, top-hat profile within r{sub 500}. All of these results are consistent with star-forming cluster galaxies being an infalling population, but one that must also survive ∼0.5–2 Gyr beyond passing within r{sub 200}. By comparing the observed distribution of star-forming galaxies in the stacked caustic diagram with predictions from the Millennium simulation, we obtain a best-fit model in which star formation rates decline exponentially on quenching timescales of 1.73 ± 0.25 Gyr upon accretion into the cluster.

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DOE OSTI2015

PRIMUS: The relationship between star formation and AGN accretion

We study the evidence for a connection between active galactic nuclei (AGNs) fueling and star formation by investigating the relationship between the X-ray luminosities of AGNs and the star formation rates (SFRs) of their host galaxies. We identify a sample of 309 AGNs with 10{sup 41}<L{sub X}<10{sup 44} erg s{sup −1} at 0.2<z<1.2 in the PRIMUS redshift survey. We find AGNs in galaxies with a wide range of SFR at a given L{sub X}. We do not find a significant correlation between SFR and the observed instantaneous L{sub X} for star-forming AGN host galaxies. However, there is a weak but significant correlation between the mean L{sub X} and SFR of detected AGNs in star-forming galaxies, which likely reflects that L{sub X} varies on shorter timescales than SFR. We find no correlation between stellar mass and L{sub X} within the AGN population. Within both populations of star-forming and quiescent galaxies, we find a similar power-law distribution in the probability of hosting an AGN as a function of specific accretion rate. Furthermore, at a given stellar mass, we find a star-forming galaxy ∼2–3 more likely than a quiescent galaxy to host an AGN of a given specific accretion rate. The probability of a galaxy hosting an AGN is constant across the main sequence of star formation. These results indicate that there is an underlying connection between star formation and the presence of AGNs, but AGNs are often hosted by quiescent galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Linking galaxies to dark matter halos at z ∼ 1: Dependence of galaxy clustering on stellar mass and specific star formation rate

We study the dependence of angular two-point correlation functions on stellar mass (M{sub *}) and specific star formation rate (sSFR) of M{sub ∗}>10{sup 10}M{sub ⊙} galaxies at z∼1. The data from the UK Infrared Telescope Infrared Deep Sky Survey Deep eXtragalactic Survey and Canada–France–Hawaii Telescope Legacy Survey cover 8.2 deg{sup 2} sample scales larger than 100 h{sup −1} Mpc at z∼1, allowing us to investigate the correlation between clustering, M{sub *}, and star formation through halo modeling. Based on halo occupation distributions (HODs) of M{sub *} threshold samples, we derive HODs for M{sub *} binned galaxies, and then calculate the M{sub ∗}/M{sub halo} ratio. The ratio for central galaxies shows a peak at M{sub halo}∼10{sup 12}h{sup −1}M{sub ⊙}, and satellites predominantly contribute to the total stellar mass in cluster environments with M{sub ∗}/M{sub halo} values of 0.01–0.02. Using star-forming galaxies split by sSFR, we find that main sequence galaxies (log sSFR/yr{sup −1}∼−9) are mainly central galaxies in ∼10{sup 12.5}h{sup −1}M{sub ⊙} halos with the lowest clustering amplitude, while lower sSFR galaxies consist of a mixture of both central and satellite galaxies where those with the lowest M{sub *} are predominantly satellites influenced by their environment. Considering the lowest M{sub halo} samples in each M{sub *} bin, massive central galaxies reside in more massive halos with lower sSFRs than low mass ones, indicating star-forming central galaxies evolve from a low M{sub *}–high sSFR to a high M{sub *}–low sSFR regime. We also find that the most rapidly star-forming galaxies (log sSFR/yr{sup −1}>−8.5) are in more massive halos than main sequence ones, possibly implying galaxy mergers in dense environments are driving the active star formation. These results support the conclusion that the majority of star-forming galaxies follow secular evolution through the sustained but decreasing formation of stars.

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DOE OSTI2015

Dusty starbursts and the formation of elliptical galaxies: A SCUBA-2 survey of a z = 1.46 cluster

We report the results of a deep SCUBA-2 850 and 450 μm survey for dust-obscured ultra-/luminous infrared galaxies (U/LIRGs) in the field of the z = 1.46 cluster XCS J2215.9−1738. We detect a striking overdensity of submillimeter sources coincident with the core of this cluster: ∼3–4 × higher than expected in a blank field. We use the likely radio and mid-infrared counterparts to show that the bulk of these submillimeter sources have spectroscopic or photometric redshifts that place them in the cluster and that their multiwavelength properties are consistent with this association. The average far-infrared luminosities of these galaxies are (1.0 ± 0.1) × 10{sup 12} L{sub ⊙}, placing them on the U/LIRG boundary. Using the total star formation occurring in the obscured U/LIRG population within the cluster, we show that the resulting mass-normalized star formation rate for this system supports previous claims of a rapid increase in star formation activity in cluster cores out to z∼1.5, which must be associated with the ongoing formation of the early-type galaxies that reside in massive clusters today.

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DOE OSTI2015

ON THE IMF IN A TRIGGERED STAR FORMATION CONTEXT

The origin of the stellar initial mass function (IMF) is a fundamental issue in the theory of star formation. It is generally fit with a composite power law. Some clues on the progenitors can be found in dense starless cores that have a core mass function (CMF) with a similar shape. In the low-mass end, these mass functions increase with mass, albeit the sample may be somewhat incomplete; in the high-mass end, the mass functions decrease with mass. There is an offset in the turn-over mass between the two mass distributions. The stellar mass for the IMF peak is lower than the corresponding core mass for the CMF peak in the Pipe Nebula by about a factor of three. Smaller offsets are found between the IMF and the CMFs in other nebulae. We suggest that the offset is likely induced during a starburst episode of global star formation which is triggered by the formation of a few O/B stars in the multi-phase media, which naturally emerged through the onset of thermal instability in the cloud-core formation process. We consider the scenario that the ignition of a few massive stars photoionizes the warm medium between the cores, increases the external pressure, reduces their Bonnor–Ebert mass, and triggers the collapse of some previously stable cores. We quantitatively reproduce the IMF in the low-mass end with the assumption of additional rotational fragmentation.

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DOE OSTI2015

Signatures of young star formation activity within two parsecs of Sgr A*

We present radio and infrared observations indicating ongoing star formation activity inside the ∼2–5 pc circumnuclear ring at the Galactic center. Collectively these measurements suggest a continued disk-based mode of ongoing star formation has taken place near Sgr A* over the last few million years. First, Very Large Array observations with spatial resolution 2.″17 × 0.″81 reveal 13 water masers, several of which have multiple velocity components. The presence of interstellar water masers suggests gas densities that are sufficient for self-gravity to overcome the tidal shear of the 4×10{sup 6} M{sub ⊙} black hole. Second, spectral energy distribution modeling of stellar sources indicates massive young stellar object (YSO) candidates interior to the molecular ring, supporting in situ star formation near Sgr A* and appear to show a distribution similar to that of the counter-rotating disks of ∼100 OB stars orbiting Sgr A*. Some YSO candidates (e.g., IRS 5) have bow shock structures, suggesting that they have gaseous disks that are phototoevaporated and photoionized by the strong radiation field. Third, we detect clumps of SiO (2-1) and (5-4) line emission in the ring based on Combined Array for Research in Millimeter-wave Astronomy and Sub-Millimeter Array observations. The FWHM and luminosity of the SiO emission is consistent with shocked protostellar outflows. Fourth, two linear ionized features with an extent of ∼0.8 pc show blue and redshifted velocities between +50 and −40 km s{sup −1}, suggesting protostellar jet driven outflows with mass-loss rates of ∼5×10{sup −5} M{sub ⊙} yr{sup −1}. Finally, we present the imprint of radio dark clouds at 44 GHz, representing a reservoir of molecular gas that feeds star formation activity close to Sgr A*.

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DOE OSTI2015

Herschel observations of the W3 GMC (II): Clues to the formation of clusters of high-mass stars

The W3 giant molecular cloud is a prime target for investigating the formation of high-mass stars and clusters. This second study of W3 within the HOBYS Key Program provides a comparative analysis of subfields within W3 to further constrain the processes leading to the observed structures and stellar population. Probability density functions (PDFs) and cumulative mass distributions (CMDs) were created from dust column density maps, quantified as extinction A{sub V}. The shape of the PDF, typically represented with a lognormal function at low A{sub V} “breaking” to a power-law tail at high A{sub V}, is influenced by various processes including turbulence and self-gravity. The breaks can also be identified, often more readily, in the CMDs. The PDF break from lognormal (A{sub V}(SF) ≈ 6–10 mag) appears to shift to higher A{sub V} by stellar feedback, so that high-mass star-forming regions tend to have higher PDF breaks. A second break at A{sub V}>50 mag traces structures formed or influenced by a dynamic process. Because such a process has been suggested to drive high-mass star formation in W3, this second break might then identify regions with potential for hosting high-mass stars/clusters. Stellar feedback appears to be a major mechanism driving the local evolution and state of regions within W3. A high initial star formation efficiency in a dense medium could result in a self-enhancing process, leading to more compression and favorable star formation conditions (e.g., colliding flows), a richer stellar content, and massive stars. This scenario would be compatible with the “convergent constructive feedback” model introduced in our previous Herschel study.

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DOE OSTI2015

THE FORMATION OF IRIS DIAGNOSTICS. VI. THE DIAGNOSTIC POTENTIAL OF THE C ii LINES AT 133.5 nm IN THE SOLAR ATMOSPHERE

We use 3D radiation magnetohydrodynamic models to investigate how the thermodynamic quantities in the simulation are encoded in observable quantities, thus exploring the diagnostic potential of the C ii 133.5 nm lines. We find that the line core intensity is correlated with the temperature at the formation height but the correlation is rather weak, especially when the lines are strong. The line core Doppler shift is a good measure of the line-of-sight velocity at the formation height. The line width is both dependent on the width of the absorption profile (thermal and non-thermal width) and an opacity broadening factor of 1.2–4 due to the optically thick line formation with a larger broadening for double peak profiles. The C ii 133.5 nm lines can be formed both higher and lower than the core of the Mg ii k line depending on the amount of plasma in the 14–50 kK temperature range. More plasma in this temperature range gives a higher C ii 133.5 nm formation height relative to the Mg ii k line core. The synthetic line profiles have been compared with Interface Region Imaging Spectrograph observations. The derived parameters from the simulated line profiles cover the parameter range seen in observations but, on average, the synthetic profiles are too narrow. We interpret this discrepancy as a combination of a lack of plasma at chromospheric temperatures in the simulation box and too small non-thermal velocities. The large differences in the distribution of properties between the synthetic profiles and the observed ones show that the C ii 133.5 nm lines are powerful diagnostics of the upper chromosphere and lower transition region.

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DOE OSTI2015

STAR FORMATION ACTIVITY IN CLASH BRIGHTEST CLUSTER GALAXIES

The CLASH X-ray selected sample of 20 galaxy clusters contains 10 brightest cluster galaxies (BCGs) that exhibit significant (>5σ) extinction-corrected star formation rates (SFRs). Star formation activity is inferred from photometric estimates of UV and Hα+[N ii] emission in knots and filaments detected in CLASH Hubble Space Telescope ACS and WFC3 observations. UV-derived SFRs in these BCGs span two orders of magnitude, including two with a SFR ≳ 100 M{sub ⊙} yr{sup −1}. These measurements are supplemented with [O ii], [O iii], and Hβ fluxes measured from spectra obtained with the SOAR telescope. We confirm that photoionization from ongoing star formation powers the line emission nebulae in these BCGs, although in many BCGs there is also evidence of a LINER-like contribution to the line emission. Coupling these data with Chandra X-ray measurements, we infer that the star formation occurs exclusively in low-entropy cluster cores and exhibits a correlation with gas properties related to cooling. We also perform an in-depth study of the starburst history of the BCG in the cluster RXJ1532.9+3021, and create 2D maps of stellar properties on scales down to ∼350 pc. These maps reveal evidence for an ongoing burst occurring in elongated filaments, generally on ∼0.5–1.0 Gyr timescales, although some filaments are consistent with much younger (≲100 Myr) burst timescales and may be correlated with recent activity from the active galactic nucleus. The relationship between BCG SFRs and the surrounding intracluster medium gas properties provide new support for the process of feedback-regulated cooling in galaxy clusters and is consistent with recent theoretical predictions.

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DOE OSTI2015

IMPLICATIONS FOR THE FORMATION OF BLUE STRAGGLER STARS FROM HST ULTRAVIOLET OBSERVATIONS OF NGC 188

We present results of a Hubble Space Telescope (HST) far-ultraviolet (FUV) survey searching for white dwarf (WD) companions to blue straggler stars (BSSs) in open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined binaries with properties suggestive of mass-transfer formation via Roche lobe overflow, specifically through an asymptotic giant branch star transferring mass to a main sequence secondary, yielding a BSS binary with a WD companion. In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a WD companion that is hot and luminous enough to be directly detected as a FUV photometric excess with HST. Comparing expected BSS FUV emission to observed photometry reveals four BSSs with WD companions above 12,000 K (younger than 250 Myr) and three WD companions with temperatures between 11,000 and 12,000 K. These BSS+WD binaries all formed through recent mass transfer. The location of the young BSSs in an optical color–magnitude diagram (CMD) indicates that distance from the zero-age main sequence does not necessarily correlate with BSS age. There is no clear CMD separation between mass transfer-formed BSSs and those likely formed through other mechanisms, such as collisions. The seven detected WD companions place a lower limit on the mass-transfer formation frequency of 33%. We consider other possible formation mechanisms by comparing properties of the BSS population to theoretical predictions. We conclude that 14 BSS binaries likely formed from mass transfer, resulting in an inferred mass-transfer formation frequency of approximately 67%.

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