UV Star Formation Rates in the Local Universe Salim, Samir; Rich, R. Michael; Charlot, Stéphane ...
The Astrophysical journal. Supplement series,
12/2007, Letnik:
173, Številka:
2
Journal Article
Recenzirano
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We measure star formation rates (SFRs) of approximately 50,000 optically selected galaxies in the local universe (z approximately 0.1)--from gas-rich dwarfs to massive ellipticals. We obtain ...dust-corrected SFRs by fitting the GALEX (ultraviolet) and SDSS photometry to a library of dust-attenuated population synthesis models. For star-forming galaxies, our UV-based SFRs compare remarkably well with those from SDSS-measured emission lines (H alpha ). Deviations from perfect agreement are shown to be due to differences in the dust attenuation estimates. In contrast to H alpha measurements, UV provides reliable SFRs for galaxies with weak H alpha , and where H alpha is contaminated with AGN emission (1/2 of the sample). Using full-SED SFRs, we calibrate a simple prescription that uses GALEX far- and near-UV magnitudes to produce dust-corrected SFRs for normal star-forming galaxies. The specific SFR is considered as a function of stellar mass for (1) star-forming galaxies with no AGNs, (2) those hosting an AGN, and (3) galaxies without Ha emission. We find that the three have distinct star formation histories, with AGNs lying Intermediate between the star-forming and the quiescent galaxies. Star-forming galaxies without an AGN lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. On the other hand, weak AGNs, while also massive, have lower SFRs, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent galaxies via strong and weak AGNs. We confirm that some galaxies with no H alpha show signs of star formation in the UV. We derive a cosmic star formation density at z = 0.1 with significantly smaller total error than previous measurements.
Abstract
We present high spectral resolution (R ∼ 110 000) Ca ii K-line (3933 Å) absorption profiles observed towards 20 nearby A-type stars thought to possess circumstellar debris discs. The stars ...were repeatedly observed over three nights on the 3.6m ESO telescope (Chile) during 2016 November. These spectra have revealed four stars (HD 24966, HD 38056, HD 79469 and HD 225200) that exhibit transient and weak absorption features at velocities in excess of 15 km s−1 from the cirumstellar radial velocity. These absorption features mostly appear at redshifted velocities with respect to the stellar radial velocity, some evolving towards bluer velocities as a function of time. We identify these absorption events with the evaporation of gas liberated during the infall of star grazing exocomets / planetesimals, a process often termed that of ‘falling evaporating bodies’ (FEBs) (Beust et al. 1990). Five of the stars (HD 2262, HD 30422, HD 45557, HD 181296 and HD 188228) exhibit weak absorption features in at least one of their nightly spectra. However, none of these targets show any significant absorption centred near the stellar radial velocity that could be confidently associated with a circumstellar gas disc. The origin of these sporadic absorptions is unclear at present. Finally, we present a list of 22 stars with known FEB absorption components in their Ca ii K-line spectra that will hopefully prove useful as a target list for the possible detection, by high contrast imaging, of exoplanets in orbit around young A-type stars.
ABSTRACT We present medium spectral resolution (R ∼ 60,000) observations of the CaII K-line (3,933 Å) absorption profiles observed toward 21 nearby A-type stars thought to possess circumstellar gas ...debris disks. The stars were repeatedly observed over two observing runs on the 2.1 m Otto Struve telescope at the McDonald Observatory, Texas in 2011 May and 2012 November. Nightly changes in the absorption strength of the CaII K-line near the stellar radial velocity were observed in four of the stars (HD 21620, HD 110411, HD 145964 and HD 183324). This type of absorption variability indicates the presence of a circumstellar gas disk around these stars. We also have detected weak absorption features that sporadically appear with velocities in the range ± 100 km s-1 of the main circumstellar K-line in the spectra of HD 21620, HD 42111, HD 110411 and HD 145964. Due to the known presence of both gas and dust disks surrounding these four stars, these transient absorption features are most probably associated with the presence of Falling Evaporated Bodies (FEBs, or exocomets) that are thought to liberate gas on their grazing trajectory toward and around the central star. This now brings the total number of A-type stars in which the evaporation of CaII gas from protoplanetary bodies (i.e., exocomets) has been observed to vary on a nightly basis to 10 systems. A statistical analysis of the 10 A-stars showing FEB-activity near the CaII K-line compared to 21 A-type stars that exhibit no measurable variability reveals that FEB-activity occurs in significantly younger stellar systems that also exhibit chemical peculiarities. The presence of FEB-activity does not appear to be associated with a strong mid-IR excess. This is probably linked to the disk inclination angle, since unless the viewing angle is favorable the detection of time-variable absorption may be unlikely. Additionally, if the systems are more evolved then the evaporation of gas due to FEB activity could have ceased, whereas the circumstellar dust disk may still remain.
We introduce a new quantity, the mass flux density of galaxies evolving from the blue sequence to the red sequence. We propose a simple technique for constraining this mass flux using the ...volume-corrected number density in the extinction-corrected UV-optical color-magnitude distribution, the stellar age indexes H delta sub(A) and unk(4000), and a simple prescription for spectral evolution using a quenched star formation history. We exploit the excellent separation of red and blue sequences in the NUV - r band Hess function. The final value we measure, rho T = 0.033 m unk yr super(-1) Mpc super(-3), is strictly speaking an upper limit due to the possible contributions of bursting, composite, and extincted galaxies. However, it compares favorably with estimates of the average mass flux that we make based on the red luminosity function evolution derived from the DEEP2 and COMBO-17 surveys, rho R - +0034 M unk yr super(-1) Mpc super(-3). We find that the blue sequence mass has remained roughly constant since z = 1 ( rho B unk 0.01 M unk yr super(-1) Mpc super(-3), but the average on-going star formation of rho SF unk 0.037 M unk yr super(-1) Mpc super(-3) over 0 < z < 1 is balanced by mass flux off the blue sequence. We explore the nature of the galaxies in the transition zone with particular attention to the frequency and impact of active galactic nuclei (AGNs). The AGN fraction peaks in the transition zone. We find circumstantial, albeit weak evidence that the quench rates are higher in higher luminosity AGNs.
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 (SFR/M unk) and stellar mass (M unk). 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, capital sigma sub(SFR), 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 l-band Sersic index and find that disk-dominated galaxies occupy a very tight locus in SFR/M unk vs. M unk space, while bulge-dominated galaxies display a much larger spread of SFR/M unk at fixed stellar mass. A significant fraction of galaxies with SFR/M unk and capital sigma sub(SFR) 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." We find that this is a plausible scenario for bulge-dominated galaxies near the characteristic transition mass under reasonable assumptions regarding quenching timescales. Similarly, we use this technique to estimate the rate of mergers/starbursts that take galaxies off of the star-forming sequence and show that the implied merger rates are consistent with local measurements.
We present an archival study of 27 circumstellar Fe i (λ3860 ) and Ca ii (λ3933 ) absorption spectra of the β Pictoris system recorded over the 2003-2014 timeframe. We have detected several transient ...absorption events at velocities red-shifted by >+20 km s−1 from the main central absorption line profiles of both Fe i and Ca ii. Such events can be attributed to the presence of kilometer-sized infalling evaporating bodies (i.e., exocomets) on their grazing approach to the central star. The majority of the transient absorption events detected in the Fe i profiles occur at velocities in the +35 to +50 km s−1 range. This is consistent with that found for Ca ii gas that has been sublimated from the "D" family of β Pictoris exocomets recently found by Kiefer et al. These spectra also reveal that the strength of the main component of the Fe i absorption line at Vhelio ∼ +21 km s−1 has weakened by ∼30% since 2011. Since neutrals, when ionized, are the main source of the ion-braking mechanism of Brandeker for circumstellar gas in the β Pictoris system, then this may have some measurable effect on the size and/or location of the main circumstellar gas disk. Finally we note that we have failed to detect any circumstellar Fe i absorption in our previously reported spectra of similar gas disks surrounding 28 young A-type stars. Thus, it would appear that the β Pictoris is anomalous with regards to circumstellar Fe i absorption.
The Calibration and Data Products of GALEX Morrissey, Patrick; Conrow, Tim; Barlow, Tom A ...
The Astrophysical journal. Supplement series,
12/2007, Letnik:
173, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX). These releases have identical pipeline calibrations that are ...significantly improved over the GR1 data release. GALEX continues to survey the sky in the far-ultraviolet (FUV, similar to 154 nm) and near-ultraviolet (NUV, similar to 232 nm) bands, providing simultaneous imaging with a pair of photon-counting, microchannel plate, delay line readout detectors. These 1.25 degree field of view detectors are well suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations, and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 m sub(AB) in the FUV and NUV, respectively. We have detected a long-term drift of order 1% FUV and 6% NUV over the mission. Astrometric precision is of order 0.5" rms in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.
Abstract
We present both archival and new high-spectral resolution visible observations of the δ Scuti/λ Boötis star HD 183324 recorded over the 2009–2013 timeframe. Previous observations of this ...A0V-type star have hinted at the presence of a circumstellar disc of gas surrounding the star. These new spectral data unequivocally show a remarkably high level of variable absorption activity, whose behaviour is typical of a stellar system in which gas is added to a circumstellar disc on a sporadic basis due to the evaporation of planetesimal-like objects (i.e. ‘exocomets’). This planetesimal material most probably resides in an IR-emitting debris disc of dust and gas, similar to that of the β Pictoris exoplanet system that has recently been shown to harbor an exoplanet.
ABSTRACT We present medium resolution (R = 60,000) absorption measurements of the interstellar Ca II K line observed towards five nearby A-type stars (49 Ceti, 5 Vul, ι Cyg, 2 And, and HD 223884) ...suspected of possessing circumstellar gas debris disks. The stars were observed on a nightly basis during a six night observing run on the 2.1-meter Otto Struve telescope at the McDonald Observatory, Texas. We have detected nightly changes in the absorption strength of the Ca II K line observed near the stellar radial velocity in three of the stars (49 Ceti, i Cyg and HD 223884). Such changes in absorption suggest the presence of a circumstellar (atomic) gas disk around these stars. In addition to the absorption changes in the main Ca II K line profile, we have also observed weak transient absorption features that randomly appear at redshifted velocities in the spectra of 49 Ceti, 5 Vul, and 2 And. These absorption features are most probably associated with the presence of falling evaporated bodies (exo-comets) that liberate evaporating gas on their approach to the central star. This now brings the total number of systems in which exocomet activity has been observed at Ca II or Na I wavelengths on a nightly basis to seven (β Pic, HR 10, HD 85905, β Car, 49 Ceti, 5 Vul, and 2 And), with 2 And exhibiting weaker and less frequent changes. All of the disk systems presently known to exhibit either type of short-term variability in Ca II K line absorption are rapidly rotating A-type stars (V sin i > 120 km s-1). Most exhibit mid-IR excesses, and many of them are very young (< 20 Myr), thus supporting the argument that many of them are transitional objects between Herbig Ae and "Vega-like" A-type stars with more tenuous circumstellar disks. No mid-IR excess (due to the presence of a dust disk) has yet been detected around either 2 And or HD 223884, both of which have been classified as λ Boötis-type stars. This may indicate that the observed changes in gas absorption for these two stars may not be due to circumstellar activity, but may instead be associated with the stars' episodic mass loss and passage though low-density interstellar clouds.
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) sub(0.1) versus M sub(r.01) galaxy color-magnitude diagram, the galaxies separate into two well-defined blue and red sequences. The (NUV - r) sub(0.1) color distribution at each M sub(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 sub(r.0.1) similar to -23. The r sub(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 similar to 10 super(10.5) M unk, galaxies with low ratios of current to past averaged star formation rate begin to dominate.
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