Browsing by Subject "forming galaxies"

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The HETDEX Pilot Survey. IV. The Evolution Of O II Emitting Galaxies From Z Similar To 0.5 To Z Similar To 0
(2013-05) Ciardullo, Robin; Gronwall, Caryl; Adams, Joshua J.; Blanc, Guillermo A.; Gebhardt, Karl; Finkelstein, Steven L.; Jogee, Shardha; Hill, Gary J.; Drory, Niv; Hopp, Ulrich; Schneider, Donald P.; Zeimann, Gregory R.; Dalton, Gavin B.; Adams, Joshua J.; Blanc, Guillermo A.; Gebhardt, Karl; Finkelstein, Steven L.; Jogee, Shardha
We present an analysis of the luminosities and equivalent widths of the 284 z < 0.56 [O II]-emitting galaxies found in the 169 arcmin(2) pilot survey for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). By combining emission-line fluxes obtained from the Mitchell spectrograph on the McDonald 2.7 m telescope with deep broadband photometry from archival data, we derive each galaxy's dereddened [O II] lambda 3727 luminosity and calculate its total star formation rate. We show that over the last similar to 5Gyr of cosmic time, there has been substantial evolution in the [O II] emission-line luminosity function, with L* decreasing by similar to 0.6 +/- 0.2 dex in the observed function, and by similar to 0.9 +/- 0.2 dex in the dereddened relation. Accompanying this decline is a significant shift in the distribution of [O II] equivalent widths, with the fraction of high equivalent-width emitters declining dramatically with time. Overall, the data imply that the relative intensity of star formation within galaxies has decreased over the past similar to 5Gyr, and that the star formation rate density of the universe has declined by a factor of similar to 2.5 between z similar to 0.5 and z similar to 0. These observations represent the first [O II]-based star formation rate density measurements in this redshift range, and foreshadow the advancements which will be generated by the main HETDEX survey.
Less Than 10 Percent Of Star Formation In Z Similar To 0.6 Massive Galaxies Is Triggered By Major Interactions
(2009-10) Robaina, Aday R.; Bell, Eric F.; Skelton, Rosalind E.; McIntosh, Daniel H.; Somerville, Rachel S.; Zheng, Xianzhong; Rix, Hans-Walter; Bacon, David; Balogh, Michael; Barazza, Fabio D.; Barden, Marco; Boehm, Asmus; Caldwell, John A. R.; Gallazzi, Anna; Gray, Meghan E.; Haeussler, Boris; Heymans, Catherine; Jahnke, Knud; Jogee, Shardha; van Kampen, Eelco; Lane, Kyle; Meisenheimer, Klaus; Papovich, Casey; Peng, Chien Y.; Sanchez, Sebastian F.; Skibba, Ramin; Taylor, Andy; Wisotzki, Lutz; Wolf, Christian; Jogee, Shardha
Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses, and UV SFRs from COMBO-17, 24 mu m SFRs from Spitzer, and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically selected very close pairs (separation <2 '') and merger remnants from the HST imaging. Our analysis confirms that the most intensely star-forming systems are indeed interacting or merging. Yet, for massive (M(*) >= 10(10) M(circle dot)) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios <= 1:4 and separations <= 40 kpc) are only 1.80 +/- 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. Our results also agree with hydrodynamical simulations of galaxy interactions, which produce some mergers with large bursts of star formation on similar to 100 Myr timescales, but only a modest SFR enhancement when averaged over the entire merger timescale. We demonstrate that these results imply that only less than or similar to 10% of star formation at 0.4 <= z <= 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z = 1.
Massive Z Similar To 1.3 Evolved Galaxies Revealed
(2003-01) Saracco, P.; Longhetti, M.; Severgnini, P.; Della Ceca, R.; Mannucci, F.; Bender, R.; Drory, N.; Feulner, G.; Ghinassi, F.; Hopp, U.; Maraston, C.; Drory, N.
We present the results of TNG near-IR low resolution spectroscopy of two (S7175_254 and S7F5_45) sources belonging to a complete sample of 15 EROs with K' < 18 and R - K' > 5 selected from the MUNICS Survey. Both the spectra show a sharp drop in the continuum which can be ascribed only to the Balmer break. This places them at 1.2 < z < 1.5. Their rest-frame z = 1.2 K-band absolute magnitude is M-K similar or equal to -26.6 (L - 7L*). The comparison of the spectra and the photometric data with a grid of synthetic template spectra provides a redshift z similar or equal to 1.22(+0.02)(-0.07) for S7175_254 and z similar or equal to 1.46 +/- 0.02 for S7175_45. The resulting lower limits to their stellar mass are M-stars(min) = 6 x 10(11) M. and M-stars(min) = 4 x 10(11) M.. The minimum age of the last burst of star formation in S7F5_254 is 3.5 Gyr while it is 0.5 Gyr in S7F5_45 implying a minimum formation redshift z(f) greater than or similar to 3.5 and z(f) greater than or similar to 2 for the two EROs respectively.
A New Z=0 Metagalactic Ultraviolet Background Limit
(2011-02) Adams, Joshua J.; Uson, Juan M.; Hill, Gary J.; MacQueen, Phillip J.; Adams, Joshua J.; Hill, Gary J.; MacQueen, Phillip J.
We present new integral-field spectroscopy in the outskirts of two nearby, edge-on, late-type galaxies to search for the H alpha emission that is expected from the exposure of their hydrogen gas to the metagalactic ultraviolet background (UVB). Despite the sensitivity of the VIRUS-P spectrograph on the McDonald 2.7 m telescope to low surface brightness emission and the large field of view, we do not detect H alpha to 5 sigma upper limits of 6.4 x 10(-19) erg s(-1) cm(-2) arcsec(-2) in UGC 7321 and of 25 x 10(-19) erg s(-1) cm(-2) arcsec(-2) in UGC 1281 in each of the hundreds of independent spatial elements (fibers). We fit gas distribution models from overlapping 21 cm data of HI, extrapolate one scale length beyond the HI data, and estimate predicted H alpha surface brightness maps. We analyze three types of limits from the data with stacks formed from increasingly large spatial regions and compare to the model predictions: (1) single fibers, (2) convolution of the fiber grid with a Gaussian, circular kernel (10('') full width at half-maximum), and (3) the co-added spectra from a few hundred fibers over the brightest model regions. None of these methods produce a significant detection (>5 sigma) with the most stringent constraints on the Hi photoionization rate of Gamma(z = 0) < 1.7 x 10(-14) s(-1) in UGC 7321 and Gamma(z = 0) < 14 x 10(-14) s(-1) in UGC 1281. The UGC 7321 limit is below previous measurement limits and also below current theoretical models. Restricting the analysis to the fibers bound by the HI data leads to a comparable limit; the limit is Gamma(z = 0) < 2.3 x 10(-14) s(-1) in UGC 7321. We discuss how a low Lyman limit escape fraction in z similar to 0 redshift star-forming galaxies might explain this lower than predicted UVB strength and the prospects of deeper data to make a direct detection.
A Population Of Massive, Luminous Galaxies Hosting Heavily Dust-Obscured Gamma-Ray Bursts: Implications For The Use Of Grbs As Tracers Of Cosmic Star Formation
(2013-12) Perley, D. A.; Levan, A. J.; Tanvir, N. R.; Cenko, S. Bradley; Bloom, Joshua S.; Hjorth, J.; Kruhler, T.; Filippenko, Alexei V.; Fruchter, A.; Fynbo, Johan P. U.; Jakobsson, Pall; Kalirai, J.; Milvang-Jensen, B.; Morgan, A. N.; Prochaska, J. X.; Silverman, Jeffrey M.; Silverman, Jeffrey M.
We present observations and analysis of the host galaxies of 23 heavily dust-obscured gamma-ray bursts (GRBs) observed by the Swift satellite during the years 2005-2009, representing all GRBs with an unambiguous host-frame extinction of AV >1 mag from this period. Deep observations with Keck, Gemini, Very Large Telescope, Hubble Space Telescope, and Spitzer successfully detect the host galaxies and establish spectroscopic or photometric redshifts for all 23 events, enabling us to provide measurements of the intrinsic host star formation rates, stellar masses, and mean extinctions. Compared to the hosts of unobscured GRBs at similar redshifts, we find that the hosts of dust-obscured GRBs are (on average) more massive by about an order of magnitude and also more rapidly star forming and dust obscured. While this demonstrates that GRBs populate all types of star-forming galaxies, including the most massive, luminous systems at z approximate to 2, at redshifts below 1.5 the overall GRB population continues to show a highly significant aversion to massive galaxies and a preference for low-mass systems relative to what would be expected given a purely star-formation-rate-selected galaxy sample. This supports the notion that the GRB rate is strongly dependent on metallicity, and may suggest that the most massive galaxies in the universe underwent a transition in their chemical properties similar to 9 Gyr ago. We also conclude that, based on the absence of unobscured GRBs in massive galaxies and the absence of obscured GRBs in low-mass galaxies, the dust distributions of the lowest-mass and the highest-mass galaxies are relatively homogeneous, while intermediate-mass galaxies (similar to 10(9)M(circle dot)) have diverse internal properties.
Probing The Star Formation History And Initial Mass Function Of The Z Similar To 2.5 Lensed Galaxy Smm J163554.2+661225 With Herschel
(2011-12) Finkelstein, Keely D.; Papovich, Casey; Finkelstein, Steven L.; Willmer, Christopher N. A.; Rigby, Jane R.; Rudnick, Gregory; Egami, Eiichi; Rieke, M.; Smith, J. D. T.; Finkelstein, Keely D.; Finkelstein, Steven L.
We present the analysis of Herschel Spectral and Photometric Imaging Receiver far-infrared (FIR) observations of the z = 2.515 lensed galaxy SMM J163554.2+661225. Combining new 250, 350, and 500 mu m observations with existing data, we make an improved fit to the FIR spectral energy distribution of this galaxy. We find a total infrared (IR) luminosity of L(8-1000 mu m) = 6.9 +/- 0.6 x 10(11) L-circle dot, a factor of three more precise over previous L-IR estimates for this galaxy, and one of the most accurate measurements for any galaxy at these redshifts. This FIR luminosity implies an unlensed star formation rate (SFR) for this galaxy of 119 +/- 10 M-circle dot yr(-1), which is a factor of 1.9 +/- 0.35 lower than the SFR derived from the nebular Pa alpha emission line (a 2.5 sigma discrepancy). Both SFR indicators assume an identical Salpeter initial mass function (IMF) with slope Gamma = 2.35 over a mass range of 0.1-100 M-circle dot; thus this discrepancy suggests that more ionizing photons may be necessary to account for the higher Pa alpha-derived SFR. We examine a number of scenarios and find that the observations can be explained with a varying star formation history (SFH) due to an increasing SFR, paired with a slight flattening of the IMF. If the SFR is constant in time, then larger changes need to be made to the IMF by either increasing the upper mass cutoff to similar to 200 M-circle dot, or a flattening of the IMF slope to 1.9 +/- 0.15, or a combination of the two. These scenarios result in up to double the number of stars with masses above 20 M-circle dot, which produce the requisite increase in ionizing photons over a Salpeter IMF with a constant SFH.
The Spatially Resolved Star Formation Law From Integral Field Spectroscopy: Virus-P Observations Of NGC 5194
(2009-10) Blanc, Guillermo A.; Heiderman, Amanda; Gebhardt, Karl; Evans, Neal J.; Adams, Joshua; Blanc, Guillermo A.; Heiderman, Amanda; Gebhardt, Karl; Evans, Neal J.; Adams, Joshua
We investigate the relation between the star formation rate (SFR) surface density (Sigma(SFR)) and the mass surface density of gas (Sigma(gas)) in NGC 5194 (a.k.a. M51a, Whirlpool Galaxy). Visible Integral field Replicable Unit Spectrograph Prototype (VIRUS-P) integral field spectroscopy of the central 4.1 x 4.1 kpc(2) of the galaxy is used to measure H alpha, H beta, [O III]lambda 5007, [N II]lambda lambda 6548,6584, and [S II]lambda lambda 6717,6731 emission line fluxes for 735 regions similar to 170 pc in diameter. We use the Balmer decrement to calculate nebular dust extinctions, and correct the observed fluxes in order to accurately measure Sigma(SFR) in each region. Archival HI 21 cm and CO maps with spatial resolution similar to that of VIRUS-P are used to measure the atomic and molecular 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 in both Sigma(gas) and Sigma(SFR), and is free of the systematics involved in performing linear regressions over incomplete data in logarithmic space. After rejecting regions whose nebular spectrum is affected by the central active galactic nucleus in NGC 5194, we use the [S II]/H alpha ratio to separate spectroscopically the contribution from the diffuse ionized gas ( DIG) in the galaxy, which has a different temperature and ionization Statefrom those of H II regions in the disk. The DIG only accounts for 11% of the total H alpha luminosity integrated over the whole central region, but on local scales it can account for up to a 100% of the H alpha emission, especially in the inter-arm regions. After removing the DIG contribution from the H alpha fluxes, 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 = Sigma(HI+H2)/Sigma(SFR) approximate to 2 Gyr, in good agreement with recent measurements by Bigiel et al. The atomic gas density shows no correlation with the SFR, and the total gas SFL in the sampled density range closely follows the molecular gas SFL. Integral field spectroscopy allows a much cleaner measurement of H alpha emission line fluxes than narrow-band imaging, since it is free of the systematics introduced by continuum subtraction, underlying photospheric absorption, and contamination by the [N II] doublet. We assess the validity of different corrections usually applied in narrow-band measurements to overcome these issues and find that while systematics are introduced by these corrections, they are only dominant in the low surface brightness regime. The disagreement with the previous measurement of a super-linear molecular SFL by Kennicutt et al. is most likely due to differences in the fitting method. Our results support the recent evidence for a low, and close to constant, star formation efficiency (SFE = tau(-1)) in the molecular component of the interstellar medium. The data show an excellent agreement with the recently proposed model of the SFL by Krumholz et al. The large intrinsic scatter observed may imply the existence of other parameters, beyond the availability of gas, which are important in setting the SFR.
To Stack Or Not To Stack: Spectral Energy Distribution Properties Of Ly Alpha-Emitting Galaxies At Z=2.1
(2014-03) Vargas, Carlos J.; Bish, Hannah; Acquaviva, Viviana; Gawiser, Eric; Finkelstein, Steven L.; Ciardullo, Robin; Ashby, Matthew L. N.; Feldmeier, John; Ferguson, Henry; Gronwall, Caryl; Guaita, Lucia; Hagen, Alex; Koekemoer, Anton; Kurczynski, Peter; Newman, Jeffrey A.; Padilla, Nelson; Finkelstein, Steven L.
We use the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) GOODS-S multi-wavelength catalog to identify counterparts for 20 Ly alpha emitting (LAE) galaxies at z = 2.1. We build several types of stacked spectral energy distributions (SEDs) of these objects. We combine photometry to form average and median flux-stacked SEDs, and postage-stamp images to form average and median image-stacked SEDs. We also introduce scaled flux stacks that eliminate the influence of variation in overall brightness. We use the SED fitting code SpeedyMC to constrain the physical properties of individual objects and stacks. Our LAEs at z = 2.1 have stellar masses ranging from 2 x 10(7) M-circle dot to 8 x 10(9) M-circle dot (median = 3 x 10(8) M-circle dot), ages ranging from 4 Myr to 500 Myr (median = 100 Myr), and E(B - V) between 0.02 and 0.24 (median = 0.12). Although still low, this represents significantly more dust reddening than has been reported for LAEs at higher redshifts. We do not observe strong correlations between Ly alpha equivalent width (EW) and age or E(B - V). The Ly alpha radiative transfer (q) factors of our sample are predominantly close to one and do not correlate strongly with EW or E(B - V). The absence of strong correlations with EW or q implies that Ly alpha radiative transfer is highly anisotropic and/or prevents Ly alpha photons from scattering in dusty regions. The SED parameters of the flux stacks match the average and median values of the individual objects, with the flux-scaled median SED performing best with uncertainties reduced by a factor of two. Median image-stacked SEDs provide a poor representation of the median individual object, and none of the stacking methods capture the large dispersion of LAE properties.