arXiv subjects

Luciana Bianchi

Publications and source records attributed to Luciana Bianchi.

At least 73 records · Page 4Linked to original sources

arXiv2009

Spatial Clustering from GALEX-SDSS samples: Star Formation History and large-scale clustering

We measure the projected spatial correlation function w_p(r_p) from a large sample combining GALEX ultraviolet imaging with the SDSS spectroscopic sample. We study the dependence of the clustering strength for samples selected on (NUV - r)_abs color, specific star formation rate (SSFR), and stellar mass. We find that there is a smooth transition in the clustering of galaxies as a function of this color from weak clustering among blue galaxies to stronger clustering for red galaxies. The clustering of galaxies within the "green valley" has an intermediate strength, and is consistent with that expected from galaxy groups. The results are robust to the correction for dust extinction. The comparison with simple analytical modeling suggests that the halo occupation number increases with older star formation epochs. When splitting according to SSFR, we find that the SSFR is a more sensitive tracer of environment than stellar mass.

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arXiv2009

The Star Formation Law at Low Surface Density

We investigate the nature of the star formation law at low gas surface densities using a sample of 19 low surface brightness (LSB) galaxies with existing HI maps in the literature, UV imaging from the Galaxy Evolution Explorer satellite, and optical images from the Sloan Digital Sky Survey. All of the LSB galaxies have (NUV-r) colors similar to those for higher surface brightness star-forming galaxies of similar luminosity indicating that their average star formation histories are not very different. Based upon four LSB galaxies with both UV and FIR data, we find FIR/UV ratios significantly less than one, implying low amounts of internal UV extinction in LSB galaxies. We use the UV images and HI maps to measure the star formation rate and hydrogen gas surface densities within the same region for all of the galaxies. The LSB galaxy star formation rate surface densities lie below the extrapolation of the power law fit to the star formation rate surface density as a function of the total gas density for higher surface brightness galaxies. Although there is more scatter, the LSB galaxies also lie below a second version of the star formation law in which the star formation rate surface density is correlated with the gas density divided by the orbital time in the disk. The downturn seen in both star formation laws is consistent with theoretical models that predict lower star formation efficiencies in LSB galaxies due to the declining molecular fraction with decreasing density.

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arXiv2009

Extreme UV QSOs

We present a sample of spectroscopically confirmed QSOs with FUV-NUV color (as measured by GALEX photometry) bluer than canonical QSO templates and than the majority of known QSOs. We analyze their FUV to NIR colors, luminosities and optical spectra. The sample includes a group of 150 objects at low redshift (z $<$ 0.5), and a group of 21 objects with redshift 1.7$<$z$<$2.6. For the low redshift objects, the "blue" FUV-NUV color may be caused by enhanced Ly$α$ emission, since Ly$α$ transits the GALEX FUV band from z=0.1 to z=0.47. Synthetic QSO templates constructed with Ly$α$ up to 3 times stronger than in standard templates match the observed UV colors of our low redshift sample. The H$α$ emission increases, and the optical spectra become bluer, with increasing absolute UV luminosity. The UV-blue QSOs at redshift about 2, where the GALEX bands sample restframe about 450-590A (FUV) and about 590-940A(NUV), are fainter than the average of UV-normal QSOs at similar redshift in NUV, while they have comparable luminosities in other bands. Therefore we speculate that their observed FUV-NUV color may be explained by a combination of steep flux rise towards short wavelengths and dust absorption below the Lyman limit, such as from small grains or crystalline carbon. The ratio of Ly$α$ to CIV could be measured in 10 objects; it is higher (30% on average) than for UV-normal QSOs, and close to the value expected for shock or collisional ionization. FULL VERSION AVAILABLE FROM AUTHOR'S WEB SITE: http://dolomiti.pha.jhu.edu/papers/2009_AJ_Extreme_UV_QSOs.pdf

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arXiv2008

GALEX Observations of Diffuse UV Radiation at High Spatial Resolution from the Sandage Nebulosity

Using the GALEX ultraviolet imagers we have observed a region of nebulosity first identified as starlight scattered by interstellar dust by Sandage (1976). Apart from airglow and zodiacal emission, we have found a diffuse UV background of between 500 and 800 \phunit in both the \galex FUV (1350 -- 1750 Å) and NUV (1750 -- 2850 Å). Of this emission, up to 250 \phunit is due to \htwo fluorescent emission in the FUV band; the remainder is consistent with scattering from interstellar dust. We have estimated the optical constants to be $a = 0.3; g = 0.7$ in the FUV and $a = 0.5; g = 0.7$ in the NUV, implying highly forward scattering grains, plus an extragalactic contribution of as much as 150 \phunit. These are the highest spatial resolution observations of the diffuse UV background to date and show an intrinsic scatter beyond that expected from instrumental noise alone. Further modeling is required to understand the nature of this scatter and its implications for the ISM.

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arXiv2008

Spitzer Observations of Star Formation in the Extreme Outer Disk of M83 (NGC5236)

Spitzer IRAC observations of two fields in the XUV-disk of M83 have been recently obtained,3R_{HII} away from the center of the galaxy (R_{HII)=6.6 kpc).GALEX UV images have shown the two fields to host in-situ recent star formation.The IRAC images are used in conjunction with GALEX data and new HI imaging from THINGS to constrain stellar masses and ages of the UV clumps in the fields,and to relate the local recent star formation to the reservoir of available gas. multi wavelength photometry in the UV and mid-IR bands of 136 UV clumps(spatial resolution >220pc) identified in the two target fields, together with model fitting of the stellar UV-MIR SED,suggest that the clumps cover a range of ages between a few Myr and >1Gyr with a median value around <100Myr,and have masses in the range 10^3-3*10^6M, with a peak ~10^4.7M.The range of observed ages,for which only a small fraction of the mass in stars appears to have formed in the past ~10Myr, agrees with the dearth of Ha emission observed in these outer fiel ds. At the location of our IRAC fields, the HI map shows localized enhancement and clumping of atomic gas. A comparison of the observed star formation with the gas reservoir shows that the UV clumps follow the Schmidt--Kennicutt scaling law of star formation,and that star formation is occurring in regions with gas dens ities at approximately (within a factor of a few) the critical density value de -rived according to the Toomre Q gravitational stability criterion. The signifi cant 8 micron excess in several of the clumps (16% of the total by number accou nting for ~67% of the 8 micron flux)) provides evidence for the existence of dust in these remote fields, in agreement with results for other galaxies. Furt hermore, we observe a relatively small excess of emission at 4.5 micron in the clumps...

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arXiv2008

Lyman alpha emitting galaxies at 0.2 < z < 0.35 from GALEX spectroscopy

The GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of about 8 A in the FUV (1350 - 1750 A) and about 20 A in the NUV (1950 - 2750 A) is used for a systematic search of Ly-a emitting galaxies at low redshift. This aims at filling a gap between high-redshift surveys and a small set of objects studied in detail in the nearby universe. A blind search of 7018 spectra extracted in 5 deep exposures (5.65 sq.deg) has resulted in 96 Ly-a emitting galaxy candidates in the FUV domain, after accounting for broad-line AGNs. The Ly-a EWs (equivalent width) are consistent with stellar population model predictions and show no trends as a function of UV color or UV luminosity, except a possible decrease in the most luminous that may be due to small-number statistics. Their distribution in EW is similar to that at z about 3 but their fraction among star-forming galaxies is smaller. Avoiding uncertain candidates, a sub-sample of 66 objects in the range 0.2 < z < 0.35 has been used to build a Ly-a LF (luminosity function). The incompleteness due to objects with significant Ly-a emission but a UV continuum too low for spectral extraction has been evaluated. A comparison with H-a LF in the same redshift domain is consistent with an average Ly-a/H-a of about 1 in about 15 % of the star-forming galaxies. A comparison with high-redshift Ly-a LFs implies an increase of the Ly-a luminosity density by a factor of about 16 from z about 0.3 to z about 3. By comparison with the factor 5 increase of the UV luminosity density in the same redshift range, this suggests an increase of the average Ly-a escape fraction with redshift.

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arXiv2007

Ultraviolet through Infrared Spectral Energy Distributions from 1000 SDSS Galaxies: Dust Attenuation

The meaningful comparison of models of galaxy evolution to observations is critically dependent on the accurate treatment of dust attenuation. To investigate dust absorption and emission in galaxies we have assembled a sample of ~1000 galaxies with ultraviolet (UV) through infrared (IR) photometry from GALEX, SDSS, and Spitzer and optical spectroscopy from SDSS. The ratio of IR to UV emission (IRX) is used to constrain the dust attenuation in galaxies. We use the 4000A break as a robust and useful, although coarse, indicator of star formation history (SFH). We examine the relationship between IRX and the UV spectral slope (a common attenuation indicator at high-redshift) and find little dependence of the scatter on 4000A break strength. We construct average UV through far-IR spectral energy distributions (SEDs) for different ranges of IRX, 4000A break strength, and stellar mass (M_*) to show the variation of the entire SED with these parameters. When binned simultaneously by IRX, 4000A break strength, and M_* these SEDs allow us to determine a low resolution average attenuation curve for different ranges of M_*. The attenuation curves thus derived are consistent with a lambda^{-0.7} attenuation law, and we find no significant variations with M_*. Finally, we show the relationship between IRX and the global stellar mass surface density and gas-phase-metallicity. Among star forming galaxies we find a strong correlation between IRX and stellar mass surface density, even at constant metallicity, a result that is closely linked to the well-known correlation between IRX and star-formation rate.

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arXiv2007

A Search for Extended Ultraviolet Disk (XUV-disk) Galaxies in the Local Universe

We have initiated a search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Herein, we compare GALEX UV and visible--NIR images of 189 nearby (D$<$40 Mpc) S0--Sm galaxies included in the GALEX Atlas of Nearby Galaxies and present the first catalogue of XUV-disk galaxies. We find that XUV-disk galaxies are surprisingly common but have varied relative (UV/optical) extent and morphology. Type~1 objects ($\ga$20% incidence) have structured, UV-bright/optically-faint emission features in the outer disk, beyond the traditional star formation threshold. Type~2 XUV-disk galaxies ($\sim$10% incidence) exhibit an exceptionally large, UV-bright/optically-low-surface-brightness (LSB) zone having blue $UV-K_s$ outside the effective extent of the inner, older stellar population, but not reaching extreme galactocentric distance. If the activity occuring in XUV-disks is episodic, a higher fraction of present-day spirals could be influenced by such outer disk star formation. Type~1 disks are associated with spirals of all types, whereas Type~2 XUV-disks are predominantly found in late-type spirals. Type~2 XUV-disks are forming stars quickly enough to double their [presently low] stellar mass in the next Gyr (assuming a constant SF rate). XUV-disk galaxies of both types are systematically more gas-rich than the general galaxy population. Minor external perturbation may stimulate XUV-disk incidence, at least for Type~1 objects. XUV-disks are the most actively evolving galaxies growing via inside-out disk formation in the current epoch, and may constitute a segment of the galaxy population experiencing significant, continued gas accretion from the intergalactic medium or neighboring objects.

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arXiv2007

Ultraviolet, Optical, and Infrared Constraints on Models of Stellar Populations and Dust Attenuation

The color of galaxies is a fundamental property, easily measured, that constrains models of galaxies and their evolution. Dust attenuation and star formation history (SFH) are the dominant factors affecting the color of galaxies. Here we explore the empirical relation between SFH, attenuation, and color for a wide range of galaxies, including early types. These galaxies have been observed by GALEX, SDSS, and Spitzer, allowing the construction of measures of dust attenuation from the ratio of infrared (IR) to ultraviolet (UV) flux and measures of SFH from the strength of the 4000A break. The empirical relation between these three quantities is compared to models that separately predict the effects of dust and SFH on color. This comparison demonstrates the quantitative consistency of these simple models with the data and hints at the power of multiwavelength data for constraining these models. The UV color is a strong constraint; we find that a Milky Way extinction curve is disfavored, and that the UV emission of galaxies with large 4000A break strengths is likely to arise from evolved populations. We perform fits to the relation between SFH, attenuation, and color. This relation links the production of starlight and its absorption by dust to the subsequent reemission of the absorbed light in the IR. Galaxy models that self-consistently treat dust absorption and emission as well as stellar populations will need to reproduce these fitted relations in the low-redshift universe.

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arXiv2007

The UV-Optical Color Magnitude Diagram II: Physical Properties and Morphological Evolution On and Off of a Star-Forming Sequence

We use the UV-optical color magnitude diagram in combination with spectroscopic and photometric measurements derived from the SDSS spectroscopic sample to measure the distribution of galaxies in the local universe (z<0.25) and their physical properties as a function of specific star formation rate (SSFR) and stellar mass. Throughout this study our emphasis is on the properties of galaxies on and off of a local "star-forming sequence." We discuss how the physical characteristics of galaxies along this sequence are related to scaling relations typically derived for galaxies of different morphological types. We find, among other trends that our measure of the star formation rate surface density is nearly constant along this sequence. We discuss this result and implications for galaxies at higher redshift. For the first time, we report on measurements of the local UV luminosity function versus galaxy structural parameters as well as inclination. We also split our sample into disk-dominated and bulge-dominated subsamples using the i-band Sersic index and find that disk-dominated galaxies occupy a very tight locus in SSFR vs. stellar mass space while bulge-dominated galaxies display a much larger spread of SSFR at fixed stellar mass. A significant fraction of galaxies with SSFR and SF surface density above those on the "star-forming sequence" are bulge-dominated. We can use our derived distribution functions to ask whether a significant fraction of these galaxies may be experiencing a final episode of star formation (possibly induced by a merger or other burst), soon to be quenched, by determining whether this population can explain the growth rate of the non-star-forming galaxies on the "red sequence." (Abridged)

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arXiv2007

The Diverse Properties of the Most Ultraviolet Luminous Galaxies Discovered by the Galaxy Evolution Explorer

We report on the properties of a sample of ultraviolet luminous galaxies (UVLGs) selected by matching the Galaxy Evolution Explorer (GALEX) Surveys with the Sloan Digital Sky Survey Third Data Release. Out of 25362 galaxies between 0.0 2x10^10 L_solar at 1530 Angstroms (observed wavelength). The properties of this population are well correlated with ultraviolet surface brightness. We find that the galaxies with low UV surface brightness are primarily large spiral systems with a mixture of old and young stellar populations, while the high surface brightness galaxies consist primarily of compact starburst systems. In terms of the behavior of surface brightness with luminosity, size with luminosity, the mass-metallicity relation, and other parameters, the compact UVLGs clearly depart from the trends established by the full sample of galaxies. The subset of compact UVLGs with the highest surface brightness (``supercompact UVLGs'') have characteristics that are remarkably similar to Lyman Break Galaxies at higher redshift. They are much more luminous than typical local ultraviolet-bright starburst galaxies and blue compact dwarf galaxies. They have metallicities that are systematically lower than normal galaxies of the same stellar mass, indicating that they are less chemically evolved. In all these respects, they are the best local analogs for Lyman Break Galaxies.

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arXiv2007

Clustering Properties of restframe UV selected galaxies I: the correlation length derived from GALEX data in the local Universe

We present the first measurements of the angular correlation function of galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions. The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22, yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm 0.6 Mpc and no significant trend is seen for this value as a function of the limiting apparent magnitude or between the GALEX bands. This estimate is close to that found from samples of blue galaxies in the local universe selected in the visible, and similar to that derived at z ~ 3 for LBGs with similar rest frame selection criteria. This result supports models that predict anti-biasing of star forming galaxies at low redshift, and brings an additional clue to the downsizing of star formation at z<1.

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arXiv2007

Chandra ACIS Survey of M33 (ChASeM33): A First Look

We present an overview of the Chandra ACIS Survey of M33 (ChASeM33): A Deep Survey of the Nearest Face-on Spiral Galaxy. The 1.4 Ms survey covers the galaxy out to $R \approx 18\arcmin (\approx 4$ kpc). These data provide the most intensive, high spatial resolution assessment of the X-ray source populations available for the confused inner regions of M33. Mosaic images of the ChASeM33 observations show several hundred individual X-ray sources as well as soft diffuse emission from the hot interstellar medium. Bright, extended emission surrounds the nucleus and is also seen from the giant \hii regions NGC 604 and IC 131. Fainter extended emission and numerous individual sources appear to trace the inner spiral structure. The initial source catalog, arising from $\sim$~2/3 of the expected survey data, includes 394 sources significant at the $3σ$ confidence level or greater, down to a limiting luminosity (absorbed) of $\sim$1.6\ergs{35} (0.35 -- 8.0 keV). The hardness ratios of the sources separate those with soft, thermal spectra such as supernova remnants from those with hard, non-thermal spectra such as X-ray binaries and background active galactic nuclei. Emission extended beyond the Chandra point spread function is evident in 23 of the 394 sources. Cross-correlation of the ChASeM33 sources against previous catalogs of X-ray sources in M33 results in matches for the vast majority of the brighter sources and shows 28 ChASeM33 sources within 10\arcsec of supernova remnants identified by prior optical and radio searches. This brings the total number of such associations to 31 out of 100 known supernova remnants in M33.

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arXiv2007

The Star Formation and Extinction Co-Evolution of UV-Selected Galaxies over 0.05<z<1.2

We use a new stacking technique to obtain mean mid IR and far IR to far UV flux ratios over the rest near-UV/near-IR color-magnitude diagram. We employ COMBO-17 redshifts and COMBO-17 optical, GALEX far and near UV, Spitzer IRAC and MIPS Mid IR photometry. This technique permits us to probe infrared excess (IRX), the ratio of far IR to far UV luminosity, and specific star formation rate (SSFR) and their co-evolution over two orders of magnitude of stellar mass and redshift 0.1<z<1.2. We find that the SSFR and the characteristic mass (M_0) above which the SSFR drops increase with redshift (downsizing). At any given epoch, IRX is an increasing function of mass up to M_0. Above this mass IRX falls, suggesting gas exhaustion. In a given mass bin below M_0 IRX increases with time in a fashion consistent with enrichment. We interpret these trends using a simple model with a Schmidt-Kennicutt law and extinction that tracks gas density and enrichment. We find that the average IRX and SSFR follows a galaxy age parameter which is determined mainly by the galaxy mass and time since formation. We conclude that blue sequence galaxies have properties which show simple, systematic trends with mass and time such as the steady build-up of heavy elements in the interstellar media of evolving galaxies and the exhaustion of gas in galaxies that are evolving off the blue sequence. The IRX represents a tool for selecting galaxies at various stages of evolution.

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arXiv2007

Extinction Corrected Star Formation Rates Empirically Derived from Ultraviolet-Optical Colors

Using a sample of galaxies from the Sloan Digital Sky Survey spectroscopic catalog with measured star-formation rates (SFRs) and ultraviolet (UV) photometry from the GALEX Medium Imaging Survey, we derived empirical linear correlations between the SFR to UV luminosity ratio and the UV-optical colors of blue sequence galaxies. The relations provide a simple prescription to correct UV data for dust attenuation that best reconciles the SFRs derived from UV and emission line data. The method breaks down for the red sequence population as well as for very blue galaxies such as the local ``supercompact'' UV luminous galaxies and the majority of high redshift Lyman Break Galaxies which form a low attenuation sequence of their own.

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arXiv2007

The Young and the Dustless: Interpreting Radio Observations of UltraViolet Luminous Galaxies

Ultraviolet Luminous Galaxies (UVLGs) have been identified as intensely star-forming, nearby galaxies. A subset of these, the supercompact UVLGs, are believed to be local analogs of high redshift Lyman Break Galaxies. Here we investigate the radio continuum properties of this important population for the first time. We have observed 42 supercompact UVLGs with the VLA, all of which have extensive coverage in the UV/optical by GALEX and SDSS. Our analysis includes comparison samples of multiwavelength data from the Spitzer First Look Survey and from the SDSS-Galex matched catalogs. In addition we have Spitzer MIPS data for 24 of our galaxies and find that they fall on the radio-FIR correlation of normal star-forming galaxies. We find that our galaxies have lower radio-to-UV ratios and lower Balmer decrements than other local galaxies with similar (high) star formation rates. Optical spectra show they have lower Dn(4000) and HdeltaA indices, higher Hbeta emission-line equivalents widths, and higher [OIII]5007/Hbeta emission-line ratios than normal star forming galaxies. Comparing these results to galaxy spectral evolution models we conclude that supercompact UVLGs are distinguished from normal star forming galaxies firstly by their high specific star formation rates. Moreover, compared to other types of galaxies with similar star formation rates, they have significantly less dust attenuation. In both regards they are similar to Lyman Break Galaxies. This suggests that the process that causes star formation in the supercompact UVLGs differs from other local star forming galaxies, but may be similar to Lyman Break Galaxies.

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arXiv2007

Nitrogen Production in Starburst Galaxies Detected by GALEX

We investigate the production of nitrogen in star forming galaxies with ultraviolet (UV) radiation detected by the Galaxy Evolution Explorer Satellite (GALEX). We use a sample of 8,745 GALEX emission line galaxies matched to the Sloan Digital Sky Survey (SDSS) spectroscopic sample. We derive both gas-phase oxygen and nitrogen abundances for the sample, and apply stellar population synthesis models to derive stellar masses and star formation histories of the galaxies. We compare oxygen abundances derived using three different diagnostics. We derive the specific star formation rates of the galaxies by modeling the 7-band GALEX+SDSS photometry. We find that galaxies that have log SFR/M$_*$ > -10.0 typically have values of log N/O ~0.05 dex less than galaxies with log SFR/M$_*$ < -10.0 and similar oxygen abundances.

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arXiv2007

The UV-optical Galaxy Color-Magnitude Diagram I: Basic Properties

We have analyzed the bivariate distribution of galaxies as a function of ultraviolet-optical colors and absolute magnitudes in the local universe. The sample consists of galaxies with redshifts and optical photometry from the Sloan Digital Sky Survey (SDSS) main galaxy sample matched with detections in the near-ultraviolet (NUV) and far-ultraviolet (FUV) bands in the Medium Imaging Survey being carried out by the Galaxy Evolution Explorer (GALEX) satellite. In the (NUV-r)_{0.1} vs. M_{r,0.1} galaxy color-magnitude diagram, the galaxies separate into two well-defined blue and red sequences. The (NUV-r)_{0.1} color distribution at each M_{r,0.1} is not well fit by the sum of two Gaussians due to an excess of galaxies in between the two sequences. The peaks of both sequences become redder with increasing luminosity with a distinct blue peak visible up to M_{r,0.1}\sim-23. The r_{0.1}-band luminosity functions vary systematically with color, with the faint end slope and characteristic luminosity gradually increasing with color. After correcting for attenuation due to dust, we find that approximately one quarter of the color variation along the blue sequence is due to dust with the remainder due to star formation history and metallicity. Finally, we present the distribution of galaxies as a function of specific star formation rate and stellar mass. The specific star formation rates imply that galaxies along the blue sequence progress from low mass galaxies with star formation rates that increase somewhat with time to more massive galaxies with a more or less constant star formation rate. Above a stellar mass of ~10^10.5 M_{sun}, galaxies with low ratios of current to past averaged star formation rate begin to dominate.

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