arXiv subjects

Daniela Calzetti

Publications and source records attributed to Daniela Calzetti.

At least 91 records · Page 5Linked to original sources

arXiv2015

Radial Star Formation Histories in Fifteen Nearby Galaxies

New deep optical and near-infrared imaging is combined with archival ultraviolet and infrared data for fifteen nearby galaxies mapped in the Spitzer Extended Disk Galaxy Exploration Science survey. These images are particularly deep and thus excellent for studying the low surface brightness outskirts of these disk-dominated galaxies with stellar masses ranging between 10^8 and 10^11 Msun. The spectral energy distributions derived from this dataset are modeled to investigate the radial variations in the galaxy colors and star formation histories. Taken as a whole, the sample shows bluer and younger stars for larger radii until reversing near the optical radius, whereafter the trend is for redder and older stars for larger galacto-centric distances. These results are consistent with an inside-out disk formation scenario coupled with an old stellar outer disk population formed through radial migration and/or the cumulative history of minor mergers and accretions of satellite dwarf galaxies. However, these trends are quite modest and the variation from galaxy to galaxy is substantial. Additional data for a larger sample of galaxies are needed to confirm or dismiss these modest sample-wide trends.

astro-ph.GA↗

arXiv2015

Resolving Ionisation and Metallicity on Parsec Scales Across Mrk 71 with HST-WFC3

Blue Compact Dwarf (BCD) Galaxies in the nearby Universe provide a means for studying feedback mechanisms and star-formation processes in low-metallicity environments in great detail. Due to their vicinity, these local analogues to young galaxies are well suited for high-resolution studies that would be unfeasible for primordial galaxies in the high-redshift universe. Here we present HST-WFC3 observations of one such BCD, Mrk 71, one of the most powerful local starbursts known, in the light of [O II], He II, Hb, [O III], Ha, and [S II]. At D=3.44 Mpc, this extensive suite of emission line images enables us to explore the chemical and physical conditions of Mrk 71 on ~2 pc scales. Using these high spatial-resolution observations, we use emission line diagnostics to distinguish ionisation mechanisms on a pixel-by-pixel basis and show that despite the previously reported hypersonic gas and super-bubble blow out, the gas in Mrk 71 is photoionised, with no sign of shock-excited emission. Using strong-line metallicity diagnostics, we present the first 'metallicity image' of a galaxy, revealing chemically inhomogeneity on scales of <50 pc. We additionally demonstrate that while chemical structure can be lost at large spatial scales, metallicity-diagnostics can break down on spatial scales smaller than a HII region. HeII emission line images are used to identify up to six Wolf-Rayet stars in Mrk 71, three of which lie on the edge of blow-out region. This study not only demonstrates the benefits of high-resolution spatially-resolved observations in assessing the effects of feedback mechanisms, but also the limitations of fine spatial scales when employing emission-line diagnostics. Both aspects are especially relevant as we enter the era of extremely large telescopes, when observing structure on ~10 pc scales will no longer be limited to the local universe.

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arXiv2015

Star Cluster Properties in Two LEGUS Galaxies Computed with Stochastic Stellar Population Synthesis Models

We investigate a novel Bayesian analysis method, based on the Stochastically Lighting Up Galaxies (slug) code, to derive the masses, ages, and extinctions of star clusters from integrated light photometry. Unlike many analysis methods, slug correctly accounts for incomplete IMF sampling, and returns full posterior probability distributions rather than simply probability maxima. We apply our technique to 621 visually-confirmed clusters in two nearby galaxies, NGC 628 and NGC 7793, that are part of the Legacy Extragalactic UV Survey (LEGUS). LEGUS provides Hubble Space Telescope photometry in the NUV, U, B, V, and I bands. We analyze the sensitivity of the derived cluster properties to choices of prior probability distribution, evolutionary tracks, IMF, metallicity, treatment of nebular emission, and extinction curve. We find that slug's results for individual clusters are insensitive to most of these choices, but that the posterior probability distributions we derive are often quite broad, and sometimes multi-peaked and quite sensitive to the choice of priors. In contrast, the properties of the cluster population as a whole are relatively robust against all of these choices. We also compare our results from slug to those derived with a conventional non-stochastic fitting code, Yggdrasil. We show that slug's stochastic models are generally a better fit to the observations than the deterministic ones used by Yggdrasil. However, the overall properties of the cluster populations recovered by both codes are qualitatively similar.

astro-ph.GA↗

arXiv2015

Supernova-Driven Outflows in NGC 7552: A Comparison of H-alpha and UV Tracers

We investigate the supernova-driven galactic wind of the barred spiral galaxy NGC 7552, using both ground-based optical nebular emission lines and far-ultraviolet absorption lines measured with the Hubble Space Telescope Cosmic Origins Spectrograph. We detect broad (~300 km/s) blueshifted (-40 km/s) optical emission lines associated with the galaxy's kpc-scale star-forming ring. The broad line kinematics and diagnostic line ratios suggest that the H-alpha emission comes from clouds of high density gas entrained in a turbulent outflow. We compare the H-alpha emission line profile to the UV absorption line profile measured along a coincident sight line and find significant differences. The maximum blueshift of the H-alpha-emitting gas is ~290 km/s, whereas the UV line profile extends to blueshifts upwards of 1000 km/s. The mass outflow rate estimated from the UV is roughly nine times greater than that estimated from H-alpha. We argue that the H-alpha emission traces a cluster-scale outflow of dense, low velocity gas at the base of the large-scale wind. We suggest that UV absorption line measurements are therefore more reliable tracers of warm gas in starburst-driven outflows.

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arXiv2015

Hierarchical Star Formation across the ring galaxy NGC 6503

We present a detailed clustering analysis of the young stellar population across the star-forming ring galaxy NGC 6503, based on the deep HST photometry obtained with the Legacy ExtraGalactic UV Survey (LEGUS). We apply a contour-based map analysis technique and identify in the stellar surface density map 244 distinct star-forming structures at various levels of significance. These stellar complexes are found to be organized in a hierarchical fashion with 95% being members of three dominant super-structures located along the star-forming ring. The size distribution of the identified structures and the correlation between their radii and numbers of stellar members show power-law behaviors, as expected from scale-free processes. The self-similar distribution of young stars is further quantified from their autocorrelation function, with a fractal dimension of ~1.7 for length-scales between ~20 pc and 2.5 kpc. The young stellar radial distribution sets the extent of the star-forming ring at radial distances between 1 and 2.5 kpc. About 60% of the young stars belong to the detected stellar structures, while the remaining stars are distributed among the complexes, still inside the ring of the galaxy. The analysis of the time-dependent clustering of young populations shows a significant change from a more clustered to a more distributed behavior in a time-scale of ~60 Myr. The observed hierarchy in stellar clustering is consistent with star formation being regulated by turbulence across the ring. The rotational velocity difference between the edges of the ring suggests shear as the driving mechanism for this process. Our findings reveal the interesting case of an inner ring forming stars in a hierarchical fashion.

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arXiv2015

HST/WFC3 Observations of an Off-Nuclear Superbubble in Arp 220

We present a high spatial resolution optical and infrared study of the circumnuclear region in Arp 220, a late-stage galaxy merger. Narrowband imaging using HST/WFC3 has resolved the previously observed peak in H$α$+[NII] emission into a bubble-shaped feature. This feature measures 1.6" in diameter, or 600 pc, and is only 1" northwest of the western nucleus. The bubble is aligned with the western nucleus and the large-scale outflow axis seen in X-rays. We explore several possibilities for the bubble origin, including a jet or outflow from a hidden active galactic nucleus (AGN), outflows from high levels of star formation within the few hundred pc nuclear gas disk, or an ultraluminous X-ray source. An obscured AGN or high levels of star formation within the inner $\sim$100 pc of the nuclei are favored based on the alignment of the bubble and energetics arguments.

astro-ph.GA↗

arXiv2015

Origin of the Diffuse, Far Ultraviolet Emission in the Interarm Regions of M101

We present images from the Solar Blind Channel on HST that resolve hundreds of far ultraviolet (FUV) emitting stars in two ~1 kpc$^2$ interarm regions of the grand-design spiral M101. The luminosity functions of these stars are compared with predicted distributions from simple star formation histories, and are best reproduced when the star formation rate has declined recently (past 10-50 Myr). This pattern is consistent with stars forming within spiral arms and then streaming into the interarm regions. We measure the diffuse FUV surface brightness after subtracting all of the detected stars, clusters and background galaxies. A residual flux is found for both regions which can be explained by a mix of stars below our detection limit and scattered FUV light. The amount of scattered light required is much larger for the region immediately adjacent to a spiral arm, a bright source of FUV photons.

astro-ph.GA↗

arXiv2015

LEGUS Discovery of a Light Echo Around Supernova 2012aw

We have discovered a luminous light echo around the normal Type II-Plateau Supernova (SN) 2012aw in Messier 95 (M95; NGC 3351), detected in images obtained approximately two years after explosion with the Wide Field Channel 3 on-board the Hubble Space Telescope (HST) by the Legacy ExtraGalactic Ultraviolet Survey (LEGUS). The multi-band observations span from the near-ultraviolet through the optical (F275W, F336W, F438W, F555W, and F814W). The apparent brightness of the echo at the time was ~21--22 mag in all of these bands. The echo appears circular, although less obviously as a ring, with an inhomogeneous surface brightness, in particular, a prominent enhanced brightness to the southeast. The SN itself was still detectable, particularly in the redder bands. We are able to model the light echo as the time-integrated SN light scattered off of diffuse interstellar dust in the SN environment. We have assumed that this dust is analogous to that in the Milky Way with R_V=3.1. The SN light curves that we consider also include models of the unobserved early burst of light from the SN shock breakout. Our analysis of the echo suggests that the distance from the SN to the scattering dust elements along the echo is ~45 pc. The implied visual extinction for the echo-producing dust is consistent with estimates made previously from the SN itself. Finally, our estimate of the SN brightness in F814W is fainter than that measured for the red supergiant star at the precise SN location in pre-SN images, possibly indicating that the star has vanished and confirming it as the likely SN progenitor.

astro-ph.SR↗

arXiv2014

Dust continuum emission as a tracer of gas mass in galaxies

We use a sample of 36 galaxies from the KINGFISH (Herschel IR), HERACLES (IRAM CO), and THINGS (VLA HI) surveys to study empirical relations between Herschel infrared (IR) luminosities and the total mass of the interstellar gas (H2+HI). Such a comparison provides a simple empirical relationship without introducing the uncertainty of dust model fitting. We find tight correlations, and provide fits to these relations, between Herschel luminosities and the total gas mass integrated over entire galaxies, with the tightest, almost linear, correlation found for the longest wavelength data (SPIRE500). However, we find that accounting for the gas-phase metallicity (affecting the dust-to-gas ratio) is crucial when applying these relations to low-mass, and presumably high-redshift, galaxies. The molecular (H2) gas mass is found to be better correlated with the peak of the IR emission (e.g. PACS160), driven mostly by the correlation of stellar mass and mean dust temperature. When examining these relations as a function of galactocentric radius we find the same correlations, albeit with a larger scatter, up to a radius of 0.7 r_25 (within which most of the galaxy's baryonic mass resides). However, beyond this radius the same correlations no longer hold, with the gas mass (predominantly HI) increasing relative to the infrared emission. The tight relations found for the bulk of the galaxy's baryonic content suggest that the total gas masses of disk-like (non-merging) galaxies can be inferred from far-infrared continuum measurements in situations where only the latter are available, e.g. in ALMA continuum observations of high-redshift galaxies.

astro-ph.GA↗

arXiv2014

Early Science with the Large Millimeter Telescope: Exploring the Effect of AGN Activity on the Relationships Between Molecular Gas, Dust, and Star Formation

The molecular gas, H$_2$, that fuels star formation in galaxies is difficult to observe directly. As such, the ratio of $L_{\rm IR}$ to $L^\prime_{\rm CO}$ is an observational estimation of the star formation rate compared with the amount of molecular gas available to form stars, which is related to the star formation efficiency and the inverse of the gas consumption timescale. We test what effect an IR luminous AGN has on the ratio $L_{\rm IR}/L^\prime_{\rm CO}$ in a sample of 24 intermediate redshift galaxies from the 5 mJy Unbiased Spitzer Extragalactic Survey (5MUSES). We obtain new CO(1-0) observations with the Redshift Search Receiver on the Large Millimeter Telescope. We diagnose the presence and strength of an AGN using Spitzer IRS spectroscopy. We find that removing the AGN contribution to $L_{\rm IR}^{\rm tot}$ results in a mean $L_{\rm IR}^{\rm SF}/L^\prime_{\rm CO}$ for our entire sample consistent with the mean $L_{\rm IR}/L^\prime_{\rm CO}$ derived for a large sample of star forming galaxies from $z\sim0-3$. We also include in our comparison the relative amount of polycyclic aromatic hydrocarbon emission for our sample and a literature sample of local and high redshift Ultra Luminous Infrared Galaxies and find a consistent trend between $L_{6.2}/L_{\rm IR}^{\rm SF}$ and $L_{\rm IR}^{\rm SF}/L^\prime_{\rm CO}$, such that small dust grain emission decreases with increasing $L_{\rm IR}^{\rm SF}/L^\prime_{\rm CO}$ for both local and high redshift dusty galaxies.

astro-ph.GA↗

arXiv2014

Spitzer Local Volume Legacy (LVL) SEDs and Physical Properties

We present the panchromatic spectral energy distributions (SEDs) of the Local Volume Legacy (LVL) survey which consists of 258 nearby galaxies ($D<$11 Mpc). The wavelength coverage spans the ultraviolet to the infrared (1500 $\textrmÅ$ to 24 $μ$m) which is utilized to derive global physical properties (i.e., star formation rate, stellar mass, internal extinction due to dust.). With these data, we find color-color relationships and correlated trends between observed and physical properties (i.e., optical magnitudes and dust properties, optical color and specific star formation rate, and ultraviolet-infrared color and metallicity). The SEDs are binned by different galaxy properties to reveal how each property affects the observed shape of these SEDs. In addition, due to the volume-limited nature of LVL, we utilize the dwarf-dominated galaxy sample to test star formation relationships established with higher-mass galaxy samples. We find good agreement with the star-forming "main-sequence" relationship, but find a systematic deviation in the infrared "main-sequence" at low luminosities. This deviation is attributed to suppressed polycyclic aromatic hydrocarbon (PAH) formation in low metallicity environments and/or the destruction of PAHs in more intense radiation fields occurring near a suggested threshold in sSFR at a value of log($sSFR$) $\sim$ $-$10.2.

astro-ph.GA↗

arXiv2014

The Spitzer Local Volume Legacy (LVL) Global Optical Photometry

We present the global optical photometry of 246 galaxies in the Local Volume Legacy (LVL) survey. The full volume-limited sample consists of 258 nearby (D < 11 Mpc) galaxies whose absolute B-band magnitude span a range of -9.6 < M_B < -20.7 mag. A composite optical (UBVR) data set is constructed from observed UBVR and SDSS ugriz imaging, where the ugriz magnitudes are transformed into UBVR. We present photometry within three galaxy apertures defined at UV, optical, and IR wavelengths. Flux comparisons between these apertures reveal that the traditional optical R25 galaxy apertures do not fully encompass extended sources. Using the larger IR apertures we find color-color relationships where later-type spiral and irregular galaxies tend to be bluer than earlier-type galaxies. These data provide the missing optical emission from which future LVL studies can construct the full panchromatic (UV-optical-IR) spectral energy distributions.

astro-ph.GA↗

arXiv2014

Empirical ugri-UBVRc Transformations for Galaxies

We present empirical color transformations between Sloan Digital Sky Survey ugri and Johnson-Cousins UBVRc photometry for nearby galaxies (D < 11 Mpc). We use the Local Volume Legacy (LVL) galaxy sample where there are 90 galaxies with overlapping observational coverage for these two filter sets. The LVL galaxy sample consists of normal, non-starbursting galaxies. We also examine how well the LVL galaxy colors are described by previous transformations derived from standard calibration stars and model-based galaxy templates. We find significant galaxy color scatter around most of the previous transformation relationships. In addition, the previous transformations show systematic offsets between transformed and observed galaxy colors which are visible in observed color-color trends. The LVL-based $galaxy$ transformations show no systematic color offsets and reproduce the observed color-color galaxy trends.

astro-ph.GA↗

arXiv2014

Star Cluster Mass and Age Distributions of Two Fields in M83 Based on HST/WFC3 Observations

We study star clusters in two fields in the nearby spiral galaxy M83 using broad and narrow band optical imaging taken with the WFC3 on-board HST. We present results based on several different catalogs of star clusters in an inner and outer field, and conclude that different methods of selection do not strongly impact the results, particularly for clusters older than $\approx$10 Myr. The age distributions can be described by a power law, $dN/dτ\proptoτ^γ$, with $γ\approx -$0.84$\pm$0.12 in the inner field, and $γ\approx -$0.48$\pm$0.12 in the outer field for $τ\gtrsim$10 Myr. We bracket the difference, $Δγ$, between the two fields to be in the range 0.18$-$0.36, based on estimates of the relative star formation histories. The mass functions can also be described by a power law, $dN/dM\propto M^β$, with $β\approx -$1.98$\pm$0.14 and $β\approx $2.34$\pm$0.26 in the inner and outer fields, respectively. We conclude that the shapes of the mass and age distributions of the clusters in the two fields are similar, as predicted by the "quasi-universal" model. Any differences between the two fields are at the $\approx$2$-$3$σ(\approx$1$-$2$σ)$ level for the age (mass) distributions. Therefore any dependence of these distributions on the local environment is probably weak. We compare the shapes of the distributions with those predicted by two popular cluster disruption models, and find that both show evidence that the clusters are disrupted at a rate that is approximately independent of their mass, but that the observational results do not support the earlier disruption of lower mass clusters relative to their higher mass counterparts.

astro-ph.GA↗

arXiv2014

Untangling the Nature of Spatial Variations of Cold Dust Properties in Star Forming Galaxies

We investigate the far-infrared (IR) dust emission for 20 local star forming galaxies from the Key Insights on Nearby Galaxies: A Far-IR Survey with Herschel (KINGFISH) sample. We model the far-IR/submillimeter spectral energy distribution (SED) using images from Spitzer Space Telescope and Herschel Space Observatory. We calculate the cold dust temperature (T(cold)) and emissivity (beta) on a pixel by pixel basis (where each pixel ranges from 0.1-3 kpc^2) using a two temperature modified blackbody fitting routine. Our fitting method allows us to investigate the resolved nature of temperature and emissivity variations by modeling from the galaxy centers to the outskirts (physical scales of ~15-50 kpc, depending on the size of the galaxy). We fit each SED in two ways: (1) fit T(cold) and beta simultaneously, (2) hold beta constant and fit T(cold). We compare T(cold) and beta with star formation rates (calculated from L(Halpha) and L(24)), the luminosity of the old stellar population (traced through L(3.6), and the dust mass surface density (traced by 500 micron luminosity, L(500)). We find a significant trend between SFR/L(500) and T(cold), implying that the flux of hard UV photons relative to the amount of dust is significantly contributing to the heating of the cold, or diffuse, dust component. We also see a trend between L(3.6)/L(500) and beta, indicating that the old stellar population contributes to the heating at far-IR/submillimeter wavelengths. Finally, we find that when beta is held constant, T(cold) exhibits a strongly decreasing radial trend, illustrating that the shape of the far-IR SED is changing radially through a galaxy, thus confirming on a sample almost double in size the trends observed in Galametz et al. (2012).

astro-ph.GA↗

arXiv2014

Hierarchical Star Formation in Nearby LEGUS Galaxies

Hierarchical structure in ultraviolet images of 12 late-type LEGUS galaxies is studied by determining the numbers and fluxes of nested regions as a function of size from ~1 to ~200 pc, and the number as a function of flux. Two starburst dwarfs, NGC 1705 and NGC 5253, have steeper number-size and flux-size distributions than the others, indicating high fractions of the projected areas filled with star formation. Nine subregions in 7 galaxies have similarly steep number-size slopes, even when the whole galaxies have shallower slopes. The results suggest that hierarchically structured star-forming regions several hundred parsecs or larger represent common unit structures. Small galaxies dominated by only a few of these units tend to be starbursts. The self-similarity of young stellar structures down to parsec scales suggests that star clusters form in the densest parts of a turbulent medium that also forms loose stellar groupings on larger scales. The presence of super star clusters in two of our starburst dwarfs would follow from the observed structure if cloud and stellar subregions more readily coalesce when self-gravity in the unit cell contributes more to the total gravitational potential.

astro-ph.GA↗

arXiv2013

Extinction and Dust Geometry in M83 HII Regions: A Hubble Space Telescope/WFC3 Study

We present HST/WFC3 narrow-band imaging of the starburst galaxy M83 targeting the hydrogen recombination lines (H$β$, H$α$ and Pa$β$), which we use to investigate the dust extinction in the HII regions. We derive extinction maps with 6 parsec spatial resolution from two combinations of hydrogen lines (H$α$/H$β$ and H$α$/Pa$β$), and show that the longer wavelengths probe larger optical depths, with $A_V$ values larger by $\gtrsim$1 mag than those derived from the shorter wavelengths. This difference leads to a factor $\gtrsim$2 discrepancy in the extinction-corrected H$α$ luminosity, a significant effect when studying extragalactic HII regions. By comparing these observations to a series of simple models, we conclude that a large diversity of absorber/emitter geometric configurations can account for the data, implying a more complex physical structure than the classical foreground "dust screen" assumption. However, most data points are bracketed by the foreground screen and a model where dust and emitters are uniformly mixed. When averaged over large ($\gtrsim$100--200 pc) scales, the extinction becomes consistent with a "dust screen", suggesting that other geometries tend to be restricted to more local scales. Moreover, the extinction in any region can be described by a combination of the foreground screen and the uniform mixture model with weights of 1/3 and 2/3 in the center ($\lesssim$2 kpc), respectively, and 2/3 and 1/3 for the rest of the disk. This simple prescription significantly improves the accuracy of the dust extinction corrections and can be especially useful for pixel-based analyses of galaxies similar to M83.

astro-ph.CO↗

arXiv2013

Quantitative Method for the Optimal Subtraction of Continuum Emission from Narrow-band Images : Skewness Transition Analysis

We present an objective method to remove the stellar continuum emission from narrow-band images to derive emission-line images. The method is based on the skewness of the pixel histogram of the residual images. Specifically, we exploit a transition in the skewness of the signal in the continuum-subtracted image, which appears when the image changes from being under-subtracted to over-subtracted. Tests on one-dimensional artificial images demonstrate that the transition identifies the optimal scaling factor μ to be used on the broad-band image IB in order to produce the optimal line-emission image IE, i.e., IE =IN - μ IB, with IN the original (un-subtracted) narrow-band image. The advantage of this method is that it uses all information-bearing pixels in the final image, and not just a sub-set of those pixels (the latter being common in many traditional approaches to stellar continuum removal from narrow-band images). We apply our method to actual images, both from ground-based and space facilities, in particular to WFPC2 and ACS images from the Hubble Space Telescope, and we show that it is successful irrespective of the nature of the sources (point-like or extended). We also discuss the impact on the accuracy of the method of non-optimal images, such as those containing saturated sources or non-uniform background, and present `workarounds' for those problems.

astro-ph.IM↗