arXiv subjects

Timothy M. Heckman

Publications and source records attributed to Timothy M. Heckman.

179 records · Page 10Linked to original sources

arXiv2000

Absorption-Line Probes of Gas and Dust in Galactic Superwinds

We discuss moderate resolution spectra of the NaD absorption-line in a sample of 32 far-IR-bright starburst galaxies. In 18 cases, the line is produced primarily by interstellar gas, and in 12 of these it is blueshifted by over 100 km/s relative to the galaxy systemic velocity. The absorption-line profiles in these outflow sources span the range from near the galaxy systemic velocity to a maximum blueshift of 400 to 600 km/s. The outflows occur in galaxies systematically viewed more nearly face-on than the others. We therefore argue that the absorbing material consists of ambient interstellar gas accelerated along the minor axis of the galaxy by a hot starburst-driven superwind. The NaD lines are optically-thick, but indirect arguments imply total Hydrogen column densities of N_H = few X 10^{21} cm^{-2}. This implies that the superwind is expelling matter at a rate comparable to the star-formation rate. This outflowing material is very dusty: we find a strong correlation between the depth of the NaD profile and the line-of-sight reddening (E(B-V) = 0.3 to 1 over regions several-to-ten kpc in size). The estimated terminal velocities of superwinds inferred from these data and extant X-ray data are typically 400 to 800 km/s, are independent of the galaxy rotation speed, and are comparable to (substantially exceed) the escape velocities for $L_*$ (dwarf) galaxies. The resulting loss of metals can establish the mass-metallicity relation in spheroids, produce the observed metallicity in the ICM, and enrich a general IGM to 10$^{-1}$ solar metallicity. If the outflowing dust grains survive their journey into the IGM, their effect on observations of cosmologically-distant objects is significant.

astro-ph↗

arXiv2000

The Detection of the Diffuse Interstellar Bands in Dusty Starburst Galaxies

We report the detection of the Diffuse Interstellar Bands (``$DIBs$'') in the optical spectra of seven far-infrared-selected starburst galaxies. The $λ$6283.9 Åand $λ$ 5780.5 Åfeatures are detected with equivalent widths of $\sim$ 0.4 to 1 Åand 0.1 to 0.6 Årespectively. In the two starbursts with the highest quality spectra (M82 and NGC2146), four other weaker $DIBs$ at $λ$ 5797.0 Å, 6010.1 Å, 6203.1 Å, and 6613.6 Åare detected with equivalent widths of $\sim$ 0.1 Å. The region over which the $DIBs$ can be detected ranges from $\sim$ 1 kpc in the less powerful starbursts, to several kpc in the more powerful ones. The gas producing the $DIBs$ is more kinematically quiescent on-average than the gas producing the strongly-blueshifted $NaIλλ$5890,5896 absorption in the same starbursts. We show that the $DIBs$ in these intense starbursts are remarkably similar to those in our Galaxy: the relative strengths of the features detected are similar, and the equivalent widths follow the same dependence as Galactic $DIBs$ on $E(B-V)$ and $NaI$ column density. While the ISM in starbursts is heated by a photon and cosmic ray bath that is $\sim$ 10$^3$ times more intense than in the diffuse ISM of the Milky Way, the gas densities and pressures are also correspondingly larger in starbursts. This ``homology'' may help explain the strikingly similar $DIB$ properties.

astro-ph↗

arXiv1999

The Role of Starbursts in the Formation of Galaxies & Active Galactic Nuclei

Starbursts are episodes of intense star-formation in the central regions of galaxies, and are the sites of roughly 25% of the high-mass star-formation in the local universe. In this contribution I review the role starbursts play in the formation and evolution of galaxies, the intergalactic medium, and active galactic nuclei. Four major conclusions are drawn. 1) Starburst galaxies are good analogues (in fact, the only plausible local analogues) to the known population of star-forming galaxies at high-redshift. 2) Integrated over cosmic time, supernova-driven galactic-winds (`superwinds') play an essential role in the evolution of galaxies and the inter-galactic medium. 3) Circumnuclear starbursts are an energetically-significant component of the Seyfert phenomenon. 4) The evolution of the population of the host galaxies of radio-quiet quasars is significantly different than that of powerful radio galaxies, and is at least qualitatively consistent with the standard picture of the hierarchical assembly of massive galaxies at relatively late times.

astro-ph↗

arXiv1999

B stars as a diagnostic of star-formation at low and high redshift

We have extended the evolutionary synthesis models by Leitherer et al. (1999b) by including a new library of B stars generated from the IUE high-dispersion spectra archive. We present the library and show how the stellar spectral properties vary according to luminosity classes and spectral types. We have generated synthetic UV spectra for prototypical young stellar populations varying the IMF and the star formation law. Clear signs of age effects are seen in all models. The contribution of B stars in the UV line spectrum is clearly detected, in particular for greater ages when O stars have evolved. With the addition of the new library we are able to investigate the fraction of stellar and interstellar contributions and the variation in the spectral shapes of intense lines. We have used our models to date the spectrum of the local super star cluster NGC1705-1. Photospheric lines of CIII1247, SiIII1417, and SV1502 were used as diagnostics to date the burst of NGC 1705-1 at 10 Myr. We have selected the star-forming galaxy 1512-cB58 as a first application of the new models to high-z galaxies. This galaxy is at z=2.723, it is gravitationally lensed, and its high signal-to-noise Keck spectrum show features typical of local starburst galaxies, such as NGC 1705-1. Models with continuous star formation were found to be more adequate for 1512-cB58 since there are spectral features typical of a composite stellar population of O and B stars. A model with Z =0.4Z_solar and an IMF with alpha=2.8 reproduces the stellar features of the 1512-cB58 spectrum.

astro-ph↗

arXiv1999

HST Imaging of the Host Galaxies of High Redshift Radio-Loud Quasars

We present rest-frame UV and Ly-alpha images of spatially-resolved structures around five high-redshift radio-loud quasars obtained with the WFPC2 camera on the Hubble Space Telescope. We find that all five quasars are extended and this "fuzz" contains ~5-40% of the total continuum flux and 15-65% of the Ly-alpha flux within a radius of about 1.5 arcsec. The rest-frame UV luminosities of the hosts are log lambda P_lambda = 11.9 to 12.5 solar luminosities (assuming no internal dust extinction), comparable to the luminous radio galaxies at similar redshifts and a factor 10 higher than both radio-quiet field galaxies at z~2-3 and the most UV-luminous low redshift starburst galaxies. The Ly-alpha luminosities of the hosts are (in the log) approximately 44.3-44.9 erg/s which are also similar to the those of luminous high redshift radio galaxies and considerably larger than the Ly-alpha luminosities of high redshift field galaxies. To generate the Ly-alpha luminosities of the hosts would require roughly a few percent of the total observed ionizing luminosity of the quasar. We find good alignment between the extended Ly-alpha and the radio sources, strong evidence for jet-cloud interactions in two cases, again resembling radio galaxies, and what is possibly the most luminous radio-UV synchrotron jet in one of the hosts at z=2.110.

astro-ph↗

arXiv1999

The Taxonomy of Blue Amorphous Galaxies: II. Structure and Evolution

Dwarf galaxies play an important role in our understanding of galaxy formation and evolution, and starbursts are believed to strongly affect their structure and evolution. Here we present a study of 12 of the nearest dwarf galaxies thought to be undergoing starbursts as selected primarily by morphology. We show that these "blue amorphous galaxies" are not physically distinguishable from dwarfs selected as starbursting by other methods, such as blue compact dwarfs and HII galaxies. All these classes exhibit exponential surface brightness profiles in their outer regions but often have a central blue excess. Typically, these starbursting "cores" are young (0.01 - 0.1 Gyr) events compared to the older (1 - 10 Gyr) enveloping galaxy. The ratio of the core-to-envelope blue fluxes ranges from essentially zero to about two. These starbursts are therefore modest events involving only a few percent of the stellar mass. The envelopes have surface-brightnesses that are much higher than typical dwarf Irregular (dI) galaxies, so it is unlikely that there is a straightforward evolutionary relation between typical dIs and dwarf starburst galaxies. Instead BAGs may repeatedly cycle through starburst and quiescent phases, corresponding to the galaxies with strong and weak/absent cores respectively. Once BAGs use up the available gas (either through star-formation or galactic winds) so that star-formation is shut off, the faded remnants would strongly resemble dwarf elliptical (dE) galaxies. However, in the current cosmological epoch this is a slow process that is the aftermath of a series of many weak, recurring bursts. Present-day dEs must have experienced more rapid and intense evolution than this in the distant past. (Abridged)

astro-ph↗

arXiv1999

Setting the Stage: Ultraluminous Galaxies in a Cosmological Context

I will try to put the ultraluminous galaxy phenomenon into a broad cosmological context. Viewed from this perspective, the significance of ultraluminous galaxies and the `starburst vs. monster' debate becomes clear. Ultraluminous galaxies are fascinating in their own right, allow detailed study of the processes by which massive spheroids were built and the IGM was heated and polluted, and resemble the most luminous and dustiest galaxies at high-redshift. Ultraluminous galaxies were apparently far more common at z $\sim$ 3 than today. Recent inventories in the local universe of the cumulative effect of nuclear burning (metal production) and of monster-feeding (compact dark objects in galactic nuclei) imply that either stars or monsters could have generated the observed far-IR cosmic background. The starburst vs. monster debate has global, as well as local importance.

astro-ph↗

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).

astro-ph↗

arXiv1999

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman (1995). We have upgraded our code by implementing the latest set of stellar evolution models of the Geneva group and the model atmosphere grid compiled by Lejeune et al. (1997). Several predictions which were not included in the previous publication are shown here for the first time. The models are presented in a homogeneous way for five metallicities between Z = 0.040 and 0.001 and three choices of the initial mass function. The age coverage is 10^6 to 10^9 yr. We also show the spectral energy distributions which are used to compute colors and other quantities. The full data set is available for retrieval at http://www.stsci.edu/science/starburst99/. This website allows users to run specific models with non-standard parameters as well. We also make the source code available to the community.

astro-ph↗

arXiv1998

Calibrating UV Emissivity And Dust Absorption At z \approx 3

We detail a technique for estimating the UV extinction and luminosity of UV selected galaxies using UV quantities alone. The technique is based on a tight correlation between the ratios of far infrared (FIR) to UV flux ratios and UV color for a sample of local starbursts. A simple empirical fit to this correlation can be used to estimate UV extinction as a function of color. This method is applied to a sample of Lyman-break systems selected from the HDF and having $z \approx 3$. The resultant UV emissivity is at least nine times higher than the original Madau et al. (1996) estimate. This technique can be readily applied to other rest-frame UV surveys.

astro-ph↗

arXiv1998

Towards a Unified Model for the `Diffuse Ionized Medium' in Normal and Starburst Galaxies

We analyze H$α$ images and long-slit spectra of samples of normal and starburst galaxies to better understand the nature of the diffuse, low-surface-brightness gas in these galaxies. We find that in both samples there is a strong inverse correlation between the H$α$ surface-brightness ($Σ_{Hα}$) and the [SII]/H$α$ line ratio at a given location in the galaxy. However, the correlation for the starbursts is offset brightward by an order-of-magnitude in H$α$ surface-brightness at a given line ratio. In contrast, we find that all the galaxies (starburst and normal alike) define a universal relation between line ratio and the relative H$α$ surface brightness ($Σ_{Hα}/Σ_e$, where $Σ_e$ is the mean H$α$ surface brightness within the galaxy half-light radius). We show that such a universal correlation is a natural outcome of a model in which the DIM is photoionized gas that has a characteristic thermal pressure ($P$) that is proportional to the mean rate of star-formation per unit area in the galaxy ($Σ_{SFR}$). Good quantitative agreement with the data follows if we require the constant of proportionality to be consistent with the values of $P$ and $Σ_{SFR}$ in the local disk of the Milky Way. Such a scaling between $P$ and $Σ_{SFR}$ may arise either because feedback from massive stars heats the ISM or because $Σ_{SFR}$ is determined (or limited) by the mean gas pressure.

astro-ph↗

arXiv1998

The Ultraviolet Spectroscopic Properties of Local Starbursts: Implications at High-Redshift

We report the results of a systematic study of the vacuum-ultraviolet spectra of 45 starburst and related galaxies observed with the IUE satellite. These span broad ranges in metallicity, bolometric luminosity, and galaxy properties. The projected size of the IUE spectroscopic aperture is typically one to several kpc and therefore usually encompasses the entire starburst and is similar to the aperture-sizes used for spectroscopy of high-redshift galaxies. Our principal conclusion is that local starbursts occupy a very small fractional volume in the multi-dimensional manifold defined by such fundamental parameters as the extinction, metallicity, and vacuum-UV line strengths (both stellar and interstellar) of the starburst and the rotation speed (mass) and absolute magnitude of the starburst's `host' galaxy. More metal-rich starbursts are redder and more heavily extinguished in the UV, more luminous, have stronger vacuum-UV lines, and occur in more massive and optically-brighter host galaxies. The results on local starbursts suggest that the high-redshift `Lyman Drop-Out' galaxies are typically highly reddened and extinguished by dust (average factor of 5 to 10 in the UV), may have moderately high metallicities (0.1 to 1 times solar?), are probably building galaxies with stellar surface- mass-densities similar to present-day ellipticals, and may be suffering substantial losses of metal-enriched gas that can `pollute' the IGM.

astro-ph↗

arXiv1998

Starbursts and the High-Redshift Universe

Starbursts are important because they can serve as local analogs of the processes that were important in the origin and early evolution of galaxies and in the heating and chemical enrichment of the inter-galactic medium. They may also play an important role in the AGN phenomenon. In this contribution I review the use of UV spectroscopic diagnostics that can be `calibrated' at low-redshift and then applied at high redshift. From the analysis of the UV properties of local starbursts we have learned: 1) dust dramatically affects our view of high-mass star-formation 2) more metal-rich starbursts are redder and more heavily extincted in the UV, more luminous, have stronger vacuum- UV absorption-lines, and occur in more massive and optically-brighter host galaxies 3) the strong interstellar absorption-lines directly reflect the hydrodynamical consequences of the starburst. These results suggest that the high-redshift `Lyman Drop-Out' galaxies are are typically highly extincted by dust (average factor of 5 to 10), may have moderately high metallicities (0.1 to 1 times solar?), are probably building galaxies with stellar surface-mass-densities similar to present-day ellipticals, and may be suffering substantial losses of metal-enriched gas that can `pollute' the inter-galactic medium. I also discuss UV observations of the nuclei of type 2 Seyfert galaxies. These show that compact (100-pc-scale) heavily-reddened starbursts are the source of most of the `featureless continuum' in UV-bright Seyfert 2 nuclei, and are an energetically significant component in these objects.

astro-ph↗

arXiv1997

Starbursts: Lessons for the Origin and Evolution of Galaxies and the Inter-Galactic Medium

Starbursts are episodes of intense star-formation that occur in the central regions of galaxies, and dominate the integrated emission from the galaxy. They are a significant component of the present- day universe, being the site of $\sim$ 25% of the high-mass star- formation. They offer unique `laboratories' for testing our ideas about star-formation, the evolution of high-mass stars, and the physics of the interstellar medium. They serve as local analogs of the processes that were important in the origin and early evolution of galaxies and in the heating and chemical enrichment of the inter-galactic medium. In this contribution I review starbursts from this broad cosmogonical perspective, stressing several key lessons we have learned from starbursts: 1) Violent, transient events play a significant role in the origin and evolution of galaxies. 2) Galaxies do not evolve as `Island Universes': starbursts are triggered by galaxy interactions and produce outflows of hot chemically-enriched gas that `pollute' the inter- galactic medium. 3) Dust dramatically affects of view of high-mass star-formation in starbursts and (probably) in high-redshift galaxies. Throughout this review I emphasize the importance of space-based observations in understanding starbursts.

astro-ph↗

arXiv1997

On the Dynamical and Physical State of the `Diffuse Ionized Medium' in Nearby Spiral Galaxies

We have analyzed deep narrow-band H$α$ images and high-resolution long-slit spectra for a sample of the nearest and brightest late-type galaxies to study the morphology, physical state, and kinematics of the `Diffuse Ionized Medium' (`DIM'). We find that the DIM covers most of the star-forming disk, and is morphologically related to the presence of the giant HII regions. In addition, the DIM and the giant HII regions differ systematically in their physical and dynamical state. The DIM is characterized by enhanced emission in the low-ionization forbidden lines ([OI], [NII], and [SII]), and even the high-ionization [OIII]$λ$5007 line is moderately strong in the DIM. We verify the inference made by Lehnert & Heckman that the DIM contributes significantly to the global emission-line ratios measured in late-type galaxies. We also find that the DIM is more disturbed kinematically than the gas in the giant HII regions. The intrinsic FWHMs of the H$α$ and [NII]$λ$6584 lines range from 30 to 100 km s$^{-1}$ in the DIM compared to 20-50 km s$^{-1}$ in HII regions. The high-ionization gas in the DIM is even more kinematically disturbed than the low-ionization gas: the [OIII]$λ$5007 lines have intrinsic FWHMs of 70-150 km s$^{-1}$. The differing kinematics implies that `the DIM' is not a single monolithic phase of the ISM. Instead, it may consist of a `quiescent DIM' with a low ionization-state and small scale-height (few hundred pc) and a `disturbed DIM' with a high ionization state and moderate scale-height (0.5 to 1 kpc). We argue that the quiescent DIM is most likely photoionized by radiation leaking out of giant HII regions, while the disturbed DIM is most likely heated by the mechanical energy supplied by supernovae and stellar winds.

astro-ph↗

arXiv1997

The Nuclear Ionized Gas in the Radio Galaxy M84 (NGC 4374)

We present optical images of the nucleus of the nearby radio galaxy M84 (NGC 4374 = 3C272.1) obtained with the Wide Field/Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). Our three images cover the H$α$ + [N II] emission lines as well as the V and I continuum bands. Analysis of these images confirms that the H$α$ + [N II] emission in the central 5'' (410 pc) is elongated along position angle (P.A.) $\approx 72\arcdeg$, which is roughly parallel to two nuclear dust lanes.Our high-resolution images reveal that the H$α$ + [N II] emission has three components, namely a nuclear gas disk,an `ionization cone', and outer filaments. The nuclear disk of ionized gas has diameter $\approx 1'' = 82$ pc and major axis P.A. $\approx 58\arcdeg \pm 6\arcdeg$. On an angular scale of $0\farcs5$, the major axis of this nuclear gas disk is consistent with that of the dust. However, the minor axis of the gas disk (P.A. $\approx 148\arcdeg$) is tilted with respect to that of the filamentary H$α$ + [N II] emission at distances > 2'' from the nucleus; the minor axis of this larger scale gas is roughly aligned with the axis of the kpc-scale radio jets (P.A. $\approx 170\arcdeg$). The ionization cone (whose apex is offset by $\approx 0\farcs3$ south of the nucleus) extends 2'' from the nucleus along the axis of the southern radio jet. This feature is similar to the ionization cones seen in some Seyfert nuclei, which are also aligned with the radio axes.

astro-ph↗

arXiv1997

The Panchromatic Starburst Intensity Limit At Low And High Redshift

The integrated bolometric effective surface brightness S_e distributions of starbursts are investigated for samples observed in 1. the rest frame ultraviolet (UV), 2. the far-infrared and H-alpha, and 3. 21cm radio continuum emission. For the UV sample we exploit a tight empirical relationship between UV reddening and extinction to recover the bolometric flux. Parameterizing the S_e upper limit by the 90th percentile of the distribution, we find a mean S_{e,90} = 2.0e11 L_{sun}/kpc^2 for the three samples, with a factor of three difference between the samples. This is consistent with what is expected from the calibration uncertainties alone. We find little variation in S_{e,90} with effective radii for R_e ~ 0.1 - 10 kpc, and little evolution out to redshifts z ~ 3. The lack of a strong dependence of S_{e,90} on wavelength, and its consistency with the pressure measured in strong galactic winds, argue that it corresponds to a global star formation intensity limit (\dotΣ_{e,90} ~ 45 M_{sun}/kpc^2/yr) rather than being an opacity effect. There are several important implications of these results: 1. There is a robust physical mechanism limiting starburst intensity. We note that starbursts have S_e consistent with the expectations of gravitational instability models applied to the solid body rotation portion of galaxies. 2. Elliptical galaxies and spiral bulges can plausibly be built with maximum intensity bursts, while normal spiral disks can not. 3. The UV extinction of high-z galaxies is significant, implying that star formation in the early universe is moderately obscured. After correcting for extinction, the observed metal production rate at z ~ 3 agrees well with independent estimates made for the epoch of elliptical galaxy formation.

astro-ph↗