Gerhardt Meurer | The University of Western Australia (original) (raw)

Papers by Gerhardt Meurer

The Astrophysical Journal, 2004

We observed the brightest super-star cluster NGC 1705-1 in the nearby dwarf galaxy NGC 1705 with ... more We observed the brightest super-star cluster NGC 1705-1 in the nearby dwarf galaxy NGC 1705 with the Space Telescope Imaging Spectrograph in the echelle mode between 1200 and 3100 A. The data allow a study of the young stellar population at hitherto unprecedented spectral resolution and signal-to-noise ratios. A comprehensive list of strong and weak stellar and interstellar absorption lines is given, together with the measured line parameters. Four distinct velocity systems are identified: stellar lines at the measured H i velocity, blueshifted interstellar lines from outflowing gas, Milky Way foreground absorption, and a high-velocity cloud. Comparison with stellar template spectra indicates an equivalent spectral type of B0 to B1, with mostly dwarf and giant stars contributing. When placed on a theoretical Hertzsprung-Russell diagram, these stars constrain the age of NGC 1705-1 to 12AE 3 1 Myr. Since this age is derived purely from spectroscopy, it is independent of reddening corrections. A comparison of the observed and theoretical mass-to-light ratio for the derived age was performed. We find no significant evidence for an anomalous initial mass function at the low-mass end, contrary to suggestions found in the literature. The stellar population of NGC 1705-1 is similar to that in other massive clusters, such as 30 Doradus or NGC 1569-A, after taking into account age differences and model uncertainties. We discuss the difficulty of relating observed and theoretical mass-to-light ratios because of the unknown gas mass fraction lost by the cluster and the uncertain mass-loss rates of asymptotic giant branch stars in population synthesis models.

Dwarf galaxies play an important role in our understanding of galaxy formation and evolution, and... more 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 ...

We present a detailed study of the star cluster population detected in the galaxy NGC 922, one of... more We present a detailed study of the star cluster population detected in the galaxy NGC 922, one of the closest collisional ring galaxies known to date, using HST/WFPC2 UBVI photometry, population synthesis models, and N-body/SPH simulations. We find that 69% of the clusters are younger than 7 Myr, and that most of them are located in the ring or along the bar, consistent with the strong Hα emission. The cluster luminosity function slope of 2.1-2.3 for NGC 922 is in agreement with those of young clusters in nearby galaxies. Models of the cluster age distribution match the observations best when cluster disruption is considered. We also find clusters with ages (>50 Myr) and masses (>10 5 M ⊙) that are excellent progenitors for faint fuzzy clusters. The images also show a tidal plume pointing toward the companion. Its stellar age from our analysis is consistent with pre-existing stars that were stripped off during the passage of the companion. Finally, a comparison of the star-forming complexes observed in NGC 922 with those of a distant ring galaxy from the GOODS field indicates very similar masses and sizes, suggesting similar origins.

The 905 to 1180 Å spectral range of the Far Ultraviolet Spectroscopic Explorer (FUSE) includes nu... more The 905 to 1180 Å spectral range of the Far Ultraviolet Spectroscopic Explorer (FUSE) includes numerous transitions of molecular hydrogen, making it possible to study H2 in diffuse interstellar environments directly through absorption measurements. We have searched for H2 absorption in five starburst galaxies: NGC 1705, NGC 3310, NGC 4214, M83 (NGC 5236), and NGC 5253. We tentatively detect weak absorption by H2 in M83 and NGC 5253, and set upper limits on the H2 column density in the other galaxies. Conservative upper limits on the mass of molecular gas detected with FUSE are many orders of magnitude lower than the H2 mass inferred from CO emission measurements for the four galaxies in our sample in which CO has been detected. This indicates that almost all of the H2 is in the form of clouds with N(H2) � 10 20 cm −2 that are opaque to far-UV light and therefore cannot be probed with far-UV absorption measurements. The far-UV continuum visible in the FUSE spectra passes between the ...

Proceedings of the International Astronomical Union, 2016

The HI in galaxies often extends past their conventionally defined optical extent. I report resul... more The HI in galaxies often extends past their conventionally defined optical extent. I report results from our team which has been probing low intensity star formation in outer disks using imaging in Hα and ultraviolet. Using a sample of hundreds of HI selected galaxies, we confirm that outer disk HII regions and extended UV disks are common. Hence outer disks are not dormant but are dimly forming stars. Although the ultraviolet light in galaxies is more centrally concentrated than the HI, the UV/HI ratio (the Star Formation Efficiency) is nearly constant, with a slight dependency on surface brightness. This result is well accounted for in a model where disks maintain a constant stability parameter Q. This model also accounts for how the ISM and star formation are distributed in the bright parts of galaxies, and how HI appears to trace the distribution of dark matter in galaxy outskirts.

Astronomical Journal, 2008

The Astronomical Journal, 2004

A number of very small isolated H ii regions have been discovered at projected distances up to 30... more A number of very small isolated H ii regions have been discovered at projected distances up to 30 kpc from their nearest galaxy. These H ii regions appear as tiny emission line objects in narrow band images obtained by the NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG). We present spectroscopic confirmation of four isolated H ii regions in two systems, both systems have tidal H i features. The results are consistent with stars forming in interactive debris due to cloud-cloud collisions. The Hα luminosities of the isolated H ii regions are equivalent to the ionizing flux of only a few O stars each. They are most likely ionized by stars formed in situ, and represent atypical star formation in the low density environment of the outer parts of galaxies. A small but finite intergalactic star formation rate will enrich and ionize the surrounding medium. In one system, NGC 1533, we calculate a star formation rate of 1.5 × 10 −3 M ⊙ yr −1 , resulting in a metal enrichment of ∼ 1 × 10 −3 solar for the continuous formation of stars. Such systems may have been more common in the past and a similar enrichment level is measured for the 'metallicity floor' in damped Lyman-α absorption systems.

The Astronomical Journal, 2010

The Astronomical Journal, 2009

We present results of a search for emission-line galaxies in the Southern Fields of the Hubble Sp... more We present results of a search for emission-line galaxies in the Southern Fields of the Hubble Space Telescope PEARS (Probing Evolution And Reionization Spectroscopically) grism survey. The PEARS South Fields consist of five ACS pointings (including the Hubble Ultra Deep Field) with the G800L grism for a total of 120 orbits, revealing thousands of faint object spectra in the GOODS-South region of the sky. Emission-line galaxies (ELGs) are one subset of objects that are prevalent among the grism spectra. Using a 2-dimensional detection and extraction procedure, we find 320 emission lines orginating from 226 galaxy "knots" within 192 individual galaxies. Line identification results in 118 new grism-spectroscopic redshifts for galaxies in the GOODS-South Field. We measure emission line fluxes using standard Gaussian fitting techniques. At the resolution of the grism data, the Hβ and [O iii ] doublet are blended. However, by fitting two Gaussian components to the Hβ and [O iii ] features, we find that many of the PEARS ELGs have high [O iii ]/Hβ ratios compared to other galaxy samples of comparable luminosities. The star-formation rates (SFRs) of the ELGs are presented, as well as a sample of distinct giant star-forming regions at z∼0.1-0.5 across individual galaxies. We find that the radial distances of these HII regions in general reside near the galaxies' optical continuum half-light radii, similar to those of giant HII regions in local galaxies.

Arxiv preprint arXiv: …, 2008

We explore the design of a space mission called Project Lyman that has the goal of quantifying th... more We explore the design of a space mission called Project Lyman that has the goal of quantifying the ionization history of the universe from the present epoch to a redshift of z ∼ 3. Observations from WMAP and SDSS show that before a redshift of z ∼ > 6 the first collapsed objects, ...

We present the spectrophotometric redshifts (SPZs) of 1.5x104 galaxies observed as part of the Pr... more We present the spectrophotometric redshifts (SPZs) of 1.5x104 galaxies observed as part of the Probing Evolution and Reionization Spectroscopically (PEARS) survey. This survey is an HST/ACS/G880L grism survey in the GOODS-N and S fields. Redshifts are measured using a combination of the slitless grism spectra and available broad-band data. Using the existing sample of over 3000 ground-based spectroscopic redshifts, we

We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6 microns made possible by t... more We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6 microns made possible by the Early Release Science (ERS) observations with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. These new infrared grism data augment existing optical Advanced Camera for Surveys G800L grism data in GOODS-South. The ERS grism field was observed at 2 orbits/grism, yielding spectra

Monthly Notices of the Royal Astronomical Society, 2018

The Stellar Populations of Galaxies, 1992

The definitions of the amorphous and blue compact dwarf (BCD) classes of galaxies are very simila... more The definitions of the amorphous and blue compact dwarf (BCD) classes of galaxies are very similar. One key difference is that BCDs are often selected for their apparent compactness (i.e. a small angular size), which selects against nearby objects, whereas amorphous galaxies must be extended. We present initial results of a project to determine the population distribution in dwarf (M B < −18) amorphous galaxies (dAgs) and determine which dAgs can be classified as BCDs. We have used the 3.9m Anglo-Australian Telescope to obtain deep B and R CCD images of two dAgs: NGC 216 (M B = −17.3) and NGC 2915 (M B = −14.1). The morphology of NGC 216 is that of a dusty late-type edge-on disk galaxy, with a peculiar one-sided bar. It would not be classified as a BCD if seen face-on. However, NGC 2915 does have all the properties of a BCD, and can be classified so. It has numerous condensations near its center. Many of these are likely to be individual stars. We derive a distance to NGC 2915 of 5 Mpc if the brightest blue non-extended objects are blue supergiants, and if there is little internal extinction. Similar condensations are seen in the dAgs NGC 1705 (Meurer, et al., 1989. Astrophys. Space Sci., 156: 141) and NGC 5253 (Caldwell and Phillips, 1989. Astrophys. J., 338: 789) which are also likely to be BCDs.

The Astrophysical Journal, 2013

We present observations of a luminous galaxy at z = 6.573-the end of the reioinization epoch-whic... more We present observations of a luminous galaxy at z = 6.573-the end of the reioinization epoch-which has been spectroscopically confirmed twice. The first spectroscopic confirmation comes from slitless HST ACS grism spectra from the PEARS survey (Probing Evolution And Reionization Spectroscopically), which show a dramatic continuum break in the spectrum at restframe 1216Å. The second confirmation is done with Keck + DEIMOS. The continuum is not clearly detected with ground-based spectra, but high wavelength resolution enables the Lyman-α emission line profile to be determined. We compare the line profile to composite line profiles at z=4.5. The Lyman-α line profile shows no signature of a damping wing attenuation, confirming that the intergalactic gas is ionized at z=6.57. Spectra of Lyman breaks at yet higher redshifts will be possible using comparably deep observations with IR-sensitive grisms, even at redshifts where Lyman-α is too attenuated by the neutral IGM to be detectable using traditional spectroscopy from the ground.

The nearby and luminous QSO 3C 273 was imaged in 2002 July with the High Resolution Channel (HRC)... more The nearby and luminous QSO 3C 273 was imaged in 2002 July with the High Resolution Channel (HRC) of the Advanced Camera for Surveys (ACS) in coronagraphic mode in F475W (g), F606W (V), and F814W (I) as part of the Early Release Observations (ERO) program. After subtraction of the remaining PSF of the QSO, these images offer the most detailed view yet of the morphology and colors of the host galaxy of this QSO. We find that the central light distribution is elongated along the jet axis and its outer edge is delineated by an arc, centered on the jet at a radius of 2>6fromtheQSOandbluerthanthesurroundinggalaxy.Thissystemisembeddedinanextendedgalactichalo.Comparedwithearly−typegalaxiesofsimilarredshiftsandluminosities,thelightdistributionof3C273isflatterinthecore,likelyfromsuppressionbydust,butsimilarintheouterhalo.TheQSOisdisplacedfromtheisophotalcenterofthegalaxyby2>6 from the QSO and bluer than the surrounding galaxy. This system is embedded in an extended galactic halo. Compared with early-type galaxies of similar redshifts and luminosities, the light distribution of 3C 273 is flatter in the core, likely from suppression by dust, but similar in the outer halo. The QSO is displaced from the isophotal center of the galaxy by 2>6fromtheQSOandbluerthanthesurroundinggalaxy.Thissystemisembeddedinanextendedgalactichalo.Comparedwithearly−typegalaxiesofsimilarredshiftsandluminosities,thelightdistributionof3C273isflatterinthecore,likelyfromsuppressionbydust,butsimilarintheouterhalo.TheQSOisdisplacedfromtheisophotalcenterofthegalaxyby1>4. Previously known emission-line extensions are confirmed and new morphological features are identified, including a dramatic spiral-shaped plume, two faint filaments, a dust lane, and a knot along the jet axis. Part of the inner jet is unambiguously detected in all three bandpasses, and its morphology matches that of a MERLIN radio map. Different mechanisms that could explain the morphology of 3C 273 are considered, such as scattered QSO radiation, a face-on disk, and a merger event.

We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the... more We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ∼ 3 Myr to 10 Myr. The extreme blue V − I (F606W−F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of M V = −14.45, with a half-light radius of 161 pc, and if it is a single star cluster, would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ∼ 4-5 Myr, its derived mass is < 1.3 ×10 6 M. Thus the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intra-cluster media in the distant and younger Universe.

We present the analysis of the faint galaxy population in the Advanced Camera for Surveys (ACS) E... more We present the analysis of the faint galaxy population in the Advanced Camera for Surveys (ACS) Early Release Observation fields VV 29 (UGC 10214) and NGC 4676. These observations cover a total area of 26.3 arcmin 2 and have depths close to that of the Hubble Deep Fields in the deepest part of the VV 29 image, with 10 detection limits for point sources of 27.8, 27.6, and 27.2 AB magnitudes in the g F475W , V F606W , and I F814W bands, respectively. Measuring the faint galaxy number count distribution is a difficult task, with different groups arriving at widely varying results even on the same data set. Here we attempt to thoroughly consider all aspects relevant for faint galaxy counting and photometry, developing methods that are based on public software and that are easily reproducible by other astronomers. Using simulations we determine the best SExtractor parameters for the detection of faint galaxies in deep Hubble Space Telescope observations, paying special attention to the issue of deblending, which significantly affects the normalization and shape of the number count distribution. We confirm, as claimed by Bernstein, Freedman, & Madore, that Kron-like magnitudes, such as the ones generated by SExtractor, can miss more than half of the light of faint galaxies, what dramatically affects the slope of the number counts. We show how to correct for this effect, which depends sensitively not only on the characteristics of the observations, but also on the choice of SExtractor parameters. We present catalogs for the VV 29 and NGC 4676 fields with photometry in the F475W, F606W, and F814W bands. We also show that combining the Bayesian software BPZ with superb ACS data and new spectral templates enables us to estimate reliable photometric redshifts for a significant fraction of galaxies with as few as three filters. After correcting for selection effects, we measure slopes of 0:32 AE 0:01 for 22 < g F475W < 28, 0:34 AE 0:01 for 22 < V F606W < 27:5, and 0:33 AE 0:01 for 22 < m F814W < 27. The counts do not flatten (except perhaps in the F475W filter), up to the depth of our observations. Our results agree well with those of Bernstein, Freedman, & Madore, who used different data sets and techniques, and show that it is possible to perform consistent measurements of galaxy number counts if the selection effects are properly considered. We find that the faint counts m AB > 25:5 can be well approximated in all our filters by a passive luminosity evolution model based on the COMBO-17 luminosity function (¼ À1:5), with a strong merging rate following the prescription of Glazebrook et al., Ã / ð1 þ QzÞ, with Q ¼ 4.