Crystal L. Martin

Star Formation Thresholds in Galactic Disks

We report the first results of a detailed study of the star formation law in a sample of 32 nearby spiral galaxies with well-measured rotation curves, H I and H_2 (as traced by CO) surface density profiles, and new Hα CCD photometry. In this paper we present an atlas of Hα images and radial surface brightness profiles and describe a surface density threshold in the star formation law. Prominent breaks in the Hα surface brightness profiles are identified in nearly all of the actively star-forming disks, confirming previous claims of star formation thresholds based on lower quality data. We measure the ratio of the gas density to the critical density for local gravitational stability at the threshold radii. The outer threshold radii observed in Sab-Sdm galaxies are in general agreement with those expected from the Toomre Q stability criterion, confirming earlier work, but with a significant variation that appears to be weakly correlated with galaxy type. Such a trend could plausibly reflect variations in the relative contribution of the stellar disk to the instability of the gas disk across this range of galaxy types. Among disks with subcritical gas surface densities, and outside the threshold radius in star-forming disks, the number of isolated H II regions increases as the gas surface density approaches the critical density. At the thresholds, the gas surface densities span a wide range, and the atomic/molecular gas fraction is highest in the disks having the lowest total gas surface density. The simple Toomre condition fails to account for the active star formation in the inner disks of low-mass spirals such as NGC 2403 and M33. An alternative stability criterion based on the shear in the disk provides a better description of these disks but is a less accurate indicator of the outer edges of star-forming disks than the Toomre criterion. These results strongly support the view that the formation of gravitationally bound interstellar clouds regulates the onset of widespread star formation, at least in the outer regions of galactic disks.

Properties of Galactic Outflows: Measurements of the Feedback from Star Formation

Properties of starburst-driven outflows in dwarf galaxies are compared with those in more massive galaxies. Over a factor of ~10 in galactic rotation speed, supershells are shown to lift warm ionized gas out of the disk at rates up to several times the star formation rate. The amount of mass escaping the galactic potential, in contrast to the disk, does depend on the galactic mass. The temperature of the hottest extended X-ray emission shows little variation around ~106.7 K, and this gas has enough energy to escape from the galaxies with rotation speed less than approximately 130 km s-1.

The Metal Content of Dwarf Starburst Winds: Results from Chandra Observations of NGC 1569*

We present deep Chandra spectral imaging of the dwarf starburst galaxy NGC 1569. The unprecedented spatial resolution allows us to spatially identify the components of the integrated X-ray spectrum. Fitted spectral models require an intrinsic absorption component and higher metal abundances than previous studies indicated. Our results provide the first direct evidence for metal-enriched winds from dwarf starburst galaxies. We identify 14 X-ray point sources in NGC 1569. Most have properties consistent with those of high-mass X-ray binaries, but one is a steep-spectrum radio source that is probably a supernova remnant. The X-ray luminosity of NGC 1569 is dominated by diffuse, thermal emission from the disk (0.7 keV) and bipolar halo (0.3 keV). Photoelectric absorption from the inclined H I disk hardens the X-ray spectrum on the northern side of the disk relative to the southern side. Requiring the fitted absorption column to match the H I column measured at 21 cm implies that the metallicity of the H I disk is significantly less than solar but greater than 0.1 Z☉. Hence, much of the H I is enriched to levels comparable to the metallicity of the H II regions [O/H = 0.2(O/H)☉]. The X-ray color variations in the halo are inconsistent with a free-streaming wind and probably reveal the location of shocks created by the interaction of the wind with a gaseous halo. The X-ray spectrum of the diffuse gas presents strong emission lines from α-process elements. Fitted models require α-element abundances greater than 0.25 Zα, ☉ and ratios of α-elements to iron 2-4 times higher than the solar ratio. The best fit to the spectrum is obtained with solar mass fractions for the α-elements, 1.0 Zα, ☉, but a degeneracy between the metallicity and the spectral normalization prevents us from deriving an upper limit on the wind metallicity from the X-ray spectrum alone. We argue, however, that abundances larger than 2.0 Zα, ☉ pose awkward implications for the dynamical evolution of the wind based on our knowledge of the starburst properties. For consistency with our best-fitting abundances, the mass of interstellar gas entrained in the wind must be about 9 times the mass of stellar ejecta in the wind. Most of the oxygen carried by the wind comes from the stellar ejecta rather than entrained interstellar gas. The estimated mass of oxygen in the hot wind, 34,000 M☉, is similar to the oxygen yield of the current starburst. Apparently the wind carries nearly all the metals ejected by the starburst. These metals appear destined to contribute to the enrichment of the intergalactic medium. Much of the nucleosynthesis in NGC 1569 must have occurred during less violent periods of star formation, however, because our measurements imply that the neutral gas disk holds at least 5 times more oxygen than wind.

The Impact of Star Formation on the Interstellar Medium in Dwarf Galaxies. II. The Formation of Galactic Winds

Images and long-slit echelle spectra of the H? emission from 14 dwarf galaxies and M82 have been used to identify expanding shells of ionized gas. Supershells (radius >300 pc) are found in 12 of the dwarfs. The measured shell sizes and expansion speeds constrain the ages and power requirements of the bubbles. The dynamical age of the larger bubbles is typically about 10 Myr, and ionized shells older than 20 Myr are rare. An energy equivalent to 100-10,000 supernova explosions over this period is needed to drive the shock front that sweeps out the cavity. The current star formation rates are high enough to meet these power requirements. Many of the shells will break through the surrounding layer of H I supersonically, but the projected expansion speeds are typically less than the lower limits on the escape velocity. Some of the shell material may permanently escape from a few galaxies such as NGC 1569. Whether bound to the galaxy or not, these outflows probably play an important role in regulating the star formation rate and are expected to significantly influence the chemical evolution of the galaxies. The shells lift gas out of the disk at rates comparable to, or even greater than, the current galactic star formation rates. They will displace a substantial fraction of the interstellar gas only if their duty cycle is much longer than the rotational period of the disk.

The Impact of Star Formation on the Interstellar Medium. I. The Excitation of Diffuse, Warm Ionized Gas in Dwarf Galaxies*

I present measurements of optical emission-line ratios from long-slit spectra of 14 star-forming dwarf galaxies. All members of the sample have a prominent component of diffuse warm ionized gas (DIG). Spectra of the DIG are characterized by high [O I]/Hα intensity ratios in addition to high [O II]/[O III], [S II]/Hα, [N II]/Hα, and low [O III]/Hβ ratios relative to the H II regions. Measurements from ~350 spectra extracted at spatial intervals of ~20-150 pc show that this spectral change is gradual and continuous; hence, the boundary between H II regions and DIG is not well defined spectroscopically. In diagnostic line-ratio diagrams, the spectral transition advances along a narrow track that is distinctly different from the well-established H II region excitation sequence. The line ratios progress toward the region populated by weak-[O I] Low-Ionization Nuclear Emission-Line Regions. I show that this H II-DIG transition sequence is driven primarily by a decrease in the relative density of ionizing photons to atoms, i.e., the ionization parameter. The radial gradient in the ionization parameter is consistent with the dilution of radiation from a centralized source. This result suggests that the dominant excitation mechanism of the DIG is photoionization by massive stars associated with the main star-forming regions. In several of the galaxies, however, an additional excitation mechanism is required to explain the moderately high [O III]/Hβ at very high [S II]/Hα and the sharp increase in [O I]/Hα with only a moderate increase in [O II]/[O III]. I show that these line ratios can be explained by a contribution to the emission from shock-excited gas. The inferred shock speeds are between 60 km s-1 and 100 km s-1; the line ratios in the regions with the lowest surface brightness require the largest relative contribution from shocks, up to 30%-50% of the emission. The prevalence of a secondary emission component from shocked gas is difficult to quantify. Where the metallicity is very low and/or the shock speed is 60 km s-1, the line ratios alone cannot distinguish emission from shocked gas from that from dilute H II regions. Among the galaxies, differences in the DIG line ratios can be largely attributed to the galactic metallicity. At similar metallicity, however, spectral differences are related to the ionization parameter and ionization state of the parent H II regions. This result provides direct evidence that star formation affects the structure of the interstellar medium on a global scale. I find that much of the observed variation in H II region ionization parameters is caused by substantial differences in their filling factors of warm ionized clouds. Based on these results, I suggest that the DIG spectra reflect the porosity of the star-forming regions.

VERY STRONG EMISSION-LINE GALAXIES IN THE WFC3 INFRARED SPECTROSCOPIC PARALLEL SURVEY AND IMPLICATIONS FOR HIGH-REDSHIFT GALAXIES* , **

The WFC3 Infrared Spectroscopic Parallel Survey uses the Hubble Space Telescope (HST) infrared grism capabilities to obtain slitless spectra of thousands of galaxies over a wide redshift range including the peak of star formation history of the universe. We select a population of very strong emission-line galaxies with rest-frame equivalent widths (EWs) higher than 200 A. A total of 176 objects are found over the redshift range 0.35 < z < 2.3 in the 180 arcmin^2 area that we have analyzed so far. This population consists of young and low-mass starbursts with high specific star formation rates (sSFR). After spectroscopic follow-up of one of these galaxies with Keck/Low Resolution Imaging Spectrometer, we report the detection at z = 0.7 of an extremely metal-poor galaxy with 12 + log(O/H) =7.47 ± 0.11. After estimating the active galactic nucleus fraction in the sample, we show that the high-EW galaxies have higher sSFR than normal star-forming galaxies at any redshift. We find that the nebular emission lines can substantially affect the total broadband flux density with a median brightening of 0.3 mag, with some examples of line contamination producing brightening of up to 1 mag. We show that the presence of strong emission lines in low-z galaxies can mimic the color-selection criteria used in the z ~ 8 dropout surveys. In order to effectively remove low-redshift interlopers, deep optical imaging is needed, at least 1 mag deeper than the bands in which the objects are detected. Without deep optical data, most of the interlopers cannot be ruled out in the wide shallow HST imaging surveys. Finally, we empirically demonstrate that strong nebular lines can lead to an overestimation of the mass and the age of galaxies derived from fitting of their spectral energy distribution (SED). Without removing emission lines, the age and the stellar mass estimates are overestimated by a factor of 2 on average and up to a factor of 10 for the high-EW galaxies. Therefore, the contribution of emission lines should be systematically taken into account in SED fitting of star-forming galaxies at all redshifts.

Magnetic fields in starburst galaxies and the origin of the fir-radio correlation

We estimate minimum energy magnetic fields (Bmin) for a sample of galaxies with measured gas surface densities, spanning more than four orders of magnitude in surface density, from normal spirals to luminous starbursts. We show that the ratio of the minimum energy magnetic pressure to the total pressure in the ISM decreases substantially with increasing surface density. For the ultraluminous infrared galaxy Arp 220, this ratio is ~10-4. Therefore, if the minimum energy estimate is applicable, magnetic fields in starbursts are dynamically weak compared to gravity, in contrast to normal star-forming spiral galaxies. We argue, however, that rapid cooling of relativistic electrons in starbursts invalidates the minimum energy estimate. We assess a number of independent constraints on the magnetic field strength in starburst galaxies. In particular, we argue that the existence of the FIR-radio correlation implies that the synchrotron cooling timescale for cosmic-ray electrons is much shorter than their escape time from the galactic disk; this in turn implies that the true magnetic field in starbursts is significantly larger than Bmin. The strongest argument against such large fields is that one might expect starbursts to have steep radio spectra indicative of strong synchrotron cooling, which is not observed. However, we show that ionization and bremsstrahlung losses can flatten the nonthermal spectra of starburst galaxies even in the presence of rapid cooling, providing much better agreement with observed spectra. We further demonstrate that ionization and bremsstrahlung losses are likely to be important in shaping the radio spectra of most starbursts at GHz frequencies, thereby preserving the linearity of the FIR-radio correlation. We thus conclude that magnetic fields in starbursts are significantly larger than Bmin. We highlight several observations that can test this conclusion.

Physical properties of galactic winds using background quasars

Background quasars are potentially sensitive probes of galactic outflows provided that one can determine the origin of the absorbing material since both gaseous disks and strong bipolar outflows can contribute to the absorption cross-section. Using a dozen quasars passing near spectroscopically identified galaxies at z 0:1, we find that the azimuthal orientation of the quasar sight-lines with strong Mg II absorption (withW 2796 r > 0:3 ˚ A) is bi-modal: about half the Mg II sight-lines are aligned with the major axis and the other half are within = 30 of the minor axis, suggesting that bipolar outflows can contribute to the Mg II cross-section. This bi-modality is also present in the instantaneous star-formation rates (SFRs) of the hosts. For the sight-lines aligned along the minor axis, a simple bi-conical wind model is indeed able to reproduce the observed Mg II kinematics and the Mg II dependence with impact parameter b, (W 2796 r / b 1 ). Using our wind model, we can directly extract key wind properties such as the de-projected outflow speed Vout of the cool material traced by Mg II and the outflow rates _ Mout. The outflow speeds Vout are found to be 150-300 kms 1 , i.e. of the order of the circular velocity, and smaller than the escape velocity by a factor of 2. The outflow rates _ Mout are typically two to three times the instantaneous SFRs. Our results demonstrate how background quasars can be used to measure wind properties with high precision.

The Persistence of Cool Galactic Winds in High Stellar Mass Galaxies between z ~ 1.4 and ~1

We present an analysis of the Mg II λλ2796, 2803 and Fe II λλ2586, 2600 absorption line profiles in co-added spectra of 468 galaxies at 0.7 10 M ☉ yr–1 host strong outflows in both this and the W09 sample, we do not detect outflows in lower-SFR (i.e., log M */M ☉ 10.5) galaxies at lower redshifts. Using a simple galaxy evolution model that assumes exponentially declining SFRs, we infer that strong outflows persist in galaxies with log M */M ☉ > 10.5 as they age between z = 1.4 and z ~ 1, presumably because of their high absolute SFRs. Finally, our spectral analysis, combined with high-resolution Hubble Space Telescope/Advanced Camera for Surveys imaging, weakly suggests that outflow absorption strength increases with galaxy SFR surface density.

Evidence for Ubiquitous Collimated Galactic-Scale Outflows along the Star-Forming Sequence at z~0.5

We analyze Mg II λλ2796, 2803 and Fe II λλ2586, 2600 absorption profiles in individual spectra of 105 galaxies at 0.3 50° (edge-on). Combined with the comparatively weak dependence of wind detection rate on intrinsic galaxy properties, this implies that biconical outflows are ubiquitous in normal, star-forming galaxies at z ~ 0.5. We find that wind velocity is correlated with galaxy M * at 3.4σ significance, while outflow equivalent width is correlated with SFR at 3.5σ significance, suggesting hosts with higher SFR launch more material and/or generate a larger velocity spread for the absorbing clouds. Assuming the gas is driven into halos with isothermal density profiles, the wind velocities (~200-400 km s–1) permit escape from the halo potentials only for the lowest-M * systems in the sample. However, the gas carries sufficient energy to reach distances 50 kpc, and may therefore be a viable source of material for the massive, cool circumgalactic medium around bright galaxies at z ~ 0.