arXiv subjects

Daniela Calzetti

Publications and source records attributed to Daniela Calzetti.

At least 163 records · Page 9Linked to original sources

arXiv2006

The Spectral Energy Distribution of Dust Emission in the Edge-on Spiral Galaxy NGC 4631 as seen with Spitzer and JCMT

We explore the nature of variations in dust emission within an individual galaxy using 3.6 - 160 micron Spitzer Space Telescope observations and 450 and 850 micron James Clerk Maxwell Telescope observations of the edge-on Sd spiral galaxy NGC 4631 with the goals of understanding the relation between polycyclic aromatic hydrocarbons (PAH) and dust emission, studying the variations in the colors of the dust emission, and searching for possible excess submillimeter emission compared to what is anticipated based on dust models applied to the mid- and far-infrared data. PAH emission at 8 micron is found to correlate best with hot dust emission at 24 micron on kiloparsec scales, although the relation breaks down on scales equal to hundreds of parsecs, possibly because of differences in the mean free paths between the photons that excite the PAHs and heat the dust and possibly because the PAHs are destroyed by the hard radiation fields in the centers of some star formation regions. The ratio of 8 micron PAH emission to 160 micron cool dust emission appears to be a function of radius. The 70/160 and 160/450 micron flux density ratios are remarkably constant even though the surface brightness varies by a factor of 25 in each wave band, which suggests that the emission is from dust heated by a nearly-uniform radiation field. Globally, we find an excess of 850 micron emission relative to what would be predicted by dust models. The 850 micron excess is highest in regions with low 160 micron surface brightness, although the strength and statistical significance of this result depends on the model fit to the data. We rule out variable emissivity functions or ~4 K dust as the possible origins of this 850 micron emission, but we do discuss the other possible mechanisms that could produce the emission.

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arXiv2006

Ultraviolet Through Far-Infrared Spatially Resolved Analysis of the Recent Star Formation in M81 (NGC3031)

The recent star formation (SF) in the early-type spiral galaxy M81 is characterized using imaging observations from the far-ultraviolet (UV) to the far-infrared (IR). We compare these data with models of the stellar, gas, and dust emission for sub-galactic regions. Our results suggest the existence of a diffuse dust emission not directly linked to the recent SF. We find a radial decrease of the dust temperature and dust mass density, and in the attenuation of the stellar light. The IR emission in M81 can be modeled with three components: 1) cold dust with a temperature =18+-2 K, concentrated near the HII regions but also presenting a diffuse distribution; 2) warm dust with T_w=53+-7 K, directly linked with the HII regions; and 3) aromatic molecules, with diffuse morphology peaking around the HII regions. We derive several relationships to obtain total IR luminosities from IR monochromatic fluxes, and we compare five different star formation rate (SFR) estimators for HII regions in M81 and M51: the UV, Halpha, and three estimators based on Spitzer data. We find that the Halpha luminosity absorbed by dust correlates tightly with the 24 microns emission. The correlation with the total IR luminosity is not as good. Important variations from galaxy to galaxy are found when estimating the total SFR with the 24 microns or the total IR emission alone. The most reliable estimations of the total SFRs are obtained by combining the Halpha emission (or the UV) and an IR luminosity (especially the 24 microns emission), which probe the unobscured and obscured SF, respectively. For the entire M81 galaxy, about 50% of the total SF is obscured by dust. The percentage of obscured SF ranges from 60% in the inner regions of the galaxy to 30% in the outer zones.

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arXiv2006

Warm Dust and Spatially Variable PAH Emission in the Dwarf Starburst Galaxy NGC 1705

We present Spitzer observations of the dwarf starburst galaxy NGC 1705 obtained as part of SINGS. The galaxy morphology is very different shortward and longward of ~5 microns: short-wavelength imaging shows an underlying red stellar population, with the central super star cluster (SSC) dominating the luminosity; longer-wavelength data reveals warm dust emission arising from two off-nuclear regions offset by ~250 pc from the SSC. These regions show little extinction at optical wavelengths. The galaxy has a relatively low global dust mass (~2E5 solar masses, implying a global dust-to-gas mass ratio ~2--4 times lower than the Milky Way average). The off-nuclear dust emission appears to be powered by photons from the same stellar population responsible for the excitation of the observed H Alpha emission; these photons are unassociated with the SSC (though a contribution from embedded sources to the IR luminosity of the off-nuclear regions cannot be ruled out). Low-resolution IRS spectroscopy shows moderate-strength PAH emission in the 11.3 micron band in the eastern peak; no PAH emission is detected in the SSC or the western dust emission complex. There is significant diffuse 8 micron emission after scaling and subtracting shorter wavelength data; the spatially variable PAH emission strengths revealed by the IRS data suggest caution in the interpretation of diffuse 8 micron emission as arising from PAH carriers alone. The metallicity of NGC 1705 falls at the transition level of 35% solar found by Engelbracht and collaborators; the fact that a system at this metallicity shows spatially variable PAH emission demonstrates the complexity of interpreting diffuse 8 micron emission. A radio continuum non-detection, NGC 1705 deviates significantly from the canonical far-IR vs. radio correlation. (Abridged)

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arXiv2006

NICMOS Status

We provide an overview of the most important calibration aspects of the NICMOS instrument on board of HST. We describe the performance of the instrument after the installation of the NICMOS Cooling System, and show that the behavior of the instrument has become very stable and predictable. We detail the improvements made to the NICMOS pipeline and outline plans for future developments. The derivation of the absolute photometric zero-point calibration is described in detail. Finally, we describe and quantify a newly discovered count-rate dependent non-linearity in the NICMOS cameras. This new non-linearity is distinctly different from the total count dependent non-linearity that is well known for near-infrared detectors. We show that the non-linearity has a power law behavior, with pixels with high system, or vice versa, pixels with low count rate detecting slightly less than expected. The effect has a wavelength dependence with observations at the shortest wavelengths being the most affected (~0.05-0.1 mag per dex flux change at ~1 micron, 0.03 mag per dex at 1.6 micron).

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arXiv2005

Spitzer Observations Of The Supergiant Shell Region In IC 2574

We present spatially resolved Spitzer imaging of the supergiant shell region of the M81 group dwarf galaxy IC 2574 obtained as part of the Spitzer Infrared Nearby Galaxies Survey. This region harbors one of the best nearby examples of a kinematically distinct HI shell, with an associated remnant stellar cluster; the shell is initiating sequential star formation as it interacts with the surrounding interstellar medium. This region dominates the infrared luminosity of IC 2574 and is spatially resolved in all Spitzer imaging bands. We study the differences in dust temperature as a function of local environment and compare local star formation rates as inferred from H Alpha and total infrared luminosities. We find that the strong H Alpha sources are associated with regions of warm dust; however, the most luminous infrared and H Alpha sources are not necessarilyco-spatial. The coolest dust is found in the regions farthest from the rim of the shell; these regions show the best agreement between star formation rates derived from H Alpha and from total infrared luminosities (although discrepancies at the factor of 3-4 level still exist). There is considerable variation in the radio-far infrared correlation in different regions surrounding the shell. The low dust content of the region may influence the scatter seen in these relations; these data demonstrate that the expanding shell is dramatically affecting its surroundings by triggering star formation and altering the dust temperature.

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arXiv2005

The Stellar Content of Nearby Star-Forming Galaxies. III. Unravelling the Nature of the Diffuse Ultraviolet Light

We investigate the nature of the diffuse intra-cluster ultraviolet light seen in twelve local starburst galaxies, using long-slit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST). We take this faint intra-cluster light to be the field in each galaxy, and compare its spectroscopic signature with STARBURST99 evolutionary synthesis models and with neighboring star clusters. Our main result is that the diffuse ultraviolet light in eleven of the twelve starbursts lacks the strong O-star wind features that are clearly visible in spectra of luminous clusters in the same galaxies. The difference in stellar features dominating cluster and field spectra indicate that the field light originates primarily from a different stellar population, and not from scattering of UV photons leaking out of the massive clusters. We consider several scenarios to understand the lack of observed O stars in the field, and their implications for the origin of the field stellar population. If the field stellar populations formed in situ, the field must either have an IMF which is steeper than Salpeter (3.0-3.5) or a Salpeter slope with an upper mass cutoff of 30-50 solar masses. If star formation occurs primarily in star clusters, the field could be composed of older, faded clusters, and/or a population which is coeval with the luminous clusters but lower in mass. If the field is composed of older, dissolving clusters, we estimate that star clusters (regardless of mass) need to dissolve on timescales 7-10 Myr to create the field (abridged).

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arXiv2005

Local Starbursts: Perspectives from the Optical

The optical regime is historically the best-studied wavelenght range. Gas ionized by massive stars produces optical emission lines that have been used to derive indicators of star-formation rate, metallicity, dust reddening, and the ionization conditions of the interstellar medium. Absorptions lines have been used to measure velocity dispersions, and the 4000 A break has been shown to be a useful indicator of the mean age of stellar populations. I briefly summarize some recent work done on, specifically, star formation rate indicators, in view of their importance for understanding star-forming galaxies at high redshift.

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arXiv2004

Limits on the Optical Brightness of the Epsilon Eridani Dust Ring

The STIS/CCD camera on the {\em Hubble Space Telescope (HST)} was used to take deep optical images near the K2V main-sequence star $ε$ Eridani in an attempt to find an optical counterpart of the dust ring previously imaged by sub-mm observations. Upper limits for the optical brightness of the dust ring are determined and discussed in the context of the scattered starlight expected from plausible dust models. We find that, even if the dust is smoothly distributed in symmetrical rings, the optical surface brightness of the dust, as measured with the {\em HST}/STIS CCD clear aperture at 55 AU from the star, cannot be brighter than about 25 STMAG/"$^2$. This upper limit excludes some solid grain models for the dust ring that can fit the IR and sub-mm data. Magnitudes and positions for $\approx $59 discrete objects between 12.5" to 58" from $ε$ Eri are reported. Most if not all of these objects are likely to be background stars and galaxies.

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arXiv2004

Large- and Small-Scale Feedback in the Starburst Canters of Galaxies

The interplay between stellar populations and gas in local starburst galaxies is analyzed using images from the Hubble Space Telescope to map the ages of the young stellar components and to isolate the contribution of shocks on spatial scales ranging from a few tens of pc to ~1 kpc. The shocked gas represents a small fraction of the total ionized gas in these objects, yet it can have profound effects on the long-term evolution of the starburst, which may include the triggering of new star formation.

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arXiv2003

The Ionized Gas in Local Starburst Galaxies: Global and Small--Scale Feedback from Star Formation

Abridged: The small-- and intermediate--scale structure and the fraction of the ISM ionized by non--radiative processes is investigated in a small sample of four local starburst galaxies, imaged with the HST WFPC2. The sample comprises three dwarf galaxies, NGC3077, NGC4214, and NGC5253, and one giant spiral, NGC5236 (M83). The galaxies span a range in metallicity, luminosity, and environment, enabling the investigation of non--radiative ionization processes in a variety of galactic conditions. For this purpose, the four galaxies were imaged in the lines of H-beta(4861 A), [OIII](5007 A), H-alpha(6563 A), and [SII](6717,6731 A). The non--photoionized gas in these galaxies has been traced on scales ranging from a few tens of pc to a few hundred pc, providing a full budget for this ionized gas component. Using the `maximum starburst line' of Kewley et al. (2001) to discriminate between photoionized and non--photoionized gas, we find that in all four galaxies non--photoionization processes are responsible for a small fraction of the total H-alpha emission, at the level of 3%--4%. The starbursts in the three dwarf galaxies deposit a significant fraction, 70%-100%, of their mechanical energy into the surrounding interstellar medium and require time--extended (a few x 10^7 yr to ~10^8 yr) star formation, in order to account for the observed luminosity of the non--photoionized gas. In the massive spiral, the non--photoionized gas is concentrated in localized areas surrounded by active star--formation, with no evidence for extended structures (as those in the dwarfs). This confirms the picture that starbursts remain confined events in massive galaxies, likely due to the deep potential well.

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arXiv2001

Far-Infrared Galaxies in the Far-Ultraviolet

In an effort to better understand the UV properties of ultraluminous infrared galaxies (ULIGs), and compare them to the rest-frame UV properties of high redshift sub-mm and Lyman-break galaxies, we have obtained far- and near-UV imaging observations (lambda_{eff}=1457Å, lambda_{eff}=2364Å, respectively) of two luminous infrared galaxies (LIGs--VV 114 and IC 883) and five ULIGs (IRAS 08572+3915, Mrk 273, IRAS 15250+3609, Arp 220, and IRAS 19254--7245) using the Hubble Space Telescope. All the galaxies were detected in both channels. UV light, both diffuse and from star clusters, can be traced to within the inner kpc of the dominant near-IR nuclei. However, in general, the brightest UV sources are clearly displaced from the $I$-band and near-IR peaks by at least hundreds of pc. Further, only 0.07%-7.3% of the total near-UV light is projected within the inner 500 pc radius, even though this is the same region wherein most of the bolometric energy is generated. All nuclei are highly obscured by dust. (Abridged)

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arXiv2001

Star Formation in the Local Universe

We present preliminary results of a long-term study aimed at answering a number of open questions on the evolution of starbursts in local galaxies. The project employes mainly HST data from the ultraviolet to the red of the stellar continuum and of the nebular emission from the galaxies. Here we concentrate on NGC5253 and NGC5236 (M83), that form a dwarf--massive galaxy pair at about 4 Mpc distance. The recent star formation history of the centers of the two galaxies is investigated in order to identify similarities and differences in the evolution of their central starbursts.

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arXiv2001

The Dust Opacity of Star-Forming Galaxies

Presence of dust in galaxies removes half or more of the stellar energy from the UV-optical budget of the Universe and has profound impact on our understanding of how galaxies evolve. Measures of opacity in local galaxies are reviewed together with widely used theoretical and empirical methods for quantifying its effects. Existing evidence shows that the dust content of nearby galaxies depends not only on their morphology, but also on their luminosity and activity level. A digression is devoted to starbursts in view of their potential relevance for measures of opacity in distant galaxies. Scarcity of coherent multiwavelength datasets hampers our ability to derive reliable obscuration estimates in intermediate and high redshift galaxies. This, in turn, limits the reliability of inferred physical quantities, such as star formation rates, stellar population ages, galaxy luminosity functions, and others.

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arXiv2000

The Effects of Dust on the Spectral Energy Distributions of Star-Forming Galaxies

I review the effects of dust, both in absorption and in emission, on the spectral energy distributions (SEDs) of Local star-forming galaxies. The energy balance between the stellar light absorbed by dust at UV-optical-nearIR wavelengths and the energy re-emitted by dust in the far infrared (FIR) shows that the amount of dust extinction affecting the galaxy can be predicted with simple prescriptions applied to the UV-optical data. The implications of these results for high redshift galaxies are discussed. Arguments can be given to support the view that Local starbursts are representative of high-redshift (z>2), UV-detected, star-forming galaxies. If this is the case, the high redshift FIR emission will be generally undetected in sub-mm surveys, unless (1) the bolometric luminosity of the high-z galaxies is comparable to or larger than that of Local ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.

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arXiv2000

The HST Survey of BL~Lacertae Objects. IV. Infrared Imaging of Host Galaxies

The HST NICMOS Camera 2 was used for H-band imaging of 12 BL Lacertae objects taken from the larger sample observed with the WFPC2 in the R band (Urry et al. 2000; Scarpa et al. 2000). Ten of the 12 BL Lacs are clearly resolved, and the detected host galaxies are large, bright ellipticals with average H-band absolute magnitude M=-26.2+-0.45 mag and effective radius 10+-5 kpc. The rest-frame integrated color of the host galaxies is on average R-H=2.3+-0.3, consistent with the value for both radio galaxies and normal, non-active elliptical galaxies, and indicating the dominant stellar population is old. The host galaxies tend to be bluer in their outer regions than in their cores, with average color gradient Delta(R-H)/Delta(log r)=-0.2 mag, again consistent with results for normal non-active elliptical galaxies. The infrared Kormendy relation, derived for the first time for BL Lac host galaxies, is m(e) = 3.8*log(R)+14.8 (where m(e) is the surface brightness at the effective radius R), fully in agreement with the relation for normal ellipticals. The close similarity between BL Lac host galaxies and normal ellipticals suggests the active nucleus has surprisingly little effect on the host galaxy. This supports a picture in which all elliptical galaxies harbor black holes which can be actively accreting for some fraction of their lifetime.

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arXiv1999

Properties of High Redshift Galaxies

I review the characteristics of high redshift galaxies, with particular attention to the effects of dust obscuration on the observed light. Galaxies at redshift z~1 and at z>2 are discussed separately, as the accessible information for each redshift range are different. In the z<=1-2 redshift region, data on the H-alpha and mid/far-IR luminosity of the galaxies are becoming increasingly available, effectively complementing the restframe UV data; they offer a better handle on the amount of dust obscuration present in the galaxies, although some ambiguity remains as to the distribution of the dust within the system and the opacity correction factor this implies. At higher redshifts, most of the available data cover the far-UV region only, representing a challenge for a careful assessment of the effects of dust in galaxies. Finally, I discuss the implications of the dust corrections to the star formation rate density of the Universe as a function of redshift.

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arXiv1999

Dust Absorption And The Ultraviolet Luminosity Density At z \approx 3 As Calibrated By Local Starburst Galaxies

We refine a technique to measure the absorption corrected ultraviolet (UV) luminosity of starburst galaxies using rest frame UV quantities alone, and apply it to U-dropouts at z ~ 3 found in the Hubble Deep Field. The method is based on an observed correlation between the ratio of far infrared (FIR) to UV fluxes with spectral slope β(a UV color). A simple fit to this relation allows the UV flux absorbed by dust to be calculated, and hence the dust-free UV luminosity to be determined. IUE spectra and IRAS fluxes of local starbursts are used to calibrate the ratio in terms of 1600 Angstrom absorption A_{1600}, and the transformation from broad band color to β. We show that the recent marginal and non-detections of HDF U-dropouts in the radio and sub-mm are consistent with our calculated A_{1600}. This is also true of observations of the ratio of optical emission line flux to UV flux density in the brightest U-dropouts. This latter ratio turns out not to be a good indicator of dust extinction. In U-dropouts, absolute magnitude correlates with β: brighter galaxies are redder, as is observed in local starburst galaxies. This suggests that a mass-metallicity relationship is in place by z ~ 3. The UV luminosity function of U-dropouts extends up to M_{1600,0} ~ -24 ABmag corresponding to a star formation rate ~ 200 \Msun/yr (H_0 = 50 km/s/Mpc, q_0 = 0.5). The absorption-corrected UV luminosity density at z ~ 3 is ρ_{1600,0} => 1.4x10^{27} erg/s/Hz/Mpc^3; still a lower limit since completeness corrections have not been done and because only galaxies with A_{1600} < ~3.6 mag are blue enough to be selected as U-dropouts. The luminosity weighted mean dust absorption factor of our sample is 5.4 +/- 0.9$ at 1600Å(abridged).

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arXiv1999

UV Emission and Dust Properties of High Redshift Galaxies

The high-redshift (z>2) galaxies discovered over the last few years with the Lyman-break technique represent, in number density, a major fraction of the galaxies known in the Local Universe. Thus, understanding the properties and the nature of these high-redshift systems is instrumental to our understanding of the cosmic evolution of galaxies and their stellar content. I briefly review the observed characteristics of the Lyman-break galaxies, relate these galaxies to their most likely low-redshift counterparts, and discuss the implications of dust obscuration on the global properties of the Lyman-break population. Finally, the observational properties of the high-redshift population are set in the framework of a simple evolutionary model for the stellar, metal and dust content of galaxies, to derive the intrinsic star formation history of the Universe.

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