arXiv subjects

Crystal L. Martin

Publications and source records attributed to Crystal L. Martin.

At least 109 records · Page 6Linked to original sources

arXiv2009

Discovery of a Giant Lya Emitter Near the Reionization Epoch

We report the discovery of a giant Lya emitter (LAE) with a Spitzer/IRAC counterpart near the reionization epoch at z=6.595. The giant LAE is found from the extensive 1 deg^2 Subaru narrow-band survey for z=6.6 LAEs in the Subaru/XMM-Newton Deep Survey (SXDS) field, and subsequently identified by deep spectroscopy of Keck/DEIMOS and Magellan/IMACS. Among our 207 LAE candidates, this LAE is not only the brightest narrow-band object with L(Lya) = 3.9+/-0.2 x 10^43 erg/s in our survey volume of 10^6 Mpc^3, but also a spatially extended Lya nebula with the largest isophotal area whose major axis is at least ~3''. This object is more likely to be a large Lya nebula with a size of >~17 kpc than to be a strongly-lensed galaxy by a foreground object. Our Keck spectrum with medium-high spectral and spatial resolutions suggests that the velocity width is v(FWHM)=251+/-21 km/s, and that the line-center velocity changes by ~60 km/s in a 10-kpc range. The stellar mass and star-formation rate are estimated to be 0.9-5.0 x 10^10 Mo and >34 Mo/yr, respectively, from the combination of deep optical to infrared images of Subaru, UKIDSS-Ultra Deep Survey, and Spitzer/IRAC. Although the nature of this object is not yet clearly understood, this could be an important object for studying cooling clouds accreting onto a massive halo, or forming-massive galaxies with significant outflows contributing to cosmic reionization and metal enrichment of inter-galactic medium.

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arXiv2008

The DEEP2 Redshift Survey: Lyman Alpha Emitters in the Spectroscopic Database

We present the first results of a search for Lyman-alpha emitters (LAEs) in the DEEP2 spectroscopic database that uses a search technique that is different from but complementary to traditional narrowband imaging surveys. We have visually inspected ~20% of the available DEEP2 spectroscopic data and have found nine high-quality LAEs with clearly asymmetric line profiles and an additional ten objects of lower quality, some of which may also be LAEs. Our survey is most sensitive to LAEs at z=4.4-4.9 and that is indeed where all but one of our high-quality objects are found. We find the number density of our spectroscopically-discovered LAEs to be consistent with those found in narrowband imaging searches. The combined, averaged spectrum of our nine high-quality objects is well fit by a two-component model, with a second, lower-amplitude component redshifted by ~420 km/s with respect to the primary Lyman-alpha line, consistent with large-scale outflows from these objects. We conclude by discussing the advantages and future prospects of blank-sky spectroscopic surveys for high-z LAEs.

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arXiv2008

A Magellan IMACS Spectroscopic Search for Lyman-Alpha Emitting Galaxies at Redshift 5.7

We present results from a blind, spectroscopic survey for redshift ~5.7 Lyman-alpha-emitting galaxies using the Inamori Magellan Areal Camera and Spectrograph. A total of ~200 square arcminutes were observed in the COSMOS and LCIRS fields using a narrowband filter, which transmits between atmospheric emission lines at 8190 A, and a mask with 100 longslits. This observing technique provides higher emission-line sensitivity than narrowband imaging and probes larger volumes than strong lensing. We find 170 emission-line galaxies and identify their redshifts spectroscopically. We confirm three Lyman-alpha emitting galaxies (LAEs), the first discovered using multislit-narrowband spectroscopy. Their line profiles are narrow, but fitted models suggest instrinsic, unattenuated widths 400 km/s FWHM. The red wing of the line profiles present features consistent with galactic winds. The star formation rates of these galaxies are at least 5-7 Msun/yr and likely a factor of two higher. We estimate the number density of L .ge. 5e42 erg/s LAEs is 9.0(+12,-4)e-5 Mpc-3 at redshift 5.7 and constrain the Schechter function parameters describing this population. Galaxies fainter than our detection limit may well be the primary source of ionizing photons at redshift ~ 6. We argue, however, that the break luminosity L* is not yet well constrained. If this break luminosity is near our detection limit, and somewhat lower than previous estimates, then the detected LAE population could be responsible for ionizing the intergalactic gas at redshift ~6. We discuss the potential of multislit-narrowband spectroscopy for deeper emission-line surveys.

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arXiv2007

Coincident, 100 kpc-scale damped Lyman alpha absorption towards a binary QSO: how large are galaxies at z ~ 3?

We report coincident damped Lyman alpha (DLA) and sub-DLA absorption at z = 2.66 and z = 2.94 towards the z ~ 3 13.8 arcsecond separation binary quasar SDSS 1116+4118 AB. At the redshifts of the absorbers, this angular separation corresponds to a proper transverse separation of ~ 110 kpc. A third absorber, a sub-DLA at z = 2.47, is detected towards SDSS 1116+4118 B, but no corresponding high column density absorber is present towards SDSS 1116+4118 A. We use high resolution galaxy simulations and a clustering analysis to interpret the coincident absorption and its implications for galaxy structure at z ~ 3. We conclude that the common absorption in the two lines of sight is unlikely to arise from a single galaxy, or a galaxy plus satellite system, and is more feasibly explained by a group of two or more galaxies with separations ~ 100 kpc. The impact of these findings on single line of sight observations is also discussed; we show that abundances of DLAs may be affected by up to a few tenths of a dex by line of sight DLA blending. From a Keck ESI spectrum of the two quasars, we measure metal column densities for all five absorbers and determine abundances for the three absorbers with log N(HI) > 20. For the two highest N(HI) absorbers, we determine high levels of metal enrichment, corresponding to 1/3 and 1/5 solar. These metallicities are amongst the highest measured for DLAs at any redshift and are consistent with values measured in Lyman break galaxies at 2 < z < 3. For the DLA at z = 2.94 we also infer an approximately solar ratio of alpha-to-Fe peak elements from [S/Zn] = +0.05, and measure an upper limit for the molecular fraction in this particular line of sight of log f(H_2)< -5.5.

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arXiv2006

The Ionization State of Sodium in Galactic Winds

Roughly 80% of Ultraluminous Infrared Galaxies (ULIRGs) show blue shifted absorption in the resonance lines of neutral sodium, indicating that cool winds are common in such objects, as shown by Rupke et al and by Martin. The neutral sodium (NaI) columns indicated by these absorption lines are ~ 10^{13}-3x10^{14}/cm^2, while the bolometric luminosity varies by a factor of only four. We show that the gas in ULIRG outflows is likely to be in photoionization equilibrium. The very small ULIRG sample of Goldader et al. demonstrates that the ratio of ultraviolet flux to far infrared flux varies by a factor $\sim100$ from object to object. While the Goldader sample does not overlap with those of Rupke et al. and Martin, we show that such a large variation in ultraviolet flux will produce a similar variation in the column of neutral sodium for a fixed mass flux and density. However, if the cold gas is in pressure equilibrium with a hot outflow with a mass loss rate similar to the star formation rate, the range of ionization state is significantly smaller. Measurements of the UV flux for objects in the Martin and Rupke et al. catalogs will definitively determine if photoionization effects are responsible for the wide variation seen in the sodium columns. If they are, a determination of the gas density and mass loss rate in the cool winds will follow, with attendant improvements in our understanding of wind driving mechanisms and of the effects of galaxies on their surroundings.

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arXiv2006

Infalling Faint [OII] Emitters in Abell 851. I. Spectroscopic Confirmation of Narrowband-Selected Objects

We report on a spectroscopic confirmation of narrowband-selected [OII] emitters in Abell 851 catalogued by Martin et al. (2000). The optical spectra obtained from the Keck I Low Resolution Imaging Spectrometer (LRIS) and Keck II Deep Imaging Multi-Object Spectrograph (DEIMOS) have confirmed [OII]3727 emission in narrowband-selected cluster [OII] candidates at a 85% success rate for faint (i <~ 25) blue (g-i < 1) galaxies. The rate for the successful detection of [OII] emission is a strong function of galaxy color, generally proving the efficacy of narrowband [OII] search supplemented with broadband colors in selecting faint cluster galaxies with recent star formation. Balmer decrement-derived reddening measurements show a high degree of reddening [E(B-V) >~ 0.5] in a significant fraction of this population. Even after correcting for dust extinction, the [OII]/Ha line flux ratio for the high-E(B-V) galaxies remains generally lower by a factor of ~2 than the mean [OII]/Ha ratios reported by the studies of nearby galaxies. The strength of [OII] equivalent width shows a negative trend with galaxy luminosity while the Ha equivalent width does not appear to depend as strongly on luminosity. This in part is due to the high amount of reddening observed in luminous galaxies. Furthermore, emission line ratio diagnostics show that AGN-like galaxies are abundant in the high luminosity end of the cluster [OII]-emitting sample, with only moderately strong [OII] equivalent widths, consistent with a scenario of galaxy evolution connecting AGNs and suppression of star-forming activity in massive galaxies.

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arXiv2006

Infalling Faint [OII] Emitters in Abell 851. II. Environment, Kinematics, and Star Formation History

We report on the environments, kinematics, and SFH of [OII]-selected objects in the z=0.4 galaxy cluster A851, using Keck optical spectra. A large fraction (~55%) of cluster [OII] emitters show strong Balmer absorptions (>4A in equivalent width at Hd). These e(a) spectra have been attributed to dusty starburst galaxies, an interpretation supported by our reddening measurements, which show a high frequency of very reddened [EBV>0.5] [OII] emitters. Our spectral modeling requires starburst ages >1Gyr, shorter than the cluster crossing timescale. We argue that this starburst phase occurs during cluster infall based on the radial velocity distribution of the cluster [OII] emitters, which present a deficit of systems near the cluster systemic velocity. The spatial segregation of some redshifted and blueshifted groups strongly indicates that the accretion was recent. Throughout the cluster, the fraction in [OII] emitters is a strong function of the local galaxy density. Our analysis supports previous suggestions that dusty starburst galaxies arise at the expense of gas-rich spiral galaxies. In addition, we describe a fainter [OII]-emitting population, comprised largely of dwarf galaxies and find an even stronger suppression of [OII] emission in high density environments among this subsample, indicative of more effective destruction by harassment and/or gas-stripping. Comparison to previous morphological studies, limited to the core of A851, suggests that galaxy-galaxy interactions may trigger the starbursts. The high e(a) galaxy fraction in A851 compared to that in the field, however, suggests that some cluster-specific mechanism, likely related to the dynamical assembly of the cluster, also contributes to the high number of starbursts. (abridged)

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arXiv2006

Mapping Large-Scale Gaseous Outflows in Ultraluminous Infrared Galaxies with Keck II ESI Spectra: Spatial Extent of the Outflow

The kinematics of neutral gas and warm ionized gas have been mapped in one-dimension across ultraluminous starburst galaxies using interstellar absorption and emission lines, in Keck II ESI spectra. Blue-shifted absorption is found along more of the slit than anticipated, exceeding scales of 15 kpc across several systems. The large velocity gradient measured across some of these outflows is inconsistent with a flow diverging from the central starburst -- angular momentum conservation reduces the rotational velocity of an outflow as it expands. More widespread star formation, likely triggered by the merger, probably drives these outflows, although some models suggest the collision itself could generate a wind by shock heating interstellar gas throughout the disk. Young mergers with separated nuclei present the highest outflow masses, due mainly to the larger area over which the cool gas can be detected. In a typical ULIG, the mass carried by the cool phase of the outflow is around 10^8Msun, or a few percent of the total dynamical mass. Assuming the starburst activity has proceeded at the observed rate for the past 10 Myr, the kinetic energy of the cool outflows is a few percent of the supernova energy, consistent with starbursts powering the outflows. The cool wind is expected to be accelerated by momentum deposition, possibly from radiation pressure as well as supernovae. Such models imply turn-around radii for the cool outflows of at least 30 to 90 kpc. This cross-section presents a significant NaI absorption cross section. If most L > 0.1L* galaxies pass through a luminous starburst phase, then relics of cool outflows will create a significant redshift-path density. Galaxy formation models should include this cool phase of the outflow in addition to a hot wind in feedback models.

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arXiv2006

Magnetic Fields in Starburst Galaxies and The Origin of the FIR-Radio Correlation

We estimate minimum energy magnetic fields (B_min) for a sample of galaxies with measured gas surface densities, spanning from normal spirals to 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 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 spiral galaxies. We argue, however, that rapid cooling of relativistic electrons in starbursts invalidates the minimum energy estimate. We critically assess a number of independent constraints on the magnetic field strength in starbursts. 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 B_min. 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. We show, however, 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 B_min. We highlight several observations that can test this conclusion.

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arXiv2006

Preliminary Results from a Spectrosopic LyA Survey at Redshift 5.7 with IMACS

We describe preliminary results from an ultra-sensitive, spectroscopic emission-line survey and illustrate the challenges inherent in identifying high-redshift LyA emitters. Our multi-slit windows technique complements other types of emission-line surveys. Narrowband imaging surveys cover large areas of sky but only detect much brighter objects. Longslit spectra taken along cluster caustics yield intrinsically fainter lensed LyA emitters but probe small volumes of space. We have observed the COSMOS deep field and a field at 15h +00. To a line flux of a few times 10^-18 erg/s/cm^2, we found 150 emission-line sources (with no detectable continuum) among 4 masks. These candidates are being re-observed with broad spectral coverage to determine the line identity. To date, the interloper to LyA ratio is about 8:1. The sky positions of the LyA candidates generally do not coincide with those of foreground objects in ultra-deep r band or i' images -- consistent with the presence of a strong Lyman break.

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arXiv2004

Mapping Large-Scale Gaseous Outflows in Ultraluminous Galaxies with Keck II ESI Spectra: Variations in Outflow Velocity with Galactic Mass

Measurements of interstellar absorption lines in 18 ultraluminous infrared galaxies (ULIGs) have been combined with published data, in order to reassess the dependence of galactic outflow speeds on starburst luminosity and galactic mass. The Doppler shifts reveal outflows of relatively cool gas at $330 \pm 100$km/s. The outflow speeds increase with the star formation rate (SFR) as roughly $v \propto SFR^{0.35}$. This result is surprising since, in the traditional model for starburst-driven winds, these relatively cool gas clouds are accelerated by the ram pressure of a hot, supernova-heated wind that exhibits weak (if any) \x temperature variation with increasing galactic mass. The lack of evidence for much hotter winds is partly a sensitivity issue; but the outflow velocities in ultraluminous starbursts actually are consistent with acceleration by the tepid wind, indicating a hotter component is unlikely to dominate the momentum flux. The \nad velocities in the dwarf starburst winds do not reach the terminal velocity of a hot wind at the measured temperature of kT ~0.73 keV. A dynamically-motivated explanation is that the dwarf starburst winds simply lack enough momentum to accelerate the clouds to the velocity of the hot wind. Among the subsample of starbursts with well- constrained dynamical masses, the terminal outflow velocities approach the galactic escape velocity. If this relation, which is similar to that observed for stellar winds, arises from radiation pressure on dust and gas limiting the maximum luminosity of a starburst, then feedback will be stronger in massive galaxies than previously thought helping shape the high-mass end of the galaxy luminosity function.

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arXiv2004

A Keck/HIRES Study of Kinematics of the Cold Interstellar Medium in Dwarf Starburst Galaxies

We have obtained high resolution Echelle spectra (R = 30,000-50,000) of the Na D absorption doublet (5890A, 5896A) for six dwarf starburst galaxies and two more luminous starbursts: M82 and NGC 1614. The absorption features were separated into multiple components and separated into stellar and interstellar parts based on kinematics. We find that three of the dwarfs show outflows, with an average blueshift of 27 km/s. This is small compared to the highest velocity components in NGC 1614 and M82 (blueshifted by 150 km/s and 91 km/s, respectively); these two brighter galaxies also show more complex absorption profiles than the dwarfs. None of the outflow speeds clearly exceed the escape velocity of the host galaxy. Sightlines in NGC 2363 and NGC4214 apparently intersect expanding shells. We compare the shocked gas velocity (v_NaD) to the ionized gas velocity (v_Halpha) and interpret the velocity difference as either a trapped ionization front (NGC 4214) or a leaky HII region (NGC 2363). The dwarfs show N(NaD) = 10^(11.8-13.7) cm^-2, while the Na D columns in M82 and NGC 1614 are 10^13.7 cm^-2 and 10^14.0 cm^-2, respectively. The mass of expelled gas is highly sensitive to outflow geometry, dust depletion, and ionization fraction, but with a simple shell model we estimate neutral outflow gas masses from ~10^6 M_solar to ~10^10 M_solar.

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arXiv2003

The Space Density of Redshift 5.7 Ly-Alpha Emitters

We present results from a blind, spectroscopic search for redshift 5.7 LyA emission-line galaxies at Keck I. Using a band-limiting filter and custom slitmask, the LRIS detector was covered with low resolution spectra in the 8100 - 8250 A atmospheric window which contains no bright night sky emission lines. Nine objects with line fluxes greater than our flux limit of 6e-18 erg/s/cm2 were found in the ~5.1 square arcminute field. Follow up observations indicate none of these are z=5.7 LyA emitters -- a result which places strong limits on evolution scenarios for LyA emitters between redshift 3 and redshift 5.7. In particular, the paucity of z=5.7 LyA emitters raises the question of whether the LyA -selected population plays a significant role in maintaining the ionization of the intergalactic medium at z = 5.7. We argue that if the escape fraction of LyA from galaxies is less than $0.4 f_{LyC}$, where $f_{LyC}$ is the escape fraction of Lyman continuum photons, then the star formation rate in the \lya emitting population is high enough in the no-evolution model (our upper limit) to maintain the ionization of the IGM at z=5.7.

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arXiv2003

Chandra Observations of Expanding Shells in the Dwarf Starburst Galaxy NGC 3077

Deep Chandra observations (53 ks, ACIS-S3) of NGC 3077, a starburst dwarf galaxy in the M 81 triplet, resolve the X-ray emission from several supershells. The emission is brightest in the cavities defined by expanding shells detected previously in H alpha emission (Martin 1998). Thermal emission models fitted to the data imply temperatures ranging from ~1.3 to 4.9 x 10^(6) K and indicate that the strongest absorption is coincident with the densest clouds traced by CO emission. The fitted emission measures give pressures of P/k~10^(5-6) xi^(-0.5) f_(v)^(-0.5) K cm^(-3) (xi: metallicity of the hot gas in solar units, f_(v): volume filling factor). Despite these high pressures, the radial density profile of the hot gas is not as steep as that expected in a freely expanding wind (e.g., as seen in the neighboring starburst galaxy M 82) implying that the hot gas is still confined by the H alpha shells. The chaotic dynamical state of NGC 3077 undermines reliable estimates of the escape velocity. The more relevant quantity for the ultimate fate of the outflow is probably the gas density in the rich intragroup medium. Based on the HI distribution of NGC 3077 and a connected tidal tail we argue that the wind has the potential to leave the gravitational well of NGC 3077 to the north but not to the south. The total 0.3-6.0 keV X-ray luminosity is ~2-5 x 10^(39) erg s^(-1) (depending on the selected thermal plasma model). Most (~85%) of the X-ray luminosity in NGC 3077 comes from the hot interstellar gas; the remainder comes from six X-ray point sources. In spite of previous claims to the contrary, we do not find X-ray emission originating from the prominent tidal tail near NGC 3077.

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arXiv2002

The Hubble Deep Field South Flanking Fields

As part of the Hubble Deep Field South program, a set of shorter 2-orbit observations were obtained of the area adjacent to the deep fields. The WFPC2 flanking fields cover a contiguous solid angle of 48 square arcminutes. Parallel observations with the STIS and NICMOS instruments produce a patchwork of additional fields with optical and near-infrared (1.6 micron) response. Deeper parallel exposures with WFPC2 and NICMOS were obtained when STIS observed the NICMOS deep field. These deeper fields are offset from the rest, and an extended low surface brightness object is visible in the deeper WFPC2 flanking field. In this data paper, which serves as an archival record of the project, we discuss the observations and data reduction, and present SExtractor source catalogs and number counts derived from the data. Number counts are broadly consistent with previous surveys from both ground and space. Among other things, these flanking field observations are useful for defining slit masks for spectroscopic follow-up over a wider area around the deep fields, for studying large-scale structure that extends beyond the deep fields, for future supernova searches, and for number counts and morphological studies, but their ultimate utility will be defined by the astronomical community.

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arXiv2001

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, HI and H$_2$ (as traced by CO) surface density profiles, and new \Ha CCD photometry. Our 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.

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arXiv1998

UV Diagnostics of the Interaction between Starburst Galaxies and the Intergalactic Medium

Both galaxies and the intergalactic medium evolve dramatically between $z \approx 2$ and the present. These changes are coupled via galactic winds, cloud accretion, and ionizing radiation. Measurements of these interactions are critical for building a physical understanding of galaxy assembly. This paper reviews the role of starburst galaxies in heating, enriching, and ionizing the intergalactic medium. The strategies used to address these problems rely heavily on access to the rest-frame ultraviolet (UV) spectrum. Rather modest gains in UV sensitivity are shown to produce enormous gains in our ability to follow the very low-density gas dispersed by galactic winds and galaxy interactions. This leverage results from the sensitivity of absorption lines to low ionic columns and the very steep rise in the areal density of quasars toward fainter magnitudes.

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arXiv1998

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

Properties of starburst-driven outflows in dwarf galaxies are compared to those in more massive galaxies. Over a factor of roughly 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 emission shows little variation around $\sim 10^{6.7}$ K, and this gas has enough energy to escape from the galaxies with rotation speed less than approximately 130 km/s.

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